On the Models for Phytochrome Chromophore

The optical properties of the models for phytochrome chromophore proposed by Crespi et al. and Rüdiger-Correll, as well as those of bilidiene and bilitriene, are examined within the framework of the π-electron approximation, using the ZDO approximation of LCAO-ASMO-SCF-CI theory. The interaction between chromophore and protein moiety in phytochrome is also examined, assuming the electric field produced by the latter to be constant. Moreover, the Coulomb interaction of the chromophore with the aromatic amino acid residue is studied by the dipole approximation, regarding the residue as tryptophan. It is thus shown that the unique optical absorption of phytochrome in the visible region cannot be explained by any model so far presented, as long as their π-systems are assumed to have no ionic structure.