Interfacial biomimetic synthesis of silica nanocapsules using a recombinant catalytic modular protein

This paper reports interfacially driven synthesis of oil-core silica-shell nanocapsules using a rationally designed recombinant catalytic modular protein (ReCaMoP), in lieu of a conventional chemical surfactant. A 116-residue protein, D4S2, was designed by modularizing a surface-active protein module having four-helix bundle structure in bulk and a biosilicification-active peptide module rich in cationic residues. This modular combination design allowed the protein to be produced via the industrially relevant cell factory Escherichia coli with simplified purification conferred by thermostability engineered in design. Dynamic interfacial tension and thin film pressure balance were used to gain an overview of the protein behavior at macroscopic interfaces. Functionalities of D4S2 to make silica nanocapsules were demonstrated by facilitating formation and stabilization of pharmaceutically grade oil droplets through its surface-active module and then by directing nucleation and growth of a silica shell at the oil-water interface through its biosilicification-active module. Through these synergistic activities in D4S2, silica nanocapsules could be formed at near-neutral pH and ambient temperature without using any organic solvents that might have negative environmental and sustainability impacts. This work introduces parallelization of biomolecular, scale-up and interfacial catalytic design strategies for the ultimate development of sustainable and scalable production of a recombinant modular protein that is able to catalyze synthesis of oil-filled silica nanocapsules under environmentally friendly conditions, suitable for use as controlled-release nanocarriers of various actives in biomedical and agricultural applications.