Partial transfer of enantioselective chiralities from a-methylated amino acids, known to be of meteoritic origin, into normal amino acids

There is overwhelming evidence that meteorites bring a-methylated amino acids to earth with some L(S) enantiomeric excess. How does that get transferred into normal biological molecules? In this brief account, we show that an a-methylated amino acid, D(R)-a-methylvaline, can react with pyruvate and phenylpyruvate salts in dry mixtures to form alanine and phenylalanine with L enantiomeric excesses, under sensible prebiotic conditions. Thus the meteoritic L(S) excesses of this compound would produce excess D-alanine and D-phenylalanine, which are found in some organisms. 2006 Elsevier Ltd. All rights reserved. One of the most intriguing biological/chemical questions is the origin of homochirality on earth. Various ideas have been discussed including the possibility that there could be some enantiomeric enrichment induced in terrestrial processes by circularly polarized light or chiral minerals, but one of the most interesting recent findings is that enantiomeric preferences have been brought to our planet in organic compounds carried by meteorites. Murray and Murchison carbonaceous chondritic meteorites have been found to contain a-methylated amino acids with L(S) enantiomeric excesses (ee) up to 15%. It is generally believed that a small excess of this magnitude or lower can be amplified to the point at which the proteins of living organisms could have the known 100% excess of the L amino acids in proteins, for instance, so the enantiomeric excesses seen in the meteorites could be the seed from which our known highly homochiral biological molecules have grown. In fact, such amplification of small ees has been seen in some chemical processes, but not yet in reactions that could play a prebiotic role. a-Methylated amino acids really cannot racemize, in contrast with normal biological amino acids that have 0040-4039/$ see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2006.01.018