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

¹ A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Leninskie Gory, 119
² Vrije Universiteit Amsterdam

^* To whom correspondence should be addressed. E-mail: rienk{at}nat.vu.nl.

Submitted on January 24, 2005
Revised on February 19, 2005
Accepted on 25 May 2005

	Abstract

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 6 core pigments and 2 peripheral chlorophylls 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. In order 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. The intrinsic time constants of electron transfer between the three radical pairs vary from sub-ps to a several ps (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.

Key Words: exciton models, modified Redfield theory, photosystem II, reaction center, time-resolved fluorescence, transient absorption

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