Ab initio MO study of the chlorophyll dimer in the photosynthetic reaction center. I. A theoretical treatment of the electrostatic field created by the surrounding proteins

The charge separation occurring in the photosynthetic reaction center is the primary subject in order to understand the whole photosynthetic process. In this article, the electronic structures of the chlorophyll dimer, in which photoexcitation occurs, are investigated by using the ab initio molecular orbital scheme and pseudocharges and dipoles method which takes into account the electrostatic effect of the surrounding proteins, pigments, and aqueous solvent. As a first step, the ionization potentials of BCmp, BCIp, and the dimer of Rhodopseudomonas viridis are computed to study the characteristic of each molecule toward the elucidation of the primary charge separation. It has been found, from the present calculations, that BCmp and BClp constituting the dimer are not equivalent in their electronic ground states, since the electron could be removed more easily from BCmp than from BClp for two reasons: (1) the distorted molecular plane of Cmp, and (2) the effects of the surrounding electrostatic potentials to BCmp and BCIp. This tendency is further strengthened by the polarization of the environment, when, to the ab initio MO calculation, the contribution from the induced dipoles of the neighboring atoms in the proteins and other chromophores is included. From the present application, induced electric dipoles are found to be important to describe the molecular electronic structures affected by proteins. © 1997 John Wiley & Sons, Inc.

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