Trimethylation of the R5 Silica-Precipitating Peptide Increases Silica Particle Size by Redirecting Orthosilicate Binding.

The unmodified R5 peptide from silaffin in the diatom Cylindrotheca fusiformis rapidly precipitates silica particles from neutral aqueous solutions of orthosilicic acid. A range of post-translational modifications found in R5 contribute toward tailoring silica morphologies in a species-specific manner. We investigated the specific effect of R5 lysine side-chain trimethylation, which adds permanent positive charges, on silica particle formation. Our studies revealed that a doubly trimethylated R5K3,4me3 peptide has reduced maximum activity yet, surprisingly, generates larger silica particles. Molecular dynamics simulations of R5K3,4me3 binding by the precursor orthosilicate anion revealed that orthosilicate preferentially associates with unmodified lysine side-chain amines and the peptide N terminus. Thus, larger silica particles arise from reduced orthosilicate association with trimethylated lysine side chains and their redirection to the N terminus of the R5 peptide.

Solid-State NMR and MD Study of the Structure of the Statherin Mutant SNa15 on Mineral Surfaces.

Elucidation of the structure and interactions of proteins at native mineral interfaces is key to understanding how biological systems regulate the formation of hard tissue structures. In addition, understanding how these same proteins interact with non-native mineral surfaces has important implications for the design of medical and dental implants, chromatographic supports, diagnostic tools, and a host of other applications. Here, we combine solid-state NMR spectroscopy, isotherm measurements, and molecular dynamics simulations to study how SNa15, a peptide derived from the hydroxyapatite (HAP) recognition domain of the biomineralization protein statherin, interacts with HAP, silica (SiO2), and titania (TiO2) mineral surfaces. Adsorption isotherms are used to characterize the binding affinity of SNa15 to HAP, SiO2, and TiO2. We also apply 1D 13C CP MAS, 1D 15N CP MAS, and 2D 13C-13C DARR experiments to SNa15 samples with uniformly 13C- and 15N-enriched residues to determine backbone and side-chain chemical shifts. Different computational tools, namely TALOS-N and molecular dynamics simulations, are used to deduce secondary structure from backbone and side-chain chemical shift data. Our results show that SNa15 adopts an α-helical conformation when adsorbed to HAP and TiO2, but the helix largely unravels upon adsorption to SiO2. Interactions with HAP are mediated in general by acidic and some basic amino acids, although the specific amino acids involved in direct surface interaction vary with surface. The integrated experimental and computational approach used in this study is able to provide high-resolution insights into adsorption of proteins on interfaces.

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.

Comparative Study of Secondary Structure and Interactions of the R5 Peptide in Silicon Oxide and Titanium Oxide Coprecipitates Using Solid-State NMR Spectroscopy.

A biomimetic, peptide-mediated approach to inorganic nanostructure formation is of great interest as an alternative to industrial production methods. To investigate the role of peptide structure on silica (SiO2) and titania (TiO2) morphologies, we use the R5 peptide domain derived from the silaffin protein to produce uniform SiO2 and TiO2 nanostructures from the precursor silicic acid and titanium bis(ammonium lactato)dihydroxide, respectively. The resulting biosilica and biotitania nanostructures are characterized using scanning electron microscopy. To investigate the process of R5-mediated SiO2 and TiO2 formation, we carry out 1D and 2D solid-state NMR (ssNMR) studies on R5 samples with uniformly 13C- and 15N-labeled residues to determine the backbone and side-chain chemical shifts. 13C chemical shift data are in turn used to determine peptide backbone torsion angles and secondary structure for the R5 peptide neat, in silica, and in titania. We are thus able to assess the impact of the different mineral environments on peptide structure, and we can further elucidate from 13C chemical shifts change the degree to which various side chains are in close proximity to the mineral phases. These comparisons add to the understanding of the role of R5 and its structure in both SiO2 and TiO2 formation.