The survival of glycine in interstellar ices: A coupled investigation using NEXAFS experiments and theoretical calculations

摘要

Complex organic molecules have been observed in the interstellar medium in comets and meteorites. Among these compounds are the building blocks that may have led to prebiotic systems and ultimately to the origin of life. In the panspermia hypothesis, the survival and the transfer of the amino acids from space to planets is a necessary condition for the appearance of life, and, especially, their resistance to the solar UV radiation in ice is a key issue. The case of glycine, which is the smallest molecule in the amino acid family, is presented here. To improve our knowledge on the decomposition mechanisms of glycine, we have undertaken a coupled experimental and computational study. On the one hand, it has been performed a near-edge X-ray absorption spectroscopy (NEXAFS) study at the oxygen K-edge of solid glycine and of glycine diluted into H_2O ice, and irradiated at 30 K with soft X-rays. However, extensive quantum chemical simulations using density functional theory have been realized at the B3LYP/cc-pVQZ level. A systematic investigation of the most plausible fragmentations has been performed for neutral glycine itself, ionized glycine, doubly ionized glycine, protonated glycine, and zwitterionic glycine, the well known stable form of glycine trapped in ices. We show that glycine easily decomposes under irradiation. Water does not enhance or protect glycine from photodecomposition, but it increases the production of CO_2 in the secondary photoreactions. The concentration of glycine tends to a limit of ～30% of the initial load in the ice. The role of water ice appears to be that of a containment environment allowing the fragments to remain close, thus leading to reformation of glycine in situ. Under these conditions, it can be concluded that glycine is partially, but not entirely, protected by ice during its journey to the Earth. It is a strong point to be credited to the panspermia hypothesis.