Structure and function in the isolated reaction center complex of Photosystem II .1. Ultrafast fluorescence measurements of PSII

Time-reserved fluorescence measurements of PSII D1-D2-Cytb559 reaction center complexes were made by using an optical signal analyzer based. on a streak camera tube synchronized with subpicosecond jitter to a femtosecond 1 kHz titanium:sapphire laser system, The instrumental response function of this analyzer was 4-4.5 ps fwhm under experimental conditions, Synchronization permitted extensive signal averaging and allowed measurements to be made with low incident pulse energy (3-10 nJ) and photon flux ((1.5-5.0) x 10(13) photons/cm(2)), Fluorescence decays were obtained following excitation throughout the Q(y) band of PSII, The data are inconsistent with an effective or apparent charge separation time scale of ca. 3 ps following direct excitation of P680. Rather, the effective charge separation from a quasi-equilibrium of chromophore excited stales is either quite fast (less than or equal to 1.25 ps) or relatively slow (greater than or equal to 20 ps). The most consistent interpretation of the available data involves rapid energy redistribution processes within pools of pigments, somewhat slower energy redistribution between pools of pigments, and relatively slow multiexponential charge separation.