Serine-Lysine Peptides as Mediators for the Production of Titanium Dioxide: Investigating the Effects of Primary and Secondary Structures Using Solid-State NMR Spectroscopy and DFT Calculations.

A biomimetic approach to the formation of titania (TiO) nanostructures is desirable because of the mild conditions required in this form of production. We have identified a series of serine-lysine peptides as candidates for the biomimetic production of TiO nanostructures. We have assayed these peptides for TiO-precipitating activity upon exposure to titanium bis(ammonium lactato)dihydroxide and have characterized the resulting coprecipitates using scanning electron microscopy. A subset of these assayed peptides efficiently facilitates the production of TiO nanospheres. Here, we investigate the process of TiO nanosphere formation mediated by the S-K peptides KSSKK- and SKSKSKS using one-dimensional and two-dimensional solid-state NMR (ssNMR) on peptide samples with uniformly C-enriched residues. ssNMR is used to assign C chemical shifts (CSs) site-specifically in each free peptide and TiO-embedded peptide, which are used to derive secondary structures in the neat and TiO coprecipitated states. The backbone C CSs are used to assess secondary structural changes undergone during the coprecipitation process. Side-chain C CS changes are analyzed with density functional theory calculations and used to determine side-chain conformational changes that occur upon coprecipitation with TiO and to determine surface orientation of lysine side chains in TiO-peptide composites.