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

Abstract

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-[1-(13)C,(15)N]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 -NH(3)(+) functional group by (15)N{(1)H}SLF NMR. (29)Si{(15)N} and (15)N{(29)Si}REDOR NMR revealed intermolecular interactions between the alanine -NH(3)(+) and three to four surface Si species, predominantly Q(3), with similar internuclear N...Si distances of 4.0-4.2 A. Distinct dynamic states of the adsorbed biomolecules were identified by (15)N{(13)C}REDOR NMR, indicating both bound and free alanine populations, depending on hydration level and temperature. In the bound populations, the -NH(3)(+) 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.

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…In spite of this, the application of solid-state NMR to understand biomolecules adsorption on silica is recent. 211,[224][225][226][227][228] This is due to the low intrinsic sensitivity of the 13 C nucleus. In conjunction with the fact that the biomolecule is a minority species in the samples, it causes NMR signals to be very weak, so that long accumulation times are necessary -unless one uses expensive isotopically enriched biomolecules.…”

A Molecular Picture of the Adsorption of Glycine in Mesoporous Silica through NMR Experiments Combined with DFT-D Calculations

“…In that sense, solid-state NMR is a perfect tool of investigation for the hybrid organic−inorganic interfaces in nanomaterials. NMR spectroscopy and more particularly dipolar recoupling experiments were used recently to identify interacting moieties in alanine adsorbed on a mesoporous silica SBA-15 surface, 7 and later in glycine adsorbed on the same support. 8 In both cases, the authors show that the ammonium group interacts directly with silanols (predominantly terminal ones) on the surface while the carboxylates seem to freely reorient.…”

Recent Advances in Solid State NMR of Small Molecules in Confinement

“…In the complementary works by Amitay‐Rosen97 and Ben Shir et al. ,98 the behavior of selectively labeled alanine inside mesoporous SBA‐15 or MCM‐41 was probed by means of 1 H and 2 H MAS NMR. Ben Shir et al.…”

“…Ben Shir et al. 98 were able to detect a single population of surface‐bound alanine at a hydration state lower than four protons per nm 2 . It was found that the binding occurs between three adjacent silanol binding sites with a distance r[N⋅⋅⋅Si]=4.0–4.2 Å.…”

“…Amitay‐Rosen et al. 104 expanded their studies on alanine and glycine97–98 to methionine and dialanine and probed them confined in SBA‐15. The 1 H MAS spectra of methionine‐d 3 were measured as a function of water content and temperature and indicated results similar to the ones found for alanine.…”