Visual processes in both vertebrates and invertebrates are initiated by the photo-isomerization of retinal chromophore in the trans-membrane protein, rhodopsin. During the initial molecular events occurring within femto/picoseconds, such isomerizations ( and resultant charge separation) efficiently stores the incident light energy via chromophore-protein interactions with the surrounding protein. This storage of energy is a prerequisite for later steps in the visual cascade involving protein-protein interactions such as the binding of a G-protein (tansducin) at the rhodopsin surface. The proposed research examines the femto/picosecond photochemistry of retinal in room temperature rhodopsin in order to elucidate: (i) light- induced structural changes in the chromophore itself and (ii) retinal- protein interactions by which different retinal conformations selectively interact with the surrounding protein. The molecular mechanism by which both phenomena participate and/or an correlated with the primary photochemical events to store and transduce absorbed light energy will be studied. Samples to be examined include bovine rhodopsin (prepared with both native lipids and synthetic detergents) and artificial rhodopsin pigments synthesized by replacing the native chromophore with chemically- modified retinals. Femto/picosecond laser techniques will be used to measure changes in electronic states by transient absorption and fluorescence spectroscopy as well as changes in the polyene structure of retinal by time-resolved resonance Raman scattering. The proposed research program thereby combines femto/picosecond laser spectroscopy with advance chemical methodologies in polyene synthesis and the preparation of artificial rhodopsin pigments. In a corollary activity, the overall influence of the primary retinal and chromophore-protein events on protein-protein interactions such as G- protein binding will be explored through a collaboration in which with the fast dynamical results are correlated with G-protein activation and surfaced binding properties of selected rhodopsin samples.
