Bivalent Surface Attachment via Cysteine Thiol Results in Efficient and Stereoselective Abiotic Peptide Synthesis

Abstract

Surface-catalyzed peptide bond formation may have been an important source of peptides for abiogenesis, but model peptide synthesis reactions using the consensus set of 10 abiotic amino acids give only modest rates of peptide bond formation. Additionally, the peptides are typically limited in length to a small number of amino acids and stereoselective amino acid incorporation is weak or absent. An abiotic route for the high-yield synthesis of cysteine from serine was recently reported by Foden et al. (Science 2020, 370, 865–869), indicating that, in some environments, prebiotic cysteine may also have been available. Here, we show that the presence of cysteine dramatically increases the yields of surface-catalyzed peptide synthesis reactions in a hydrothermal vent solvent model containing achiral silicate minerals and that the reaction exhibits a strong stereoselective bias toward l-cysteine. Solid state NMR confirmed that cysteine associates bivalently with silicates at alkaline pH via both the carboxylate and the sulfur groups. Polarization-resolved IRRAS indicates that the bivalent adsorption stereospecifically orients the reactive amino group, providing a mechanism for stereoselective incorporation of l-cysteine. Stereoselective rates of peptide bond formation in surface-catalyzed peptide bond formation are expected to occur for any amino acid able to form sufficiently strong side chain–silicate interactions at alkaline pH. The high nucleophilicity of the thiol group produces unusually high reaction rates and stereoselectivity in such reactions, in addition to conferring transition metal ion binding to the peptide products. The potential benefits of reactive sulfur species for abiogenesis suggest that they may be useful biosignatures in the search for habitable extraterrestrial environments.