Formation of Glycine on Ultraviolet-Irradiated Interstellar Ice-Analog Films and Implications for Interstellar Amino Acids

Abstract

We report the synthesis of glycine on interstellar ice-analog films composed of water, methylamine (MA), and carbon dioxide under irradiation of ultraviolet (UV) photons. Analysis of the UV-irradiated ice films by in situ mass spectrometric methods revealed glycine and other isomers as photochemical products. Deuterium-labeling experiments were conducted to determine the structures of the photoproducts and to examine their formation pathways. The reactions occur via photocleavages of C-H and N-H bonds in MA, followed by subsequent reactions of the nascent H atom with CO 2 , leading to the formation of HOCO and then to glycine and carbamic acid. The photochemical synthesis of glycine occurs efficiently at the ice surfaces, and the competing photosynthesis and photodestruction processes can reach a steady-state kinetic balance at an extended UV exposure, maintaining a substantial population level of glycine. The observation suggests that interstellar amino acids can be created on ice grains, and that they can also be stored in the ices by maintaining a kinetic balance under interstellar UV irradiation. As such, the transport of amino acids in interstellar space may be possible without depleting the net abundance of amino acids in the ices but rather increasing the structural diversity of the molecules.

Adsorption of Methylamine at the Surface of Ice. A Grand Canonical Monte Carlo Simulation Study

“…The attempted insertions and deletions were accepted or rejected according to the corresponding acceptance criterion. 49,50 Searching for cavities was done along a 100 × 100 × 100 grid, and repeated in every 10 6 Monte Carlo steps of the simulations. The probability of finding an empty cavity for insertion, P cav , a value needed for applying the acceptance criterion of the cavity biased scheme, 49,50 was determined as the ratio of the number of suitable cavities found and total number of grid points tested.…”

Reactive Ion Scattering of Low Energy Cs+from Surfaces. A Technique for Surface Molecular Analysis

“…They include the RIS and LES studies of surface diffusion of water molecules, 28 ionization of electrolytes, [29][30][31] acid-base chemistry, 12,[32][33][34][35] proton transfer, [36][37][38][39][40][41] and various types of reactions that are either of fundamental interest [42][43][44] or related to environmental chemistry [25][26] and astrochemistry. 45,46 These subjects have been reviewed recently. 27,47 Ice surfaces offer a unique reaction environment, which is quite different from a liquid water or gas environment.…”

Computational Study of Interstellar Glycine Formation Occurring at Radical Surfaces of Water-Ice Dust Particles

“…For instance, VUV photolysis or cosmic rays irradiation of interstellar ice analogs (i.e., made by H 2 O, NH 3 , CH 4 , CH 3 OH, CO, CO 2 , HCN, CH 3 CN molecules) followed by warming and acidic hydrolysis of the obtained residue led to the formation of Gly and other amino acids (Bernstein et al 2002;Elsila et al 2007;Hudson et al 2008;Kobayashi et al 1995;Muñoz Caro et al 2002;Nuevo et al 2008;Takano et al 2002). Moreover, UV irradiation or electron bombardment of NH 3 and CH 3 COOH or CH 3 NH 2 and CO 2 ices brings about the Gly molecule (Holtom et al 2005;Lafosse et al 2006;Lee et al 2009), and thermal processing of CH 3 NH 2 and CO 2 in a water-dominated ice forms yielded the formation of [CH 3 (Bossa et al 2009). In spite of the experimental evidence, very little is known about the mechanistic steps through which amino acids form and about the specific role played by the icy dust particles as catalysts.…”