INTRAMEMBRANE ELECTRON-TRANSFER - PROCESSES IN THE PHOTOSYNTHETIC REACTION-CENTER

The intramembrane charge transfer has been analyzed in the framework o f the 'fixed-charge-density' formalism. For a three-layer model of mem brane, the expressions for the reorganization energy have been derived . In the case of very fast reactions, the time-evolution of the dielec tric response of a protein should be taken into account. This has been described phenomenologically in terms of a set of effective dielectri c constants, epsilon(tau), operative in different time intervals. For the charge separation process, the effects of the variable epsilon(tau ) 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 acti vationless, irrespective of the Value of epsilon(tau). On the basis of the existing experimental data, the semiquantitative estimates of eps ilon(tau) at different times were given. With these values of epsilon( tau), the activation energy close to zero was calculated for the react ions of electron transfer from bacteriopheophytin to quinone, of recom bination of the primary radical-ion pair with formation of a neutral t riplet, and of recombination of the special pair cation with quinone a nion.