The term homochirality is used to describe a group of molecules that have the same chirality. The molecules are not necessarily identical, but similar substituents are arranged in the same manner to a central atom, the center of chirality. Chirality is observed especially in living organisms. For example, all naturally occurring amino acids, the L-form. Likewise, the most biologically relevant sugars have the D-form. The corresponding other enantiomer of these molecules usually are biologically inactive, and in some cases even toxic to living organisms. The origin of this phenomenon is not fully understood. It is not even clear whether homochirality has a purpose. One theory is that homochirality reduces the Entropiebarriere in the formation of large, organized molecules. It has been demonstrated experimentally that amino acids form large aggregates in greater yield when enantiomerically pure and not as a racemate present.
Homochirality should develop in three steps: The breaking of the mirror symmetry arises a tiny enantiomeric imbalance, this is the key to homochirality. Chiral amplification leads to an enantiomeric enrichment. Chirality allows the transfer of chirality from one group to another molecule.
Break the mirror symmetry
The most difficult question to answer is how an enantiomeric imbalance is originally generated. Some scientists support the theory of extraterrestrial origin, which is based on the discovery of enantiomeric excess in various, especially alpha -methylated amino acids in the Murchison meteorite. There is evidence for the existence of circularly polarized light in space, which is probably caused by Mie scattering of aligned interstellar dust particles. The interaction of this circularly polarized light with racemic organic compounds could have triggered the formation of a small enantiomeric excess. Under simulated space conditions, it succeeded by using circularly polarized synchrotron radiation an enantiomeric excess of 2.6% in the amino acid leucine to induce.
A classic study includes a laboratory experiment. If sodium chlorate crystallized from water and the crystals obtained are examined in a polarimeter, one finds that each crystal is chiral and consists exclusively of the L-or D-form. In a normal experiment, the relative amounts of L-and D crystals are equal if statistical effects are corrected. When the sodium chlorate solution is stirred during the crystallization process, however, is obtained only L- or D- crystals. At 32 crystallization experiments yielded 14 experiments D crystals and the remaining 18 experiments L crystals. There is no clear explanation for this break of symmetry, but it is believed that it is related to the autocatalysis, which takes place during the nucleation process.
In laboratory experiments it could be demonstrated how some autocatalytic reaction systems, the presence of small amounts of the reaction product with enantiomeric excess at the beginning of the reaction can lead to the end of the reaction to a much greater enantiomeric excess. In a landmark study pyrimidine -5 -carbaldehyde (Fig. 1) was alkylated with diisopropylzinc to the corresponding pyrimidyl alcohol. The initial reaction product is an effective catalyst, thus the reaction is autocatalytic. The presence of only 0.2 equivalents of (S)- enantiomer of the formed alcohol in the beginning of the reaction is sufficient to raise the enantiomeric excess 93%.
Another study concerns the proline -catalyzed aminoxylation of propanal with nitrosobenzene ( Fig. 2). In this system, the presence of an enantiomerically enriched catalyst, also leads to one of two possible optical isomers.
Serine octamer clusters are another example. These clusters, which consist of eight serine molecules in mass spectrometry show an unusual homochiral preference, but there is no evidence that such clusters exist under non- ionizing conditions. In addition, the behavior of the amino acids is at phase boundaries prebiotic far more relevant. The observation that the partial sublimation of a 10% enantiomerically enriched sample of leucine results in an enrichment of up to 82 % in the sublimate, shows that enrichment of one enantiomer in amino acids may also be carried out in space. Partial sublimation processes can take place on the surface of meteors, where large temperature variations occur. This discovery could have implications for the development of the Mars Organics Detectors, whose launch is planned for 2013. This probe is to win small amounts of amino acids by means of a sublimation of the surface of Mars.
A high amplification of the enantiomeric excess of sugars is also observed at the amino-acid catalyzed asymmetric form hydrocarbons.
Many strategies for the stereoselective synthesis based on chirality. Particularly important is the so-called organocatalysis organic reactions by proline, for example, Mannich reactions.
Resolution upon racemic amino acids
There is no theory that enlightens the correlation between L -amino acids. Comparing, for example, alanine, which carries a small methyl group, and phenylalanine, which has a large benzyl, the simple question of the aspect of L-alanine to L-phenylalanine is more similar than the D - phenylalanine, and the type of mechanism, the selection all L- amino acids causes. It would be equally possible that natural alanine has an L - configuration, on the other hand, a phenylalanine natural D-configuration.
In 2004, it was reported that an excess of racemic D, L- asparagine ( Asn ), which spontaneously forms crystals of the pure isomers during recrystallization, the resolution of a well in solution, racemic amino acid such as arginine ( Arg), aspartic acid ( Asp), glutamine (Gln ), histidine (His), leucine (Leu ), methionine (Met), phenylalanine (Phe ), serine (Ser), valine (Val), tyrosine (Tyr ), and tryptophan (Trp), induced. The enantiomeric excess of this second amino acid is correlated almost linearly with the corresponding value of the inducing amino acid, that is asparagine. In the recrystallization of a mixture of 12 D, L- amino acids ( Ala, Asp, Arg, Glu, Gln, His, Leu, Met, Ser, Val, Phe, and Tyr ), and an excess of D, L -Asn, it was found that all amino acids having the same configuration as Asn preferably crystallized. It depended on chance whether the enrichment was performed using the L- or D- asparagine. However, once the choice was made, the co-existing amino acid with the same configuration at the α - carbon atom was preferentially involved, which can be explained by the thermodynamic stability of the crystal structures formed. The highest reported ee value was 100 %. Based on these results it is assumed that from a racemic mixture of amino acids and a spontaneous effective resolution can be carried out, even if the asymmetric synthesis of a single amino acid, without the aid of an optically active molecule can not take place.
This is the first study that explains the origin of chirality from racemic amino acids with experimental evidence.
The term homochirality was introduced by Kelvin in 1904, the same year in which he published his " Baltimore Lecture " of 1884. Lately, the term was however homochiral used in the same sense as enantiomerically pure. This is allowed in some journals, but it is not encouraged. In these journals However, the term means that in a process or system, a single one of the two optical isomers is preferred.