Structure and function of the silicifying peptide R5

The 19-mer synthetic peptide known as R5 has been used widely in studies of peptide-driven silica condensation. Despite this, the structure and function of R5 have not yet been fully characterized. Here, we present a systematic study of R5 silicification focusing on three key variables: the concentration of the peptide, the concentration of the silica precursor silicic acid, and the solution pH. Additionally, we present the first study of R5 secondary structure in the presence and absence of silicic acid and introduce one-dimensional and two-dimensional solution NMR to probe both structure and higher-order peptide aggregation. We find that R5-directed silicification is linear with regard to silicic acid and H⁺ but, unexpectedly, that silicification appears to be cooperative with respect to peptide concentration. We also find that R5 is a random coil ensemble at subsaturating silicic acid concentrations and does not spontaneously self-assemble to form discrete aggregates in solution. These data contradict a model that invokes the functional micellization of R5 and provide a framework for future studies with the R5 peptide.