| Abstract
| - Electron transport through photosystem II (PSII), measured as oxygen evolution, was investigated in isolatedPSII particles and thylakoid membranes irradiated with white light of intensities (I) of 20 to about 4000 μmolof photons/(m2·s). In steady-state conditions, the evolution of oxygen varies with I according to the hyperbolicexpression OEth = OEth(max)I/(L1/2 + I) (eq i) where OEth is the theoretical oxygen evolution, OEth(max) isthe maximum oxygen evolution, and L1/2 is the light intensity giving OEth(max)/2. In this work, the mathematicalderivation of this relationship was performed by using the Langmuir adsorption isotherm and assuming thatthe photon interaction with the chlorophyll (Chl) in the PSII reaction center is a heterogeneous reaction inwhich the light is represented as a stream of particles instead of an electromagnetic wave (see discussion inTurro, N. J. Modern Molecular Photochemistry; University Science Books: Mill Valley, CA, 1991). Inaccordance with this approximation, the Chl molecules (P680) were taken as the adsorption surfaces (orheterogeneous catalysts), and the incident (or exciting) photons as the substrate, or the reagent. Using thesenotions, we demonstrated that eq i (Langmuir equation) is a reliable interpretation of the photon-P680 interactionand the subsequent electron transfer from the excited state P680, i.e., P680*, to the oxidized pheophytin(Phe), then from Phe- to the primary quinone QA. First, eq i contains specific functional and structuralinformation that is apparent in the definition of OEth(max) as a measure of the maximal number of PSIIreaction centers open for photochemistry, and L1/2 as the equilibrium between the electron transfer from Phe-to QA and the formation of reduced Phe in the PSII reaction center by electrons in provenance from P680*.Second, a physiological control mechanism in eq i is proved by the observation that the magnitudes of OEth(max) and L1/2 are affected differently by exogenous PSII stimulators of oxygen evolution (Fragata, M.;Dudekula, S. J. Phys. Chem. B2005, 109, 14707). Finally, an unexpected new concept, implicit in eq i, is theconsideration of the photon as the substrate in the photochemical reactions taking place in the PSII reactioncenter. We conclude that the Langmuir equation (eq i) is a novel mathematical formulation of energy andelectron transfer in photosystem II.
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