Secondary structure and dynamics study of the intrinsically disordered silica-mineralizing peptide P S during silicic acid condensation and silica decondensation.

The silica forming repeat R5 of sil1 from Cylindrotheca fusiformis was the blueprint for the design of P S , a 50-residue peptide which can be produced in large amounts by recombinant bacterial expression. It contains 5 protein kinase A target sites and is highly cationic due to 10 lysine and 10 arginine residues. In the presence of supersaturated orthosilicic acid P S enhances silica-formation whereas it retards the dissolution of amorphous silica (SiO ) at globally undersaturated concentrations. The secondary structure of P S during these 2 processes was studied by circular dichroism (CD) spectroscopy, complemented by nuclear magnetic resonance (NMR) spectroscopy of the peptide in the absence of silicate. The NMR studies of dual-labeled ( C, N) P S revealed a disordered structure at pH 2.8 and 4.5. Within the pH range of 4.5-9.5 in the absence of silicic acid, the CD data showed a disordered structure with the suggestion of some polyproline II character. Upon silicic acid polymerization and during dissolution of preformed silica, the CD spectrum of P S indicated partial transition into an α-helical conformation which was transient during silica-dissolution. The secondary structural changes observed for P S correlate with the presence of oligomeric/polymeric silicic acid, presumably due to P S -silica interactions. These P S -silica interactions appear, at least in part, ionic in nature since negatively charged dodecylsulfate caused similar perturbations to the P S CD spectrum as observed with silica, while uncharged ß-d-dodecyl maltoside did not affect the CD spectrum of P S . Thus, with an associated increase in α-helical character, P S influences both the condensation of silicic acid into silica and its decondensation back to silicic acid.