Trp86 --> Phe replacement in bacteriorhodopsin affects a water molecule near Asp85 and light adaptation

Illumination of the Trp86 --> Phe mutant of bacteriorhodopsin causes anomalous light adaptation, i.e., isomerization of the retinal from all-trans to 13-cis, 15-syn. FTIR spectral analysis shows that illumination at 250 K yields two 13-cis photoproducts, the conventional 13-cis, 15-syn state, BR(C), and another termed BR(X). BR(X) is different from BR(C) because it has a lower N-H in-plane bending frequency and a higher C14-C15 stretching frequency, as well as an absence of coupling between these modes. BR(X), which is stable at 275 K, is more abundant in the photosteady state produced by longer wavelength light and detected as the only photoproduct at 170 K. Its different structural features result from distortion of the C14-C15 bond of the chromophore. In the W86F mutant protein, the small structural changes of a water molecule in the conversion between the all-trans and 13-cis, 15-syn forms and in the formation of the K photointermediate are absent, but the larger changes of water molecule(s) that normally occur in the L and M intermediates are present. We propose that Trp86, together with Asp85, is involved in binding the water molecule and in preventing the formation of the 13-cis, 15-syn photoproducts, BR(C) and BR(X), when the wild type protein is illuminated.