Entrapment of biomembranes in mesoporous metal oxide gels has proven to be a challenge, as current and previous techniques utilize liposomes as biological membrane hosts. The instability of liposomes in mesoporous gels is attributed to their size and altered structure and lipid dynamics upon entrapment within the nanometer scale pores (5-50 nm) of silica gel. We have overcome these barriers by using nanolipoprotein particles (NLPs) as biomembrane hosts. NLPs are discoidal patches of lipid bilayer that are belted by amphiphilic scaffold proteins and have an average thickness of 5 nm, with diameters ranging from 10-15 nm. To ensure proper functioning of gel-entrapped NLPs, we have investigated the phase behavior of the lipids in addition to the secondary structure, localization, and environmental polarity of the scaffold proteins. Our results indicate that gel-entrapped NLPs remain intact, with only slightly altered lipid and scaffold protein structure and dynamics. In addition, the integral membrane protein bacteriorhodopsin (BR) in its native purple membrane has been successfully encapsulated within titania gels as evidenced by substantial retention of the characteristic visible spectral absorbance over 20 days and pink-purple color. This was accomplished through our development of sol-gel processing conditions to eliminate the use of organic solvent after demonstrating that inclusion of organic solvents eliminated the characteristic BR visible spectra. Once encapsulated in the wet gel, BR was shown to reversibly convert from its dark adapted (mixed 13-cis- and all-trans-retinal cofactor) to its light-adapted (all-trans-retinal) state, and to retain its thermochromic behavior, indicating that the pores did not hinder the membrane protein’s functionality to make conformational changes in response to its environment. This suggests that BR, whether in purple membrane or in a membrane nanodisc, will retain its ability to function as a proton pump, a factor in favor of using the protein over a conventional dye for sensitizing mesoporous titania gel to permit visible light photocatalysis and photocurrent generation. Moreover, we showed that the titania gels display the characteristic titania band-gap of 3.4 eV and were photocatalytic at a level comparable with commercial crystalline nanopowders under UV irradiation. In addition to these results, our most recent progress will be presented.

Functional biomembranes entrapped within mesoporous silica and titania gels

2019 Spring National Meeting

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