The silica mineralisation properties of synthetic Silaffin-1A 1 ( syn Sil-1A 1 ).

The synthetic monodisperse pentadecapeptide syn Sil-1A 1 is a representative of the microdisperse mixture of the native silaffin nat Sil-1A 1 produced by the diatom Cylindrotheca fusiformis . The octaphosphorylated zwitterionic syn Sil-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, syn Sil-1A 1 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 syn Sil-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 syn Sil-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.