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 m easured with the RCs being in different functional states. In the phot oactive RCs at room temperature a picosecond decay with the lifetime c omponents 13 +/- 3 ps and 110 +/- 30 ps is followed by the complex nan osecond 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 kinet ic model postulating the formation of the primary radical pair in an u nrelaxed form and allowing a back-recombination from that state. If th is is correct, it is the proof that in the charge-separated state nucl ear coordinate relaxation takes place on the picosecond time-scale. Th e following conclusions considering the nature and temporal characteri stics of light excitations have been made: (i) At room temperature exc itations are in equilibrium between P680 and the accessory chlorophyll s including CP47 antenna and, therefore, only the average trapping tim e 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 tempera ture (except probably part of the 5 +/- 0.5 ns component) are of a rec ombination origin and reflect complex relaxation of the metastable rad ical pair state. (iii) At low temperatures an energetically directed e xcitation transfer, qualitatively very similar to the one observed in the core antenna of some purple bacteria takes place. This energy tran sfer 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.