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.
The results of a theoretical study of the dynamic
self-organization phenomenon in the photosynthetic
reaction
center (RC) from purple bacteria are presented accounting for the
stochastic effects an RC ensembles. The
adiabatic approximation is applied to determine the specific role of
slow structural (protein/cofactors) modes
in the correlated behavior of the electronic and structural variables.
It is shown that, at certain values of light
intensity, the system undergoes bifurcation. The bistability
region for the generalized structural variable
occurs where the system has two stable states, one characteristic for
the dark-adapted sample (i.e., the sample
under very low illumination intensity) and the other for the
light-adapted sample. The description is based
on the solution of the “forward” Kolmogorov equations using the
Markov approach. A distribution function
describing the probability of finding the electron localized on a
particular cofactor with a certain value of the
generalized structural variable is used. Modeling shows a good
agreement with the results of experimental
investigations of transient optical absorbance changes of the isolated
RCs from the purple bacterium
Rhodobacter sphaeroides. The results indicate that the
free energy difference between QA
- and
QB
- changes
substantially in different conformational states of the
protein/cofactors induced by light over a wide range of
the illuminating light intensity.
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