Escape probability and trapping mechanism in purple bacteria: revisited

Despite intensive research for decades, the trapping mechanism in the core complex of purple bacteria is still under discussion. In this article, it i s attempted to derive a conceptionally simple model that is consistent with all basic experimental observations and that allows definite conclusions o n the trapping mechanism. Some experimental data reported in the literature are conflicting or incomplete. Therefore we repeated two already published experiments like the time-resolved fluorescence decay in LH1-only purple b acteria Rhodospirillum rubrum and Rhodopseudomonas viridis chromatophores w ith open and closed (Q(A)(-)) reaction centers. Furthermore, we measured fl uorescence excitation spectra for both species under the two redox-conditio ns. These data, all measured at room temperature, were analyzed by a target analysis based on a three-state model (antenna, primary donor, and radical pair). All states were allowed to react reversibly and their decay channel s were taken into consideration. This leads to seven rate constants to be d etermined. It turns out that a unique set of numerical values of these rate constants can be found, when further experimental constraints are met simu ltaneously, i.e, the ratio of the fluorescence yields in the open and close d (Q(A)(-)) states F-m/F-o approximate to 2 and the P+H--recombination kine tics of 3-6 ns. The model allows to define and to quantify escape probabili ties and the transfer equilibrium. We conclude that trapping in LH1-only pu rple bacteria is largely transfer-to-the-trap-limited. Furthermore, the mod el predicts properties of the reaction center (RC) in its native LH1-enviro nment. Within the framework of our model, the predicted P+H--recombination kinetics are nearly indistinguishable for a hypothetically isolated IPC and an antenna-RC complex, which is in contrast to published experimental data for physically isolated RCs. Therefore RC preparations may display modifie d kinetic properties. Elsevier Science B.V. All rights reserved.