The inherent glasslike structural heterogeneity of photosynthetic antenna protein complexes is now known to result in a distribution of values for any given donor-acceptor energy gap. The width of the distribution is sufficiently broad to raise the possibility that the kinetics of D-A energy transfer might be dispersive and strongly dependent on temperature especially since the chlorophyll optical transitions are characterized by weak electron-phonon coupling. An approximate theory is presented that allows for computationally simple analysis of this problem. It is based on the familiar nonadiabatric energy-transfer theory and Condon approximation and, thus, is applicable to conventional Forster transfer involving localized phonons. The case of delocalized phonons is also treated. Calculations are presented for a model D-A system which, based on hole-burning data, can be considered to be realistic and typical of many complexes. The results reveal pronounced and temperature-dependent dispersion in the kinetics. The effect of pure electronic dephasing on the calculated results is considered. The theory is not applicable to the situation where excitonic level splittings are large relative to the pure dephasing frequencies of the levels, i.e., where one has to go beyond the Condon approximation. Nevertheless, the theory can be easily extended to cover this strong coupling case which can be expected to be important for many photosynthetic complexes at sufficiently low temperatures. The possibility that strong coupling might lead to a diminuation in the degree of dispersive kinetics is considered.
We have measured the extent of flash-induced electron transfer from the bacteriochlorophyll dimer, P, to the bacteriopheophytin in the M-subunit, HM, in reaction centers of Rhodopseudomonas viridis. This has been done by measuring the transient states produced by excitation of reaction centers trapped in the PHL (-)HM state at 90 K. Under these conditions the normal forward electron transfer to the bacteriopheophytin in the L-subunit, HL, is blocked and the yield of transient P(+)HM (-) can be estimated with respect to the lifetime of P(*). Under these conditions flash induced absorbance decreases of the bacteriochlorophyll dimer 990 nm band suggest that a transient P(+) state is formed with a quantum yield of 0.09±0.06 compared to that formed during normal photochemistry. These transient measurements provide an upper limited on the yield of a transient P(+) HM (-) state. An estimate of 0.09 as the yield of the P(+) HM (-) state is consistent with all current observations. This estimate and the lifetime of P(*) suggest that the electron transfer rate from P(*) to HM, kM, is about 5 × 10(9) sec(-1) (τM = 200ps). These measurements suggest that the a branching ratio kL/kM is on the order of 200. The large value of the branching ratio is remarkable in view of the structural symmetry of the reaction center. This measurement should be useful for electron transfer calculations based upon the reaction center structure.
Structural modification of photosynthetic reaction centers is an important approach for understanding their charge-separation processes. An unprecedented persistent structural transformation of the special pair (dimer) of bacteriochlorophyll molecules can be produced by light absorption alone. The nonphotochemical hole-burned spectra for the reaction center of Rhodopseudomonas viridis show that the phototransformation leads to a red shift of 150 wave numbers for the special pair's lowest energy absorption band, P960, and a comparable blue shift for a state at 850 nanometers, which can now be definitively assigned as being most closely associated with the upper dimer component. Additional insights on excited-state electronic structure include the identification of a new state.
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