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Abstract
A model for light-induced charge separation in a donor-acceptor system of the reaction
center of photosynthetic bacteria is described. This description is predicated on
a self-regulation of the flow of photo-activated electrons due to self-consistent,
slow structural rearrangements of the macromolecule. Effects of the interaction between
the separated charges and the slow structural modes of the biomolecule may accumulate
during multiple, sequential charge transfer events. This accumulation produces non-linear
dynamic effects on system function, providing a regulation of the charge separation
efficiency. For a biomolecule with a finite number of different charge-transfer states,
the quasi-stationary populations of these states with a localized electron on different
cofactors may deviate from a Lagmuir law dependence with actinic light intensity.
Such deviations are predicted by the model to be due to light-induced structural changes.
The theory of self-regulation developed here assumes that light-induced changes in
the effective adiabatic 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 charge-separated
state may be quite different from that of the "dark-adapted" conformation. The results
predicted by this theory agree with previously obtained experimental results on photosynthetic
reaction centers.