PHOTOCHEMISTRY IN DRIED POLYMER-FILMS INCORPORATING THE DEIONIZED BLUE MEMBRANE FORM OF BACTERIORHODOPSIN

The preparation and photochemical properties of dried deionized blue m embrane (dlbR(600); lambda(max) approximate to 600 nm, epsilon approxi mate to 54, 760 cm(-1) M-1, f approximate to 1.1) in polyvinyl alcohol films are studied. Reversible photoconversion from dlbR(600) to the p ink membrane (dlbR(485); lambda(max) approximate to 485 nm) is shown t o occur in these films under conditions of strong 647-nm laser irradia tion. The pink membrane analog, dlbR(485), has a molar extinction coef ficient of similar to 39,000 cm(-1) M-1 (f approximate to 1.2). The ra tio of pink --> blue and blue --> pink quantum efficiencies is 33 +/- 5. We observe an additional blue-shifted species (dlbR(455), lambda(ma x) approximate to 455 nm) with a very low oscillator strength (f appro ximate to 0.6, epsilon approximate to 26,000 cm(-1) M-1). This species is the product of fast thermal decay of dlbR(485). Molecular modeling indicates that charge/charge and charge/dipole interactions introduce d by the protonation of ASP(85) are responsible for lowering the excit ed-state all-trans --> 9-cis barrier to similar to 6 kcal mol(-1) whil e increasing the corresponding all-trans --> 13-cis barrier to similar to 4 kcal mol(-1). Photochemical formation of both g-cis and 13-cis p hotoproducts are now competitive. as is observed experimentally. We su ggest that dlbR(455) may be a 9-cis, 10-s-distorted species that parti ally divides the chromophore into two localized conjugated segments wi th a concomitant blue shift and decreased oscillator strength of the l ambda(max) absorption band.