Spectral substructure and excitonic interactions in the plant antenna complexes LHC II and CP29 as revealed by non-linear laser spectroscopy

Abstract

Manifestation and extent of excitonic interactions in the plant light-harvesting complexes LHC II and CP29 have been investigated by nonlinear laser spectroscopy. Nonlinear absorption of 120 fs pulses indicates an increase in absorption cross section in the red wing of the Qy-band of LHC II as compared to monomeric chlorophyll (Chl) a in organic solution. Nonlinear polarization spectroscopy in the frequency domain (NLPF) exploiting also the intensity dependence of the NLPF signal reveals that in LHC II a species emitting at 682 nm is characterized by a 2.2(± 0.8)-fold larger dipole strength than that of monomeric Chl a. NLPF experiments indicate strong excitonic coupling between Chls a and b in CP29. Moreover, the lowest Qy-transition in CP29 (in contrast to LHC II) is assigned to a non-excitonically coupled Chl a. Stepwise two-photon excitation of Chls a and b in LHC II and CP29 with 100 fs pulses in the (red) Qy-region results in a weak blue fluorescence. The dependence of the spectral shape of the blue fluorescence on excitation wavelength together with a comparison to properties of Chls in solution are consistent with the existence of strongly excitonically coupled Chl a/b-heterodimers in LHC II. Implications of the results for the refinement of structural models and intra- as well as inter-antenna energy-transfer mechanisms are discussed.