On the excitation wavelength dependence of the fluorescence of bacteriorhodopsin

Time-integrated fluorescence study of native bacteriorhodopsin as a function of excitation wavelength, reveal a violation of the mirror image between absorption and fluorescence. These observations are attributed to vibrationally hot emission, and to distortion of the excited state potential. We present a systematic study of the time-integrated fluorescence of native bacteriorhodopsin, as a function of excitation wavelength across the visible absorption band. While the fluorescence maximum is unaffected, the spectrum broadens on the high-energy side, with decreasing excitation wavelengths. In addition, there is no mirror image relation between emission and absorption, even for the longest excitation wavelengths. By comparison with the retinal cation in solution, we attribute these observations to vibrationally hot emission, and to the topology of the excited state surface on the way to isomerization.