Kinetics and Mechanism of Electron Transfer in Intact Photosystem II and in the Isolated Reaction Center: Pheophytin Is the Primary Electron Acceptor

The mechanism and kinetics of electron transfer in isolated D1/$D2-cyt_{b559}$photosystem (PS) II reaction centers (RCs) and in intact PSII cores have been studied by femtosecond transient absorption and kinetic compartment modeling. For intact PSII, a component of ≈1.5 ps reflects the dominant energy-trapping kinetics from the antenna by the RC. A 5.5-ps component reflects the apparent lifetime of primary charge separation, which is faster by a factor of 8-12 than assumed so far. The 35-ps component represents the apparent lifetime of formation of a secondary radical pair, and the ≈200-ps component represents the electron transfer to the$Q_{A}$acceptor. In isolated RCs, the apparent lifetimes of primary and secondary charge separation are ≈3 and 11 ps, respectively. It is shown (i) that pheophytin is reduced in the first step, and (ii) that the rate constants of electron transfer in the RC are identical for PSII cores and for isolated RCs. We interpret the first electron transfer step as electron donation from the primary electron donor$Chl_{acc D1}$. Thus, this mechanism, suggested earlier for isolated RCs at cryogenic temperatures, is also operative in intact PSII cores and in isolated RCs at ambient temperature. The effective rate constant of primary electron transfer from the equilibrated$RC^{*}$excited state is 170-180$ns^{-1}$, and the rate constant of secondary electron transfer is 120-130$ns^{-1}$.