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Biophys J, February 2002, p. 1030-1039, Vol. 82, No. 2

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

Axel Schubert,* Wichard J. D. Beenken,dagger Holger Stiel,dagger Bernd Voigt,dagger Dieter Leupold,dagger and Heiko Lokstein*

 *Humboldt-Universität zu Berlin, Institut für Biologie Plant Physiology, Unter den Linden 6, D-10099 Berlin, and  dagger Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany

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.

Biophys J, February 2002, p. 1030-1039, Vol. 82, No. 2
© 2002 by the Biophysical Society   0006-3495/02/02/1030/10  $2.00


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