Molecular Level Characterization of the Inorganic-Bioorganic Interface by Solid State NMR: Alanine on a Silica Surface, a Case Study

The molecular interface between bioorganics and inorganics plays a key role in diverse scientific and technological research areas including nanoelectronics, biomimetics, biomineralization, and medical applications such as drug delivery systems and implant coatings. However, the physical/chemical basis of recognition of inorganic surfaces by biomolecules remains unclear. The molecular level elucidation of specific interfacial interactions and the structural and dynamical state of the surface bound molecules is of prime scientific importance. In this study, we demonstrate the ability of solid state NMR methods to accomplish these goals. L-[I-C-13,N-15]Alanine loaded onto SBA-15 mesoporous silica with a high surface area served as a model system. The interacting alanine moiety was identified as the -NH3+ functional group by 15N CH ISLE NMR. Si-29 {N-15} and N-15{Si-29} REDOR NMR revealed intermolecular interactions between the alanine -NH3+ and three to four surface Si species, predominantly Q(3), with similar internuclear N center dot center dot center dot Si distances of 4.0-4.2 angstrom. Distinct dynamic states of the adsorbed biomolecules were identified by N-15{C-13} REDOR NMR, indicating both bound and free alanine populations, depending on hydration level and temperature. In the bound populations, the -NH3+ group is surface anchored while the free carboxylate end undergoes librations, implying the carboxylate has small or no contributions to surface binding. When surface water clusters grow bigger with increased hydration, the libration amplitude of the carboxyl end amplifies, until onset of dissolution occurs. Our measurements provide the first direct, comprehensive, molecular-level identification of the bioorganic-inorganic interface, showing binding functional groups, geometric constraints, stoichiometry, and dynamics, both for the adsorbed amino acid and the silica surface.