Pasteur’s Crystals and the Beauty of Simplicity
https://reasonandscience.catsboard.com/t2668-pasteurs-crystals-and-the-beauty-of-simplicity
Paris, 1848—A young French scientist named Louis Pasteur is investigating the shapes of crystals. He is puzzled as to why different substances form crystals of the same shape, while at the same time some identical substances form crystals of different shapes. He suspects these properties are related to the shapes of the constituent molecules. His studies of crystalline salts prepared from the by-products of wine-making thus lead Pasteur to a crucial insight about the threedimensional structures of carbon-based molecules. 1
The American Chemical Society polled experts some time ago to identify the most beautiful experiment in the history of chemistry, they responded by giving the highest ranking to Louis Pasteur’s separation of mirror-image molecular forms of tartaric acid, conducted just a year after Pasteur gained his doctorate at the Ecole Normale in Paris. The experiment apparently lays claim to a special beauty because it was conceptually simple yet painstakingly executed, and because it revealed a deep and important truth in chemistry in a way that was clear-cut and unambiguous. Pasteur’s insight was a stroke of genius that arrived as a eureka-like revelation. It may come as a disappointment, then, to learn that things did not, in all probability, happen that way. The reality was more like science as it is usually conducted: a rather mundane and clumsy affair in which the significance (or otherwise) of one’s results becomes apparent only in retrospect. It is understandable that the history books should be misleading, however, because they quite reasonably assumed that they could rely on the first-hand account of the experiment given retrospectively by Pasteur himself – a story that was subsequently embellished, with Pasteur’s blessing, by his son-in-law in the first of several hagiographies of this towering figure of French science. We’d do well to heed the advice of science historian Gerald Geison, who has carried out the detective work that reveals the real story of Pasteur’s first great discovery: ‘to be ever skeptical of participants’ retrospective accounts of scientific discovery’. While this was by no means the only case of self-mythologizing in nineteenthcentury science, few people were more adept at arranging facts to suit their own ends than Louis Pasteur.
Crystal gazing
There is no rule which says that great scientists have to be likeable, and for Pasteur that is just as well. It is not that he was especially monstrous or wicked, but his arrogance and lack of charity do not create an endearing portrait. He was energetic and skilful at selfpromotion, and somewhat calculating in the relationships he fostered (and in those he neglected). Only grudgingly did he give credit to others, but he was not slow to claim it for himself. Despite his tremendous selfassurance, he was easily stung by criticism. Yet he dished it out freely and sometimes vindictively, to the extent that one octogenarian French scientist was sufficiently enraged to challenge him to a duel (whichwas, fortunately, not enacted). Perhaps more damagingly for Pasteur’s scientific reputation, he was willing to rewrite history and was not averse to a rather biased selection of the evidence to support his ideas. But, despite all that, his genius is undeniable. It is after all commonly the case that genius must be accompanied by a degree of assertiveness if it is to be recognized at all. Although remembered by posterity primarily for his biological discoveries – he more or less founded the field of microbiology, and made pioneering contributions to immunology and the technique of vaccination – and although lauded by chemists, Pasteur actually began his career in physics. Like several later physical scientists who turned their attention to biology – Linus Pauling and Francis Crick come to mind – Pasteur’s initial enthusiasm was for crystallography, the study of crystals.
Born in Dôle in the region of Franche-Comte´, Pasteur went to Paris in 1844 to study for his doctorate at the prestigious Ecole Normale. One of his prime mentors there was Gabriel Delafosse, who had been a student of the great crystallographer Abbé René Just Hauy. When Hauy formulated his ideas about the shapes of crystals in the 1780s, it was commonly believed that these reflect the shapes or arrangements of their constituent particles – a notion that went back at least as far as Johannes Kepler’s speculations on the hexagonal symmetry of snowflakes in the early seventeenth century. Hauy believed that the form of every crystal could be traced back to the shape of a fundamental grouping of its particles, which he called an ‘integrant molecule’. This can be seen as analogous to the modern idea of a ‘unit cell’, the simplest block of atoms or molecules that repeats again and again throughout the crystal like bricks in a wall. In essence, Hauy’s idea seemed to imply that the shape of a crystal depends on the molecules from which it is made. Chemists at the beginning of the nineteenth century had no clear idea of what molecules were, but John Dalton’s atomic theory helped them to envisage compounds – substances that contained more than one element – as being composed of discrete groupings of atoms in fixed ratios. In 1818 the French chemist Michel Eugene Chevreul defined different chemical substances in terms of the kind, number and spatial arrangement of atoms in these molecules. But no one gave much thought to the spatial arrangement, for they knew of no way to investigate it; indeed, many chemists considered the molecular models of coloured wooden balls and sticks that became popular with chemistry lecturers as mere heuristic devices that should not be considered to bear any
relation to the way actual molecules looked. They fretted that such ‘toys’ would be taken too literally by the uninformed. Even in 1869, William Crookes advised students to ‘leave atoms and molecules alone for the present. Nobody knows how the atoms are arranged.’
Twisted logic
It had been known since the seventeenth century that some crystals have a curious effect on polarized light. When normal light is passed through a polarizing filter, the electromagnetic waves in the emergent rays all oscillate in the same plane, like so many snakes writhing in the horizontal plane of the ground. The Dutch scientist Christiaan Huygens, Isaac Newton’s great rival in the study of light, found that when such polarized light is passed through the transparent mineral Iceland spar, the plane of polarization is rotated by a well-defined angle (Figure below).
This behaviour became known as optical activity. Quartz has the same effect, and it was widely assumed that optical activity somehow results from the spatial arrangement of the molecules in a crystal – they act rather like a spiral staircase that turns the polarization through a certain angle. Some materials will rotate the plane of polarization to the left, some to the right: there is a ‘handedness’, an asymmetry, to the effect. In 1815 the French physicist Jean-Baptiste Biot found that some crystalline organic substances remain optically active when they are dissolved in solution. This apparently undermined the ‘spiral-staircase’ idea, showing in effect that the individual rungs of the staircase were themselves active: it wasn’t a question of how they were assembled in the solid. Biot argued that the property of optical activity must be inherent in the organic molecules themselves, and he proposed that they had a twisted configuration of their atoms. It has often been implied that Pasteur’s doctoral thesis at the Ecole Normale was an attempt to uncover the relation between optical activity and molecular ‘twistedness’. But that wasn’t so. Pasteur did indeed study this phenomenon in solutions of organic compounds, but the conclusion of his thesis was essentially a restatement of Biot’s idea:
I regard as extremely probable that the mysterious and unknown disposition of physical molecules, in a whole crystal of quartz, is also found in [optically] active bodies, but, this time, in each molecule in particular; that it is each molecule, taken separately in an active body, that must be compared, for the arrangement of its parts, with a complete crystal of quartz.
One year after he wrote these words, however, Pasteur had something much more momentous to add. Under Delafosse’s guidance, he investigated the shapes of crystals. Hau¨ y’s proposal that a crystal form is dictated by the nature of its constituent molecules was challenged by two discoveries made by the German scientist Eilhardt Mitscherlich in 1819–21. First, compounds composed of different atoms can show the same crystal shape – a
property called isomorphism. Second, some compounds can crystallize in more than one crystal form, which was known as polymorphism. It rather looked, then, as though crystalline form was independent of chemical constitution.
In the 1840s, Delafosse was trying to revise his mentor Hauy’s theory so that it could accommodate isomorphism and polymorphism, and he set Pasteur to work on the problem. One example of isomorphism that had particularly interested Mitscherlich was the case of the salts of the organic compounds tartaric and racemic acid. Tartaric acid salts, or tartrates, were long known as by-products of wine-making: they were precipitated as white solids on the walls of wine casks. Paracelsus in the sixteenth century named these precipitates ‘tartar’, and in the seventeenth century the French apothecary Pierre Seignette of La Rochelle identified one of these salts as sodium potassium tartrate.* Carl Wilhelm Scheele showed in 1770 that this ‘Seignette salt’ could be converted into an acidic substance, the first known organic acid, called tartaric acid. This mild acid became an industrially useful substance, used in textile manufacture and medicine. Wine makers began to collect and sell it as a profitable sideline.
That was how an industrialist named Kestner discovered racemic acid at his factory at Thann in Alsace. It appeared to be simply a second kind of tartaric acid, with the same elemental composition and almost identical properties, and Kestner gave some of it to the French chemist Joseph Louis Gay-Lussac to study. It was Gay-Lussac who named it after the Latin racemus: a bunch of grapes. But Jo¨ ns Jacob Berzelius, a doyen of chemical nomenclature, called this new compound paratartaric acid, and decided that it was an isomer of tartaric acid: that is to say, the molecules contained the same atoms, but differently arranged. Although Mitscherlich decided that salts of tartaric and racemic acid were isomorphous, he was hard pushed to find any real difference at all between the two substances. Their crystals looked identical, and there seemed to be only one way of distinguishing them: tartaric acid (or a solution of a tartrate) was optically active while racemic acid was not. According to Biot’s view, this must mean that molecules of tartaric acid were ‘twisted’ while those of racemic acid were not. Mitscherlich told Biot what he had found, and the Frenchman repeated his experiment and confirmed the result in 1844. Browsing in the library at the Ecole Normale in 1848, Pasteur happened to see Biot’s report, and he wondered how it could be that tartaric acid and racemic acid could give rise to salts with isomorphous crystals, identical in shape, if the molecules of the organic acids themselves had quite different shapes.
Were the crystals really identical? That’s what Mitscherlich had said. But Pasteur decided to check. This is how Pasteur began his first great experiment. What was he really looking for, and what did he expect to find? In a retrospective lecture in 1860, Pasteur explained that he wanted to discover why it was that the two acids differed in their optical activity. But Geison has inspected his lab books, which initially make no mention of optical
activity. Instead, Pasteur is caught up with a recondite question about how the isomorphism of salts of the two acids might be connected to the number of water molecules that get ‘locked up’ in their crystalline lattices. There is no grand hypothesis – his notes look like the typical records of a young researcher, rather dull collections of tables and measurements accompanied by marginal notes that signify a desperate quest to find some logical order to it all.
Pasteur used a microscope to inspect the shapes of crystals of sodium ammonium tartrate and ‘paratartrate’ (that is, the corresponding salt of racemic acid). He had learnt how to recognize and classify the facets of crystals, and he found that, just as Mitscherlich had said, these salt crystals were ‘asymmetric’: they had a handedness. If you start with a symmetrical crystal shape like a cube, and cut off some corners but not others, you can produce left- or right-handed shapes that are mirror images of one another. Such crystal forms are said to be hemihedral. The tartrate salt crystals were always of the right-handed hemihedral form. But Pasteur recorded in his notebook that for the paratartrate salt, ‘The crystals are frequently hemihedral to the left, frequently to the right’ (Figure below).
He added that sometimes the truncated corners ‘repeat’, so that left-handedness and right-handedness cancel one another out to produce symmetrical crystals. But he later crossed out this comment. It looks like a routine observation – puzzling, perhaps, but just a part of the wider puzzle. Compare this with the version of events that Pasteur later gave as the ‘official’ record. He asserted that, knowing that tartrate was optically active but paratartrate was not, he suspected that Mitscherlich had been wrong to say their crystals were identical. He thought that, on the contrary, the paratartrate crystals would prove to be symmetrical, with no handedness. When he looked and found that they were after all asymmetric, Pasteur claimed that ‘for an instant my heart stopped beating’. But then he looked more closely and noticed that they were asymmetric in both senses: some were lefthanded, and some right-handed, in contrast to the single handedness of tartrate. And so (Pasteur said), the answer to this riddle struck him in a flash. Right-handed tartrate revolved the plane of polarized light. Paratartrate, seemingly a mixture of left-handed and right-handed forms, left the light unchanged. Could this be because the effects of the two forms cancel each other out? That is to say, maybe the righthanded paratartrate crystals were composed of right-handed molecules, and the left-handed crystals of left-handed molecules, and the two were present in solutions of paratartrate in equal measure. In that case, the molecules in the left-handed crystals would, if separated and dissolved, rotate polarized light one way, and the righthanded molecules would rotate it equally in the other direction. All one had to do was separate the two types of crystal. Pasteur did this by hand, wielding tweezers to patiently sort the crystals into two piles. Then he redissolved them – and there it was, the very result he’d anticipated. In one account, he is said to have exclaimed ‘Tout est trouve´!’ – all is found. Other reports are yet more colourful: Rene´ Vallery-Radot, Pasteur’s son-in-law, claims that Pasteur rushed from his lab, grabbed a hapless curator in the corridors of the Ecole, and dragged him into the Luxembourg Gardens to tell him about the great discovery. Pasteur was not a whimsical man; he maintained an air of stiff bourgeois respectability, sharpened by a rather imperious and abrupt manner. It is hard to imagine him, even in his youth, prancing about gaily with excitement. But Pasteur was happy for Vallery-Radot to tell history this way (he saw the proofs before the biography was published), and if it is hyperbole then it is not so very unusual for its time. Victorian scientists frequently indulged in romanticized retelling of their discoveries: they seemed to feel that a bit of mythologizing would do science (and them) no harm. We have to be comparably cautious about Pasteur’s description of subsequent events. The head of his laboratory, Jerome Balard, conveyed the remarkable discovery to Biot, who asked to see the young man responsible for it. Brought into the presence of the great French scientist at the Colle`ge de France, Pasteur was requested to repeat his experiment before Biot’s eyes. He did so, whereupon (according to Pasteur) Biot, ‘visibly moved, took me by the arm and said: ‘‘My dear boy, I have loved science so much all my life that this stirs my heart.’’’ Yet even if the exact words were not so epigrammatic, the sentiment may have been much the same, for Biot subsequently regarded Pasteur as his prote´ge´. It is not in fact clear that Pasteur had given any thought to the relationship between optical activity and crystal shape in the tartrates and paratartrates until he performed his laborious separation of crystals – certainly, when he looked closely at their shapes, he does not appear to have formulated a strong hypothesis about whether paratartrate would have symmetric or asymmetric crystals. So we must doubt that his heart truly skipped a beat at what the microscope revealed. In addition, having prepared his two solutions of right- and left-handed paratartrate, his measurements of optical activity were not, so to speak, crystal-clear: the solutions did rotate polarized light in opposite senses, but not to identical amounts. Reasonably enough, Pasteur put this down to his limited ability to distinguish the two crystal types under the microscope*, but that cool comment doesn’t exactly support the idea of a eureka-like epiphany.
Mirror matter
Be that as it may, Pasteur saw with perfect clarity what his observations implied. Paratartaric or racemic acid was not a single molecular substance, but two: it was a mixture of mirror-image molecules. The right-handed versions were identical to the molecules in tartaric acid; the left-handed molecules were in effect tartaric acid twisted the other way, an isomer with equal but opposite optical activity. Pasteur introduced the word ‘racemic’ as a general term for an equal mixture of both isomers of an optically active compound. He also understood what the notion of handedness must imply for a molecule’s structure. One possibility was that the atoms were arranged in a screw-like pattern, a spiral staircase turning to the right or the left. This is indeed the atomic arrangement that makes quartz (silicon dioxide) optically active. But Pasteur realised that there were other options for organic molecules, as he explained in 1860:
Are the atoms of the right-handed acid grouped along the turns of a right-handed helix, or situated at the vertices of an irregular tetrahedron, or disposed according to such and such a fixed asymmetric [dissymmetrique] arrangement? We would not know how to answer these questions. But there can be no doubt that there is a grouping of the atoms of an asymmetric type that is not superposable on its mirror image.
In 1874 the Dutch scientist Jacobus van’t Hoff did offer an answer. He suggested that the stick diagrams that chemists had begun to use to depict organic molecular structures as an arrangement of atoms in space should be extended into the third dimension, out of the flat printed page. Such drawings had evolved from a mere graphical representation of molecular categories – a way of showing how different molecules were related to one another – to the point where they were regarded (by some researchers, at least) as showing the actual shapes of molecules. The atoms were represented by chemical symbols – C for carbon, H for hydrogen, O for oxygen – and the links between them were shown as lines or dashes. It was recognized since the work of Friedrich August Kekule´ in 1858 that carbon atoms generally form bonds to four other atoms. (The Scottish scientist Archibald Couper had the same idea independently, and would have published it first had his paper not been delayed by editors.) But van’t Hoff proposed that, instead of having their four bonds arranged as a flat cross, carbon atoms disport their bonds pointing to the corners of a tetrahedron:
For carbon atoms that have four different atoms or groups of atoms attached (and this is true of the two central carbons in tartaric acid) there are two different ways to arrange the substituents in space, which are not superimposable. These are mirror images of one another:
This, said van’t Hoff, is how organic compounds can acquire a handedness that results in their optical activity. Just a month after he published this idea, Joseph Le Bel in Paris clarified what it implies: any molecule with an asymmetric carbon atom – one with four different groups attached – will be optically active unless there is another equivalent grouping in the molecule that cancels its effect. In the latter case, the molecule becomes symmetric, so that it is its own mirror image: it is no longer ‘handed’, and no longer rotates polarized light. Tartaric acid has such a molecular form, called the meso form, as well as the optically active right- and left-handed forms. A molecule’s ‘stereochemistry’ refers to the arrangement of its atoms in three-dimensional space: molecules containing the same atoms and the same connections between them, but different three-dimensional shapes, are known as stereoisomers. Two stereoisomers that are mirror images of one another are called enantiomers (from the Greek enantios, opposite). Lord Kelvin decided that the property of molecular handedness should itself be dignified with a classical name, and in 1904 he derived one from the Greek word for hand, kheir: this sort of asymmetry is called chirality. Thus the two optically active forms of tartaric acid are chiral molecules, and enantiomers.
Pasteur’s famous dictum that ‘luck favours the prepared mind’ is often quoted, but it is possible to read it as a defence against the accusation that he just got lucky himself. He elected to study tartaric and racemic acids because Mitscherlich had discussed them as an example of isomorphism. He could not have known that racemic acid is one of the very few chiral organic compounds that will spontaneously separate into its two enantiomers as it crystallizes – most other such compounds will forms crystals in which the two enantiomers are mixed together. Moreover, few other organic compounds will form crystals so large that they can be readily inspected with a nineteenth-century microscope. And even racemic acid requires the right conditions to separate in this way: it happens only at relatively low temperatures, and so it was lucky that Pasteur happened to be carrying out his experiments in an unheated Paris laboratory in February. Had it been July, the result might have been quite different.
Race for the racemate
In late 1848 Pasteur left Paris to take up a teaching position at the lyce´e in Dijon, but this seemed to him to be a remote backwater, and he eagerly accepted a subsequent offer from the University of Strasbourg, where he began work in January 1849. He was sure that his discovery of what would later be called molecular chirality had yet more fruits to bear. He asked Kestner at Thann for more racemic acid, which the industrialist duly provided; Kestner subsequently became Pasteur’s trusted supplier and correspondent. Pasteur figured that Kestner’s winemaking process first produced only right-handed tartaric acid, which subsequently became converted into racemic acid. But how could that happen? It meant that precisely half of the molecules had to acquire an opposite twist. ‘It would be necessary’, he said, ‘to find an agent capable of producing this molecular modification on tartaric acid.’ But how do you alter the twist of a molecule? The problem was so profound that in 1851 the Socie´te´ de Pharmacie in Paris offered a prize of 1500 francs to anyone who could explain the origin of racemic acid. The ambitious Pasteur was less interested in the money than in the fact that winning this prize would enhance his fame. So he set out to crack the puzzle. During 1852 he travelled all over Europe, visiting laboratories and industrial plants to gather clues. Yet the quest seemed hopeless, and in January of 1853 Pasteur decided that the transformation of tartaric into racemic acid looked impossible. Only six months later, however, he had his answer. He studied several other optically active organic compounds to look for a relation between molecular asymmetry and crystal form (hemihedry). The similarity between crystals of tartaric acid and those of an organic compound known as malic acids suggested to him that their molecular structures might be related. That is to say, they might both contain the same chiral ‘core’. ‘If there exists a common molecular grouping between the right-handed tartaric acid and the known malic acid of the mountain ash’, Pasteur wrote:
" There is necessarily a common molecular grouping between the lefthanded tartaric acid and a malic acid as yet unknown, and which would be to the malic acid known to chemists, what left-handed tartaric acid is to right-handed tartaric acid. "
Maybe, then, nature made racemic acid not by reversing half of right-handed tartaric acid but by making both left- and right-handed forms from corresponding precursor molecules (such as malic acid) and then mixing them. But what about the precursors – where did their handedness come from? Pasteur came across an extraordinary claim by a chemist named Victor Dessaignes at Vendoˆme, who said that he had made malic acid from two other organic acids, maleic and fumaric acids. But those starting materials were optically inactive, whereas malic acid was optically active. ‘Now to this day’, Pasteur commented, ‘no optically active substance has ever been prepared by laboratory procedures, starting with substances which are not optically active.’ So could Dessaignes be mistaken? Pasteur repeated his experiment, and found that it produced optically inactive malic acid. He concluded that this form of the compound was somehow ‘untwisted’: he called this mesomalic acid, introducing the ‘meso’ terminology. Now, as it happens, malic acid does, like tartaric acid, have an optically inactive meso form. But that’s not what Pasteur made using Dessaignes’ procedure – rather, he made a racemic mixture of left- and right-handed malic acid. Unaware of this, he set out to see whether he could make the tartaric acid analogue of what he deemed to be his mesomalic acid. During these studies, he found by chance that the optically active compound of tartaric acid, combined with a substance called cinchonia – an extract of the bark of the cinchona tree, which was the source of the antimalarial drug quinine – would, when heated, become transformed into racemic acid. By luck (and, of course, mental preparation) he had stumbled onto the solution that won him the Socie´te´ de Pharmacie prize.
Chirality and life
These experiments hint at how Pasteur was starting to think about molecular chirality. At first, his claim that a chiral molecule could not be prepared from an achiral one rested simply on careful reasoning based on what was known about such chemical transformations (notice that his investigation of Dessaignes’ claim did not in the end seem to him to contradict this notion). Gradually, however, this property of chirality became for Pasteur an article of faith. He was convinced that there was an impermeable barrier between chiral and achiral molecular forms – and, moreover, he thought that this same barrier separated the living from the inanimate world. Molecular chirality, he supposed, was uniquely a property of living systems.
‘Every chemical substance’, Pasteur wrote:
" whether natural or artificial, falls into one of two major categories, according to a spatial characteristic of its form. The distinction is between those substances that have a plane of symmetry and those that do not. The former belong to the mineral, the latter to the living world. "
This strong assertion touched on the protean notion of vitalism – the idea that there was a ‘vital force’ that quickened life, and which made living or organic matter fundamentally different from inorganic matter. By the mid-nineteenth century, the vitalistic hypothesis was looking distinctly embattled. But the debate was far from settled, despite later claims about the influence of Friedrich Wo¨hler’s synthesis of urea in 1828. Pasteur was no vitalist – indeed, his own work on microbes helped to dispel the belief that a vital force caused the spontaneous generation of organisms in decaying matter – but he was keen to make a fundamental distinction between the organic and the inorganic, based on the idea of chirality.
So where did this handedness of life come from in the first place? Pasteur reasoned that it must be impressed on organic matter during its formation by the action of some all-pervasive chiral force. He illustrated how such a mechanical force might be manifested by analogy with a left- or right-handed screw being driven into a wooden block ‘in which the fibres themselves have a right or left spiral arrangement’. In one case the screw would go in easily; in the other, only with difficulty. ‘Is it not necessary and sufficient’, he concluded, ‘to admit that at the moment of the elaboration of the immediate principles in the vegetable organism [in other words, at the moment the molecules are formed], an asymmetric force is present?’ Pasteur speculated that this force could be imposed by the environment, perhaps as an electric or magnetic field:
" Do such asymmetric agencies arise from the cosmic influences light, electricity, magnetism, heat? Do they perhaps stand in relation with the movement of the earth, with the electric currents by means of which physicists explain the earth’s magnetic poles? "
If so, then maybe these forces could be reversed, inverting the chirality of natural organic molecules. Consequently, in 1853 Pasteur embarked on a series of experiments that now look decidedly cranky: crystallizing substances in magnetic fields, growing plants from seeds irradiated with sunlight that was ‘inverted’ by reflection from mirrors. Biot implored him to abandon this strange quest, and indeed Pasteur himself admitted that ‘One has to be a little mad to undertake what I am trying to do now.’ Yet chirality led him in more productive directions too. In 1854 he left Strasbourg to work at Lille, where there was an active brewing and distilling industry, and he was asked to help with some of the problems the industrialists encountered during the fermentation process. Fermentation was itself a disputed matter at that time. In 1837 Baron Charles Cagniard de la Tour proposed that the fermentation of sugar, by which means it is converted into alcohol, depends on the action of yeast: that is, of microscopic organisms. But others disagreed. The eminent German chemist Justus von Liebig suggested that yeast might be just a by-product of fermentation, and was not essential to what was basically a chemical process. Berzelius argued that the role of yeast was simply to ‘awaken slumbering affinities’ between the chemical compounds involved in fermentation: he gave this ‘assisted’ breakdown of substances a new name: catalysis. (Liebig’s colleague Oerhardt complained that ‘Calling the phenomenon catalytic does not explain it, it only replaces a name of ordinary language by a Greek word.’) Pasteur was drawn to study fermentation partly because one of the products, amyl alcohol, was chiral.
This, he thought, could only be produced by the action of living organisms: fermentation could not be simply ‘chemical’. He observed that the fermentation of sour milk into lactic acid – which happens spontaneously if milk is left to go off – produces tiny grey particles that, if extracted from the ferment, are then capable of converting sugar into lactic acid too. He decided that this ‘lactic yeast’ was a microorganism that multiplied as a result of fermentation, from which it derived sustenance. In other words, different kinds of microorganism gave rise to different modes of fermentation. This sowed the seeds for the discipline of microbiology, as well as paving the way to the discovery, at the end of the nineteenth century, of nature’s molecular catalysts: enzymes, the key agents of biochemistry. In late 1857, Pasteur studied the fermentation of tartaric acid when it was left to age. He found that, with racemic acid, only the righthanded isomer was fermented. This showed him that living organisms made precise distinctions between chiral molecules: ‘We see here the character of molecular asymmetry peculiar to organic substances intervene as a modifier of affinity [that is, of chemical reactivity].
’ Pasteur discovered that our own physiology makes similar distinctions: one enantiomer of a chiral sugar may produce a sensation of sweetness while the other barely registers. ‘When active asymmetric substances are involved in producing an impression on the nerves’, he said, ‘their effort is translated by sweet taste in one case and almost no taste in the other.’ We now know that this exquisite discrimination in molecular physiology is governed by the ability of enzymes to ‘feel’ the difference between mirror-image molecular shapes. One consequence is that our bodies can only metabolize right-handed sugars, which are the naturally occurring forms. Because our sweet taste buds can nevertheless be stimulated by some left-handed sugars (such as left-handed glucose), these are manufactured industrially as artificial sweeteners, which give the taste without the calories of normal sugar. The sensitivity of enzymes to chirality also accounts for nature’s enviable ability to generate just one of the two enantiomeric forms of a chiral compound. Chemists today still find it a tremendous challenge to design synthetic catalysts that can effect comparably selective chemical transformations.
Yet it is the very selectivity of the body’s biochemistry that creates a demand for such catalysts. While some chiral compounds are simply physiologically inactive in one enantiomeric form, others can elicit more troublesome responses. The most notorious example is the drug thalidomide, which was administered to pregnant women in the 1950s and 1960s to suppress morning sickness. While the right-handed form acts as a sedative and an anti-inflammatory agent, the left-handed form causes fetal growth abnormalities. Because the marketed drug contained a mixture of the two enantiomers*, it led to a wide incidence of birth defects in babies born in the early 1960s. The thalidomide case made it clear that one must distinguish carefully between the physiological effects of both enantiomers of chiral drugs, and many drugs now have to be prepared in enantiomerically pure form. This has led to the development of methods of separating enantiomers (something that Pasteur also pioneered), although the ideal and most economical solution is to make only one enantiomer in the first place: hence the demand for enantiomerically selective catalysts. Several such manufacturing processes use natural enzymes,either extracted from microorganisms or within whole cells bred in fermentation vats, to conduct these delicate operations – for if Pasteur was wrong to think that only nature can generate chiral molecules, nevertheless it remains true that she is a lot better at it than we are.
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https://reasonandscience.catsboard.com/t2668-pasteurs-crystals-and-the-beauty-of-simplicity
Paris, 1848—A young French scientist named Louis Pasteur is investigating the shapes of crystals. He is puzzled as to why different substances form crystals of the same shape, while at the same time some identical substances form crystals of different shapes. He suspects these properties are related to the shapes of the constituent molecules. His studies of crystalline salts prepared from the by-products of wine-making thus lead Pasteur to a crucial insight about the threedimensional structures of carbon-based molecules. 1
The American Chemical Society polled experts some time ago to identify the most beautiful experiment in the history of chemistry, they responded by giving the highest ranking to Louis Pasteur’s separation of mirror-image molecular forms of tartaric acid, conducted just a year after Pasteur gained his doctorate at the Ecole Normale in Paris. The experiment apparently lays claim to a special beauty because it was conceptually simple yet painstakingly executed, and because it revealed a deep and important truth in chemistry in a way that was clear-cut and unambiguous. Pasteur’s insight was a stroke of genius that arrived as a eureka-like revelation. It may come as a disappointment, then, to learn that things did not, in all probability, happen that way. The reality was more like science as it is usually conducted: a rather mundane and clumsy affair in which the significance (or otherwise) of one’s results becomes apparent only in retrospect. It is understandable that the history books should be misleading, however, because they quite reasonably assumed that they could rely on the first-hand account of the experiment given retrospectively by Pasteur himself – a story that was subsequently embellished, with Pasteur’s blessing, by his son-in-law in the first of several hagiographies of this towering figure of French science. We’d do well to heed the advice of science historian Gerald Geison, who has carried out the detective work that reveals the real story of Pasteur’s first great discovery: ‘to be ever skeptical of participants’ retrospective accounts of scientific discovery’. While this was by no means the only case of self-mythologizing in nineteenthcentury science, few people were more adept at arranging facts to suit their own ends than Louis Pasteur.
Crystal gazing
There is no rule which says that great scientists have to be likeable, and for Pasteur that is just as well. It is not that he was especially monstrous or wicked, but his arrogance and lack of charity do not create an endearing portrait. He was energetic and skilful at selfpromotion, and somewhat calculating in the relationships he fostered (and in those he neglected). Only grudgingly did he give credit to others, but he was not slow to claim it for himself. Despite his tremendous selfassurance, he was easily stung by criticism. Yet he dished it out freely and sometimes vindictively, to the extent that one octogenarian French scientist was sufficiently enraged to challenge him to a duel (whichwas, fortunately, not enacted). Perhaps more damagingly for Pasteur’s scientific reputation, he was willing to rewrite history and was not averse to a rather biased selection of the evidence to support his ideas. But, despite all that, his genius is undeniable. It is after all commonly the case that genius must be accompanied by a degree of assertiveness if it is to be recognized at all. Although remembered by posterity primarily for his biological discoveries – he more or less founded the field of microbiology, and made pioneering contributions to immunology and the technique of vaccination – and although lauded by chemists, Pasteur actually began his career in physics. Like several later physical scientists who turned their attention to biology – Linus Pauling and Francis Crick come to mind – Pasteur’s initial enthusiasm was for crystallography, the study of crystals.
Born in Dôle in the region of Franche-Comte´, Pasteur went to Paris in 1844 to study for his doctorate at the prestigious Ecole Normale. One of his prime mentors there was Gabriel Delafosse, who had been a student of the great crystallographer Abbé René Just Hauy. When Hauy formulated his ideas about the shapes of crystals in the 1780s, it was commonly believed that these reflect the shapes or arrangements of their constituent particles – a notion that went back at least as far as Johannes Kepler’s speculations on the hexagonal symmetry of snowflakes in the early seventeenth century. Hauy believed that the form of every crystal could be traced back to the shape of a fundamental grouping of its particles, which he called an ‘integrant molecule’. This can be seen as analogous to the modern idea of a ‘unit cell’, the simplest block of atoms or molecules that repeats again and again throughout the crystal like bricks in a wall. In essence, Hauy’s idea seemed to imply that the shape of a crystal depends on the molecules from which it is made. Chemists at the beginning of the nineteenth century had no clear idea of what molecules were, but John Dalton’s atomic theory helped them to envisage compounds – substances that contained more than one element – as being composed of discrete groupings of atoms in fixed ratios. In 1818 the French chemist Michel Eugene Chevreul defined different chemical substances in terms of the kind, number and spatial arrangement of atoms in these molecules. But no one gave much thought to the spatial arrangement, for they knew of no way to investigate it; indeed, many chemists considered the molecular models of coloured wooden balls and sticks that became popular with chemistry lecturers as mere heuristic devices that should not be considered to bear any
relation to the way actual molecules looked. They fretted that such ‘toys’ would be taken too literally by the uninformed. Even in 1869, William Crookes advised students to ‘leave atoms and molecules alone for the present. Nobody knows how the atoms are arranged.’
Twisted logic
It had been known since the seventeenth century that some crystals have a curious effect on polarized light. When normal light is passed through a polarizing filter, the electromagnetic waves in the emergent rays all oscillate in the same plane, like so many snakes writhing in the horizontal plane of the ground. The Dutch scientist Christiaan Huygens, Isaac Newton’s great rival in the study of light, found that when such polarized light is passed through the transparent mineral Iceland spar, the plane of polarization is rotated by a well-defined angle (Figure below).
This behaviour became known as optical activity. Quartz has the same effect, and it was widely assumed that optical activity somehow results from the spatial arrangement of the molecules in a crystal – they act rather like a spiral staircase that turns the polarization through a certain angle. Some materials will rotate the plane of polarization to the left, some to the right: there is a ‘handedness’, an asymmetry, to the effect. In 1815 the French physicist Jean-Baptiste Biot found that some crystalline organic substances remain optically active when they are dissolved in solution. This apparently undermined the ‘spiral-staircase’ idea, showing in effect that the individual rungs of the staircase were themselves active: it wasn’t a question of how they were assembled in the solid. Biot argued that the property of optical activity must be inherent in the organic molecules themselves, and he proposed that they had a twisted configuration of their atoms. It has often been implied that Pasteur’s doctoral thesis at the Ecole Normale was an attempt to uncover the relation between optical activity and molecular ‘twistedness’. But that wasn’t so. Pasteur did indeed study this phenomenon in solutions of organic compounds, but the conclusion of his thesis was essentially a restatement of Biot’s idea:
I regard as extremely probable that the mysterious and unknown disposition of physical molecules, in a whole crystal of quartz, is also found in [optically] active bodies, but, this time, in each molecule in particular; that it is each molecule, taken separately in an active body, that must be compared, for the arrangement of its parts, with a complete crystal of quartz.
One year after he wrote these words, however, Pasteur had something much more momentous to add. Under Delafosse’s guidance, he investigated the shapes of crystals. Hau¨ y’s proposal that a crystal form is dictated by the nature of its constituent molecules was challenged by two discoveries made by the German scientist Eilhardt Mitscherlich in 1819–21. First, compounds composed of different atoms can show the same crystal shape – a
property called isomorphism. Second, some compounds can crystallize in more than one crystal form, which was known as polymorphism. It rather looked, then, as though crystalline form was independent of chemical constitution.
In the 1840s, Delafosse was trying to revise his mentor Hauy’s theory so that it could accommodate isomorphism and polymorphism, and he set Pasteur to work on the problem. One example of isomorphism that had particularly interested Mitscherlich was the case of the salts of the organic compounds tartaric and racemic acid. Tartaric acid salts, or tartrates, were long known as by-products of wine-making: they were precipitated as white solids on the walls of wine casks. Paracelsus in the sixteenth century named these precipitates ‘tartar’, and in the seventeenth century the French apothecary Pierre Seignette of La Rochelle identified one of these salts as sodium potassium tartrate.* Carl Wilhelm Scheele showed in 1770 that this ‘Seignette salt’ could be converted into an acidic substance, the first known organic acid, called tartaric acid. This mild acid became an industrially useful substance, used in textile manufacture and medicine. Wine makers began to collect and sell it as a profitable sideline.
That was how an industrialist named Kestner discovered racemic acid at his factory at Thann in Alsace. It appeared to be simply a second kind of tartaric acid, with the same elemental composition and almost identical properties, and Kestner gave some of it to the French chemist Joseph Louis Gay-Lussac to study. It was Gay-Lussac who named it after the Latin racemus: a bunch of grapes. But Jo¨ ns Jacob Berzelius, a doyen of chemical nomenclature, called this new compound paratartaric acid, and decided that it was an isomer of tartaric acid: that is to say, the molecules contained the same atoms, but differently arranged. Although Mitscherlich decided that salts of tartaric and racemic acid were isomorphous, he was hard pushed to find any real difference at all between the two substances. Their crystals looked identical, and there seemed to be only one way of distinguishing them: tartaric acid (or a solution of a tartrate) was optically active while racemic acid was not. According to Biot’s view, this must mean that molecules of tartaric acid were ‘twisted’ while those of racemic acid were not. Mitscherlich told Biot what he had found, and the Frenchman repeated his experiment and confirmed the result in 1844. Browsing in the library at the Ecole Normale in 1848, Pasteur happened to see Biot’s report, and he wondered how it could be that tartaric acid and racemic acid could give rise to salts with isomorphous crystals, identical in shape, if the molecules of the organic acids themselves had quite different shapes.
Were the crystals really identical? That’s what Mitscherlich had said. But Pasteur decided to check. This is how Pasteur began his first great experiment. What was he really looking for, and what did he expect to find? In a retrospective lecture in 1860, Pasteur explained that he wanted to discover why it was that the two acids differed in their optical activity. But Geison has inspected his lab books, which initially make no mention of optical
activity. Instead, Pasteur is caught up with a recondite question about how the isomorphism of salts of the two acids might be connected to the number of water molecules that get ‘locked up’ in their crystalline lattices. There is no grand hypothesis – his notes look like the typical records of a young researcher, rather dull collections of tables and measurements accompanied by marginal notes that signify a desperate quest to find some logical order to it all.
Pasteur used a microscope to inspect the shapes of crystals of sodium ammonium tartrate and ‘paratartrate’ (that is, the corresponding salt of racemic acid). He had learnt how to recognize and classify the facets of crystals, and he found that, just as Mitscherlich had said, these salt crystals were ‘asymmetric’: they had a handedness. If you start with a symmetrical crystal shape like a cube, and cut off some corners but not others, you can produce left- or right-handed shapes that are mirror images of one another. Such crystal forms are said to be hemihedral. The tartrate salt crystals were always of the right-handed hemihedral form. But Pasteur recorded in his notebook that for the paratartrate salt, ‘The crystals are frequently hemihedral to the left, frequently to the right’ (Figure below).
He added that sometimes the truncated corners ‘repeat’, so that left-handedness and right-handedness cancel one another out to produce symmetrical crystals. But he later crossed out this comment. It looks like a routine observation – puzzling, perhaps, but just a part of the wider puzzle. Compare this with the version of events that Pasteur later gave as the ‘official’ record. He asserted that, knowing that tartrate was optically active but paratartrate was not, he suspected that Mitscherlich had been wrong to say their crystals were identical. He thought that, on the contrary, the paratartrate crystals would prove to be symmetrical, with no handedness. When he looked and found that they were after all asymmetric, Pasteur claimed that ‘for an instant my heart stopped beating’. But then he looked more closely and noticed that they were asymmetric in both senses: some were lefthanded, and some right-handed, in contrast to the single handedness of tartrate. And so (Pasteur said), the answer to this riddle struck him in a flash. Right-handed tartrate revolved the plane of polarized light. Paratartrate, seemingly a mixture of left-handed and right-handed forms, left the light unchanged. Could this be because the effects of the two forms cancel each other out? That is to say, maybe the righthanded paratartrate crystals were composed of right-handed molecules, and the left-handed crystals of left-handed molecules, and the two were present in solutions of paratartrate in equal measure. In that case, the molecules in the left-handed crystals would, if separated and dissolved, rotate polarized light one way, and the righthanded molecules would rotate it equally in the other direction. All one had to do was separate the two types of crystal. Pasteur did this by hand, wielding tweezers to patiently sort the crystals into two piles. Then he redissolved them – and there it was, the very result he’d anticipated. In one account, he is said to have exclaimed ‘Tout est trouve´!’ – all is found. Other reports are yet more colourful: Rene´ Vallery-Radot, Pasteur’s son-in-law, claims that Pasteur rushed from his lab, grabbed a hapless curator in the corridors of the Ecole, and dragged him into the Luxembourg Gardens to tell him about the great discovery. Pasteur was not a whimsical man; he maintained an air of stiff bourgeois respectability, sharpened by a rather imperious and abrupt manner. It is hard to imagine him, even in his youth, prancing about gaily with excitement. But Pasteur was happy for Vallery-Radot to tell history this way (he saw the proofs before the biography was published), and if it is hyperbole then it is not so very unusual for its time. Victorian scientists frequently indulged in romanticized retelling of their discoveries: they seemed to feel that a bit of mythologizing would do science (and them) no harm. We have to be comparably cautious about Pasteur’s description of subsequent events. The head of his laboratory, Jerome Balard, conveyed the remarkable discovery to Biot, who asked to see the young man responsible for it. Brought into the presence of the great French scientist at the Colle`ge de France, Pasteur was requested to repeat his experiment before Biot’s eyes. He did so, whereupon (according to Pasteur) Biot, ‘visibly moved, took me by the arm and said: ‘‘My dear boy, I have loved science so much all my life that this stirs my heart.’’’ Yet even if the exact words were not so epigrammatic, the sentiment may have been much the same, for Biot subsequently regarded Pasteur as his prote´ge´. It is not in fact clear that Pasteur had given any thought to the relationship between optical activity and crystal shape in the tartrates and paratartrates until he performed his laborious separation of crystals – certainly, when he looked closely at their shapes, he does not appear to have formulated a strong hypothesis about whether paratartrate would have symmetric or asymmetric crystals. So we must doubt that his heart truly skipped a beat at what the microscope revealed. In addition, having prepared his two solutions of right- and left-handed paratartrate, his measurements of optical activity were not, so to speak, crystal-clear: the solutions did rotate polarized light in opposite senses, but not to identical amounts. Reasonably enough, Pasteur put this down to his limited ability to distinguish the two crystal types under the microscope*, but that cool comment doesn’t exactly support the idea of a eureka-like epiphany.
Mirror matter
Be that as it may, Pasteur saw with perfect clarity what his observations implied. Paratartaric or racemic acid was not a single molecular substance, but two: it was a mixture of mirror-image molecules. The right-handed versions were identical to the molecules in tartaric acid; the left-handed molecules were in effect tartaric acid twisted the other way, an isomer with equal but opposite optical activity. Pasteur introduced the word ‘racemic’ as a general term for an equal mixture of both isomers of an optically active compound. He also understood what the notion of handedness must imply for a molecule’s structure. One possibility was that the atoms were arranged in a screw-like pattern, a spiral staircase turning to the right or the left. This is indeed the atomic arrangement that makes quartz (silicon dioxide) optically active. But Pasteur realised that there were other options for organic molecules, as he explained in 1860:
Are the atoms of the right-handed acid grouped along the turns of a right-handed helix, or situated at the vertices of an irregular tetrahedron, or disposed according to such and such a fixed asymmetric [dissymmetrique] arrangement? We would not know how to answer these questions. But there can be no doubt that there is a grouping of the atoms of an asymmetric type that is not superposable on its mirror image.
In 1874 the Dutch scientist Jacobus van’t Hoff did offer an answer. He suggested that the stick diagrams that chemists had begun to use to depict organic molecular structures as an arrangement of atoms in space should be extended into the third dimension, out of the flat printed page. Such drawings had evolved from a mere graphical representation of molecular categories – a way of showing how different molecules were related to one another – to the point where they were regarded (by some researchers, at least) as showing the actual shapes of molecules. The atoms were represented by chemical symbols – C for carbon, H for hydrogen, O for oxygen – and the links between them were shown as lines or dashes. It was recognized since the work of Friedrich August Kekule´ in 1858 that carbon atoms generally form bonds to four other atoms. (The Scottish scientist Archibald Couper had the same idea independently, and would have published it first had his paper not been delayed by editors.) But van’t Hoff proposed that, instead of having their four bonds arranged as a flat cross, carbon atoms disport their bonds pointing to the corners of a tetrahedron:
For carbon atoms that have four different atoms or groups of atoms attached (and this is true of the two central carbons in tartaric acid) there are two different ways to arrange the substituents in space, which are not superimposable. These are mirror images of one another:
This, said van’t Hoff, is how organic compounds can acquire a handedness that results in their optical activity. Just a month after he published this idea, Joseph Le Bel in Paris clarified what it implies: any molecule with an asymmetric carbon atom – one with four different groups attached – will be optically active unless there is another equivalent grouping in the molecule that cancels its effect. In the latter case, the molecule becomes symmetric, so that it is its own mirror image: it is no longer ‘handed’, and no longer rotates polarized light. Tartaric acid has such a molecular form, called the meso form, as well as the optically active right- and left-handed forms. A molecule’s ‘stereochemistry’ refers to the arrangement of its atoms in three-dimensional space: molecules containing the same atoms and the same connections between them, but different three-dimensional shapes, are known as stereoisomers. Two stereoisomers that are mirror images of one another are called enantiomers (from the Greek enantios, opposite). Lord Kelvin decided that the property of molecular handedness should itself be dignified with a classical name, and in 1904 he derived one from the Greek word for hand, kheir: this sort of asymmetry is called chirality. Thus the two optically active forms of tartaric acid are chiral molecules, and enantiomers.
Pasteur’s famous dictum that ‘luck favours the prepared mind’ is often quoted, but it is possible to read it as a defence against the accusation that he just got lucky himself. He elected to study tartaric and racemic acids because Mitscherlich had discussed them as an example of isomorphism. He could not have known that racemic acid is one of the very few chiral organic compounds that will spontaneously separate into its two enantiomers as it crystallizes – most other such compounds will forms crystals in which the two enantiomers are mixed together. Moreover, few other organic compounds will form crystals so large that they can be readily inspected with a nineteenth-century microscope. And even racemic acid requires the right conditions to separate in this way: it happens only at relatively low temperatures, and so it was lucky that Pasteur happened to be carrying out his experiments in an unheated Paris laboratory in February. Had it been July, the result might have been quite different.
Race for the racemate
In late 1848 Pasteur left Paris to take up a teaching position at the lyce´e in Dijon, but this seemed to him to be a remote backwater, and he eagerly accepted a subsequent offer from the University of Strasbourg, where he began work in January 1849. He was sure that his discovery of what would later be called molecular chirality had yet more fruits to bear. He asked Kestner at Thann for more racemic acid, which the industrialist duly provided; Kestner subsequently became Pasteur’s trusted supplier and correspondent. Pasteur figured that Kestner’s winemaking process first produced only right-handed tartaric acid, which subsequently became converted into racemic acid. But how could that happen? It meant that precisely half of the molecules had to acquire an opposite twist. ‘It would be necessary’, he said, ‘to find an agent capable of producing this molecular modification on tartaric acid.’ But how do you alter the twist of a molecule? The problem was so profound that in 1851 the Socie´te´ de Pharmacie in Paris offered a prize of 1500 francs to anyone who could explain the origin of racemic acid. The ambitious Pasteur was less interested in the money than in the fact that winning this prize would enhance his fame. So he set out to crack the puzzle. During 1852 he travelled all over Europe, visiting laboratories and industrial plants to gather clues. Yet the quest seemed hopeless, and in January of 1853 Pasteur decided that the transformation of tartaric into racemic acid looked impossible. Only six months later, however, he had his answer. He studied several other optically active organic compounds to look for a relation between molecular asymmetry and crystal form (hemihedry). The similarity between crystals of tartaric acid and those of an organic compound known as malic acids suggested to him that their molecular structures might be related. That is to say, they might both contain the same chiral ‘core’. ‘If there exists a common molecular grouping between the right-handed tartaric acid and the known malic acid of the mountain ash’, Pasteur wrote:
" There is necessarily a common molecular grouping between the lefthanded tartaric acid and a malic acid as yet unknown, and which would be to the malic acid known to chemists, what left-handed tartaric acid is to right-handed tartaric acid. "
Maybe, then, nature made racemic acid not by reversing half of right-handed tartaric acid but by making both left- and right-handed forms from corresponding precursor molecules (such as malic acid) and then mixing them. But what about the precursors – where did their handedness come from? Pasteur came across an extraordinary claim by a chemist named Victor Dessaignes at Vendoˆme, who said that he had made malic acid from two other organic acids, maleic and fumaric acids. But those starting materials were optically inactive, whereas malic acid was optically active. ‘Now to this day’, Pasteur commented, ‘no optically active substance has ever been prepared by laboratory procedures, starting with substances which are not optically active.’ So could Dessaignes be mistaken? Pasteur repeated his experiment, and found that it produced optically inactive malic acid. He concluded that this form of the compound was somehow ‘untwisted’: he called this mesomalic acid, introducing the ‘meso’ terminology. Now, as it happens, malic acid does, like tartaric acid, have an optically inactive meso form. But that’s not what Pasteur made using Dessaignes’ procedure – rather, he made a racemic mixture of left- and right-handed malic acid. Unaware of this, he set out to see whether he could make the tartaric acid analogue of what he deemed to be his mesomalic acid. During these studies, he found by chance that the optically active compound of tartaric acid, combined with a substance called cinchonia – an extract of the bark of the cinchona tree, which was the source of the antimalarial drug quinine – would, when heated, become transformed into racemic acid. By luck (and, of course, mental preparation) he had stumbled onto the solution that won him the Socie´te´ de Pharmacie prize.
Chirality and life
These experiments hint at how Pasteur was starting to think about molecular chirality. At first, his claim that a chiral molecule could not be prepared from an achiral one rested simply on careful reasoning based on what was known about such chemical transformations (notice that his investigation of Dessaignes’ claim did not in the end seem to him to contradict this notion). Gradually, however, this property of chirality became for Pasteur an article of faith. He was convinced that there was an impermeable barrier between chiral and achiral molecular forms – and, moreover, he thought that this same barrier separated the living from the inanimate world. Molecular chirality, he supposed, was uniquely a property of living systems.
‘Every chemical substance’, Pasteur wrote:
" whether natural or artificial, falls into one of two major categories, according to a spatial characteristic of its form. The distinction is between those substances that have a plane of symmetry and those that do not. The former belong to the mineral, the latter to the living world. "
This strong assertion touched on the protean notion of vitalism – the idea that there was a ‘vital force’ that quickened life, and which made living or organic matter fundamentally different from inorganic matter. By the mid-nineteenth century, the vitalistic hypothesis was looking distinctly embattled. But the debate was far from settled, despite later claims about the influence of Friedrich Wo¨hler’s synthesis of urea in 1828. Pasteur was no vitalist – indeed, his own work on microbes helped to dispel the belief that a vital force caused the spontaneous generation of organisms in decaying matter – but he was keen to make a fundamental distinction between the organic and the inorganic, based on the idea of chirality.
So where did this handedness of life come from in the first place? Pasteur reasoned that it must be impressed on organic matter during its formation by the action of some all-pervasive chiral force. He illustrated how such a mechanical force might be manifested by analogy with a left- or right-handed screw being driven into a wooden block ‘in which the fibres themselves have a right or left spiral arrangement’. In one case the screw would go in easily; in the other, only with difficulty. ‘Is it not necessary and sufficient’, he concluded, ‘to admit that at the moment of the elaboration of the immediate principles in the vegetable organism [in other words, at the moment the molecules are formed], an asymmetric force is present?’ Pasteur speculated that this force could be imposed by the environment, perhaps as an electric or magnetic field:
" Do such asymmetric agencies arise from the cosmic influences light, electricity, magnetism, heat? Do they perhaps stand in relation with the movement of the earth, with the electric currents by means of which physicists explain the earth’s magnetic poles? "
If so, then maybe these forces could be reversed, inverting the chirality of natural organic molecules. Consequently, in 1853 Pasteur embarked on a series of experiments that now look decidedly cranky: crystallizing substances in magnetic fields, growing plants from seeds irradiated with sunlight that was ‘inverted’ by reflection from mirrors. Biot implored him to abandon this strange quest, and indeed Pasteur himself admitted that ‘One has to be a little mad to undertake what I am trying to do now.’ Yet chirality led him in more productive directions too. In 1854 he left Strasbourg to work at Lille, where there was an active brewing and distilling industry, and he was asked to help with some of the problems the industrialists encountered during the fermentation process. Fermentation was itself a disputed matter at that time. In 1837 Baron Charles Cagniard de la Tour proposed that the fermentation of sugar, by which means it is converted into alcohol, depends on the action of yeast: that is, of microscopic organisms. But others disagreed. The eminent German chemist Justus von Liebig suggested that yeast might be just a by-product of fermentation, and was not essential to what was basically a chemical process. Berzelius argued that the role of yeast was simply to ‘awaken slumbering affinities’ between the chemical compounds involved in fermentation: he gave this ‘assisted’ breakdown of substances a new name: catalysis. (Liebig’s colleague Oerhardt complained that ‘Calling the phenomenon catalytic does not explain it, it only replaces a name of ordinary language by a Greek word.’) Pasteur was drawn to study fermentation partly because one of the products, amyl alcohol, was chiral.
This, he thought, could only be produced by the action of living organisms: fermentation could not be simply ‘chemical’. He observed that the fermentation of sour milk into lactic acid – which happens spontaneously if milk is left to go off – produces tiny grey particles that, if extracted from the ferment, are then capable of converting sugar into lactic acid too. He decided that this ‘lactic yeast’ was a microorganism that multiplied as a result of fermentation, from which it derived sustenance. In other words, different kinds of microorganism gave rise to different modes of fermentation. This sowed the seeds for the discipline of microbiology, as well as paving the way to the discovery, at the end of the nineteenth century, of nature’s molecular catalysts: enzymes, the key agents of biochemistry. In late 1857, Pasteur studied the fermentation of tartaric acid when it was left to age. He found that, with racemic acid, only the righthanded isomer was fermented. This showed him that living organisms made precise distinctions between chiral molecules: ‘We see here the character of molecular asymmetry peculiar to organic substances intervene as a modifier of affinity [that is, of chemical reactivity].
’ Pasteur discovered that our own physiology makes similar distinctions: one enantiomer of a chiral sugar may produce a sensation of sweetness while the other barely registers. ‘When active asymmetric substances are involved in producing an impression on the nerves’, he said, ‘their effort is translated by sweet taste in one case and almost no taste in the other.’ We now know that this exquisite discrimination in molecular physiology is governed by the ability of enzymes to ‘feel’ the difference between mirror-image molecular shapes. One consequence is that our bodies can only metabolize right-handed sugars, which are the naturally occurring forms. Because our sweet taste buds can nevertheless be stimulated by some left-handed sugars (such as left-handed glucose), these are manufactured industrially as artificial sweeteners, which give the taste without the calories of normal sugar. The sensitivity of enzymes to chirality also accounts for nature’s enviable ability to generate just one of the two enantiomeric forms of a chiral compound. Chemists today still find it a tremendous challenge to design synthetic catalysts that can effect comparably selective chemical transformations.
Yet it is the very selectivity of the body’s biochemistry that creates a demand for such catalysts. While some chiral compounds are simply physiologically inactive in one enantiomeric form, others can elicit more troublesome responses. The most notorious example is the drug thalidomide, which was administered to pregnant women in the 1950s and 1960s to suppress morning sickness. While the right-handed form acts as a sedative and an anti-inflammatory agent, the left-handed form causes fetal growth abnormalities. Because the marketed drug contained a mixture of the two enantiomers*, it led to a wide incidence of birth defects in babies born in the early 1960s. The thalidomide case made it clear that one must distinguish carefully between the physiological effects of both enantiomers of chiral drugs, and many drugs now have to be prepared in enantiomerically pure form. This has led to the development of methods of separating enantiomers (something that Pasteur also pioneered), although the ideal and most economical solution is to make only one enantiomer in the first place: hence the demand for enantiomerically selective catalysts. Several such manufacturing processes use natural enzymes,either extracted from microorganisms or within whole cells bred in fermentation vats, to conduct these delicate operations – for if Pasteur was wrong to think that only nature can generate chiral molecules, nevertheless it remains true that she is a lot better at it than we are.
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Elegant Solutions - Ten Beautiful Experiments in Chemistry Philip Ball, page 101
