A combined experimental and theoretical study on the formation of the amino acid glycine (NH2CH2COOH) and its isomer (CH3NHCOOH) in extraterrestrial ices

We have investigated the synthesis of the simplest amino acid, glycine, by Galactic cosmic-ray particles in extraterrestrial ices. Laboratory experiments combined with electronic structure calculations showed that a methylamine molecule [CH3NH2(X(1)A ')] can be dissociated through interaction with energetic electrons in the track of a cosmic-ray particle to form atomic hydrogen and the radicals CH2NH2(X(2)A ') and CH3NH(X(2)A '). Hydrogen atoms with sufficient kinetic energy could overcome the entrance barrier to add to a carbon dioxide molecule [CO2(X(1)Sigma(g)(+))], yielding a trans-hydroxycarbonyl radical, HOCO(X(2)A '). Neighboring radicals with the correct geometric orientation then recombine to form glycine, NH2CH2COOH(X(1)A), and also its isomer, CH3NHCOOH(X(1)A). These findings expose for the first time detailed reaction mechanisms of how the simplest amino acid glycine and its isomer can be synthesized via nonequilibrium chemistry in interstellar and cometary ices. Our results offer an important alternative to aqueous and photon-induced formation of amino acids in comets and in molecular clouds. These results also predict the existence of a hitherto undetected isomer of glycine in the interstellar medium, suggest that glycine should be observable on Saturn's moon Titan, and help to account for the synthesis of more complex amino acids in the Murchison and Orgueil meteorites.