Characterization of the Structure and Dynamics of Biomimetic Peptides by Solid-State NMR

Abstract

Diatoms and sponges use proteins, long chain polyamines, and other biomolecules to assemble silica structures of controlled morphology. Investigated here are biosilicification peptides. Under mild conditions, these peptides produce silica nanoparticles from solutions of silicic acid, whereas harsh methods are currently employed to produce these nanoparticles commercially. Biomimetic precipitation studies have shown that LKα14 (Ac-LKKLLKLLKKLLKL- C), an amphiphilic lysine/leucine repeat peptide with an α-helical secondary structure at polar/apolar interfaces, co-precipitates with silica to form nanospheres. Previous work con- firmed the α-helical secondary structure in both the neat and silica-complexed states of the peptide and suggested that the tetrameric bundles of peptide that are known to form in solution persisted in the silica-complexed form. To further investigate the peptide aggregation, deuterium solid-state nuclear magnetic resonance (2H ssNMR) was used to establish how the site-specific leucine side-chain dynamics of LKα14 differ in the neat state, buffered state, and silica-precipitated form. Modeling the 2H ssNMR line shapes using code developed in-house helped probe the mechanisms of peptide pre-aggregation and silica co-precipitation. For the silica samples, the model was verified by fitting three types of 2H NMR data: static quadrupolar echo (QE), magic angle spinning (MAS), and T1 inversion recovery (T1IR) spectra. The resulting spectra demonstrate the presence of a tetrameric bundle in the neat state, which is disrupted with the introduction of buffer and then by silica co-precipitation. This work describes the development of the simulation framework used to model these types of experimental data, strategies for fitting these types of data, and finally, the expansion of the model beyond leucine for use with isoleucine.