(691g) Harnessing Membrane Protein Functionalities in Mesostructured Silica

Membrane proteins are crucial for various cellular functions such as enzymatic activity, signal transduction, and importantly, selective ion transport within living organisms. The diverse and intricate functionalities of these proteins make them desirable as guest species in synthetic host materials for technological applications. However, the incorporation of functionally active membrane protein molecules into robust abiotic hosts at high concentrations has been exceedingly challenging, due in part to the narrow range of processing conditions required to maintain the functional activity and stability of membrane protein molecules within abiotic host environments.

Mesostructured silica-surfactant films are excellent host materials for incorporating membrane proteins, where the surfactant species stabilize the membrane protein molecules and co-assemble into ordered mesophases, with the cross-linked silica network imparting robust mechanical and thermal stabilities. Here, we report the incorporation of proteorhodopsin (PR), a transmembrane protein that actively transports H⁺ ions in response to green light, into mesostructured silica-surfactant film. Judicious selection of synthetic conditions and compositions allows the functionally active PR molecules to be loaded at very high concentrations of up to 44 wt% protein. Small-angle X-ray scattering and transmission electron microscopy analyses show that these materials exhibit the most mesostructurally-ordered membrane protein-hybrid films to date, as prepared under the relatively benign pH conditions that are required to preserve the functionality of the protein guests. Furthermore, the complicated transient phenomena during co-assembly of the surfactant-silica-membrane protein species can be controlled by using semipermeable poly(dimethylsiloxane) molds to orientationally order PR-containing surfactant-silica mesochannels on a macroscopic scale. Solid-state, two-dimensional ²⁹Si{¹H} NMR correlation analyses reveal nanoscale interactions between silica surface moieties and surfactant head groups, which confirm the dual protein-stabilizing and structure-directing roles of the surfactants. Time-resolved UV/vis spectroscopy measurements establish that the PR molecules in the abiotic silica-surfactant hosts exhibit native-like photocycle kinetics, which are influenced by interactions between PR and the structure-directing surfactant species. The functional activity of the PR molecules in the confined silica mesochannel environments is furthermore maintained to over 100 ^oC. In addition, nanoindentation analyses show that the co-assembled films exhibit excellent mechanical properties, as quantified by the modulus and hardness, which increase with the extent of mesoscale order and PR loading. The results reveal interrelationships among the molecular-level compositions, structures, dynamics, and properties of membrane proteins in inorganic-organic host materials from which new biomimetic design criteria emerge.