PNNL

Abstract

The silica forming repeat R5 of sil1 from Cylindrotheca fusiformis was the blueprint for the design of P5S3, a 50-residue peptide which can be produced in large amounts by recombinant bacterial expression. It contains five protein kinase A target sites and is highly cationic due to 10 lysine and 10 arginine residues. In the presence of supersaturated ortho silicic acid P5S3 strongly enhances silica-formation whereas it retards the dissolution of amorphous silica (SiO2) at globally undersaturated concentrations. The secondary structure of P5S3 during these different functions was studied by circular dichroism (CD), complemented by nuclear magnetic resonance (NMR) studies of the peptide in the absence of silicate. The NMR studies of dual-labeled (13C, 15N) P5S3 revealed a disordered structure at pH 2.8 and 4.5. Within the pH range of 4.5 to 9.5, the CD data verified the disordered secondary structure but also suggested the presence of some polyproline II character in the absence of silicic acid. Upon silicic acid polymerization and during dissolution of preformed silica, the CD spectrum of P5S3 indicated partial transition into an a-helical conformation which was transient during silica-dissolution. Consequently, the secondary structural changes observed for P5S3 correlate with the presence of oli-gomeric/polymeric silicic acid, presumably due to P5S3-silicic acid interactions. These interactions appear, at least in part, ionic in nature since dodecylsulfate micelles, which are negatively charged, cause similar conformational shifts to P5S3 in the absence of silica while ß-D-dodecyl maltoside micelles, which are neutral, do not. Thus, P5S3 influences both the condensation of silicic acid into silica and its decondensation back to silicic acid. Moreover, the dynamics of these pro-cesses may be indirectly monitored by following structural changes to P5S3 with CD spectroscopy.