Does silica surface catalyse peptide bond formation? New insights from first-principles calculations

The role that silica surface could have played in prebiotic chemistry as a catalyst for peptide bond formation has been addressed at the BRYP/6-31 + G(d,p) level for a model reaction involving glycine and ammonia on a silica cluster mimicking an isolated terminal silanol group present at the silica surface. Hydrogen-bond complexation between glycine and the silanol is followed by the formation of the mixed surface anhydride Si-surf-O-C(=O)-R, which has been suggested in the literature to activate the C=O bond towards nucleophilic attack by a second glycine molecule, here simulated by the simpler NH3 molecule. However, B3LYP/6-31 + G(dp) calculations show that formation of the surface mixed anhydride SisurfO-C(=O)-R is disfavoured (Delta(r)G(298)approximate to 6 kcal mol(-1)), and that the surface bond only moderately lowers the free-energy barrier of the nucleophilic attack responsible for peptide bond formation (Delta G(298)(double dagger) kcal mol(-1)) in comparison with the uncatalysed reaction (Delta G(298)(double dagger) approximate to 52 kcal mol(-1)). A further decrease of the free-energy barrier of peptide bond formation (Delta G(298)(double dagger) approximate to 41 kcal mol(-1)) is achieved by a single water molecule close to the reaction centre acting as a proton-transfer helper in the activated complex. A possible role of strained silica surface defects on the formation of the surface mixed anhydride Si-surf-O-C(=O)-R has also been addressed