A THEORETICAL-STUDY OF THE EFFECT OF CHARGE RECOMBINATION ON THE TRANSFER AND TRAPPING OF EXCITATION-ENERGY IN PHOTOSYNTHESIS

Photosynthetic organisms absorb solar energy using chlorophyll-containing antennas and convert it to chemical energy in the form of charge-separated radical pairs. Experimental studies of photosystem 2 from plants and cyanobacteria are often interpreted in terms of a model in which the excited antenna is in equilibrium with the charge-separated states. Assigning a single state for the excited antenna is a simplification since it is known that the excitation energy migrates over the antenna for some time until it is trapped in special reaction centers where charge separation takes place. In the present work, existing random-walk models for excitation-energy transfer are extended to a case where charge recombination is allowed for. The treatment is based on Laplace and discrete Fourier transform techniques. It is shown that the simple equilibrium model approximates the situation well, provided that the overall trapping and detrapping rates are appropriately scaled to the rate constants for charge separation and recombination in the reaction center. Expressions for the quantum yields of charge separation and of fluorescence are derived and the consistency between the present random-walk model and the general notion of antenna entropy is demonstrated.