Structural influences on the direction of electron transfer in the charge separation process of the photosynthetic reaction centers of Rhodopseudomonas viridis and Rhodobacter sphaeroides are studied using quantum chemical models to calculate the electronic factor. Our results support the sequential mechanism for the primary charge separation. Using crystallographic coordinates refined in 1994, we find a larger coupling between the special pair D and the accessory bacteriochlorophyll BA of the A branch than between D and BB of the inactive B branch. We have been able to localize the coupling to the acetyl group of ring I of DB and the methyl group of ring III of BA. The corresponding contact between DA and BB has less coupling, apparently due to a distorting hydrogen bond between the acetyl group on DA and the imidazole side group of HisL168. The coupling between BA and bacteriopheophytin ΦA is accomplished by two methyl groups, directly connected to the conjugated π system of the chromophore.

The electronic structure and spectrum of several models of the binuclear metal site in soluble CuA domains of cytochrome-c oxidase have been calculated by the use of an extended version of the complete neglect of differential overlap/spectroscopic method. The experimental spectra have two strong transitions of nearly equal intensity around 500 nm and a near-IR transition close to 800 nm. The model that best reproduces these features consists of a dimer of two blue (type 1) copper centers, in which each Cu atom replaces the missing imidazole on the other Cu atom. Thus, both Cu atoms have one cysteine sulfur atom and one imidazole nitrogen atom as ligands, and there are no bridging ligands but a direct Cu-Cu bond. According to the calculations, the two strong bands in the visible region originate from exciton coupling of the dipoles of the two copper monomers, and the near-IR band is a charge-transfer transition between the two Cu atoms. The known amino acid sequence has been used to construct a molecular model of the CuA site by the use of a template and energy minimization. In this model, the two ligand cysteine residues are in one turn of an a-helix, whereas one ligand histidine is in a loop following this helix and the other one is in a a-strand.

The electron paramagnetic resonance (EPR) spectrum of the binuclear CuA center in the water-soluble subunit II fragment from cytochrome ba3 of Thermus thermophilus was recorded at 3.93, 9.45, and 34.03 GHz, and the EPR parameters were determined by computer simulations. The frequency and M1 dependence of the linewidth was discussed in terms of g strain superimposed on a correlation between the A and g values. The g values were found to be gx = 1.996, gy = 2.011, gz = 2.187, and the two Cu ions contribute nearly equally to the hyperfine structure, with magnitude of Ax magnitude of approximately 15 G, magnitude of Ay magnitude = 29 G, and magnitude of Az magnitude of = 28.5 G (65Cu). Theoretical CNDO/S calculations, based on the x-ray structure of the Paracoccus denitrificans enzyme, yield a singly occupied antibonding orbital in which each Cu is pi*-bonded to one S and sigma*-bonded to the other. In contrast to the equal spin distribution suggested by the EPR simulations, the calculated contributions from the Cu ions differ by a factor of 2. However, only small changes in the ligand geometry are needed to reproduce the experimental results.

We have studied a double-bond bridged porphyrin dimer, trans-1,2-bis(meso-octaethylporphyrinyl)ethene (tbisOEP), which in solutions exhibits new spectral properties:  (i) pronounced absorption bands in addition to the monomer ones are observed in the 480−500 and 600−900 nm regions; (ii) a broad-band fluorescence with a viscosity-dependent intensity and spectral position is detected in the near-IR region (750−1100 nm). An investigation of fluorescence excitation spectra, combined with semiempirical quantum chemical calculations and geometry optimizations suggest that in solution tbisOEP exists in two main conformations, which we name conformers P and U. The ratio of their concentrations was estimated to be approximately 5:1 in toluene at room temperature. The P conformer was found to be responsible for the “usual” monomer-type absorption bands (Soret and Q) in the tbisOEP absorption spectrum, whereas the U conformer is responsible for the additional absorptions in the 480−500 and 600−900 nm regions. The unusual near-IR fluorescence originates from the U conformer and its quantum yield (Φ) is 6 × 10-4 in toluene. Increase of solvent viscosity results in a strong blue-shift of the near-IR emission and increase of its intensity (Φ = 4 × 10-3 in paraffin oil). Both conformers P and U were found to exhibit very short exited-state lifetimes (<10 ps in toluene) which become significantly longer in more viscous solvents. The calculations suggest that the peculiar ground-state spectral properties of the U conformer result from its particular geometrical structure favoring a common conjugation between the π orbitals of the porphyrin rings and the ethylene bond, whereas only excitonic interactions exist in the P conformer. Therefore, the U conformer can be considered as a real supermolecule rather than two interacting separate porphyrin macrocycles. Our results point to the key role of the connecting bridge in the formation of the optical properties of the ethylene-bridged porphyrin dimers.