Molecular Details of Amorphous Silica Surfaces Determine Binding Specificity to Small Amino Acids

Molecular details of glycine adsorbed on mesoporous MCM-41 silica surfaces were determined, and their comparison to binding onto SBA-15 surfaces provided a correlation between surface structure and reactivity. Employing solid-state NMR techniques, the interfacial interactions and structural and dynamic states of surface-bound glycine and l-alanine were revealed as a function of hydration and temperature. These small amino acids with nonpolar side-chains show a general pattern of interactions with silica surfaces via their −NH3+ group with pendent carboxylate. While SBA-15 uses specific surface binding sites consisting of closely spaced 3–4 silanols, MCM-41 exhibits weaker and therefore less specific binding employing fewer surface silanols and/or longer Si···N distances, as is also manifested by enhanced temperature-dependent dynamics. A single population of likely bound amino acids exists when the surfaces are dehydrated. Upon minute hydration and/or temperature increase, new populations form in which th...