A REDOR ssNMR Investigation of the Role of an N-Terminus Lysine in R5 Silica Recognition.

Diatoms are unicellular algae that construct cell walls called frustules by the precipitation of silica, using special proteins that order the silica into a wide variety of nanostructures. The diatom species Cylindrotheca fusiformis contains proteins called silaffins within its frustules, which are believed to assemble into supramolecular matrices that serve as both accelerators and templates for silica deposition. Studying the properties of these biosilicification proteins has allowed the design of new protein and peptide systems that generate customizable silica nanostructures, with potential generalization to other mineral systems. It is essential to understand the mechanisms of aggregation of the protein and its coprecipitation with silica. We continue previous investigations into the peptide R5, derived from silaffin protein sil1p, shown to independently catalyze the precipitation of silica nanospheres in vitro. We used the solid-state NMR technique 13C{29Si} and 15N{29Si} REDOR to investigate the structure and interactions of R5 in complex with coprecipitated silica. These experiments are sensitive to the strength of magnetic dipole-dipole interactions between the 13C nuclei in R5 and the 29Si nuclei in the silica and thus yield distance between parts of R5 and 29Si in silica. Our data show strong interactions and short internuclear distances of 3.74 ± 0.20 Å between 13C═O Lys3 and silica. On the other hand, the Cα and Cβ nuclei show little or no interaction with 29Si. This selective proximity between the K3 C═O and the silica supports a previously proposed mechanism of rapid silicification of the antimicrobial peptide KSL (KKVVFKVKFK) through an imidate intermediate. This study reports for the first time a direct interaction between the N-terminus of R5 and silica, leading us to believe that the N-terminus of R5 is a key component in the molecular recognition process and a major factor in silica morphogenesis.