Homochirality
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'''Homochirality''' is a term used to refer to a group of molecules that possess the same sense of [[Chirality (chemistry)|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. Virtually all active forms of amino acids are of the [[enantiomer|L-form]] ([[serine#Signaling|d-serine]] being a notable exception) 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. It is even unclear if homochirality has a purpose. One suggestion is that it reduces [[entropy]] barriers in the formation of large organized molecules. It has been experimentally verified that amino acids form large aggregates in larger abundance from [[enantiopure]] substrates than from [[racemic]] ones. <ref>''Do Homochiral Aggregates Have an Entropic Advantage?'' Julian, R. R.; Myung, S.; Clemmer, D. E. J. Phys. Chem. B.; (Article); '''2005'''; 109(1); 440-444. {{DOI|10.1021/jp046478x}} </ref>.
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.
It is also entirely possible that homochirality is simply a result of the natural autoamplification process of life -- that either the formation of life as preferring one chirality or the other was a chance rare event which happened to occur with the chiralities we observe, or that all chiralities of life emerged rapidly but due to catastrophic events and strong competition, the other unobserved chiral preferences were wiped out by the preponderance and metabolic, enantiomeric enrichment from the 'winning' chirality choices {{Fact|date=March 2008}}.
==Mirror-symmetry breaking==
Explaining how an enantiomeric imbalance is created in the first place is a difficult question to answer. The discovery of an enantiomeric imbalance in molecules in the [[Murchison meteorite]] support an extraterrestrial origin. There is evidence for the existence of [[Circular polarization|circularly polarized light]] originating from [[white dwarf]]s) which may trigger the formation of optical isomers in space. Another speculation suggests that fundamental chirality of physical processes such as that of the beta decay (see [[Parity (physics)|Parity violation]]) lead to slightly different half-lives of biologically relevant molecules.
==Chiral amplification==
Laboratory experiments exist demonstrating how in certain [[autocatalysis|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,<ref name=shibata>{{cite journal|title=Highly Enantioselective Catalytic Asymmetric Automultiplication of Chiral Pyrimidyl Alcohol| author= Takanori Shibata, Hiroshi Morioka, Tadakatsu Hayase, Kaori Choji, and Kenso Soai| journal = [[J. Am. Chem. Soc.]] | date=1996| volume= 118 | issue=2| pages=471 - 472 | doi=10.1021/ja953066g}}</ref> pyrimidine-5-carb[[aldehyde]] (''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%.
[[Image:Soai autocatalysis.png|center|500px|Scheme 1. Soai autocatalysis]]
Another study <ref name=mathew>{{cite journal|title=Amplification of Enantiomeric Excess in a Proline-Mediated Reaction| author= Suju P. Mathew, Hiroshi Iwamura and [[Donna G. Blackmond]] | journal = [[Angewandte Chemie International Edition]] | volume = 43| issue = 25 | date=21 Jun 2004| pages= 3317-3321}}</ref> 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.
[[Image:Homochiralproline.png|center|500px|Scheme 2. Blackmond autocatalysis]]
[[Serine octamer cluster]]s <ref>{{cite journal | title = Chiroselective Self-Directed Octamerization of Serine: Implications for Homochirogenesis | author = Cooks, R. G., Zhang, D., Koch, K. J. | journal = [[Anal. Chem.]] | volume'''2001''' | volume = 73 | issue = 15) | pages = 3646-3655 | doi = 10.1021/ac010284l}}</ref><ref>{{cite journal | title = Serine Octamers: Cluster Formation, Reactions, and Implications for Biomolecule Homochirality | author = Nanita, S., Cooks, R. G. | journal = [[Angew. Chem. Int. Ed.]] | year = 2006 | volume = 45 | issue = 4 | pages = 554-569 | doi = 10.1002/anie.200501328}}</ref> are also contenders. These clusters of 8 serine molecules appear in mass spectroscopy with an unusual homochiral preference, however there is no evidence that such clusters exist under non-ionizing conditions and amino acid phase behavior is far more prebiotically relevant <ref>{{cite journal | title = Spoilt for choice: assessing phase behaviour models for the evolution of homochirality | author = Donna G. Blackmond and Martin Klussmann | journal = [[Chem. Commun.]] | year = 2007 | pages = 3990 - 3996 | doi = 10.1039/b709314b}}</ref>. The recent observation that partial [[sublimation]] of a 10% enantioenriched sample of [[leucine]] results in up to 82% enrichment in the sublimate shows that enantioenrichment of amino acids could occur in space <ref>{{cite journal | title = An astrophysically relevant mechanism for amino acid enantiomer enrichment | author = Stephen P. Fletcher, Richard B. C. Jagt and Ben L. Feringa | journal = [[Chem. Commun.]] | volume = 2007 | year = 2007 | pages = 2578 - 2580 | doi = 10.1039/b702882b}}</ref>. Partial sublimation processes can take place on the surface of meteors where large variations in temperature exist. This finding may have consequences for the development of the [[Mars Organic Detector]] scheduled for lauch in 2013 which aims to recover trace amounts of amino acids from the Mars surface exactly by a sublimation technique.
A high asymmetric amplification of the [[enantiomeric excess]] of sugars are also present in the [[amino acid]] catalyzed asymmetric formation of [[carbohydrates]]<ref name=Engqvist>{{cite journal| title=Plausible origins of homochirality in the amino acid catalyzed neogenesis of carbohydrates| author= Armando Córdova, Magnus Engqvist, Ismail Ibrahem, Jesús Casas, Henrik Sundén | journal=[[Chem. Commun.]] | volume=15| pages= 2047 - 2049 |date=2005}}</ref>
One classic study involves an experiment that takes place in the laboratory.<ref name=kondepudi>{{cite journal|title=Chiral Symmetry Breaking in Sodium Chlorate Crystallization| author= Kondepudi, D. K., Kaufman, R. J. & Singh, N.| journal= [[Science (journal)|Science]] | volume = 250 | pages = 975-976|date=1990}}</ref> When [[sodium chlorate]] is allowed to [[crystallization|crystallize]] from water and the collected crystals examined in a [[polarimeter]], each crystal turns out to be chiral and either the [[Levorotation|L]] form or the [[Dextrorotation|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.
In a related experiment, a crystal suspension of a racemic [[amino acid]] derivative contineously stirred, results in a 100% crystal phase of one of the enantiomers because the enantiomeric pair is able to equilibrate in solution (compare with [[dynamic kinetic resolution]]) <ref>''Emergence of a Single Solid Chiral State from a Nearly Racemic Amino Acid Derivative'' Wim L. Noorduin, Toshiko Izumi, Alessia Millemaggi, Michel Leeman, Hugo Meekes, Willem J. P. Van Enckevort, Richard M. Kellogg, Bernard Kaptein, Elias Vlieg, and Donna G. Blackmond [[J. Am. Chem. Soc.]]; '''2008'''; 130(4) pp 1158 - 1159; (Communication) {{DOI|10.1021/ja7106349}}</ref>
==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 reaction]]s.
==Optical resolution in racemic amino acids==
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<ref name=kojo>{{cite journal|author=S. Kojo, H. Uchino, M. Yoshimura, and K. Tanaka |title = 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.| journal = [[Chem. Comm.]] | pages= 2146 - 2147 | date= 2004|doi = 10.1039/b409941a}}</ref> in 2004 that excess racemic [[asparagine|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.<ref name=kojo/> 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.
==History==
This term was introduced by [[William Thomson, 1st Baron Kelvin|Kelvin]] in 1904, the year that published his Baltimore Lecture of 1884.<ref name=morris>''Stereochemistry'' David G. Morris, Cambridge : Royal Society of Chemistry, '''2001''', p30.</ref><ref name=Engqvist/> Recently, however, homochiral has been used in the same sense as [[enantiomer]]ically 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.
==See also==
* [[stereochemistry]]
* [[Unsolved problems in chemistry]]
* [[CIP system]]
==References==
{{reflist}}
==External links==
* ''On the Genesis of Homochirality '' A. Maureen Rouhi [[Chemical & Engineering News]] June 17, '''2004''' [http://pubs.acs.org/cen/news/8224/8225earlyscicon.html Link]
* ''Observations Support Homochirality Theory '' Photonics TechnologyWorld November '''1998''' [http://www.photonics.com/spectra/tech/XQ/ASP/techid.443/QX/read.htm Link]
* Scienceweek digest '''1998''' [http://scienceweek.com/1998/sw980828.htm 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''' [http://www.imperial.ac.uk/P5343.htm Link]
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