The role of defective silica surfaces in exogenous delivery of prebiotic compounds: clues from first principles calculations

Abstract

The reaction of glycine (Gly) with a strained (SiO)₂ four-membered ring defect (D2) at the surface of an interstellar silica grain dust has been studied at ONIOM2[B3LYP/6-31+G(d,p):MNDO] level within a cluster approach in the context of hypothetical reactions occurring in the interstellar medium. The D2 opens up exothermically for reaction with Gly (Δ_rU₀=−26.3 kcal mol⁻¹) to give a surface mixed anhydride S_surf–O–C(O)–CH₂NH₂ as a product. The reaction barriers, ΔU^≠₀, are 0.1 and 10.4 kcal mol⁻¹ for reactive channels involving COOH and NH₂ as attacking groups, respectively. Calculations show the surface mixed anhydride to be rather stable under the action of interstellar processes, such as reactions with isolated H₂O and NH₃ molecules or the exposure to cosmic rays and UV radiation. The hydrolysis of the surface mixed anhydride to release again Gly was modelled by microsolvation (from 1 to 4 H₂O molecules) mimicking what could have happened to the interstellar dust after seeding the primordial ocean in the early Earth. Results for these calculations show that the reaction is exergonic and activated, the Δ_rG₂₉₈ becoming more negative and the ΔG^≠₂₉₈ being dramatically reduced as a function of increasing number of H₂O molecules. The present results are relevant because they show that defects present at interstellar dust surfaces could have played a significant role in capturing, protecting and delivering essential prebiotic compounds on the early Earth.