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

Abstract

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 the pendent carboxylates reorient, and their motional amplitude increases as the water cluster at the binding site grows larger. Onset of dissolution manifested by rapid isotropic reorientation is reached when only one or two water molecules are present at the binding site. This study demonstrates the unique sensitivity of solid-state NMR to probe surface binding strength, structure, and dynamics.