Time-resolved fluorescence measurements of photosystem - II: The effect of quenching by oxidized chlorophyll Z

Chlorophyll Z (Chl(Z)) is a unique redox-active chlorophyll bound to His 118 in the D1 subunit of photosystem II (PSII). Previous work has shown that Chl(Z)(+) is a potent quencher of fluorescence. In this study, we have used time-resolved fluorescence spectroscopy to determine the effect of Chl(Z)(+) on the decay components in PSII at low temperature. Both EPR and fluorescence measurements were made to ascertain the redox state of the sample under the conditions of the fluorescence measurements. We have compared spinach PSII membranes, which contain both the core antenna and the peripheral LHCII antennae, with PSII core complexes from Synechocystis PCC 6803, which contain only the core antenna. The presence of Chl(Z)(+) decreases the steady-state fluorescence intensity by about 10-fold, and this decrease is associated with a large decrease in the yield of the nanosecond decay components. In samples containing a fractional amount of Chl(Z)(+), there is a good correlation between the fluorescence decay time and the amount of Chl(Z)(+).. Chl(Z)(+) is an effective quencher of emission from all of the core antenna chlorophylls. This result is consistent with "rapid exciton equilibration" within the PSII core antenna. However, emission from the peripheral LHCII antenna was less effectively quenched by Chl(Z)(+), indicating that exciton equilibration is incomplete among the core and peripheral antennas, at least at 77 K.