Chirality (chemistry)

the concept of the chirality also exists in other fields.

Chiral molecule

The chirality can be compared with a simple problem of shoes. All the children were already confronted with a problem of chirality by putting the right foot in the left shoe and conversely. A shoe is a chiral object because it is not superposable with its image in a mirror. Just like hands. The distribution of different elements in space, for example around a point, can lead to nonidentical situations, therefore different objects. Thus the dice to be played are chiral objects: the rule of construction wants that the sum of the opposite faces is equal to Sept. Posons the six on the higher face and consequently the a on the lower face, then the five in front of thus the two behind. There remain two nonequivalent ways to finish: the four on the left and the three on the right, or conversely. One obtains two enantiomers images one of the other in the mirror.

In the same way, the amino-acid are chiral Molécule S: an amino-acid and its image in a mirror are not superposable because the molecule contains an asymmetrical carbon atom (related to 4 atoms or groupings of different atoms).

Carbon Tétravalent can accept 4 different substituents on each one of its 4 connections:

Here the example with the amino-acid Alanine (two named stereophony-isomers D and L):

It is thus the Symétrie of the molecule which determines if it is chiral or not chiral (it is then called achirale ). In practice, a molecule chiral if and only if it is deprived of any unsuitable axis of rotation of order N (N whole strictly positive), is noted S $n$ . It is about the rotation of the molecule of nπ around the axis, followed by a reflection compared to a plan perpendicular to this axis. An unsuitable axis of rotation of order 1 (S $1$ ) is a center of symmetry (noted $i$ ) and an unsuitable axis of rotation of order 2 (S $2$ ) is a symmetry plane (noted $\ sigma$ ).

Historical aspects

A solution making turn the plan of polarization of a beam of light polarized in the opposite direction of the needles of a watch contains an optical isomer (−) (it is the opposite for an optical isomer (+)). The magneto-optical effect is at the origin of this phenomenon. This property was observed for the first time by Jean-Baptiste Biot in 1815 and acquired a considerable importance in the industry of sugars, the analytical Chimie and the drug company. Louis Pasteur showed into 1848 that the optical activity is related to the chirality (Criterion of Pasteur).

A chiral object, two enantiomers

A chiral object has only two different forms possible called enantiomers (of the “opposite” Greek enantios). Thus a shoe is a chiral object with two enantiomers: the right shoe and the left shoe.

It is exactly similar for the molecules: as soon as a molecule has a sufficiently low Symétrie, it necessarily exists in the shape of two enantiomers which have the same physical properties symmetrical (solubility, boiling point, etc); they can however be differentiated by an optical property, the deviation of the polarized light: one of made up deviates it on the right and the other on the left.

Both enantiomer S of the Lactic acid : lévo-lactic acid and dextro-lactic acid.

Nomenclature

A word on the nomenclature used to indicate the configuration (arrangement in the space of the substituents of carbon): the chemists say R/S “Rectus, right” or “Sinister, left” (official nomenclature), but the biologists continues to use old system D/L. The first system is founded an agreed set of priorities of the four various substituents of asymmetrical carbon; system D/L uses, him, of the complicated chemical correlations starting from the Glycéraldéhyde.

There is no relation between the two system baseds on completely different criteria. Moreover, in both cases, there is no relation between the optical configuration and properties (dextrogyre or laevogyrous).

Nomenclature R/S is known as absolute configuration since it is applicable to all types of molecules, on the basis of not-ambiguous criterion, the rules of Cahn, Ingold and Prelog. To each asymmetrical carbon (C*) a descriptor R or S is allotted by giving to each one of its substituents a priority according to the atomic number of the atom related to C* (greatest priority being granted to the titular substituent of the atomic number highest). If two substituents are related to C* by the same atom, it is necessary, to allot the priority, to refer to the other atoms related to this last, and so on.

Then, it is a question of knowing if the three priority substituents are presented in a time way (while conventionally placing the priority substituent with the back) or anti-clockwise. The isomer R is thus towards the line, that is to say a configuration where the priority substituents “turn” clockwise. The isomer S corresponds to anti-clockwise rotation. Several carbons can be asymmetrical, therefore to present the name of the molecule, one adds, in front of the name of molecule (XR, YS) -, where X and Y are the numbers of asymmetrical carbon and R and S the possible isomers. One can thus have (2S, 5R) - 5-chlorohexan-2-ol, (2S, 5S) - 5-chlorohexan-2-ol, (2R, 5R) - 5-chlorohexan-2-ol or of (2R, 5S) - chlorohexan-2-ol.

Right or left enantiomer?

At the 19th century, one showed that some compounds in solution deviate the plan of polarization of the polarized light. There are some who deviate it on the left (angle - α) and others on the right (angle +α). And it is the case for our enantiomers: there is of them one which deviates the light on the right, it is known as dextrogyre ( D ), and the other which deviates it on the left, it is laevogyrous ( L ). This makes it possible to differentiate them. The nomenclature D/L refers it to the absolute configuration, i.e. with the space arrangement of composed by reference to alanine, one of the natural amino-acids.

Where is the problem for the life? Today, the amino-acids “natural” which constitute the living beings are all L whereas when one synthesizes them under symmetrical conditions (experiment of Miller for example) one obtains a mixture 50/50 of the forms L and D: it is a racemic mixture. It is the reverse for sugars: all the natural Glucide S are series D. In a general way, in alive, only one enantiomer was selected with each time the problem arose.

The Vitamin C: only the form L is comparable by the organization.

Why and how the Vie it systematically privileged one of the two forms? How, it is a mystery. But one can easily include/understand why: it is a saving in means. The life of the every day offers of them successful examples and other failures to us. Thus, except very special cases, it is a large saving in means which, all over the world, all the screws and all the nuts are of right step (thus chiral objects); let us imagine the disorder if the screws and the nuts were sold in racemic mixture (50/50)…

For the same reason of economy the two engines of a twin-engine aircraft turn in the same direction. On the other hand the military navy offers the luxury to equip its buildings with pairs of propellers turning in opposite direction.

In the two countries most populated in the world, India and China, one does not lead a same side: on the left in India and on the right in China. For a foreign manufacturer, it will thus be necessary to envisage two assembly lines to export the same model of car. What constitutes an enormous wasting that nature knew to avoid.

A right hand in a left glove!

Two enantiomers have identical properties in a symmetrical environment. Thus an oven glove flat and symmetrical will be appropriate in the same way on a right hand or left. On the other hand a single-breasted glove is not appropriate for a left hand: a receiver itself chiral thus discriminates two enantiomers. However, we are built with chiral building blocs (Amino-acid, Sucre S, etc) and, as regards alive, the consequences of this situation are multiple:

two enantiomers can have simply different properties: the odor characteristic of the Fennel and dill is due to the one of the enantiomers of the Carvone, while the flavor of Spearmint is due to the different one. The two enantiomers of the Citronellal are the orange and lemon flavors.
an enantiomer can be gifted of an interesting, insecticidal biological property for example whereas the other remains completely inactive. For example the Deltaméthrine.
an enantiomer can be an effective drug and the other a highly toxic poison. This had not been included/understood until in the years 1970: for reasons of cost it was current to synthesize drugs in racemic form, while postulating that the bad enantiomer does not have any activity. For example the Thalidomide (Analgesic), was marketed in racemic form, but the supposed enantiomer inactive being proved to cause malformations (Tératogène).

All depends on the molecule and its biological receiver.

References

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