COMPUTATIONAL STUDY OF INTERSTELLAR GLYCINE FORMATION OCCURRING AT RADICAL SURFACES OF WATER-ICE DUST PARTICLES

Glycine is the simplest amino acid and due to the significant astrobiological implications that suppose its detection, its search in the interstellar medium (ISM), meteorites, and comets is intensively investigated. In the present work, quantum mechanical calculations based on density functional theory have been used to model the glycine formation on water-ice clusters present in the ISM. The removal of either one H atom or one electron from the water-ice cluster has been considered to simulate the effect of photolytic radiation and of ionizing particles, respectively, which lead to the formation of OH • radical and H 3 O + surface defects. The coupling of incoming CO molecules with the surface OH • radicals on the ice clusters yields the formation of the COOH radicals via ZPE-corrected energy barriers and reaction energies of about 4–5 kcal mol − 1 and − 22 kcal mol − 1 , respectively. The COOH radicals couple with incoming NH = CH 2 molecules (experimentally detected in the ISM) to form the NHCH 2 COOH • radical glycine through energy barriers of 12 kcal mol − 1 , exceedingly high at ISM cryogenic temperatures. Nonetheless, when H 3 O + is present, one proton may be barrierless transferred to NH = CH 2 to give NH 2 = CH 2 + . This latter may react with the COOH • radical to give the NH 2 CH 2 COOH + • glycine radical cation which can then be transformed into the NH 2 CHC(OH) 2 + • species (the most stable form of glycine in its radical cation state) or into the NH 2 CHCOOH • neutral radical glycine. Estimated rate constants of these events suggest that they are kinetically feasible at temperatures of 100–200 K, which indicate that their occurrence may take place in hot molecular cores or in comets exposed to warmer regions of solar systems. Present results provide quantum chemical evidences that defects formed on water ices due to the harsh-physical conditions of the ISM may trigger reactions of cosmochemical interest. The relevance of surface H 3 O + ions to facilitate chemical processes by proton transfer (i.e., acting as acidic catalysts) is highlighted, and plausible ways of their formation at the water-ice surface in the ISM are also discussed.