Binding Specificity of Amino Acids to Amorphous Silica Surfaces: Solid-State NMR of Glycine on SBA-15

Abstract

The interactions between bioorganic molecules and inorganic surfaces play a key role in a wide range of multidisciplinary phenomena, among which are catalysis, biomineralization, separation methods, and surface functionalization. Binding of amino acids to inorganic surfaces is of special interest due to their significant role in protein–surface recognition; however, direct experimental evidence on the molecular details of these is scant and often inconclusive. Herein, [1-13C,15N]glycine interactions with amorphous silica surface of SBA-15 were comprehensively characterized using multinuclear, solid-state NMR techniques (REDOR, TEDOR, SLF, 2D-HETCOR). Glycine’s ammonium group is shown to interact directly with a specific surface site of a well-defined geometry and stoichiometry: −NH3+ interacts with 3–4 approximately equidistant (r[N···Si] = 4.1 ± 0.3 Å) silanols, predominantly Q3’s, arranged in a triangular or square geometry (r[Si···Si] ∼ 5 Å). While the −NH3+ group is surface anchored, the pendent carboxylate reorients with small amplitude with a minor or no contribution to binding. The role of water molecules was studied by increasing surface hydration and temperature and monitoring bound glycine dynamics. Glycine populations with increasing reorientation amplitudes, through isotropic motion of dissolved glycine, coexist, reflecting binding sites solvated by larger water clusters. The similarity of the specific silica site and of the interactions and dynamic modes of bound glycine to those previously reported for l-alanine on SBA-15 suggests we evidence a general binding pattern of amino acids with nonpolar side chains to amorphous silica surfaces. Although loaded from unsaturated aqueous solution, competing with the sparse surface binding, surface-induced crystallization of the α and β polymorphs occurred. Tailored solid-state NMR methodology yields direct, quantitative experimental evidence that enables molecular-functional description of the interfacial interactions and further demonstrates the importance of this class of techniques in the wide field of surface science.