<b>SPECIAL PHYSICS LECTURE</b><br><i>Energy and Electron Transfer in the Photosystem II Reaction Center: Modeling of the Exciton and Radical Pair Dynamics</i>

The D1-D2-cyt_b559 complex is the reaction center (RC) of photosystem II (PSII) which is responsible for light-driven water splitting in oxygenic photosynthesis.  We have measured the exciton and charge separation kinetics in the isolated RC complex by femtosecond transient absorption spectroscopy at 278 K using 120 fs excitation pulses centered at 670 nm and 681 nm, respectively.

The experimental dynamics of the system have been modeled theoretically using modified Redfield theory to calculate the exciton density matrix evolution of the core RC. The properties of the radical pair states have been determined by simultaneous fitting of the modeled exciton dynamics and the radical pair kinetics and transient spectra to the experimental data. A Genetic Algorithm employing the Differential Evolution method has been applied for the optimization of the parameters. The modeling yields the transition (site) energies of the pigments, the exciton relaxation times, the charge separation and recombination rates, the inhomogeneous broadening of the pigments, and the spectra of the radical pairs.

The resulting charge separation rates are substantially slower than what has been proposed in recent work on this complex and we do not find evidence for a heterogeneous charge separation initiation.

The relevance of these data and conclusions for the understanding of charge separation dynamics in both the isolated RC as well as the intact PSII complex will be discussed.