The nature of the chromophore binding sites and the primary photochemical events of rhodopsin, bacteriorhodopsin and the Xenopus violet cone pigment, viodopsin, will be studied by using spectroscopic and theoretical techniques. The goals are to understand the photophysical properties of the bound chromophores. The principal spectroscopic methods to be used in these studies include two-photon spectroscopy, Stark effect spectroscopy, Fourier transform infra-red spectroscopy, microwave spectroscopy, and pulsed laser photocalorimetry. The principal chemical studies to be undertaken include organic cation and chromophore analog substitutions, as well as site directed mutagenesis. The theoretical methods include semiempirical molecular orbital theory and molecular dynamics theory. The goal is to combine experiment and theory in a synergistic program which enhances both. In addition to the more global goals outlined above, Dr. Birge will seek to answer the following specific questions: (1) What is the principal mechanism of wavelength modulation in the blue and violet cones? (2) Where are the cation binding sites in bacteriorhodopsin, and how do these sites mediate the properties of the bound chromophore? (3) Is there a chloride binding site in viodopsin, and what impact does this site have on the photophysical properties of the chromophore? (4) What are the molecular origins of energy storage in the primary events of these three proteins? (5) What is the principal molecular mechanism of dark noise in vertebrate and invertebrate vision? (6) What are the molecular origins of the unusual photochemical properties of the 4-keto retinal bacteriorhodopsin analog? (7) Can one improve the accuracy of the MNDO-PSDCI semi-empirical molecular orbital theory by using ab-initio effective Hamiltonian parameterizaion?
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