Primary Processes and Structure of the Photosystem II Reaction Center. 5. Modeling of the Fluorescence Kinetics of the D1-D2-cyt-b559 Complex at 77 K†

The fluorescence kinetics of the photosystem II reaction center (D1-D2-cyt-b559 complex, PSII-RC) at a temperature of 77 K has been analyzed. A kinetic model is presented that takes into account the inhomogeneous broadening of the pigment spectra and the heterogeneity in the pigment composition of the sample. This work is based on the spectral analysis that was presented in a recent study to describe the absorption properties of the PSII-RC (Konermann, L.; Holzwarth, A. R. Biochemistry 1996, 35, 829). For the kinetic model, the Förster theory was applied to calculate the rate constants for pairwise energy transfer. Due to the inhomogeneous broadening of the spectra the energy transfer rates show a pronounced dispersion which has severe consequences for the excited state kinetics. We tested different models for radical pair formation and recombination to describe the data. Only with a model that includes several sequential radical pairs we could obtain acceptable agreement with the experimental fluorescence kinetics. A model with a static Gaussian distribution for the free energy difference of charge separation did not fit the data. The occurrence of sequential radical pair states is interpreted as a dynamic relaxation process of the protein surrounding triggered by the sudden creation of a pair of ions. The temperature dependence of the stationary fluorescence quantum yield and the temperature dependence of the stationary spectrum, as well as other experimental data predicted by the kinetic model, is in good agreement with experimental results