Effects of Aggregation on Trimeric Light-Harvesting Complex II of Green Plants: A Hole-Burning Study

Abstract

Low-temperature absorption, fluorescence, and persistent hole-burned spectra are reported for aggregates of the trimeric light-harvesting antenna complex of photosystem II (LHC II) . The lowest energy Qy-state was found to lie at 681.5 nm on the basis of hole spectra, which corresponds to a 2 nm red 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 ∼ 0.8 and coupling to phonons with a mean frequency of ∼ 20 cm-1 which is very similar to that of the isolated trimer. This coupling is consistent with the 4.2 K Stokes shift of the fluorescence originating from the 681.5 nm state. An adjacent state at 680.0 nm is assigned. On the basis of the results of Pieper et al. for the isolated trimer, a state at ∼ 678.5 nm is inferred. These three lowest energy Qy-states are associated with the lowest energy chlorophyll a state of the subunit of the isolated LHC II trimer. Their degeneracy is removed because of structural heterogeneity. The hole-burning results indicate that, aside from a quite uniform and small red shifting, aggregation has little effect on the excitonic level structure and intrinsic dynamics 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 temperature dependence of aggregation-induced fluorescence quenching between 4.2 and 80 K (Ruban et al. Biochim. Biophys. Acta 1992, 1102, 30).