LH1-RC light-harvesting photocycle under realistic light-matter conditions

Quantum master equations are used to simulate the photocycle of the light-harvesting complex 1 (LH1) and the associated reaction center (RC) in purple bacteria excited with natural incoherent light. The influence of the radiation and protein environments and the full photocycle of the complexes, including the charge separation and RC recovery processes, are taken into account. Particular emphasis is placed on the steady state excitation energy transfer rate between the LH1 and the RC and the steady state dependence on the light intensity. The transfer rate is shown to scale linearly with light intensity near the value in the natural habitat and at higher light intensities is found to be bounded by the rate-determining step of the photocycle, the RC recovery rate. Transient (e.g., pulsed laser induced) dynamics, however, shows rates higher than the steady state value and continues to scale linearly with the intensity. The results show a correlation between the transfer rate and the manner in which the donor state is prepared. In addition, the transition from the transient to the steady state results can be understood as a cascade of ever slower rate-determining steps and quasi-stationary states inherent in multi-scale sequential processes. This type of transition of rates is relevant in most light-induced biological machinery.