Vibrational Spectroscopy of Bacteriorhodopsin Mutants: Chromophore Isomerization Perturbs Tryptophan-86

Fourier transform infrared difference spectra have been obtained for the bR → K and bR → M photoreactions of bacteriorhodopsin mutants with Phe replacements for Trp residues 10, 12, 80, 86, 138, 182, and 189 and Cys replacements for Trp residues 137 and 138. None of the tryptophan mutations caused a significant shift in the retinylidene C=C or C—C stretching frequencies of the light-adapted bR₅₇₀ state. Since these frequencies are known to be strongly correlated with the visible absorption maximum of the chromophore, it is concluded that none of the tryptophan residues are essential for forming a normal bR₅₇₀ chromophore. However, a 742-cm^-1 negative peak attributed previously to the perturbation of a tryptophan residue during the bR → K photoreaction was found to be absent in the bR → K and bR → M difference spectra of the Trp-86 mutant. On this basis, we conclude that the structure or environment of Trp-86 is altered during the bR → K photoreaction. All of the other Trp → Phe mutants exhibited this band, although its frequency was altered in the Trp-189 → Phe mutant. In addition, the Trp-182 → Phe mutant exhibited much reduced formation of normal photoproducts relative to the other mutants, as well as peaks indicative of the presence of additional chromophore conformations. A model of bR is discussed in which Trp-86, Trp-182, and Trp-189 form part of a retinal binding pocket. One likely function of these tryptophan groups is to provide the structural constraints needed to prevent chromophore photoisomerization other than at the C₁₃=C₁₄ double bond.