Structure and Function in the Isolated Reaction-Center Complex of Photosystem II. 2. Models for Energy Relaxation and Charge Separation in a Protein Matrix

We have developed a model that describes the energy and charge transfer in photosystem II reaction-center complexes, using parameters with clear physical interpretations. The input parameters are deduced from independent experimental work and from the theoretical constraints of the system. This model is comprised of exitonically coupled chlorophyll electronic states, including charge-transfer states, which are coupled to the protein bath. The dynamics of the system are calculated using a master-equation approach, and the results are converted into spectroscopic observables for comparison with experimental data. The model accurately depicts the emission kinetics observed for both bacterial reaction centers from Rhodobacter sphaeroides and photosystem II from spinach. The model is discussed with regard to the wavelength dependence of energy transfer, the effects of static disorder in the system, and the initial ultrafast dynamics of the charge separation.