Perturbation Effects On The Tryptophan Fluorescence In Bacteriorhodopsin

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The picosecond fluorescence kinetics of tryptophan residues in bacteriorhodopsin (bR) and some perturbed analogues are measured to study the different tryptophan environments and their changes upon metal cation removal, partial delipidation, retinal removal, and M412 trapping. In bR, the emission shows four decay components designated C1R, C2R, C3R, and C4R in order of increasing lifetimes. The emission wavelength of C3R and C4R is near that found in aqueous solution, while that of C1R is the shortest. The removal of retinal triples the total emission intensity and reduces the number of components to two, suggesting that the observed variation of the lifetimes in bR results from the variation of the energy transfer efficiency between different tryptophans and retinal. We conclude that the C1R and C2R emission is from the closest tryptophans to the retinal. The quenching of the C3R emission by all metal cations, including those that cannot act as energy acceptors, e.g. Cali-, is attributed to protein conformation changes caused by metal cation binding which leads to a stronger energy transfer coupling between tryptophans and retinal. The additional quenching of the C2R emission in Eu3+ bound bR is proposed to result from direct energy transfer between tryptophans and Eu3+. No effect on tryptophan emission decay kinetics and intensity by adding metal cations to deionized delipidated bR indicates that the protein conformation is changed by delipidation. But the similar kinetics between bR and delipidated bR suggest that the delipidation provides a proper protein conformation to deprotonate the protonated Schiff base without metal cations. The formation of M412 at low temperature in glycerol leads to the quenching of only the faster emission decay component.