Amino Acids in Carbonaceous Chondrites

Abstract

For almost 20 years laboratory experiments have advanced the concepts of chemical evolution, particularly with regard to formation of the amino acids. What has been generally lacking is concrete natural evidence for this chemical evolution hypothesis. The recent development of sophisticated analytical techniques and availability of carbonaceous chondrites with a minimum of terrestrial contamination has resulted in the identification of amino acids which provide strong evidence for a natural extraterrestrial chemical synthesis. Since the initial find in the Murchison meteorite (a type II carbonaceous chondrite) of both protein and nonprotein amino acids and amino acids with nearly equal abundances of D and L isomers, further studies have been carried out. These studies have revealed the presence of at least 35 amino acids; the population consists of a wide variety of linear, cyclic and polyfunctional amino acids which shows a trend of decreasing concentration with increasing carbon number. Investigations of the Murray meteorite (a type II carbonaceous chondrite) has produced similar results, but studies of the Orgueil meteorite (a type I carbonaceous chondrite) show only a limited suite of amino acids, some of which appear to be indigenous while others appear to be terrestrial contaminants. A sample of the Murchison meteorite was extracted with D₂O and in addition to ‘free’ amino acids, showing no deuterium incorporation, some amino acids showed the presence of deuterium suggesting either a ‘precursor(s)’ or hydrogen-deuterium exchange which require(s) formation of carbon-hydrogen bonds.