Mechanisms of the Opsin Shift

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The shift of the absorption maximum of a protonated retinal Schiff base upon binding to the apoprotein, the so-called opsin-shift, is the most conspicuous evidence of the special interaction between the chromophore and its environment in the protein binding pocket. Tuning the sensitivity of the visual pigment to different spectral regions is necessary for survival in different habitats; it is also prerequisite for color vision. Mechanisms which have been proposed in the past to explain the origin of the opsin shift could not be tested properly, since the structures were largely hypothetical and computational tools for quantitative calculation of the spectra were not available. With the recent X-ray structures of retinal binding proteins and the development of reliable quantum mechanical methods the calculation of excited states of the retinal chromophore including parts of the protein binding pocket has become feasible. We propose the non-empirical calculation of the excited states of chromophore models of retinal binding proteins on the basis of CASPT2 corrected multiconfigurational SCF wavefunctions. Chromophore geometries will be optimised subject to the constraints of the known X-ray data. The environment will be included in the calculations. On the basis of these calculations we will analyse different mechanisms of the opsinshift and evaluate their relative contributions. The calculations will support, via the UV/Vis spectra, the structure elucidation of the intermediates of the rhodopsin and the bR photocycles.