Pathways and timescales of primary charge separation in the photosystem II reaction center as revealed by a simultaneous fit of time-resolved fluorescence and transient absorption

We model the dynamics of energy transfer and primary charge separation in isolated photosystem II ( PSII) reaction centers. Different exciton models with specific site energies of the six core pigments and two peripheral chlorophylls (Chls) in combination with different charge transfer schemes have been compared using a simultaneous fit of the absorption, linear dichroism, circular dichroism, steady-state fluorescence, transient absorption upon different excitation wavelengths, and time-resolved fluorescence. To obtain a quantitative fit of the data we use the modified Redfield theory, with the experimental spectral density including coupling to low-frequency phonons and 48 high-frequency vibrations. The best fit has been obtained with a model implying that the final charge separation occurs via an intermediate state with charge separation within the special pair (RP1). This state is weakly dipole-allowed, due to mixing with the exciton states, and can be populated directly or via 100-fs energy transfer from the core-pigments. The RP1 and next two radical pairs with the electron transfer to the accessory Chl (RP2) and to the pheophytin (RP3) are characterized by increased electron-phonon coupling and energetic disorder. In the RP3 state, the hole is delocalized within the special pair, with a predominant localization at the inactive-branch Chl. The intrinsic time constants of electron transfer between the three radical pairs vary from subpicoseconds to several picoseconds ( depending on the realization of the disorder). The equilibration between RP1 and RP2 is reached within 5 ps at room temperature. During the 5 - 100-ps period the equilibrated core pigments and radical pairs RP1 and RP2 are slowly populated from peripheral chlorophylls and depopulated due to the formation of the third radical pair, RP3. The effective time constant of the RP3 formation is 7.5 ps. The calculated dynamics of the pheophytin absorption at 545 nm displays an instantaneous bleach (30% of the total amplitude) followed by a slow increase of the bleaching amplitude with time constants of 15 and 12 ps for blue ( 662 nm) and red ( 695 nm) excitation, respectively.