Ultrafast energy relaxation in bacteriorhodopsin studied by time-integrated fluorescence

Abstract

Time-integrated fluorescence experiments on native bacteriorhodopsin and on its non-isomerizing form bR5.12 are reported. The experimental set-up was designed such as to observe emission exclusively from the excited state intermediate I-460. We obtain the first systematic investigation of the fluorescence spectra as a function of the excitation wavelength tuned throughout the entire absorption band of bR. An important finding is that the position of the fluorescence maximum does not show a systematic shift when the excitation wavelength is shortened. For excitation with high excess energy, we observe a broadening of the blue wing of the bR fluorescence, indicating incomplete vibrational energy relaxation on the time scale of the lifetime of I-460. Due to a much longer excited state lifetime, vibrational energy relaxation is more effective in bR5.12 and the fluorescence spectra are much less dependent on excitation wavelength. The results are placed in the general framework of thermalization between the retinal chromophore and the protein environment, and are compared with information obtained by femtosecond experiments.