Homochirality
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Homochirality is a term used to refer to a group of molecules that possess the same sense of chirality. Molecules involved are not necessarily the same compound, but similar groups are arranged in the same way around a central atom. In biology homochirality is found inside living organisms. Active forms of amino acids are all of the L-form and most biologically relevant sugars are of the D-form. Typically, the alternative form is inactive and sometimes even toxic to living things. The origin of this phenomenon is not clearly understood. Homochirality is said to evolve in three distinct steps: mirror-symmetry breaking creates a minute enantiomeric imbalance and is key to homochirality, chiral amplification is a process of enantiomeric enrichment and chiral transmission allows the transfer of chirality of one set of molecules to another.
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[edit] Mirror-symmetry breaking
Explaining how an enantiomeric imbalance is created is the first place is the most difficult question to answer. Supporters exist for an extraterrestrial origin based on findings relating to the Murchison meteorite. There is evidence for the existence of circularly polarized light in space which may trigger the formation of optical isomers.
One classic study involves an experiment that takes place in the laboratory.[1] When sodium chlorate is allowed to crystallize from water and the collected crystals examined in a polarimeter, each crystal turns out the be chiral and either the L form or the D form. In an ordinary experiment the amount of L crystals collected equals the amount of D crystals (corrected for statistical effects). However when the sodium chlorate solution is stirred during the crystallization process the crystals are either exclusively L or exclusively D. In 32 consecutive crystallization experiments 14 experiments deliver D-crystals and 18 others L-crystals. The explanation for this symmetry breaking is unclear but is related to autocatalysis taking place in the nucleation process.
[edit] Chiral amplification
Laboratory experiments exist demonstrating how in certain autocatalytic reaction systems the presence of a small amount of reaction product with enantiomeric excess at the start of the reaction can result in a much larger enantiomeric excess at the end of the reaction. In one pioneering study,[2] pyrimidine-5-carbaldehyde (Scheme 1) is alkylated by diisopropylzinc to the corresponding pyrimidyl alcohol. Because the initial reaction product is also an effective catalyst the reaction is autocatalytic. The presence of just 0.2 equivalent of the alcohol S-enantiomer at the start of the reaction is sufficient to amplify the enantiomeric excess to 93%.
Another study [3] concerns the proline catalyzed aminoxylation of propionaldehyde by nitrosobenzene (scheme 2). In this system too the presence of enantioenriched catalyst drives the reaction towards one of the two possible optical isomers.
Serine octamer clusters are also contenders. Serine is also unique in catalyzing the asymmetric aldol reaction between ketones and aldehydes with a significant asymmetric amplification.[4]
A high asymmetric amplification of the enantiomeric excess of sugars are also present in the amino acid catalyzed asymmetric formation of carbohydrates[5]
[edit] Chiral transmission
Many strategies in asymmetric synthesis are built on chiral transmission. Especially important is the so-called organocatalysis of organic reactions by proline for example in Mannich reactions.
[edit] Theory of spontaneous optical resolution from a mixture of racemic amino acids
Although many efforts have been devoted to elucidate the origin of the homochirality, especially L-amino acids, no clear solution has been obtained. Recently the extraterrestrial L-amino acids are suggested to be the origin of L-amino acids on the earth. During long history of organic chemistry, it has been fully shown that chiral molecule is necessary for asymmetric synthesis. Since studies on chemical evolution demonstrate that racemic amino acids (D,L-amino acids) were formed on the prebiotic ocean, the central question must be the mechanism how chirality is formed from racemic amino acids.
In addition it must be pointed out that there exists no theory elucidating correlations among L-amino acids. If one takes, for example, alanine, which has a small methyl group, and phenylalanine, which has a big benzyl group, a simple question is in what aspect, L-alanine resembles L-phenylalanine more than D-phenylalanine, and what kind of mechanism causes the selection of all L-amino acids. Because it might be possible that alanine was L and phenylalanine was D.
It was reported[6] in 2004 that excess racemic D,L-asparagine (Asn), which spontaneously forms crystals of either isomer during recrystallization, induces asymmetric resolution of a co-existing 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). The enantiomeric excess {ee=100x(L-D)/(L+D)} of these amino acids was correlated almost linearly with that of the inducer, i.e., Asn. When recrystallizations from a mixture of 12 D,L-amino acids (Ala, Asp, Arg, Glu, Gln, His, Leu, Met, Ser, Val, Phe, and Tyr) and excess D,L-Asn were made, all amino acids with the same configuration with Asn were preferentially co-crystallized.[6] It was incidental whether the enrichment took place in L- or D-Asn, however, once the selection was made, the co-existing amino acid with the same configuration at the α-carbon was preferentially involved because of thermodynamic stability in the crystal formation. The maximal ee was reported to be 100%. Based on these results, it is proposed that a mixture of racemic amino acids causes spontaneous and effective optical resolution, even if asymmetric synthesis of a single amino acid does not occur without an aid of an optically active molecule.
This is the first study elucidating reasonably the formation of chirality from racemic amino acids with experimental evidences.
[edit] History
This term was introduced by Kelvin in 1904, the year that published his Baltimore Lecture of 1884.[7][5] Recently, however, homochiral has been used in the same sense as enantiomerically pure. This is permitted in some journals (but not encouraged), its meaning changing into the preference of a process or system for a single optical isomer in a pair of isomers in these journals.
[edit] See also
[edit] References
- ^ Kondepudi, D. K., Kaufman, R. J. & Singh, N. (1990). "Chiral Symmetry Breaking in Sodium Chlorate Crystallization". Science 250: 975-976.
- ^ Takanori Shibata, Hiroshi Morioka, Tadakatsu Hayase, Kaori Choji, and Kenso Soai (1996). "Highly Enantioselective Catalytic Asymmetric Automultiplication of Chiral Pyrimidyl Alcohol". J. Am. Chem. Soc. 118 (2): 471 - 472. DOI:10.1021/ja953066g.
- ^ Suju P. Mathew, Hiroshi Iwamura and Donna G. Blackmond (21 Jun 2004). "Amplification of Enantiomeric Excess in a Proline-Mediated Reaction". Angewandte Chemie International Edition 43 (25): 3317-3321.
- ^ Armando Córdova , Weibiao Zou, Pawel Dziedzic, Ismail Ibrahem, Efraim Reyes, Yongmei Xu (2006). "Direct asymmetric intermolecular aldol reactions catalyzed by amino acids and small peptides". Chem. Eur. J. 12. DOI:10.1002/chem.200501639.
- ^ a b Armando Córdova, Magnus Engqvist, Ismail Ibrahem, Jesús Casas, Henrik Sundén (2005). "Plausible origins of homochirality in the amino acid catalyzed neogenesis of carbohydrates". Chem. Commun. 15: 2047 - 2049.
- ^ a b S. Kojo, H. Uchino, M. Yoshimura, and K. Tanaka (2004). "Racemic D,L-asparagine causes enantiomeric excess of other coexisting racemic D,L-amino acids during recrystallization: a hypothesis accounting for the origin of L-amino acids in the biosphere.". Chem. Comm.: 2146 - 2147. DOI:10.1039/b409941a.
- ^ Stereochemistry David G. Morris, Cambridge : Royal Society of Chemistry, 2001, p30.
[edit] External links
- On the Genesis of Homochirality A. Maureen Rouhi Chemical & Engineering News June 17, 2004 Link
- Observations Support Homochirality Theory Photonics TechnologyWorld November 1998 Link
- Scienceweek digest 1998 Link
- How left-handed amino acids got ahead: a demonstration of the evolution of biological homochirality in the lab Press release Imperial College London 2004 Link
