A comparative study of the catalysis of peptide bond formation by oxide surfaces

It is well-known that amino acids deposited on some inorganic oxides undergo peptidic condensation. It is seldom realised however that a large diversity of behaviours can be observed in such systems. Here we use the apparently simple case of glycine-non-porous silica as a reference system, in which glycine (Gly) dimerisation to diketopiperazine (DKP) is easy to evidence, especially when using TG in combination with NMR. We then proceed to compare it with other AA deposited on the same support on the one hand, with Gly deposited on other mineral surfaces on the other hand. In a final section, we provide more detailed mechanistic information on the glycine condensation process on silica, including kinetic data and a C-13 solid-state NMR follow up of the species at various stages of thermal condensation. The best mechanism to rationalise these data involves a crucial step of isomerisation from zwitter-ion to neutral glycine, and the participation of several distinct types of surface sites probably consisting of silanol ensembles.