Self-regulation phenomena in bacterial reaction centers. I. General theory

A model for light-induced charge separation in a donor-acceptor system of t he reaction center of photosynthetic bacteria is described. This descriptio n is predicated on a self-regulation of the flow of photo-activated electro ns due to self-consistent, slow structural rearrangements of the macromolec ule. Effects of the interaction between the separated charges and the slow structural modes of the biomolecule may accumulate during multiple, sequent ial charge transfer events. This accumulation produces non-linear dynamic e ffects on system function, providing a regulation of the charge separation efficiency. For a biomolecule with a finite number of different charge-tran sfer states, the quasi-stationary populations of these states with a locali zed electron on different cofactors may deviate from a Lagmuir law dependen ce with actinic light intensity. Such deviations are predicted by the model to be due to light-induced structural changes. The theory of self-regulati on developed here assumes that light-induced changes in the effective adiab atic potential occur along a slow structural coordinate. In this model, a " light-adapted" conformational state appears when bifurcation produces a new minimum in the adiabatic potential. in this state, the lifetime of the cha rge-separated state may be quite different from that of the "dark-adapted" conformation. The results predicted by this theory agree with previously ob tained experimental results on photosynthetic reaction centers.