Dynamic quenching in single photosystem II supercomplexes.

Photosystem II (PSII) is a huge pigment-protein supercomplex responsible for the primary steps of photosynthesis in green plants. Its light-harvesting antenna exhibits efficient transfer of the absorbed excitation energy to the reaction center and also contains a well-regulated protection mechanism against over-excitation in strong light conditions. The latter is based on conformational changes in antenna complexes that open up excitation decay channels resulting in considerable fluorescence quenching. Meanwhile, fluorescence blinking, observed in single antennas, is likely caused by a similar mechanism. Thus the question arises whether this effect is also present in and relevant to the native supramolecular organization of a fully assembled PSII. To further investigate energy transfer and quenching in single PSII, we performed single-molecule experiments on PSII supercomplexes at 5 °C. Analysis of the fluorescence intensity and mean lifetime allowed us to distinguish detached antennas and specifically analyze PSII supercomplexes. The average fluorescence lifetime in PSII of about 100-150 ps, measured under our extreme excitation conditions, is surprisingly similar to published ensemble lifetime data of photochemical quenching in PSII of a similar size. In our case, this lifetime is nevertheless caused by either one or multiple quenched antennas or by a quencher in the reaction center. The observed reversible light-induced changes in fluorescence intensity on a millisecond timescale are reminiscent of blinking subunits. Our results therefore directly illustrate how environmental control over a fluctuating antenna can regulate light-harvesting in plant photosynthesis.