PARENT SESSION
Posters P4Aa Chlorophyll and bilin based antenna systems. Abstracts (239-271)

Manifestation and significance of excitonic interactions in the plant antenna complexes, LHC II and CP29. Heiko Lokstein^*,1, Bernd Voigt², Maria Krikunova^{2, 3}, Axel Schubert¹, Klaus Teuchner², Klaus-Dieter Irrgang⁴, Gernot Renger⁴, Dieter Leupold², ¹ Institut für Biologie/Pflanzenphysiologie, Berlin, Germany² Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Berlin, Germany³ Present address: MPI für Biophysikalische Chemie, Göttingen, Germany⁴

ABSTRACT- Manifestation of excitonic interactions in plant light-harvesting complexes, LHC II and CP29, has been investigated by nonlinear laser spectroscopy. Nonlinear absorption of 120 fs pulses reveals a subband in the red wing of the Q_y-band of LHC II with an about twofold increased absorption cross section - as compared to monomeric chlorophyll (Chl) a in solution. Intensity dependent nonlinear polarization spectroscopy in the frequency domain (NLPF) corroborates that a subband emitting at 682 nm in LHC II has a 2.2(+0.8)-fold larger dipole strength than Chl a. Moreover, NLPF indicates significant excitonic coupling between Chls a and b in LHC II and CP29. The lowest Q_y-transition in CP29, however, is assigned to a non-coupled Chl a. Stepwise two-photon excitation of Chls a and b in LHC II and CP29 with 100 fs pulses in the Q_y-region results in a weak blue fluorescence. The dependence of the spectral shape of this emission on excitation wavelength is consistent with the existence of strong Chl a/b-coupling in LHC II. Based on additional information from site-directed mutagenesis studies the interacting Chl molecules have been assigned. The results obtained with LHC II are corroborated by a recent improved structural model: The longest wavelength Chl a is part of a strongly excitonically coupled homo-dimer, two further Chl a/b-hetero-dimers exist. Implications for intra- as well as inter-antenna excitation energy-transfer and thus, for overall efficiency of photosynthetic energy conversion are discussed.

KEY WORDS: antenna complex, CP29, excitonic coupling, LHC II