PICOSECOND AND NANOSECOND FLUORESCENCE KINETICS OF PHOTOSYSTEM-II REACTION-CENTER AND ITS COMPLEX WITH CP47 ANTENNA

Spectrally resolved pico- and nanosecond fluorescence kinetics of two types of Photosystem II core complex: D1/D2/cyt b559 reaction centres (RCs) and RCs together with CP47 proximal antenna have been studied at room temperature and at 77 K. The kinetics at room temperature were measured with the RCs being in different functional states. In the photoactive RCs at room temperature a picosecond decay with the lifetime components 13 +/- 3 ps and 110 +/- 30 ps is followed by the complex nanosecond kinetics. In the case of RC + CP47 complexes picosecond decays are slower: 25 +/- 10 ps and 190 +/- 30 ps, but nanosecond decay has similar behaviour. The data are analyzed by a simple three-state kinetic model postulating the formation of the primary radical pair in an unrelaxed form and allowing a back-recombination from that state. If this is correct, it is the proof that in the charge-separated state nuclear coordinate relaxation takes place on the picosecond time-scale. The following conclusions considering the nature and temporal characteristics of light excitations have been made: (i) At room temperature excitations are in equilibrium between P680 and the accessory chlorophylls including CP47 antenna and, therefore, only the average trapping time could be observed. This time is equal to 13 +/- 3 ps in RCs and 25 +/- 10 ps in RC + CP47 complexes. (ii) All other decays at room temperature (except probably part of the 5 +/- 0.5 ns component) are of a recombination origin and reflect complex relaxation of the metastable radical pair state. (iii) At low temperatures an energetically directed excitation transfer, qualitatively very similar to the one observed in the core antenna of some purple bacteria takes place. This energy transfer is relatively slow with an apparent transfer time 10-20 ps at 77 K. (iv) Not only P680, but also P(+)680 and Pheo(-) are very efficient quenchers of excitations.