Relationship between excitation energy transfer, trapping, and antenna size in photosystem II

We present a systematic study of the effect of antenna size on energy trans fer and trapping in photosystem II. Time-resolved fluorescence experiments have been used to probe a range of particles isolated from both higher plan ts and the cyanobacterium Synechocystis 6803. The isolated reaction center dynamics are represented by a quasi-phenomenological model that fits the ex tensive time-resolved data from photosystem II reaction centers and reactio n center mutants. This representation of the photosystem II "trapping engin e" is found to correctly predict the extent of, and time scale for, charge separation in a range of photosystem II particles of varying antenna size ( 8-250 chlorins). This work shows that the presence of the shallow trap and slow charge separation kinetics, observed in isolated D1/D2/cyt b559 reacti on centers, are indeed retained in larger particles and that these properti es are reflected in the trapping dynamics of all larger photosystem II prep arations. A shallow equilibrium between the antennae and reaction center in photosystem II will certainly facilitate regulation via nonphotochemical q uenching, and one possible interpretation of these findings is therefore th at photosystem II is optimized for regulation rather than for efficiency.