Excitonic Coupling of Chlorophylls in the Plant Light-Harvesting Complex LHC-II

ABSTRACT Manifestation and extent of excitonic interactions in the red Chl-absorption region (Qy band) of trimeric LHC-II were investigated using two complementary nonlinear laser-spectroscopic techniques. Nonlinear absorption of 120-fs pulses indicates an increased absorption cross section in the red wing of the Qy band as compared to monomeric Chl a in organic solution. Additionally, the dependence of a nonlinear polarization response on the pump-field intensity was investigated. This approach reveals that one emitting spectral form, characterized by a 2.3(0.8)-fold larger dipole strength than monomeric Chl a, dominates the fluorescence spectrum of LHC-II. Considering available structural and spectroscopic data, these results can be consistently explained assuming the existence of an excitonically coupled dimer located at Chl-bindings sites a2 and b2 (referring to the original notation of W. Kühlbrandt, D. N. Wang, and Y. Fujiyoshi, Nature, 1994, 367:614–621), which must not necessarily correspond to Chls a and b). This fluorescent dimer, terminating the excitation energy-transfer chain of the LHC-II monomeric subunit, is discussed with respect to its relevance for intra- and inter-antenna excitation energy transfer