The preparation and photochemical properties of dried deionized blue membrane (dlbR600; λ(max) ≃ 600 nm, ε ≃ 54, 760 cm-1 M-1, f ≃ 1.1) in polyvinyl alcohol films are studied. Reversible photoconversion from dlbR600 to the pink membrane (dlbR485; λ(max) ≃ 485 nm) is shown to occur in these films under conditions of strong 647-nm laser irradiation. The pink membrane analog, dlbR485, has a molar extinction coefficient of ~39,000 cm-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 (dlbR455, λ(max) ≃ 455 nm) with a very low oscillator strength (f ≃ 0.6, ε ≃ 26,000 cm-1 M-1). This species is the product of fast thermal decay of dlbR485. Molecular modeling indicates that charge/charge and charge/dipole interactions introduced by the protonation of ASP85 are responsible for lowering the excited-state all-trans → 9-cis barrier to ~6 kcal mol-1 while increasing the corresponding all-trans → 13-cis barrier to ~4 kcal mol-1. Photochemical formation of both 9-cis and 13-cis photoproducts are now competitive, as is observed experimentally. We suggest that dlbR455 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.
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