A Young Chemist Called Pasteur

A Young Chemist Called Pasteur

Jean Jacques,
Collège de France, Paris

A panegyric to Pasteur would be superfluous. He was eulogized in his own lifetime, and in this centennial year of his death it would be difficult not to repeat what has already been said over and over again. External signs of mankind's gratitude have never failed him in the whole world. In the cities and villages of his own country, France, he is one of the scientists most often called upon to give his name to a street. Admittedly, it was not as a chemist and crystallographer that called the attention of city councils upon Pasteur's name. Yet even one less partial than me must admit that chemistry was the catalyst in a series of chain reactions which ultimately led to bacteriology and to Pasteur's undying glory. The first main work the young post-doctoral student published while still in the Ecole Normale was entitled Research on Dimorphism. With this, he was broaching a far from evident topic: why, and under which conditions can a specific chemical substance --either a simple body or a complex molecule-- engender two different species of crystals? His now old-fashioned but still valid answer does not concern us here; but in so doing he learned crystallization techniques and the patience that numerous and boring measurements of angles call for. He observes scores of crystals; their facets, their similarities, and their differences no longer hold any secrets for him. He is thus well prepared to take on the challenge of the seemingly total similitude between crystals of the ammonium and sodium salts of natural tartaric acid which deviate polarized light and those of its isomer, the racemic acid, which to the contrary have no rotatory power. Despite what has often been asserted, this issue was not a major one for the chemists of that decade. Yet the young student of the Ecole Normale had guessed that this experimental fact --discovered by Mitscherlich in 1843 and introduced by Biot in France only three years later--raised a fundamental issue which concerns the very structure of matter. Pasteur discovered that the crystals derived from racemic acid are chiral as one says nowadays; two species coexist, mirror-images of one another. One half of these crystals are identical in aspect and rotatory power to those he had observed while studying the salts of natural tartaric acid. The second half deviates polarized light in the opposite way to the first set. This spontaneous resolution of sodium and ammonium racemates is an exceptional phenomenon in more way than one. Specifically, we know today that this resolution takes place for less than 10% of all racemic products. Pasteur thus had good reason to write later that fortune smiles only on minds prepared for it.

Even though the concept of chemical structure did not yet exist, Pasteur immediately postulated that the dissymmetry of the crystals he had observed revealed the dissymmetry of the molecules of which they were made. By this brilliant intuition he laid the foundation stone of what would later be stereochemistry, the chemistry in space with which we associate the names of Le Bel and van't Hoff.

Yet, even for the experienced crystallographer that Pasteur was, hemihedrism which revealed this dissymmetry was not always present or easily observed in all the crystals of optically active compounds. Pasteur long sought to find a method which would unfailingly reveal its presence. In 1853, he wrote: "I thought that it would suffice to modify conditions of crystallization to force hemihedral facets to always appear." He knew, for example, that Sea Salt yields different crystals in pure water and in water containing urea. Derivatives of tartaric acid were therefore crystallized in the presence of as diverse a set of substances as possible. Among these, the natural alkaloids, such as cinchonine and strychnine, lead him through a sideroad to two fundamental and probably unintended discoveries. First, Pasteur observed that the identical behavior of the two right and left-handed tartaric acids "ceases to exist in the presence of substances which have any molecular action on the plane of polarisation of light." In other words, he invented --unawaredly at first-- a new method for the resolution of racemic compounds based on the solubility differences of those chemical combinations we now call diastereoisomers. He noticed for the first time the existence of a quasi-racemic association, which would only be rediscovered almost one hundred years later. Second, Pasteur realized that by heating the cinchonine tartrate he transformed all at once the alkaloid into conchonicine and the tartaric acid into racemic acid. As we know, this specific isomerisation is one instance of a more general phenomenon not limited to this case which gave it its name, racemisation.

Logic and chance, the two determining factors throughout Pasteur's life and work, would lead him to direct his research towards fermentation, that is, ultimately, towards microbes and glory. He himself recounted how already in 1849 Biot had made him aware of the amylic alcohol, a product then commercially available under the name of fusel oil. It was the product of the fermentation of either potato starch or Sugar beet extract. This substance is a mixture of two alcohols, only one of which has rotatory power. Despite stubborn efforts, Pasteur failed to separate them through some of their crystalline derivatives. He wrote: "My research has long been determined by the idea that the constitution of bodies, as seen from the standpoint of its molecular dissymmetry or non-dissymmetry (all other factors being equal) plays an eminent role in the most minute laws organizing living beings and is a factor in their most hidden and most minute properties. This idea was for me the cause and reason of my experiments on fermentation." Thanks to a German manufacturer of chemicals, Pasteur knew that "impure lime tartrate, contaminated by organic substances and left underwater during the summer, can ferment and yield various products." Given this, he said, "I left the ordinary right-handed ammonium tartrate to ferment." The final step was now just around the corner: it was fully logical to try to make ferment the corresponding salt of the racemic acid. He could then observe that the at first inactive solution had acquired a left-handed rotatory power. The natural acid only had been consumed by his voracious yeast.

From that point on, Pasteur was no longer the young chemist we have watched maturing. He was already at the gates of another legendary realm. Still, he would often later return (and went the rumor, with nostalgia) on these prestigious formative years and on the issue of the origins of this dissymmetry specific to natural substances. This issue obsessed him. This is now part of his legacy, a legacy which perpetuates his memory. Tweet