The formation of hierarchically nano- to microstructured silica in diatoms occurs in lipid bilayer-bound vesicles and involves the interplay between soluble polycationic long-chain polyamines (LCPAs), and highly phosphorylated peptides (silaffins, silacidins). The chemical synthesis and functional characterization of the silica forming phosphopeptides involved in this process is the focus of this project. For that purpose, we will develop efficient methods for the synthesis of peptides which are site-specifically functionalized with several oligopropyleneimine side chains and phosphorylation sites. This will enable with unprecedented detail studies on the structure-function correlation of these molecules avoiding the structural microheterogeneity of the natural isolates. The chemical structure of the peptides will be correlated with the pH dependence of silica formation. A detailed conformational analysis of silaffin-type peptides will identify how the distribution of positive and negative charges influences the peptide structure.
This will be investigated by CD spectroscopy and in more detail by high-resolution solution NMR spectroscopy. Furthermore, synthetic methods will be established to enable the site specific labeling of LCPAs and phosphopeptides with fluorophores and desired isotopes. This will (in collaboration with other groups of this Research Unit) allow for the analysis by fluorescence microscopy and solid state NMR spectroscopy, respectively, of biomolecule-biomolecule and biomolecule-mineral interactions during silica formation. Site-specific isotope labels will be particularly useful for identification of the functional groups at the organic-inorganic interface. The insights obtained from such studies will directly influence the design of the next generation of synthetic molecules for the analysis of the influence of individual functional groups/posttranslational modifications on peptide self-assembly and silica morphogenesis.