Photochemistry in Dried Polymer Films Incorporating the Deionized Blue Membrane Form of Bacteriorhodopsin

Abstract The preparation and photochemical properties of dried deionized blue membrane (dIbR 600 ; λ max ≈600nm, ϵ≈54, 760cm −1 M −1 , f≈1.1) in polyvinyl alcohol films are studied. Reversible photoconversion from dIbR 600 to the pink membrane (dIbR 485 ; λ max ≈485nm) is shown to occur in these films under conditions of strong 647-nm laser irradiation. The pink membrane analog, dIbR 485 , has a molar extinction coefficient of ∼39,000cm −1 M −1 (f≈1.2). The ratio of pink→blue and blue→pink quantum efficiencies is 33±5. We observe an additional blue-shifted species (dIbR 455 , λ max ≈455nm) with a very low oscillator strength (f≈0.6, ϵ≈26,000cm −1 M −1 ). This species is the product of fast thermal decay of dIbR 485 . Molecular modeling indicates that charge/charge and charge/dipole interactions introduced by the protonation of ASP 85 are responsible for lowering the excited-state all- trans →9- cis barrier to ∼6kcal mol −1 while increasing the corresponding all- trans →13- cis barrier to ∼4kcal mol −1 . Photochemical formation of both 9- cis and 13- cis photoproducts are now competitive, as is observed experimentally. We suggest that dIbR 455 may be a 9- cis , 10-s-distorted species that partially divides the chromophore into two localized conjugated segments with a concomitant blue shift and decreased oscillator strength of the λ max absorption band.