The silica mineralisation properties of synthetic Silaffin-1A1 (synSil-1A1)

Abstract

The synthetic monodisperse pentadecapeptide synSil-1A₁ is a representative of the microdisperse mixture of the native silaffin natSil-1A₁ produced by the diatom Cylindrotheca fusiformis. The octaphosphorylated zwitterionic synSil-1A₁ is able to mineralise silica under slightly acidic conditions at pH 5.5, which is the physiologically relevant pH range assumed. Like the posttranslational modifications of the native silaffins, synSil-1A₁ is functionalised on all four lysine and phosphorylated on all seven serine residues. We describe the synthesis of a trimethyl-δ-hydroxy-L-lysine building block, the incorporation of this choline-type amino acid in peptide synthesis and its phosphorylation, together with all further posttranslational modifications observed in the native silaffins. Quantitative structure–activity relationships from silicification experiments at high dilution reveal the unique mineralisation properties of the hyperphosphorylated peptide as a single substance and in interaction with long-chain polyamines (LCPA). Diffusion-ordered spectroscopy (DOSY) experiments reveal the formation of polyelectrolyte complexes (PEC) between synSil-1A₁ and long-chain polyamines, which promotes the silicification process. The microdroplets have an overall balanced ratio of 100–150 cationic and the same number of anionic charges. The unique zwitterionic synSil-1A₁ confirms the prevailing molecular model of biosilicification and validates it with quantitative data based on a single phosphopeptide species, avoiding the usual unphysiologically high concentrations of phosphate of many previous in vitro silicification experiments.