Fractal-like R5 assembly promote the condensation of silicic acid into silica particles

• Published in: Journal of colloid and interface science 2021-09, Vol.598, p.206-212
• Main Authors: Gascoigne, Levena, Magana, Jose Rodrigo, Atkins, Dylan Luke, Sproncken, Christian C.M., Gumi-Audenis, Berta, Schoenmakers, Sandra M.C., Wakeham, Deborah, Wanless, Erica J., Voets, Ilja Karina
• Format: Article
• Language: English
• Subjects: Biomineralization; Peptides; Self-assembly; Silaffin; Silica; Supramolecular chemistry; Template synthesis
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2021.04.030

A systematic scattering investigation of R5 (SSKKSGSYSGKSGSKRRIL) peptides establishes that self-assembly of large aggregates occurs due to phosphate bridging and at elevated ionic strengths. Static light scattering, small angle X-ray and small angle neutron scattering measurements reveal peptides conform into spheres with radius ~1 nm which assemble into large mass-fractal aggregates. Further, R5 aggregates are found to act as nucleation sites for the condensation of silicic acid oligomers while phosphate ions assist in driving bio-silification. [Display omitted] Despite advances in understanding the R5 (SSKKSGSYSGKSGSKRRIL) peptide-driven bio-silica process, there remains significant discrepancies regarding the physicochemical characterization and the self-assembling mechanistic driving forces of the supramolecular R5 template. This paper investigates the self-assembly of R5 as a function of monovalent (sodium chloride) and multivalent salt (phosphate) to determine if assembly is phosphate ion concentration dependent. Additionally, we hypothesize that the assembled R5 aggregates do not resemble a micelle or unimer structure as proposed in current literature. R5 peptides were synthesized, and aggregates evaluated for their size, morphology, and association state as a function of salt and ionic strength concentration via dynamic and static light scattering, small angle X-ray and neutron scattering and cryogenic transmission electron microscopy. Furthermore, we compare the proposed R5 template to precipitated silica by scanning electron microscopy. R5 peptides assemble into large aggregates due to multivalence bridging and the decrease in electrostatic repulsion due to ionic strength. We elucidate the structure of R5 aggregates as mass-fractals composed of small spherical aggregates. Moreover, we discover that phosphate ions not only have a significant role in driving the growth of the R5 scaffold, but additionally in driving the polycondensation of silicic acid during the bio-silification process via electrostatic interactions.