Effects of aggregation on trimeric light-harvesting complex II of green plants: A hole-burning study

Low-temperature absorption, fluorescence, and persistent hole-burned spectr a are reported for aggregates of the trimeric light-harvesting antenna comp lex of photosystem II (LHC II). The lowest energy Q(y)-state was found to l ie at 681.5 nm on the basis of hole spectra, which corresponds to a 3 nm re d shift relative to the isolated LHC II trimer (Pieper et al. J. Phys. Chem . B 1999,2412, accompanying paper). The electron-phonon coupling of the 681 .5 nm state is characterized by S similar to 0.8 and coupling to phonons wi th a mean frequency of similar to 20 cm(-1) which is very similar to that o f the isolated trimer. This coupling is consistent with the 4.2 K Stokes sh ift of the fluorescence originating from the 681.5 nm state. An adjacent st ate at 680.0 nm is assigned. On the basis of the results of Pieper et al. f or the isolated trimer, a state at similar to 678.5 nm is inferred. These t hree lowest energy Q(y)-states are associated with the lowest energy chloro phyll a state of the subunit of the isolated LHC II trimer. Their degenerac y is removed because of structural heterogeneity. The hole-burning results indicate that, aside from a quite uniform and small red shifting, aggregati on has little effect on the excitonic level structure and intrinsic dynamic s of the isolated trimer, Taken together, the results presented here and in Pieper et al. lead to a model that qualitatively accounts for the strong t emperature dependence of aggregation-induced fluorescence quenching between 4.2 and 80 K (Ruban et al. Biochim. Biophys. Acta 1992, 1102, 30).