The long-term objectives of this research project are to assign the structure of the chromophore binding sites in rhodopsin and light-adapted bacteriorhodopsin and to define the molecular electronic details of the primary photochemical events in these two protein systems. Three projects will be carried out in order to accomplish these goals: (1) Two-photon spectroscopy will be used to identify the location and photophysical properties of the low-lying """"""""forbidden"""""""" pi pi* states in various visual chromophore analogs in solution and in the binding sites of the proteins. This information, when combined with the one-photon spectroscopic data, will provide insights into the nature and magnitude of the electrostatic and dispersive perturbations induced by the protein binding sites. (2) All-valence electron (INDO-PSDCI) molecular orbital theory will be used to help interpret the spectral data and calculate the ground and excited state potential surfaces, and excited state reaction paths, for double bond isomerization for various models of the binding sites. Semiclassical molecular dynamics theory will be used to calculate the trajectories and the quantum yields of the primary photochemical events. The effect of the protein will be included in the above calculations using classical force field procedures to determine the equilibrium geometry of the amino acid residues on the alpha helices in the vicinity of the binding site. Various models for the binding site can then be tested by comparing the calculated results with the data obtained in the experimental portions of this program as well as from the literature. (3) Energy storage in the primary events in artificial rhodopsin and bacterio-rhodopsin analogs will be measured using pulsed laser photocalorimetry. By changing the position of methyl groups along the polyene chain and determining the energy storage associated with the photoisomerization it should be possible to map out the geometry of the binding sites. These data will provide information concerning the geometry of the binding site. These studies will provide new insights into the molecular basis of vertebrate visual transduction.
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