Photoisomerization acceleration in retinal protonated Schiff-base models.

The results of new and recently reported CASSCF/6-31G* photoisomerization path computations of a series of models of the 11-cis retinal chromophore of the visual pigment rhodopsin are discussed. The results indicate that, with respect to the chromophore in vacuo, certain structural, intramolecular and environmental factors are capable of speeding up the excited-state decay associated with the cis --> trans isomerization motion. Using suitable protonated Schiff-base models, it is shown that three structural factors can potentially speed up the isomerization: (i) reducing the length of the conjugated chain, (ii) twisting of the hydrocarbon end of the conjugated chain with respect to the protonated Schiff-base end and (iii) ring locking of the conjugated chain with an eight-membered ring. All these factors operate through increasing the slope of the excited-state energy surface and enhancing the coupling between stretching and torsional modes. We argue that the protein catalysis seen in rhodopsin may, at least partly, exploit the same principles.