Mechanisms of the Opsin Shift

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The shift of the absorption maximum of a protonated retinal Schiff base in solution 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. Mechanisms which have been proposed in the past to expiain 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 availability of 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.The objective of the proposal is the non-empirical calculation of the excited states of the retinal chromophore in different protein environments an the basis of CASPT2 corrected muiticonfigurational SCF wavefunctions. Chromophore geometries will be optimized subject to the constraints of the known X-ray data. The environment will be included in the calculations depending an the computational resources. On the basis of these calculations the different mechanisms of the opsin shift will be studied and their relative contributions evaluated. The calculations will be used to validate, via the UV/Vis and CD spectra, the structures propesed for the intermediates of the rhodopsin and the bR photocycles.