Racemization of meteoritic amino acids

Meteorites may have contributed amino acids to the prebiotic Earth, affecti ng the global ratio of right-handed to left-handed (D/L) molecules. We calc ulate D/L ratios for seven biological, alpha-hydrogen, protein amino acids over a variety of plausible parent body thermal histories, based on meteori te evidence and asteroid modeling. We show that amino acids in meteorites d o not necessarily undergo complete racemization by the time they are recove red on Earth. If the mechanism of amino acid formation imposes some enantio meric preference on the amino acids, a chiral signature can be retained thr ough the entire history of the meteorite. Original enantiomeric excesses in meteorites such as Murchison, which have undergone apparently short and co ol alteration scenarios, should have persisted to the present time. Of the seven amino acids for which relevant data are available, we expect glutamic acid, isoleucine, and valine, respectively, to be the most likely to retai n an initial enantiomeric excess, and phenylalanine, aspartic acid, and ala nine the least. Were the D/L ratio initially identical in each amino acid, final D/L ratios could be used to constrain the initial ratio and the therm al history experienced by the whole suite. (C) 2000 Academic Press.