Thermodynamics of the Primary Reactions of Photosynthesis

It appears to be widely believed that fundamental thermodynamic limitations prevent the products of the primary electron transfer reactions of photosynthesis from exceeding the reactants in energy by more than about 0.7 hv 0 where v 0 is the frequency of the lowest energy absorption band of the reaction center. Specifically, in photosynthetic bacteria, where hv 0 ? 1.34 eV, it is often said that the midpoint redox potentials of the primary electron donor and acceptor cannot differ by more than about 0.9 eV. This is incorrect. A simple expression is developed for δμ, the increase in the partial molecular free energy of an absorber under illumination. The magnitude of δμ gives one no information about the nature of the photochemical reactions that can result from excitation of the absorber. It puts no limits on the midpoint redox potentials of the reactants. However, knowing δμ does allow one to calculate the concentrations of the products if the system comes to equilibrium during illumination.