Using 1,3-butadiene and 1,3,5-hexatriene to model the cis-trans isomerization of retinal, the chromophore in the visual pigment rhodopsin

The short polyenes 1,3-butadiene and 1,3,5-hexatriene are used to model the cis-trans isomerization of the protonated Schiff base of retinal (PSBR) in rhodopsin (Rh). We employed the complete active space self-consistent field (CASSCF) method for calculation of the potential energy surfaces (PESs) in C, symmetry. In the calculations, the central bond was twisted from 0 to 180 degrees in the first singly excited singlet state (S-se), i.e., the state dominated by a configuration with one electron excited from HOMO to LUMO. It was found that the PES of 1,3-butadiene has a maximum whereas the PES of 1,3,5-hexatriene has a minimum for a twist angle of 90 degrees. This is explained by a shift in border of single and double bonds in the S-se state. The first step in the cis-trans isomerization of PSBR, which is the formation of the C6-C7 (see Scheme 1 for numbering) twisted PSBR in the first excited singlet state (S-1), inside the protein binding pocket of the visual pigment Rh is modeled using crystal coordinates and the calculations performed on 1,3-butadiene and 1,3,5-hexatriene. More specifically, a plausible approximate structure is calculated in a geometric way for the C6-C7 90 degrees twisted PSBR, which fits into the protein binding pocket in the best possible way. It has been shown earlier that PSBR has an energy minimum for this angle in S-1. The CASSCF method was used to investigate the wave function of the calculated structure of PSBR. (C) 2002 Wiley Periodicals, Inc.