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, the search for it 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 center dot radical and H3O+ surface defects. The coupling of incoming CO molecules with the surface OH center dot radicals on the ice clusters yields the formation of the COOH center dot radicals via ZPE-corrected energy barriers and reaction energies of about 4-5 kcal mol(-1) and -22 kcal mol(-1), respectively. The COOH center dot radicals couple with incoming NH=CH2 molecules (experimentally detected in the ISM) to form the NHCH2COOH center dot radical glycine through energy barriers of 12 kcal mol(-1), exceedingly high at ISM cryogenic temperatures. Nonetheless, when H3O+ is present, one proton may be barrierless transferred to NH=CH2 to give NH2=CH2+. This latter may react with the COOH center dot radical to give the NH2CH2COOH+center dot glycine radical cation which can then be transformed into the NH2CHC(OH)(2)(+center dot) species (the most stable form of glycine in its radical cation state) or into the NH2CHCOOH center dot 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 evidence 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 H3O+ 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.