In living organisms, chiral compounds such as amino acids and sugars exist as single enantiomers. What is the origin of this exclusive preference? Japanese scientists believe that achiral molecules from space could provide the answer.
One theory for the presence of single enantiomers in nature is that slight excesses of one enantiomer in the prebiotic world could have been increased further by autocatalysis, a reaction where the product promotes its own formation. These slight initial excesses may be extraterrestrial in origin, as they have been observed for a variety of chiral molecules found in meteorites.
The achiral molecules in meteorites may have been made chiral by deuterium
However, Kenso Soai and co-workers from the Tokyo University of Science claim that the high deuterium content in meteoritic environments could mean that achiral compounds are just as important. 'Even the most simple achiral amino acid, glycine, becomes chiral when one of the methylene group's hydrogen atoms is substituted by deuterium,' explains Soai. 'Meteoritic chiral amino acids due to deuterium substitution may exist and may constitute the extraterrestrial origin of chirality.'
Detecting enantiomers formed by isotope substitution is difficult though, says Soai. Despite this, the team found that deuterium-enriched glycine and alpha-methylalanine, achiral amino acids that were made chiral by deuterium substitution, efficiently catalysed reactions between aldehydes and organozinc compounds. These reactions produced enantioenriched alcohols in high excesses, the chirality of which was dependent on that of the amino acid catalyst.
Soai's team is the first to show that isotopically chiral amino acids can promote such a highly specific reaction. John Brown, an expert in asymmetric catalysis at the University of Oxford, UK, describes the work as interesting and stimulating. 'Substantive interest lies in analysis of meteorites with a small enantiomeric excess in amino acid content,' he says. 'This chemistry could be used to discover whether the known deuterium excess in meteorites is correlated with asymmetry in otherwise achiral amino acids.'
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