Self-Regulation Phenomena Applied to Bacterial Reaction Centers

Abstract

Experimental and theoretical results in support of nonlinear dynamic behavior of photosynthetic reaction centers under light-activated conditions are presented. Different conditions of light adaptation allow for preparation of reaction centers in either of two different conformational states. These states were detected both by short actinic flashes and by the switching of the actinic illumination level between different stationary state values. In the second method, the equilibration kinetics of reaction centers isolated from Rhodobacter sphaeroides were shown to be inherently biphasic. The fast and slow equilibration kinetics are shown to correspond to electron transfer (charge separation) at a fixed structure and to combined electron-conformational transitions governed by the bounded diffusion along the potential surface, respectively. The primary donor recovery kinetics after an actinic flash revealed a pronounced dependence on the time interval (deltat) between cessation of a lengthy preillumination of a sample and the actinic flash. A pronounced slow relaxation component with a decay half time of more than 50 s was measured for deltat > 10 s. This component corresponds to charge recombination in reaction centers for which light-induced structural changes have not relaxed completely before the flash. The amplitude of this component depended on the conditions of the sample preparation, specifically on the type of detergent used in the preparation. The redox potential parameters as well as the structural diffusion constants were estimated for samples prepared in different ways.

Light-Induced Equilibration Kinetics in Membrane-Bound Photosynthetic Reaction Centers: Nonlinear Dynamic Effects in Multiple Scattering Media

“…The rate of electron recombination from the secondary quinone acceptor Q B is written in eq 1 as k r (x) ) k AP e -x . Such a dependence for RCs has been discussed in details previously 1,2,19,20,22 and was based on the fact that the rate constant of electron transfer from Q B to P at normal physiological conditions is given by By assumption that we obtain the assumed expression for k r (x). In the above consideration, we took into account that the slow component of structural rearrangements, triggered by electron localization on Q B causes slow changes in the ∆G AB value due to interaction with the polar surroundings of the secondary electron acceptor in RCs.…”

“…As we have recently shown, structural memory effects induced by adaptation to a charge-separated state in isolated RCs may be probed in experiments using a sequence of actinic flashes applied to a preilluminated sample. 1,2 The results of similar experiments on membrane-bound RCs are presented in this section. Figure 6a shows the transient transmittance of a sample of membrane-bound RCs with myxothiazol in a 1-cm path length cuvette.…”

“…Note that these RC equilibration kinetics are described within a formalism of nonequilibrium dynamics with a slow structural diffusion along the surface of an adiabatic potential. 1,2 Under the reasonable assumption of an equal-absorption oscillator strength at 865 nm for both isolated and membrane bound RCs, the difference in bleaching kinetics of the two systems can be explained by assuming that the excitation rate for each macromolecule, at the same incident actinic light intensity, is higher for membrane-bound RCs than for isolated RCs. To check this assumption, we studied the dependence of absorption bleaching kinetics with an excitation beam cross section at a fixed photoexcitation intensity I ex .…”

“…[1][2][3] In particular, a pronounced, slow relaxation phase following flash excitation is prominent in the equilibration kinetics of RCs isolated from the purple bacteria Rhodobacter (Rb.) sphaeroides after dialysis against an excess of two different detergents, either Triton X-100 (0.05%, pH 7.5) or sodium (Na) cholate (0.1%, pH 8.0).…”

“…The structural diffusion constant of the charge neutral state was determined to be among the main parameters responsible for the emergence of a slow relaxation component in the kinetics of isolated RCs. 1,2 Despite the obvious interest in a comparison of results from studies of nonequilibrium dynamic effects in isolated RCs with those in chromatophores, the latter studies have not previously been performed because of apparent difficulties in deducing an unambiguous interpretation of the results of light-induced effects on photosynthetic membranes. The interpretation of experimental results on the dynamics of membrane-bound RCs is confounded by significant overlap of RC absorption with that of antennae complexes in these systems.…”