Theory of color tuning in biological photoreceptors: From structure to function
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Photoreceptors are small pigment-protein complexes containing a single chromophore per protein chain, with various important physiological roles in all biological kingdoms, ranging from phototaxis, gene regulation, proton pumping to the complex process of dim-light and color perception. To optimize the functionality of these systems, the protein matrix tunes the local optical transition energies (site energies) of individual pigments. According to our theoretical results, which are in very good agreement with experimental data, a general tuning mechanism through electrostatic interactions between the protein matrix and the chromophores exists. Results were obtained using the charge-density coupling method (CDC) , which is a quantum chemical/electrostatic two-step approach developed in the Renger group. First CDC was applied to BLUF photoreceptors, which mediate gene regulation in various photosynthetic bacterial organisms. Based on our structure-based calculations the identities of certain functional states, which are discussed controversially in the literature, could be unambiguously revealed . Recent applications concern bovine rhodopsin , which is in charge of dim-light perception, and green and red cone pigments of the monkey retina, which are responsible for color perception . We provide a microscopic explanation of these phenomena.
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 F. Collette, T. Renger, F. Müh, and M. Schmidt am Busch, in preparation.