On the Models for Phytochrome Chromophore. II

Abstract

On the model for phytochrome chromophore proposed by Crespi et al. , the light-absorbing property is examined using the ZDO approximation of the LCAO-ASMO-SCF-CI theory of π-electrons. All the bond lengths together with the bond angles of the pyrrole rings with cyclic π-system are improved in each stage of the iteration processes for solving MO's, and a sufficient number of configurations are included in the CI calculation. The interactions of the model chromophore with a negative point-charge and with a point-dipole are also examined on the basis of the stationary perturbation theory for degenerate states, and it is shown that, among the models so far proposed, only the P _fr -form by Crespi et al. can reproduce the observed absorption spectra of phytochrome, provided its interaction with the electric field caused by protein moiety is taken into account.