A new approach to understanding the initial step in visual transduction.

Data from picosecond spectroscopic studies of the formation kinetics of bathorhodopsin upon photolysis of rhodopsin and isorhodopsin was analyzed in terms of the Englman-Jortner theory of radiationless transitions. It was found that low frequency vibrations of the protein and/or chromophore are important in coupling bathorhodopsin to its precursor. The results were consistent with a mechanism for bathorhodopsin formation involving only a simple chromophore isomerization. A similar analysis of the formation kinetics of the K state of bacteriorhodopsin showed that different low frequency vibrations than those calculated for rhodopsin couple it to its precursor. The frequency of these vibrations increases upon deuteration for rhodopsin, while it decreases upon deuteration for bacteriorhodopsin. This points out the importance the specific protein matrix has on the primary photolysis reaction.