Photochemistry and fluorescence of bacteriorhodopsin excited in its 280-nm absorption band.

Photochemical and fluorescence studies are carried out, exciting bacteriorhodopsin (BR) in its 280-nm absorption band. The data indicate that energy transfer takes place, with a quantum yield of 0.7--0.8, from excited tyrosines and tryptophans to the retinyl chromophore. All of the tyrosine and five to six tryptophan residues are completely quenched by the transfer process while one tryptophan is unquenched and one is partially (approximately 80%) quenched. Energy transfer to the chromophore leads to a photocycle identical with that triggered in (light adapted) bacteriorhodopsin by excitation within the visible absorption bands of the chromophore. The emissive properties of BR in the intact membrane are found equal to those of Triton X-100 solubilized BR monomer. The energy transfer data are discussed in terms of the available amino acid sequence and the electron density map of bacteriorhodopsin. Although such data cannot suggest a single fit between the sequence and the density map (one out of the 7! = 5040 possibilities), they do provide a criterion for testing any specific model for the structure of bacteriorhodopsin.