The structural organization of the interface characterizing binding, assembly and recognition of biomolecules on inorganic surfaces has attracted considerable attention in the domains of catalysis and prebiotic chemistry. In the context of origins-of-life chemistry, this study is focused on the catalytic effect of a silica surface on amino acids condensation, more precisely on two amino acids, Leucine and Glutamic acid, adsorbed on silica nanoparticles. It takes into consideration the important effect of hydration on the system. Solid state NMR is a choice technique for surface characterization and observation of bonding at the atomic level, as well as local proximities between the amino acid and the silica surface sites.
13C and 15N CP-MAS NMR and 2D-HETCOR experiments allowed to characterize the structure of adsorption complexes at the interface, involving amino acids, surface groups, and water molecules. At high surface coverages, both crystalline and adsorbed Leucine exist in the samples, while only adsorbed forms of leucine were observed at rather low surface coverage (3%Leu/SiO2). The same type of approach was applied to Glutamic Acid, but the adsorbed form was predominant only at much lesser loadings (0.3%Glu/SiO2) than those observed for Leucine. The sensitivity of NMR compared to other characterization techniques is low; however, we were able to use 13C and 15N-enriched amino acids which gave us information on the evolution of chemical shifts, after adsorption and upon drying in careful conditions, even for such low loadings.
Hydration was a key parameter in our experiments and samples with varying degrees of hydration were studied experimentally by NMR, and also by computational methods such as DFT simulation. Water turned out to mediate the interaction of molecules with silanols of the silica surface.