Potentially Prebiotic Synthesis of α-Amino Thioacids in Water

Although thioacids have been proposed as acylating agents and energy carriers in the context of prebiotic chemical evolution, there has been little exploration of their synthesis in an aqueous, abiotic setting. Here we report that a-amino thioacids are generated from a-amino acids in water in the presence of carbonyl sulfide (COS), a component of volcanic and deep sea vent emissions. Traces of thioacid are formed when amino acids react with COS alone in water, with the required sulfide presumably originating from COS decomposition. Higher yields of thioacid are observed when sulfide ions are added exogenously to the reactions. The proposed mechanism involves activation of the amino acid to an N-carboxyanhydride (NCA) mediated by COS. Our data shows that COS also catalyzes the hydrolysis of a-amino thioacids, so the observed yields are a net of thioacid synthesis and breakdown. Although the NCA has been a topic of research for decades, this is the first report to our knowledge of NCA-mediated thioacid formation in water. These findings broaden the scope of reactions mediated by COS, suggesting a mechanism by which peptide formation, phosphate activation and phosphorylation, and the syntheses of aminoacyl thioacids, could have shared a common pathway on the prebiotic Earth.