Co-Assembly of Functionally Active Proteorhodopsin Membrane Protein Molecules in Mesostructured Silica-Surfactant Films

A combination of nonionic, cationic, and zwitterionic surfactants is shown both to stabilize the transmembrane protein proteorhodopsin, as well as to direct coassembly into robust transparent mesostructured silica-surfactant films containing high loadings of functionally active protein guests. Proteorhodopsin is a transmembrane protein that exhibits light-activated H⁺ transport properties, the photocycle kinetics of which are quantified by time-resolved UV-visible spectroscopy and demonstrated to be similar to proteorhodopsin in the abiotic mesostructured films compared to native-like lipids. The surfactants mediate the pK_a of a key ion-channel residue, leading to an expanded pH functional range for proteorhodopsin in mesostructured silica-surfactant host materials. Small-angle X-ray diffraction results for 100-μm films show high extents of mesoscale order with protein loadings up to 25 wt % and wormlike mesostructural order for 44 wt % proteorhodopsin. Solid-state ¹H, ¹³C, and ²⁹Si NMR analyses provide atomic-scale insights into the compositions and interactions at the mesochannel surfaces, which account for the structure-directing roles of surfactant species. Nanoindentation measurements reveal the mechanical robustness of the films, which interestingly increases with proteorhodopsin loading for the compositions examined. Heat treatment analyses show improved thermal stability for proteorhodopsin to 110 °C within mesostructurally ordered films. The results establish closely correlated relationships between the compositions, nano- and mesoscale structures, photocycle kinetics, and macroscopic mechanical properties and thermal stabilities of the silica-surfactant- proteorhodopsin films, providing key biomimetic design criteria.