Excitation Wavelength Dependence of Energy Transfer and Charge Separation in Reaction Centers from Rhodobacter sphaeroides:  Evidence for Adiabatic Electron Transfer

Abstract

Femtosecond transient absorption spectroscopy has been used to investigate the excitation wavelength dependence of energy transfer and initial charge separation processes in reaction centers of the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides (R-26) at room temperature. The Q Y transition bands of the bacteriopheophytins (H), bacteriochlorophyll monomers (B), and special pair (P) were selectively excited with pulses of 150 fs duration and 5 nm spectral bandwidth. Absorbance changes were analyzed over the entire wavelength region from 700 to 1000 nm. From this analysis we concluded the following: (1) As seen by others, energy transfer between H, B, and P is extremely fast, occurring on the 100-300 fs time scale.(2) The spectral evolution of the system is excitation wavelength dependent for picoseconds after excitation, implying that vibrational relaxation is not complete on the time scale of either energy transfer or charge separation and suggesting that the pathway of charge separation may be excitation wavelength dependent.(3) The absorbance change spectra of the initial excited states of B and H are not consistent with intensity borrowing between these bands, reopening the question of what gives rise to the complex absorbance changes normally associated with the H A -state. (4) The 10-20 ps component of the stimulated emission decay is excitation wavelength dependent and spectrally different from the dominant 2-3 ps decay of the stimulated emission. This component is unlikely to represent a static conformational heterogeneity in the reaction center charge separation rate. These conclusions lead to the proposal of the following model for energy and electron transfer in the reaction center. Energy transfer in this system is very fast because it is mediated by electron exchange interactions between cofactors (implying relatively strong electronic coupling for electron transfer) and because there is little nuclear displacement between donor and acceptor potential surfaces during energy transfer. Electron transfer is slower than energy transfer because the nuclear displacement is larger, and the rate is limited by movement along the reaction coordinate. Thus, initial electron transfer occurs in the near adiabatic limit before vibrational relaxation is complete. This model would explain many issues which have been difficult to resolve using standard electron transfer models including the difficulty in identifying the P + B A -intermediate and the insensitivity of the initial electron transfer rate to temperature and driving force.

Primary Electron Transfer Kinetics in Membrane-Bound Rhodobacter sphaeroides Reaction Centers: A Global and Target Analysis

“…sphaeroides RCs of the YM210W mutant (23,24), of the heterodimer and of the wild type (Van Brederode et al, unpublished observations), we added a parallel B* f P + B L -path with a rate of 1/ps. Such a process would then also be consistent with the observed 15% decrease of P* emission upon 800 nm excitation (relative to 860 nm excitation) measured at room temperature (20). The SADS of this target analysis ( Figure 7) were similar to those from Figure 6, except for an increase of the P*-stimulated emission and changes of the presumed P* and P + B L -bands in the 800 nm region.…”

“…On the basis of the time-gated spectra directly after the 0.2 ps ultrafast B* relaxation process we suggested that in this electron transfer pathway P + B L -and B + H L -are directly formed from B* which are then converted to P + H L -. Similar electron transfer mechanisms might be operating in wild type RCs at room temperature, as was suggested by (20) on the basis of 15-20% less P* stimulated emission formed upon excitation in the H and B band compared to excitation of the P band.…”

STRUCTURE, DYNAMICS, AND ENERGETICS OF THE PRIMARY PHOTOCHEMISTRY OF PHOTOSYSTEM II OF OXYGENIC PHOTOSYNTHESIS

“…These observations have led a number of authors to suggest that primary charge separation might occur differently in PSII, potentially initiated by excitation of B A rather than P A /P B . A number of groups have shown in bacterial reaction centers that it is possible to observe, under certain conditions, the formation of radical pair states upon direct excitation of B A (69,128,130,142). Dekker & van Grondelle (24) and Rutherford and coworkers (98,99) have suggested that in PSII the lack of spectral differentiation and the orientation of 3 P, respectively, might be consistent with a contribution of B * A to primary radical formation.…”

Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in Rhodobacter capsulatus Reaction Centers

“…Contributions to complex kinetic profiles from dynamic effects involving motions of the chromophores, the protein, or both accompanying or following the decay of P* or P + H L -also have been discussed. 18,22,65,74,[104][105][106][107] In our previous work, we proposed the scheme for D LL given in Figure 2B, wherein the charge-separated intermediates (states other than P + Q A -and P + Q B -) lie above P* in free energy. This scheme derived from (1) the current view of the free energies of the intermediates in wild type, (2) the measured P/P + potential in D LL being at least 100 meV higher than in wild type, and (3) the lack of spectral signatures for formation of a chargeseparated state from P* in D LL at 295 K. In the simplest interpretation, the data presented here for D LL in either Deriphat glass or LDAO glass at 77 K are similar to the results for D LL at 295 K and can be reconciled with the model shown in Figure 2B, namely, with the charge-separated intermediates lying above P* and P* f ground state (via internal conversion) being the only decay route of P*.…”