(357as) Controlling the Properties of the Light Responsive Transmembrane Protein Proteorhodopsin in Mesostructured Silica-Surfactant Hybrid Materials

The physicochemical properties of membrane proteins, such as for selective catalysis, biosensing, or ion transport, are highly desirable for technological applications, although they are exceedingly challenging to harness in synthetic abiotic host materials. This is due, in part, to the challenges of incorporating relatively fragile protein molecules at sufficient concentrations in robust host environments in which the functionalities of the membrane proteins are retained. Nevertheless, judicious selections of self-assembling surfactants, solvent, silica precursors, and synthesis conditions enable high concentrations of functionally active membrane proteins to be stabilized in solution and in robust mesostructured inorganic-organic host matrices. Specifically, light-activated proteorhodopsin (PR), a transmembrane protein that pumps H⁺ ions in green light, has been incorporated at exceptionally high loadings (29 wt%) into transparent silica-surfactant films with high extents of mesostructural order. Small-angle X-ray scattering patterns show that these materials represent the most ordered surfactant-directed mesostructured inorganic-organic hybrid materials to date that have been synthesized under the benign pH conditions required for incorporating functional protein guest species. The results point to the complicated and closely coupled relationships among the compositions, structures, and dynamics of PR in the mesostructured composite films. This is especially the case for the complicated influences that the stabilizing and structure-directing surfactants have on the function of PR and the formation of robust ordered host matrices. For example, the inclusion of certain phospholipids interestingly leads to improved mesostructural order, as well as enabling the pKa value of a key ion-channel residue to be tuned to extend the pH range over which proteorhodopsin molecules function. Transient UV−visible spectroscopy analyses furthermore show that the proteorhodopsin molecules in mesostructured silica-surfactant films exhibit native-like dynamics, as well as enhanced stability compared to surfactant or lipid environments. The results correlate the nano-scale compositions, structures and conformational dynamics of the proteorhodopsin molecules and surfactant-silica host materials with their macroscopic properties, leading to the establishment of key biomimetic design criteria. The light absorbance properties and light-activated conformational changes of the proteorhodopsin guest molecules in mesostructured silica films are consistent with those associated with the native H⁺-pumping mechanism of these biomolecules, which have potential applications for solar-to-electrochemical energy conversion.

Research Interests: Open