On the reported optical activity of amino acids in the Murchison meteorite

In analyses of extracts from the Murchison meteorite (a carbonaceous chondrite), Engel and Nagy1 reported an excess of L-enantiomers for several protein amino acids but found that the non-protein amino acids were racemic. They suggested that the excess of L-isomers might have resulted from an asymmetric synthesis or decomposition. Their results disagree with those obtained previously2–4 and they claim this is due to improved methodology. In fact, their extraction method and analytical procedure (gas chromatography–mass spectrometry, GC–MS) was similar to those used in the original report2 of amino acids in the Murchison meteorite except that they used specific ion monitoring in the GC–MS measurements. We found the results of Engel and Nagy odd in that likely contaminants (the protein amino acids ala, leu, glu, asp and pro) were nonracemic while unlikely contaminants (isovaline and α-amino-n-butyric acid) were racemic. For example, Engel and Nagy report that the leucine is ∼90% L-enantiomer in the water-extracted sample whereas isovaline (α-methyl-α-aminobutyric acid) is racemic. It would be most unusual for an abiotic stereoselective decomposition or synthesis of amino acids to occur with protein amino acids but not with non-protein amino acids. We now show here that the explanation of terrestrial contamination is consistent with their results and is much more probable.