Energy transfer and trapping in the CP47RC complex of photosystem II

Abstract

The CP47RC complex of Photosystem II (PS II) differs from the isolated reaction center (RC) by the presence of a single antenna unit (CP47). Both complexes can be prepared in such a way that the primary quinone acceptor (QA) is absent, implying that secondary electron transfer can not occur in these complexes. The excited state kinetics of RC has previously been investigated by several groups, but such kinetics of CP47RC have hardly been measured. The only additional process upon excitation of the CP47RC complex compared to the RC complex is energy migration from the antenna to RC and back. Our goal was to determine the fluorescence kinetics of CP47RC complex and to analyze the influence of the CP47 antenna on the kinetics. We have measured time-resolved fluorescence kinetics of CP47RC complexes at room temperature using a synchroscan streak camera with a time resolution of about 3 ps. The intensity of the excitation laser pulses was low, which prevented significant annihilation contributions. Global analysis of the data revealed that the excited state kinetics of CP47RC has a 4-exponential behavior. We consider a model in which at room temperature excitations reach equilibrium in CP47 within less than 2 ps (see the abstract of F.L.de Weerd et.al.) and in which the energy transfer from CP47 to RC is represented by single rate constant. The possible ways of excitation energy migration and trapping in the CP47RC complex are discussed.