Decay Kinetics and Quantum Yields of Fluorescence in Photosystem I from Synechococcus elongatus with P700 in the Reduced and Oxidized State: Are the Kinetics of Excited State Decay Trap-Limited or Transfer-Limited?

Transfer and trapping of excitation energy in photosystem I (PS I) trimers isolated from Synechococcus elongatus have been studied by an approach combining fluorescence induction experiments with picosecond time-resolved fluorescence measurements, both at room temperature (RT) and at low temperature (5 K). Special attention was paid to the influence of the oxidation state of the primary electron donor P700. A fluorescence induction effect has been observed, showing a ∼12% increase in fluorescence quantum yield upon P700 oxidation at RT, whereas at temperatures below 160 K oxidation of P700 leads to a decrease in fluorescence quantum yield (∼50% at 5 K). The fluorescence quantum yield for open PS I (with P700 reduced) at 5 K is increased by ∼20-fold and that for closed PS I (with P700 oxidized) is increased by ∼10-fold, as compared to RT. Picosecond fluorescence decay kinetics at RT reveal a difference in lifetime of the main decay component: 34 ± 1 ps for open PS I and 37 ± 1 ps for closed PS I. At 5 K the fluorescence yield is mainly associated with long-lived components (lifetimes of 401 ps and 1.5 ns in closed PS I and of 377 ps, 1.3 ns, and 4.1 ns in samples containing ∼50% open and 50% closed PS I). The spectra associated with energy transfer and the steady-state emission spectra suggest that the excitation energy is not completely thermally equilibrated over the core-antenna-RC complex before being trapped. Structure-based modeling indicates that the so-called red antenna pigments (A708 and A720, i.e., those with absorption maxima at 708 nm and 720 nm, respectively) play a decisive role in the observed fluorescence kinetics. The A720 are preferentially located at the periphery of the PS I core-antenna-RC complex; the A708 must essentially connect the A720 to the reaction center. The excited-state decay kinetics turn out to be neither purely trap limited nor purely transfer (to the trap) limited, but seem to be rather balanced.