Identification of the HO•CHC(O)NH2 Radical Intermediate in the Reaction of H + Glycolamide in Solid Para-Hydrogen and Its Implication to the Interstellar Formation of Higher-Order Amides and Polypeptides

Abstract

Glycolamide [HOCH₂C(O)NH₂, GAm], the only isomer of glycine [H₂NCH₂C(O)OH] detected in the interstellar medium (ISM) to date, consists of an inherent peptide bond [C(O)NH] that is fundamental to protein synthesis and the origin of life. Despite its importance in ISM, detailed investigations on the reactivity of GAm under cosmic circumstances remain rather limited. In the present study, we performed the reaction involving H atoms and GAm in solid para-hydrogen (p-H₂) at 3.2 K and observed the exclusive formation of 2-amino-1-hydroxy-2-oxoethyl radical [HO^•CHC(O)NH₂] via H abstraction on the CH₂ moiety of GAm. We successfully characterized the infrared spectra of both Cis–cis (Cc)- and Trans–trans (Tt)-conformers of HO^•CHC(O)NH₂ from the reactions of H + Cc-GAm and H + Tt-GAm in a single experiment; the ratio of Cc:Tt conformers of GAm in the deposited matrix was estimated to be ∼3:2, and, after H abstraction, that of HO^•CHC(O)NH₂ remains approximately the same. In darkness, the increase in the infrared intensities of both conformers of HO^•CHC(O)NH₂ indicated that these radicals were formed from the reaction H + Cc-/Tt-GAm through tunneling, a possible route in dark interstellar clouds. This radical intermediate, HO^•CHC(O)NH₂, may serve as a potential precursor in the formation of higher-order amides bearing a chiral center, including lactamide and glyceramide (glycerol amide), after reactions with ^•CH₃ and ^•CH₂OH, respectively. In addition to the conventional condensation reaction in the process of polymerization for the formation of dipeptides such as malonamide in the dark regions of the ISM, the radical–radical reaction of HO^•CHC(O)NH₂ with NH₂^•CO might serve as an alternative pathway.