Abstract
The silica forming repeat R5 of sil1 from Cylindrotheca fusiformis was the blueprint for the design of P(5) S(3) , 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(5) S(3) enhances silica-formation whereas it retards the dissolution of amorphous silica (SiO(2) ) at globally undersaturated concentrations. The secondary structure of P(5) S(3) 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 ((13) C, (15) N) P(5) S(3) 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(5) S(3) indicated partial transition into an α-helical conformation which was transient during silica-dissolution. The secondary structural changes observed for P(5) S(3) correlate with the presence of oligomeric/polymeric silicic acid, presumably due to P(5) S(3) -silica interactions. These P(5) S(3) -silica interactions appear, at least in part, ionic in nature since negatively charged dodecylsulfate caused similar perturbations to the P(5) S(3) CD spectrum as observed with silica, while uncharged ß-d-dodecyl maltoside did not affect the CD spectrum of P(5) S(3) . Thus, with an associated increase in α-helical character, P(5) S(3) influences both the condensation of silicic acid into silica and its decondensation back to silicic acid.