A computational study of the gas-phase interstellar formose-like reactions

Abstract

Understanding the extraterrestrial origin of ribose, as one of the sub-units of ribonucleic acid (RNA), is crucial to anticipating the formation of the building blocks of life under interstellar medium (ISM) conditions. Under ordinary atmospheric conditions, the formation of sugar is suggested to occur through the formose reaction, where formaldehyde is solely used as the precursor in each of the iterative steps of the formation process. On the other hand, sugar synthesis under ISM conditions has evidently been shown to be significantly different from the terrestrial formose reactions due to the extremely low density of molecules and the very low temperature. In this study, we theoretically investigate the formose-like reactions for ribose formation catalyzed by a single proton through the mean of computer simulation based on first-principles density functional theory (DFT). The ribose formation reactions are modelled in the absence of solvation effects at absolute zero temperature to mimic the extremely low pressure and temperature found in ISM. We observe that the presence of a proton gives rise to a more thermodynamically stable complex of the two reactive carbonyl compounds by bridging their oxygen atoms as required for the iterative formose reaction to proceed. In order to form a new carbon-carbon bond through an iterative process, only the region where the additional formaldehyde attaches to the existed protonated carbonyl compound is of great importance. The energy barriers for all the iterative steps during the ribose formation are similar, i.e., ∼67 kcal mol−1. Our findings show that a single proton can act as an efficient catalyst for the gas-phase ribose formation reactions in the absence of solvent effects. The results also suggest that ribose formation can preferentially occur via the formose pathway in the presence of protons, which are abundantly presented in ISM.