Nature (1978) 272: 443-444.
Published: 30 March 1978.
A variety of indigenous organic compounds have been detected in carbonaceous meteorites, and of these, the amino acids have been the most extensively studied. In addition to the amino acids commonly found in proteins of terrestrial organisms, a large number of nonprotein amino acids have also been detected. Moreover, the amino acids in meteorites exist as a racemic mixture, demonstrating their abiotic origin. The general abundance pattern of amino acids in the Murchison meteorite is similar to that obtained in a typical prebiotic electric discharge experiment. The mechanism of formation of amino acids in the electric discharge is by way of the Strecker synthesis, that is, the reaction of hydrogen cyanide, aldehydes and ammonia in aqueous solution, producing amino nitriles, which yield amino acids following acid hydrolysis. In the electric discharge experiment, in addition to amino acid formation, the corresponding alpha-hydroxycarboxylic acids (hydroxy acids) are also produced by the reaction of hydrogen cyanide and aldehydes (the cyanohydrin synthesis). The ratio of hydroxy to amino acids depends on the amount of ammonia present in the system. At high ammonia concentrations, amino acids are preferentially synthesized, whereas if ammonia is absent, only hydroxy acids are produced. Thus, the detection of hydroxy acids in carbonaceous meteorites would provide evidence in support of the hypothesis that the Strecker mechanism is responsible for the natural extraterrestrial synthesis of amino acids found in meteorites. In addition, the ratio of hydroxy to amino acids would provide some indication as to the ammonia content of the environment in which these compounds were synthesized. We report here the presence of seven alpha-hydroxycarboxylic acids in the Murchison meteorite, a type II carbonaceous chondrite.
We thank Dr D. MacDougall for providing the Murchison meteorite sample, Dr S. L. Miller for the sample from the electric discharge experiment and for helpful discussions and encouragement, and E. Hoopes for assistance in the enantiomeric gas chromatographic analyses. JLB is an Alfred P. Sloan Fellow.