Abiotic Peptide-Like Bond Formation in Gas-Phase Nitrile-Water Clusters Driven by Vacuum Ultraviolet Photoionization

The abiotic formation of prebiotic molecules with peptide or peptide-like bonds in the cold gaseous interstellar medium (ISM) and (exo)planetary atmospheric environments remains a critical unsolved question for elucidating cosmic chemical evolution and the origin of life. To explore the potential formation pathway, we combined vacuum ultraviolet-infrared (VUV-IR) photodissociation spectroscopy with ab initio quantum chemical calculations to investigate the ion-molecule reactions of three unsaturated nitriles (acrylonitrile, AN; 3-butenenitrile, 3BN; and 4-pentenenitrile, 4PN) with water under 118 nm photoionization. Experimental spectra reveal that the reaction products show a pronounced dependence on the alkyl chain length of the nitrile precursors. When short-chain AN reacts with water, it exclusively forms a kinetically trapped imine-alcohol cation (H⁺-N≡C-R-OH). In contrast, longer-chain 3BN and 4PN undergo efficient barrierless cyclization upon reaction with water, forming stable five- or six-membered cyclic cations with peptide-like bonds, respectively. The conclusions were supported by theoretical calculations of reaction pathways, which further reveal that all these reactions initially proceed via a mechanism analogous to Michael addition under basic conditions. These findings provide direct evidence for an ionization-induced intracluster covalent bond formation (ICBF) pathway to prebiotic peptide-like bonds under astrochemically relevant conditions, highlighting the critical role of molecular structure in governing its reaction selectivity.