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 .

Photosystem I (PSI) is a large pigment-protein complex that unites a reaction center (RC) at the core with ∼100 core antenna chlorophylls surrounding it. The RC is composed of two cofactor branches related by a pseudo-C2 symmetry axis. The ultimate electron donor, P₇₀₀(a pair of chlorophylls), and the tertiary acceptor, F_X(a Fe₄S₄cluster), are both located on this axis, while each of the two branches is made up of a pair of chlorophylls (ec2 and ec3) and a phylloquinone (PhQ). Based on the observed biphasic reduction of F_X, it has been suggested that both branches in PSI are competent for electron transfer (ET), but the nature and rate of the initial electron transfer steps have not been established. We report an ultrafast transient absorption study ofChlamydomonas reinhardtiimutants in which specific amino acids donating H-bonds to the 13¹-keto oxygen of either ec3_A(PsaA-Tyr696) or ec3_B(PsaB-Tyr676) are converted to Phe, thus breaking the H-bond to a specific ec3 cofactor. We find that the rate of primary charge separation (CS) is lowered in both mutants, providing direct evidence that the primary ET event can be initiated independently in each branch. Furthermore, the data provide further support for the previously published model in which the initial CS event occurs within an ec2/ec3 pair, generating a primary ec2⁺ec3^-radical pair, followed by rapid reduction by P₇₀₀in the second ET step. A unique kinetic modeling approach allows estimation of the individual ET rates within the two cofactor branches.