Charge stabilization mechanism in the visual and purple membrane pigments

The effects of charged groups of rhodopsin and bacteriorhodopsin on the potential energy surface of their chromophore are examined, taking into account the protein dielectric effect. It is found that the barriers for twisting double bonds of an isolated chromophore can be drastically reduced when the chromophore interacts with the protein charges. New types of local minima are found in the ground-state potential surface of the protein-chromophore complex. These minima correspond to "charge-stabilized intermediates" which are formed when a shift of the chromophore positive charge to the ring is stabilized by the ionization of a properly placed acidic group of the protein and by partial alternation of the bond lengths of the chromophore. It is suggested that the absorption of light by rhodopsin and bacteriorhodopsin may be used not only for isomerization about double bonds, but also for trapping such charge-stabilized intermediates. Thus, for example, it is concluded that prelumirhodopsin might be still in the cis configuration. Both the mechanism of the proton pump system of the purple membrane and the dark reaction of the visual and purple membrane pigments are considered. The connection between the finding of the present work and the mechanism of storage of light energy in photobiology is indicated.