Synthesis of Amino Acids from Aldehydes and Ammonia: Implications for Organic Reactions in Carbonaceous Chondrite Parent Bodies

The Strecker cyanohydrin synthesis is a known mechanism for the formation of amino acids from aldehydes, ammonia, and cyanide. An alternative mechanism for amino acid synthesis called the ammonia-involved formose-like reaction uses aldehydes and ammonia, and it has been demonstrated to explain the amino acid distributions in carbonaceous meteorites. In this study, experiments were performed to synthesize amino acids from 13C-labeled aldehydes and ammonia and to investigate the detailed formation pathways of the formose-like reaction. The effects of the initial aldehyde compositions and free oxygen on the reaction were also examined. The production of amino acids, including glycine, significantly increased in the air compared to in a nitrogen atmosphere, which indicates that oxygen in the air promotes amino acid synthesis. A precursor compound of glycine was identified as N-oxalylglycine, and it can be synthesized from ammonia and glyoxylic acid produced by the oxidation of glycolaldehyde. This formation mechanism indicates that the oxidation of aldehydes is crucial for amino acid synthesis by the formose-like reaction. Based on the formation pathway of glycine via N-oxalylglycine, α-, β-, and γ-oxo acids can be hypothesized to react with ammonia to produce the various structural isomers of amino acids found in carbonaceous chondrites, including α-, β-, and γ-hydroxy amino acids.