Structure and Function in the Isolated Reaction Center Complex of Photosystem II. 1. Ultrafast Fluorescence Measurements of PSII

Abstract

Time-resolved 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) × 1013 photons/cm2). Fluorescence decays were obtained following excitation throughout the Qy 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 states is either quite fast (≤1.25 ps) or relatively slow (≥20 ps). The most consistent interpretation of the available data involves rapid energy redistribution processes wi...