Quantifying the excitation migration time in photosystem II. Consequences for primary and secondary charge separation rates and the corresponding drop in free energy

The fluorescence decay kinetics of Photosystem II (PSII) membranes with open reaction centers (RCs), was compared after excitation at 420 and 484 nm. These excitation wavelengths lead to preferential excitation of Chl a and Chl b, respectively, which causes different initial excitation populations in the inner and outer antenna system. The non-exponential fluorescence decay appears to be 6.6 ± 0.8 ps slower upon 484 nm excitation for preparations that contain on average 2.3 LHCII (light-harvesting complex II) trimers per RC. Using a recently introduced method (Biophysical J 91:3776-3786, 2006) it is concluded that the average migration time of an excitation towards the RC contributes 34 ± 7% to the overall trapping time. This demonstrates that the exciton-radical pair equilibrium (ERPE) model that describes the kinetics of samples without outer antenna [Proc. Natl. Acad. Sci .USA 84, 1987], is not applicable for systems with outer antenna. This prompts us to introduce the MiCS model, which includes appreciable contributions from both the Mi(gration) time and the trapping or C(harge) S(eparation) time to the overall decay. It is conluded that the effective rate of primary charge separation of the entire RC (i.e. not only the primary donor) is (3.7 ± 0.5 ps)-1, the rate of secondary charge separation is (155 ± 42 ps)-1 and the drop in free energy upon primary charge separation is (825±106) cm-1. This large drop in energy occurs faster than generally found for systems without outer antenna.