DFTB QM/MM calculations of retinal protein excited states

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Although the proteins bR and Rh contain the same chromophore, a protonated retinal Schiff base, the absorption maxima are shifted differently in the different protein matrices compared to the spectrum of the isolated retinal ("opsin shift"). This is due to the interaction of the chromophore with various amino acids in the different protein pockets. Further, the photo-isomerization in the protein differs significantly from that in solution, concerning its velocity, reaction coordinate and quantum yield. In this project we investigate the specific interactions of the proteins with the chromophore in its electronically excited state. These investigations are now feasible, since only recently crystal structures with sufficient resolution became available, and appropriate theoretical methods have been developed. The goal of this proposal is to investigate the influence of steric and electrostatic interactions of amino acid residues from the protein pocket on the optical properties of the chromophore. Further, the spectral changes due to rearrangement of the protein during the photo cycle and the changes due to mutations of the amino acid sequence will be investigated.