FAST-ELECTRON TRANSFERS IN PHOTOSYNTHETIC REACTION-CENTER - EFFECT OF THE TIME-EVOLUTION OF DIELECTRIC RESPONSE

The total dielectric response of a protein in due to different kinds of motion each of them having its own, and often very different characteristic time. This situation has been described phenomenologically in terms of a set of effective dielectric constants epsilon tau operative in the corresponding time intervals. The effects of this time dependence on the reaction free energy Delta G and on the medium reorganization energy E(s) were considered employing the models of an infinite homogeneous dielectric and of a three-layer membrane. For the primary charge separation, the effects of the variable epsilon tau on both these energies, Delta G and E(s), are practically equal and opposite in sign, and hence compensate each other. As a result, the primary charge separation proves to be activationless irrelevant to the value of epsilon tau. On the basis of the existing experimental data, the semiquantitative estimates of epsilon tau at different times were given. With these values of epsilon tau the activation energy close to zero was calculated for the fast reactions of electron transfer from bacteriopheophytin to quinone and of recombination of the primary radical-ion pair with formation of a neutral triplet.