Origin of the bathochromic shift in the early photointermediates of the rhodopsin visual cycle: A CASSCF/CASPT2 study

CASSCF/CASPT2 calculations have been applied to study the excited-state properties of the chromophore in rhodopsin and in the early photointermediate, photorhodopsin. For rhodopsin, the chromophore geometry was taken from an optimized X-ray structure white for photorhodopsin the structure proposed by Bifone et al. (J Phys Chem B 1997, 101, 2954) was employed. The experimentally observed bathochromic shift of 72 nm is faithfully reproduced by the calculation. Analysis of a model chromophore in which the transition between the geometries is performed stepwise reveals that this shift is predominantly caused by double bond torsion. (C) 2003 Wiley Periodicals, Inc.