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 (TiO₂) 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 TiO₂ nanostructures. We have assayed these peptides for TiO₂-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 TiO₂ nanospheres. Here, we investigate the process of TiO₂ nanosphere formation mediated by the S–K peptides KSSKK- and SKSK₃SKS using one-dimensional and two-dimensional solid-state NMR (ssNMR) on peptide samples with uniformly ¹³C-enriched residues. ssNMR is used to assign ¹³C chemical shifts (CSs) site-specifically in each free peptide and TiO₂-embedded peptide, which are used to derive secondary structures in the neat and TiO₂ coprecipitated states. The backbone ¹³C CSs are used to assess secondary structural changes undergone during the coprecipitation process. Side-chain ¹³C CS changes are analyzed with density functional theory calculations and used to determine side-chain conformational changes that occur upon coprecipitation with TiO₂ and to determine surface orientation of lysine side chains in TiO₂–peptide composites