Molecular and chiral analyses of some protein amino acid derivatives in the Murchison and Murray meteorites

The varied organic suite extracted from the Murchison meteorite contains several amino acids that are common to the biosphere. Some of these have been found to be non-racemic, but the indigenous nature of their L-enantiomeric excesses has been subject to debate in view of possible terrestrial contamination. We have investigated two amino acids of common terrestrial and meteoritic occurrence, alanine and glutamic acid, and assessed their indigenous enantiomeric ratios in the Murchison and Murray meteorites through the ratios of some of their derivatives. Analyzed were N-acetyl alanine, alpha-imino propioacetic acid, N-acetyl glutamic acid and pyroglutamic acid. Both alanine derivatives were found to be racemic, while those of glutamic acid showed L-enantiomeric excesses varying from 16% to 47.2% for pyroglutamic acid, and from 8.6% to 41% for N-acetyl glutamic acid. The delta C-13 was determined for the two enantiomers of Murchison pyroglutamic acid both before and after acid hydrolysis to glutamic acid. The values of +27.7 parts per thousand (D-pyro), +10.0 parts per thousand (L-pyro), +32.2 parts per thousand (D-glu) and +14.6 parts per thousand (L-glu) were obtained. The racemic nature of alanine derivatives strongly suggests that alanine itself, as indigenous to the meteorite, is racemic. The explanation of the L-enantiomeric excesses found for glutamic acid derivatives is less direct; however, the variability of the enantiomeric ratios for these compounds and the distinctly lower delta C-13 values determined for pyroglutamic L-enantiomer point to a terrestrial contamination, possibly dating to the time of fall.