Pasteur and Molecular Chirality

Pasteur studied tartaric acids (wine fermentation) and discovered the two chiral forms, the levorotatory form and the dextrorotatory form; he thus explains – which has not been included – the optical neutrality of mixing two components. By laying the foundations of molecular chirality, Pasteur unveiled one of the first main discoveries in chemistry and biology. Since this pioneering work, chirality has been central to material sciences. Its applications are numerous, ranging from the synthesis of active molecules to the properties of components involved in imaging. Also worth mentioning is the recent science, astrochemistry, where research of chiral precursors of terrestrial molecules is more relevant than ever. The big question is this: in what way has nature chosen a single enantiomeric form of sugars and amino acids from which we are made?


Pasteur and Molecular Chirality by Alain Sevin Emeritus Research Director at CNRS (French National Centre for Scientific
Research) The term chirality is derived from the Greek keir (hand).It describes the fact that some figures or bodies are not superimposable to their mirror image, no matter the scale on which the phenomenon takes place.Chirality has become central to physics, chemistry and biology, and the chirality of macroscopic objects has been recognised and discussed since Ancient Greece.Nevertheless, we are indebted to Pasteur for having studied this fundamental asymmetry of nature, and for defining the link between the measurable macroscopic properties observed and the molecular structure of the bodies considered.His work has launched an evergrowing number of developments in chemistry and biology. 1

THE CONCEPT OF CHIRALITY BEFORE THE NINETEENTH CENTURY
We are indebted to Plato for the first scientific remark on chirality.In Timaeus, he proposes a theoretical analysis of the mechanism of vision, saying, As for the origin of the images given by mirrors and all the shiny and polished surfaces [...] But then what is to the left appears to the righ.
Aristotle was the first to implicitly use chirality in his study on the movement of stars.He defines the "natural movement" as going from the bottom up, from back to front and from right to left.Chirality appears in his final criterion.
Considering then the apparent movement of the sun, he deduces a logical consequence from the fact that the east is lit before the west.However, for the Greek observer, this would "seem" as if the sun had an anti-natural movement (i.e., going from left to right)!Let's save the phenomenon!(sauzein ta phainomena) 3 : the sun does not in fact have this movement.The solution is simple: the world's axis is oriented from our head to our feet.And with this convention, it is fine for us to lower our headssince everything falls into place!
As a matter of a fact, the perpendicular of something is the starting point for local movement, and the starting point for circular movement is the area in which stars rise, in such a way that it would be to the right, and the area in which they fall to the left.Therefore, if stars begin their journey from the left, in order to go in a circular movement to the right, then the highest point has to be the invisible pole, because if it were the visible pole, the movement would be to the leftand this is not the case.Kant stresses the intuitive nature of chirality, that simple volutes are chiral: So, we can no longer understand the difference between two things that are similar and equal and yet unalike (for example, inversely coiled volutes) by any concept, but only by the relationship between the right and left hand, which is instantly to do with intuition.[...] Thus, what is the solution?These objects are not representations of themselves, per se, and as pure understanding would know them, but are sensible intuitions, i.e., phenomena whose possibility is based on the relationship between certain unknown things, per se, and something elsenamely our sensitivity.

THE DISCOVERY OF MOLECULAR CHIRALITY
The basis of Pasteur' work on chirality is mainly the use of plane-polarised light for studying solutions of natural compounds.Among the forerunners are:  Augustin Fresnel (1788-1827).He was the first to create polarised light circularly.He proposed a simple vector construction of plane-polarised light 6 .
 Jean-Baptiste Biot (1777-1862).Around 1815, he developed and used the saccharometer which employs the deflection of the plane of polarisation of light for studying and measuring sugar solutions.He formed the simple law: where , measured in degrees, is the observed deviation of the plane of light, [0] is the specific deviation of the studied compound, l the length in dm of the tank that is penetrated by light, containing the sample in solution and c its concentration in g mL -1 .[0] is set to temperature T and the wavelength  is given, and, in addition, with the nature of the solvent being equally precise.@@@@@@@ Another image that is firmly linked to Pasteur is that of microbiology, and of the discovery of germs (or microbes).We forget that if he had indeed created this new science, he was originally by training and by practice a chemisthence his work on the polarity of molecules.Pasteur was a student of the physicist Jean-Baptiste Biot (1774-1862).

Louis Pasteur (1822-1895), Active Chemist from 1840
From the 1840s, Pasteur studied the products formed in fermentation, paying particular attention to salts, which he called "paratartrates", and which are salts of tartaric acid with several varieties, as shown in the following figure.his turn.The paratartrate of soda and ammonia deviate on the plane of polarisation; it is only among the crystals that come from the same sample that some deflect the plane of polarisation to the left, and others to the right; when there are as many of one type as the other, the solution is not active, and the two complete deviations are compensated.
Pasteur deduced that the observed macroscopic property comes from a different spatial arrangement of atoms at the molecular level.This proposal is therefore of great valuethis crucial discovery would be supported later on by the work of renowned scientists Joseph Achille Le Bel and Jacobus Henricus van 't Hoff, who in 1874 independently introduced the tetrahedral structure of carbon in saturated organic compounds.Thanks to their findings, Pasteur's hypothesis can quite easily be confirmed and justified, which was based on the macroscopic structure of crystals, though in the 1850s it had not yet been confirmed at the molecular level, due to a lack of essential knowledge about molecular structures.
Note that the variety of tartrates, known as "meso" earlier, are not active in polarised light as they are shared by a plane of symmetry, passing through the middle of the C-C bond.This molecule is therefore superimposable to its image; you only have to place it in coincidence with the plane of molecular symmetry.
Three basic consequences ensue from the separation of crystalline forms:  Enantiomer-type crystals correspond to molecules that have the same physical properties, and only differ by the absolute spatial configuration of their components.This arrangement produces a different behaviour for each enantiomer when opposite the polarised light;  The mixture in equal amounts of these two forms leads to a neutralisation of properties when opposite the plane of polarised light;  If a compound has a plane of molecular symmetry, then it is superimposable to its image and is not active in polarised light.

Explanation of Pasteur's Discovery
We shall use this fundamental result of Le Bel and van t'Hoff.A carbon atom with four different substituents (alternatively, it would have a plane of symmetry) is not superimposable to its mirror image.This is shown in the following diagram where we can see that the central carbon has four different substituents, which are marked as A, B, D, E.
I and its mirror image, II, are not superimposable.The spatial arrangement of the substituents of the central atom is called "absolute configuration".To define it without ambiguity, an observer is needed who by nature is chiral, i.e., their right hand is not superimposable to their left hand 8 .The new molecules I and II, of opposite spatial configurations are called enantiomers.Mixing them in equal amounts is what is known as a racemix mixture, as previously stated by Pasteur.
@@@@@@@ It is worth distinguishing the configuration, with an absolute nature (which can only be changed by destroying the molecule), from its simple conformation, resulting in a free rotation around a C-C axis; these conformations do not change the molecule's nature (e.g., below).

THE POSTERITY OF PASTEUR'S WORKS: THE ROLE OF CHIRALITY IN BIOLOGY
This sort of concludes a long chapter in the history of chemistry that Pasteur addressed his colleagues from the Chemical Society of Paris in a meeting, 1883: […] The principles of molecular dissymmetry 9 were based [on polarimetric measurements] […] Gentlemen, to be perfectly honest, it is understood that things are this way.Perhaps you never commented on this when I first brought it to your 8.This definition is tautological!We cannot do anything about that.9. Today we use the term "asymmetry".these conditions, it would be practically impossible to produce the same compound twice: duplication would be unthinkable, whatever the means considered.There is only one way to be sure of obtaining in each duplication a compound identical to the model: using only one enantiomer of each amino acid, always the same.This condition is scrupulously followed by the living and we see that, without chirality, life would not be able to reproduce or even simply exist.

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Notice the perfect coherence of the reasoning that is based on the postulation of natural movement.The definition's arbitrariness is only apparent.The opposite convention would be equally arbitrary.The definition of chirality is relative to an observer who has already put their right hand over their left hand: for to be able to understand and transmit chirality, you must already possess it.

Figure 1 :
Figure 1: 1a.The apple is whole; 1b.A quarter is cut out; 1c.A second quarter is cut out.The half-apple (two quarters of the apple have been removed) in 1cis chiral (image from Dézarnaud-Dandine & Sevin, cf.NbdP 1)

Figure 2 :
Figure 2: Banknote of 5 French francs, put into circulation in 1966.Pasteur is the only scientist to have his face on a French banknote, aside from Marie Curie on the 500 French francs banknote, put into circulation in 1994.The original building of the Pasteur Institute can be seen here (founded in 1895 in Paris, 15th arrondissement), still in existence today.

Figure 3 :
Figure 3: isomers, or tartrate salts.Tartaric acid (with the chemical formula C4H6O6) is an organic compound present in many plants, including grapevine; the salts that come from tartaric acid, or tartrates, are collected during the fermentation of wine.3a(above): The first pair of tartrates, denoted (+) (dextrorotatory) and (-) (levorotatory) is composed of two enantiomers (a geometric mirror image).The third compound, known as a "meso compound", made up of a plane of symmetry, is not active when opposite polarised light.3b (below):The chemical formula developed of the two enantiomers (to the left: the levorotatory; and to the right: the dextrorotatory)

Figure 4 :
Figure 4: Conformations of the same molecule.Unlike I and II, which are two different molecules, here the same molecule, III, can adopt different geometries, such as IIIa and IIIb.

10Figure 5 :
Figure 5: Common structure, -helix, of a protein (image WikiCommons).Amino acids (combination of carboxyls-COOH and amines-NH2) are formed in a helix, with the creation of hydrogen bonds that reduce the internal energy of the molecule, stabilising it.
To conclude, by laying the foundations of molecular chirality, Pasteur unveiled one of the first main discoveries in chemistry and biology.Since this pioneering work, chirality has been central to material sciences.Its applications are numerous, ranging from the synthesis of active molecules to the properties of components involved in imaging.Also worth mentioning is the recent science, astrochemistry, where research of chiral precursors of terrestrial molecules is more relevant than ever.The big question is this: in what way has nature chosen a single enantiomer form of sugars and amino acids from which we are made?(January 2012) (Translated in English by John Moran, published April 2015) attention.Consider any object, be it natural or artificial, of the mineral or organic kingdom, living or dead, created by life or constructed by man, a mineral, a plant, this table, a chair, the sky, the earthwhich is to say, any object whatsoever.Just consider the form of all these objects, their exterior appearance and the same thing for similar objects, if they exist, and what you will discover is that all of them can be put into two major categories: the first category would include all those that have a plane of symmetry.There can be several planes of symmetry for the one objectit is a question of being able to share this plane, so that what you find to your left is also what you find to your right.[…] On the other hand, there also are bodies that have no plane of symmetry.Let a plane cross through your hand and you will see that what is to the left and what is to the right are not the same.This is the same for an eye, an ear, a spiral staircase, a helix, and a spiral shell.All these object, and others, do not have plane of symmetry; they are such that if were you to place them in front of a mirror, you would be able to see that their image is not superimposable.