Possible interstellar formation of glycine from the reaction of CH₂NH, CO and H₂O: catalysis by extra water molecules through the hydrogen relay transport

Abstract

"How the fundamental life elements are created in the interstellar medium (ISM)?" is one of the intriguing questions related to the genesis of life. Using computational calculations, we have discussed the reaction of CH2=NH, CO and H2O for the formation of glycine, the simplest life element. This reaction proceeds through a concerted mechanism with reasonably large barriers for the cases with one and two water molecules as reactants. For the two water case we found that the extra water molecule exhibits some catalytic role through the hydrogen transport relay effect and the barrier height is reduced substantially compared to the case with one water molecule. These two cases can be treated as ideal cases for the hot-core formation of the interstellar glycine. With an increasing number of water molecules as the reactants, we found that when the numbers of water molecules are three or more than three, the barrier height reduced so drastically that the transition states were more stable than the reactants. Such a situation gives a clear indication that with excess water molecules as the reactants, this reaction will be feasible even under the low temperature conditions existing in the cold interstellar clouds and the exothermic nature of the reaction will be the driving force.

Classical dynamics simulations of interstellar glycine formation via $$\hbox {CH}_{2} = \hbox {NH} + \hbox {CO} + \hbox {H}_{2}\hbox {O}$$ CH 2 = NH + CO + H 2 O reaction

“…and it was found that all the reactions proceed through formation of a pre-reaction complex involving all the reactant molecules i.e., reactant complexes formed from 4, 5, and 6 reactant molecules for n = 2, 3, and 4, respectively. With increase in n, the barrier height for Gly formation decreases and for n = 4, energy of the respective transition state is lower 28 as comparison to the reactants. This decrease in the barrier height with increasing number of water molecules has been viewed as an indicator of catalytic effect by the excess water molecules for reaction 1.…”

“…For the above reaction, detailed density functional electronic structure calculations at the B3LYP/6-31++g (3df, 2pd) level of theory (and other methods) were reported by Nhlabatsi et al 28 Reaction 1 is exoergic and proceeds via formation of a stable hydrogen bonded reactant complex involving all three reactants. This ternary complex passes through a five-membered transition state forming Gly molecule.…”

“…This ternary complex passes through a five-membered transition state forming Gly molecule. The calculations were also performed at MP2 level of theory and the results 28 showed only minor changes to the potential energy profile of the reaction. This work indicated a concerted type of mechanism for the formation of Gly through the CH 2 = NH+CO+H 2 O reaction as opposed to a stepwise mechanism proposed earlier by Rimola et al, 29 for the same reaction.…”

“…clusters -thus indicating a catalytic role 29 exhibited by the water-ice clusters for the formation of Gly in the ISM. Following this work, Nhlabatsi et al, 28 also investigated the effect of excess water molecules on reaction 1 at the same B3LYP/6-31++g(3df,2pd) level of electronic structure theory. Calculations were carried out for the reactions…”

“…This effect was discussed in terms of an efficient proton transfer to the CH 2 = NH reactant molecule by the additional water molecules through a relay transport mechanism. 28 Several earlier studies 30,31 in the literature have also described the ability of water molecules to help transfer proton to a reaction site.…”