Picosecond kinetics of the initial photochemical electron-transfer reaction in bacterial photosynthetic reaction centers

Abstract

The absorption changes that occur in reaction centers of the photosynthetic bacterium Rhodopseudomonas sphaeroides during the initial photochemical electron-transfer reaction have been examined. Measurements were made between 740 and 1300 nm at 295 and 80 K by using a pulse-probe technique with 610-nm, 0.8-ps flashes. An excited singlet state of the bacteriochlorophyll dimer P* was found to give rise to stimulated emission with a spectrum similar to that determined previously for fluorescence from reaction centers. The stimulated emission was used to follow the decay of P*; its lifetime was 4.1 +/- 0.2 ps at 295 K and 2.2 +/- 0.1 ps at 80 K. Within the experimental uncertainty, the absorption changes associated with the formation of a bacteriopheophytin anion, Bph-, develop in concert with the decay of P* at both temperatures, as does the absorption increase near 1250 nm due to the formation of the cation of P, P+. No evidence was found for the formation of a bacteriochlorophyll anion, Bchl-, prior to the formation of Bph-. This is surprising, because in the crystal structure of the Rhodopseudomonas viridis reaction center [Deisenhofer, J., Epp, O., Miki, K., Huber, R., & Michel, H. (1984) J. Mol. Biol. 180, 385-398] a Bchl is located approximately in between P and the Bph. It is possible that Bchl- (or Bchl+) is formed but, due to kinetic or thermodynamic constraints, is never present at a sufficient concentration for us to observe. Alternatively, a virtual charge-transfer state, such as P+Bchl-Bph or PBchl+Bph-, could serve to lower the energy barrier for direct electron transfer between P* and the Bph.

Chlorophyll a fluorescence lifetime distributions in open and closed Photosystem II reaction center preparations

“…2). These kinetics are almost identical to the analogous reaction in photosynthetic bacterial reaction center [17][18][19]. Takahashi et al [201 and Danielius et al [211 have measured at 5 °C a 32 ns lifetime for P680 ÷ Pheo-(Eqn.…”

Electron transfer rates and Franck–Condon factors: an application to the early electron transfer steps in photosynthetic reaction centers

“…These results would suggest that, provided that small energy fluctuations, much smaller than a thermal quantum k b T (k b Boltzmann constant), take place, ET from BChl − to BPh and from BPh − to UQ can be mechanistically modeled as a nonradiative transition which mainly involves the intramolecular modes of vibrations of the two redox cofactors, without the assistance of the low-frequency modes of the medium. This conclusion is in good agreement with the general finding that the rates of most ET processes occurring in photosynthetic reaction centers (P * → BPh, BPh − →UQ, UQ − → P + ) are only moderately temperature-dependent [28][29][30][31][32]. The dynamics of ET from BPh − to UQ have been successfully examined in a previous paper [18]; by considering only the intramolecular modes of the two mojeties we obtained a transition time of ca.…”

Subpicosecond characterization of the optical properties of the primary electron donor and the mechanism of the initial electron transfer in Rhodobacter capsulatus reaction centers

“…sphaeroides and Rps. viridis RCs have been based largely on the finding of identical kinetics for the decay of P* and the formation of P+BPhF~ [4][5][6][7][8]. Our spectral data provide a new measure of the extent to which the conversion of P* to P+BPhF~ may involve an intermediate state.…”