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 His118 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 10 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.