Theoretical study of prebiotic precursors-2: about glycine, its N-carboxyanhydride and their protonated ions

This study was motivated by the fact that some amino acids are currently identified in carbonaceous chondrites but none are yet observed in the interstellar medium. The question of the relative stability of these prebiotic compounds with respect to the other possible species of the same chemical formula is addressed by means of quantum chemical simulations based on density functional theory (DFT). A preliminary investigation of molecules with the formula C2H5O2N shows that glycine (H2NCH2COOH) is not the most stable compound that can be formed: it is similar to 10 kcal mol(-1) higher in energy than CH3NHCOOH, which becomes the most likely candidate for a search in space. The same argument of relative stability used to explained why formamide H2N-HC=O was identified in the interstellar medium (rather than any of its isomers) provides here a rationalization of the permanent debate about the detection of glycine. A second investigation of protonated derivatives of the same series that may result from ion molecule chemistry shows that under its nitrogen protonated form (H3N+CH2COOH) glycine does become the most stable compound. This suggests that, if glycine presently detected in chondrites has an interstellar origin, it must have been synthetized as its protonated form by ion-molecule processes. A similar study was carried out on the corresponding glycine N-carboxyanhydride. The calculated rotational constants (GHz) of glycine A = 10.3603; B = 3.8590; C = 2.9037 (Exp: 10.3418; 3.8762; 2.9123, respectively) served as the benchmark to derive the correcting factors necessary to estimate the rotational constants of the possible candidates within a few tenths of a percent. The dipole moments were determined and the values in the range 4-6 Debye obtained for the protonated species should be large enough to initiate laboratory experiments and/or observations. Theoretical infrared spectra are also provided to assist in the identification of these exotic species. Received 29 July 2006, accepted 29 October 2006