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.

Geometry, reorganization energy, and vibrational and electronic spectra of pigment molecules in bacterial photosynthetic reaction centers (Rhodobacter sphaeroides) are determined using quantum chemical methods. B3LYP calculations on a slightly truncated form of bacteriochlorophyll a give vibrational modes at 23, 40, 64, and 128 cm -1 , which we conclude correspond to those seen in resonance Raman (RR) spectra of reaction centers with labeled cofactor atoms, and, as oscillations, in optical transient spectra. On heavy atom substitution at the axial imidazole connection to the protein, a mode with considerable contribution of bending deformations of the whole imidazole is transformed to predominantly rocking modes of the whole macrocycle at 10 cm -1 . Other modes, with significant character of out-of-plane, torsion, and in-plane motion of the acetyl side group of ring I, are less changed. We show that there is a connection between the mentioned modes, particularly the torsion mode of the acetyl group of ring I, and electron transfer and intensity in RR spectra. Coupling energies, calculated at a transition state between P*B A and P + B A -, are found to agree reasonably well with experimental data. The lower activity of the B-side compared to the A-side is caused partly by less electronic overlap between P A and B B than between P B and B A and partly by higher energy of the P + B Bstate than of the P + B Astate. Finally, we compare to results for other species and for mutants. Measured rates correlate well with the energies of the P*B A (and PB A *) states relative to the energy of the intermediary P + B Astate.

The excited states of a structurally well-determined photosystem II (PSII) reaction center are obtained using an effective Hamiltonian for the interaction between the Q(y) states. The latter are calculated using the time-dependent density functional theory (DFT) method in DFT-optimized geometries, but with conserved side group orientations. Of particular importance is the orientation of the vinyl group of ring I. Couplings are calculated using actual transition charge distributions via the INDO/S model. Good agreement with experimental spectra is obtained. The lowest excited state is mainly located on the inactive B-side, but with a large component on P(A) too, making charge separation to H(A) possible at low temperature. The "trap state" and triplet state are localized on the inactive B-side. Since the spin singlet Q(y) states of the reaction center are all within a rather small energy range, the state with the highest component of B(A)*, on the blue side of the Q(y) absorption, has a rather high Boltzmann population at room temperature. The charge-transfer states, however, have a rather large spread and cannot be calculated accurately at present. The orientation of the phytyl chains is important and has as a consequence that the energy for the charge-separated B(A)+ H(A)- state is significantly lower than the corresponding state on the B-side. It follows that the B(A)* and P(A)* states are both possible origins for a fast charge separation in PSII.

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