Formation of Amino Acid Precursors in the Interstellar Medium. A DFT Study of Some Gas-Phase Reactions Starting with Methylenimine

Abstract

To evaluate feasibility of the formation of amino acid precursors in dense interstellar clouds from methylenimine, HCN, ‚CN radical, water, and ‚OH radical, stationary points for four gas-phase reactions have been computed at the B3LYP/6-311++G(d,p) level of theory. All the reactions are exothermic. For the reaction HNdCH 2 + HsCtN f H 2 NsCH 2 sCtN, three high-energy transition states have been found (energies of ca. 23-33 kcal mol -1 relative to the reactant level) which do not allow this reaction to occur as a purely gas-phase process. The same conclusion has been done for the further glycine amide formation according to the pathway H 2 NsCH 2 sCtN + H 2 O f H 2 NsCH 2 sC(dO)sNH 2 , where three transitions states have been obtained as well, one of them having relative energy of ca. 52 kcal mol -1 . Reaction HNd CH 2 + ‚CtN f H 2 NsC(‚)HsCtN (>70 kcal mol -1 exothermic) exhibits no positive barriers at the theoretical level employed; it is the only one unconditionally feasible in the gas-phase. Despite the fact that for reaction H 2 NsCH 2 sCtN + ‚OH f H 2 NsC(‚)HsC(dO)sNH 2 (ca. 60 kcal mol -1 exothermic) four transition states have been found, it might also appear plausible under the interstellar conditions for the following reasons: all the barriers are associated with exclusively proton-transfer processes; only one of them is positive, and three lay below the level of reactants; for the only positive barrier, proton tunneling might facilitate the reaction. Radical products H 2 NC(‚)HCN and H 2 NC(‚)HC(dO)NH 2 forming in the latter two cases have a free valence at the carbon atom corresponding to R-C atom in the molecules of biological amino acids. Combining them with a vast variety of open-shell interstellar species is suggested as a possible route to diverse amino acid derivatives in dense molecular clouds.

Is the reaction between formic acid and protonated aminomethanol a possible source of glycine precursors in the interstellar medium?

“…Several proposals of different neutral-neutral reactions have been made (Hoyle 1976;Maeda & Ohno 2004a,b, 2006Basiuk & Kobayashi 2004;Andreazza et al 2006). In some cases theoretical studies have been carried out (Basiuk & Bogillo 2000;Basiuk 2001) that conclude that these processes are not feasible under interstellar conditions. Ion-molecule reactions are more promising, since in many cases they are more likely to proceed without involving energy barriers.…”

The reaction between NH3+and CH3COOH: a possible process for the formation of glycine precursors in the interstellar medium

“…Hoyle & Wickramasinghe (1976) suggested the formation of glycine from the reaction of methanimine (HNCH 2 ) and formic acid (HCOOH) since these compounds had been observed in dense molecular clouds of comparable abundance. In this context, Basiuk & Bogillo (2000), Basiuk (2001), and Basiuk & Kobayashi (2002) carried out a theoretical study of amino-acid-precursor formation in the interstellar medium considering different pathways that include the participation of methanimine and methanimine-related cations, HNCH + and H 2 NCH 2 + . Their studies demonstrated that although the reactions considered are exothermic they are not feasible under the interstellar conditions because of the high energy transition states and the possibility of competitive formation of side products.…”

Computational study on the addition of HCN to methanimine catalyzed by formamidine and formamide

“…HNC, the isomer of HCN, was also included as a direct reactant since it is more reactive though less stable [18]. Formamide is dissymmetric and may tautomerize to formamidic acid, which has been also investigated as a catalyst in the addition (Scheme 2).…”