Intramembrane electron transfer: processes in the photosynthetic reaction center

AbstractThe intramembrane charge transfer has been analyzed in the framework of the ‘fixed-charge-density’ formalism. For a three-layer model of membrane, the expressions for the reorganization energy have been derived. In the case of very fast reactions, the time-evolution of the dielectric response of a protein should be taken into account. This has been described phenomenologically in terms of a set of effective dielectric constants, ετ, operative in different time intervals. For the charge separation process, the effects of the variable ετ on the reaction free energy and on the medium reorganization energy are equal and opposite in sign, and hence compensate each other. As a result, the photosynthetic primary charge separation proves to be activationless, irrespective of the value of ετ. On the basis of the existing experimental data, the semiquantitative estimates of ετ at different times were given. With these values of ετ, the activation energy close to zero was calculated for the reactions of electron transfer from bacteriopheophytin to quinone, of recombination of the primary radical-ion pair with formation of a neutral triplet, and of recombination of the special pair cation with quinone anion