Thermodynamics and kinetics of model photosynthetic reaction centers

Photosynthetic quantum conversion, modeled as a Markov process, is simulated on a digital computer to determine what thermodynamic losses are a necessary consequence of specific assumptions about reactions along a chain of molecules. The behavior of model systems with one primary acceptor is comparatively independent of whether there is one or an infinite number of molecules that can donate electrons to the reaction center, and whether there is one or an infinite number of secondary electron acceptors. Maximal free energy yield requires that all forward rate constants be at least 10² times the rate of light absorption, and that all reverse rate constants be at least the rate of light absorption. Maximal yield also requires that all dark reactions be in near equilibrium, so the potential of all half cells on the donor side of the light act is the same, and the potential of all half cells on the acceptor side is the same. A system having no triplet state can convert light energy with a nearly ideal efficiency, provided that the midpoint potentials of the reaction center and the primary acceptor half cells are precisely located with respect to one another and to the potentials of the donor and acceptor pools. While not necessary for near maximal free energy yield, a triplet or other metastable intermediate of the reaction center allows a flexibility in the choice of half cell potentials which is not possible in the absence of such an intermediate. For systems, either with or without a triplet intermediate, having a single donor to the reaction center, the standard potential of the half cell for the donor can vary widely without substantially hurting the free energy yield.