Abstract
Bacteriorhodopsin (bR) is an efficient light-driven proton pump which shows a trans-cis isomerization reaction of its retinal chromophore after light absorption. BR exhibits a large reorganization energy λ of 2520 cm-1 on optical excitation. In this paper, we have studied the nature, origin, and dynamical aspects of this extensive reorganization. We report the results of a femtosecond three-pulse echo peak shift (3PEPS), transient grating (TG) and transient absorption (TA) study, complemented with those of steady-state absorption and fluorescence spectroscopy in wild-type bR and the D85S mutant in its blue and purple, halide-pumping forms. We have simulated the results in the context of the multimode Brownian oscillator (MBO) formalism. A simple model that incorporates retinal's known intramolecular vibrations, which represent 1094 cm-1 or reorganization energy, and a single Gaussian protein relaxation with a decay of 50 fs representing 1430 cm-1 of reorganization energy, yielded satisfactory results for ...
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