Solvent effects on the ultrafast dynamics of the retinal chromophore of bacteriorhodopsin

Photo-induced excited-state reactions stand in the center of function of the photosensitive biological systems. These reactions can be accompanied by structural changes of the chromophore (for example isomerization) which are influenced by the nearest environment of the chromophore, and vice versa. In bacteriorhodopsin (bR), the effects of the protein environment are crucial to assure high rate and outstanding bond selectivity of isomerization. In order to identify if these effects are of steric or electrostatic origin, we carried out an extensive femtosecond time resolved fluorescence study on the retinal chromophore of bR in a large class of solvents. The latter differ in viscosity, dielectric constant, polarizability and hydrogen bonding abilities. To carry out this study a novel experimental setup – the polychromatic fluorescence up-conversion, has been designed and constructed that allows broad band detection of the fluorescence spectra, with the time resolution of 100 fs. It is found that in all studied solvents here essential part of the ultrafast excited-state dynamics is dominated by intramolecular processes. Indeed, the relaxation times and the period of the vibrational coherences, which are the markers of the protein-solvent difference, do not show significant dependence with respect to viscosity and/or to dielectric constant of studied solvents. Additionally, we find that in solvents that are able to evacuate the excess energy from the Franck-Condon zone, the chromophore is less likely to take a non-reactive path (return to the initial state). Consequently, the photoisomerization efficiency gets enhanced. Nevertheless, this solvent-induced enhancement is far smaller than the enhancement induced by the photocatalytic effect of the protein binding pocket. These observations lead us to conclude that in fact the dynamics of isomerization in protein are essentially determined by steric effects.

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