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Spectral Tuning in Sensory Rhodopsin II

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Perception of light and color permits humans to enjoy art, though the sense evolved more likely to better find ripe fruits. The recognition, centuries ago, that the wondrous sense of color comes from three visual receptors to which is added in the eye a fourth black & white one is one of the major achievements in the history of science. The visual receptors of all animals rely on one molecule for light absorption, retinal. How do the receptors tune then their spectral sensitivity? Exploiting a similarity of visual receptors to proteins in an archaebacterium, Natronobacterium pharaonis, researchers have finally been given an opportunity to answer this question quantitatively. In the bacterium, two structurally almost identical proteins absorb maximally light of 497 nm and 568 nm wavelength. X-ray crystallography and advanced quantum chemical studies could explain the difference and pinpoint to the protein side groups (see figure) that actually tune the spectra.