Excited states of free base chlorin (FBC), free base Bacteriochlorin (FBBC), pheophytin a (Pheo a), and chlorophyll a (Chlo a), which are derivatives of free base porphine (FBP), were calculated by the SAC (symmetry adapted cluster)/SAC-CI (configuration interaction) method. The results reproduced well the experimentally determined excitation energies. The reduction of the outer double bonds in the porphine ring in the order of FBP, FBC, and FBBC causes a breakdown of the symmetry and a narrowing of the HOMO-LUMO gap, which result in a red shift of the Q x band and an increase of its intensity. In the change from Pheo a to Chlo a, the Mg coordination reduces the quasidegeneracy in the Q x state and then increases the spectral intensity. The disappearance of the Q y humps from the absorption spectrum of Pheo a, compared with that of Chlo a, is due to the red shift of the Q y state.

The excitation spectrum of the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) Viridis is assigned by using the SAC(symmetry adapted cluster)-CI(configuration interaction) method. All the chromophores included in the PSRC, bacteriochlorophyll b dimer (special pair, P), bacteriochlorophyll b in L-and M-branches (B L and B M ), bacteriopheophytin b in L-and M-branches (H L and H M ), menaquinone (MQ), ubiquinone (UQ), and four different hemes, c-552, c-554, c-556, and c-559 in c-type cytochrome subunit, were calculated within the environment of proteins, waters, and the other chromophores which were dealt with by the point-charge electrostatic model. We have assigned successfully all the peaks in the experimental spectrum in the energy range from 1.2 to 2.5 eV. The assignment was done by comparing the SAC-CI theoretical spectrum with the experimental one in excitation energy, oscillator strength, linear dichroism data (angle of transition moment), and other experimental information available. Almost all the peaks were red shifted due to the effect of proteins. The present assignment of the spectrum would give a basis for future photoexperimental studies of the PSRC.

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ABSTRACT:The pathways of the electron transfers (ETs) within the c-type cytochrome subunit and from the cytochrome subunit to the oxidized special pair (P + ) in the reaction center of Rhodopseudomonas viridis were studied by calculating the transfer integrals among the chromophores and the bridging amino acid residues by ab initio molecular orbital method. In the cytochrome subunit, the candidate of the bridge molecules was selected by the criterion of the distance from the two neighboring hemes, and the calculated results indicate that the ET occurs directly from a heme to the next heme. From c559, the proximate heme to the special pair, to P + , the ET occurs mainly through TYR L162, which lies halfway between c559 and P + , because of its proper location. Furthermore, the mutation experiments in which TYR L162 was replaced by phenylalanine and threonine were examined by the same theoretical method, and it was shown that the result of the mutation experiment was understood by the difference in the spatial distribution of the MOs between the wild type and mutants, and not by the energy difference of the MOs between donor (or acceptor) and bridge, though the latter factor had often been considered as the main factor controlling the rate of the ET.

The excited states of four hemes, c‐552, c‐554, c‐556, and c‐559, in a c‐type cytochrome subunit of the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) viridis were studied using the symmetry‐adapted cluster (SAC)‐configuration interaction (CI) method. c‐552 has two imidazole ligands, while the other three hemes have imidazole and methionine ligands. The electronic states of these four hemes are affected significantly by both the ligands and protein and water environments. The ligand effect classifies the four hemes into two groups, while the environmental effect is unique in each heme. The HOMO–LUMO energy separation of c‐552 is much smaller than those of the other hemes, and therefore, the low‐lying excited states of c‐552 are more stable than those of the other hemes. The low‐lying excited states and their oscillator strengths of c‐559 are significantly modified by the environment when compared with c‐556 and c‐554. Some observed UV peaks of the PSRC of Rps. viridis were found to have been assigned reasonably to the excited states of these four hemes in a previous report. The calculated linear dichroism angles also supported our assignment. Copyright © 2001 John Wiley & Sons, Ltd.

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