About the detectability of glycine in the interstellar medium

Abstract

Context. Glycine, the simplest of aminoacids, has been found in several carbonaceous meteorites. It remains unclear, however, wether glycine is formed in the interstellar medium (ISM) and therefore available everywhere in the Universe. For this reason, radioastronomers have searched for many years unsuccessfully to detect glycine in the ISM. Aims. We provide possible guidelines to optimize the return of these searches. Since, for most of the species observed so far in the ISM, the most abundant isomer of a given generic chemical formula is the most stable one (minimum energy principle (MEP)), we assess whether neutral glycine is the best molecule to search for or whether one of its isomers/conformers or ionic, protonated, or zwitterionic derivatives would have a higher probability of being detected. Methods. The question of the relative stability of these different species is addressed by means of quantum density functional theory (DFT) simulations within the hybrid B3LYP formalism. Each fully optimized structure is verified as a stationary point by means of a vibrational analysis. A comprehensive screening of 32 isomers/conformers of the C 2 H 5 O 2 N chemical formula (neutral, negative, and positive ions together with the corresponding protonated species and the possible zwitterionic structures) is carried out. In the sensitive case of the neutral compounds, more accurate relative energies were obtained by means of high level post Hartree-Fock coupled cluster calculations with large basis sets (CCSD(T)/cc-pVQZ). Results. We find that neutral glycine is not the most stable isomer and, therefore, probably not the most abundant one, which might explain why it has escaped detection so far. We find instead that N-methyl carbamic acid and methyl carbamate are the two most stable isomers and, therefore, probably the two most abundant ones. Among the non-neutral forms, we found that glycine is the most stable isomer only if protonated or zwitterionic if present in interstellar ices. Conclusions. Assuming that MEP can be applied to optimize our search for glycine, our conclusion is that this search will remain extremely difficult with the present instruments and we propose searching instead for other examples among the most stable isomers.

Possible survival of simple amino acids to X-ray irradiation in ice: the case of glycine

“…Several cases of highenergy isomers have indeed been found, such as HNC for HCN, cyc-C 2 H 4 O for CH 3 CHO, or CH 2 OHCHO for CH 3 COOH. It simply suggests that the glycine abundance should be very low and probably too small to be detected by the present instruments (Lattelais et al 2011b). On the other hand, the identification of various amino acids in meteorites and the recent synthesis of prebiotic materials in artificial comets (Meinert et al 2012) support the hypothesis of the formation of glycine in icy environments, which may apply to interstellar, as well as to outer solar system, conditions.…”

“…This is due to the additional OH reaction that is known to convert CO into CO 2 + H (Laffon et al 2010). It is very efficient in the water ice mixture because it increases the CO 2 /CO ratio by three in the water mixture compared to pure glycine (Lattelais et al 2011b). …”

“…In a recent theoretical study we have shown that glycine may not exist under the unique form of a neutral species, NH 2 CH 2 COOH, depending on the environment (Lattelais et al 2011b). The possible fragmentations of glycine were investigated accordingly under different forms in order to cover all possibilities, namely, ionized, doubly ionized, protonated, and zwitterionic, which potentially exist in one or another interstellar environment.…”

C5H9N isomers: pointers to possible branched chain interstellar molecules

“…Thus, before proposing to observe any particular species in the interstellar region, it is essential to know the relative energies between all its and their conformers and also to point out the lowest energy conformer of the species of interest [32][33]. Based on our calculations (enthalpy of formation) it is evident that Tert-butylnitrile, Isobutyl cyanide and n-butyl cyanide are the three most stable isomers of the C 5 H 9 N isomeric group.…”

Quantum-chemical approach to serine formation in the interstellar medium: A possible reaction pathway

“…These results also strengthen the possibility that glycine and its precursors exist in the ISM. Protonated glycine is predicted to be the most stable isomer of the family, although its abundance is expected to be very low (Lattelais et al 2011). This indicates another possibility that NH 2 CHCOOH is present in the ISM.…”