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

The synthetic monodisperse pentadecapeptide synSil-1A(1) is a representative of the microdisperse mixture of the native silaffin natSil-1A(1) produced by the diatom Cylindrotheca fusiformis. The octaphosphorylated zwitterionic synSil-1A(1) 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-1A(1) is functionalised on all four lysine and phosphorylated on all seven serine residues. We describe the synthesis of a trimethyl-delta-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-1A(1) 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-1A(1) 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.

Automated summary

The synthetic monodisperse pentadecapeptide synSil-1A(1) is a representative of the microdisperse mixture of the native silaffin natSil-1A(1) produced by the diatom Cylindrotheca fusiformis. The unique mineralisation properties of synSil-1A(1) were confirmed through quantitative experiments, providing validation for the prevailing molecular model of biosilicification.