title: 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 (Q(y) 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 Q(y) 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. Nühlbrandt, 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.
authors.affiliation: Humboldt-Universität zu Berlin, Institut für Biologie Plant Physiology, Unter den Linden 6, D-10099 Berlin, Germany. axel.schubert@chemphys.lu.se
authors.forename: Axel
authors.initials: A
authors.lastname: Schubert
authors.forename: Wichard J D
authors.initials: WJ
authors.lastname: Beenken
authors.forename: Holger
authors.initials: H
authors.lastname: Stiel
authors.forename: Bernd
authors.initials: B
authors.lastname: Voigt
authors.forename: Dieter
authors.initials: D
authors.lastname: Leupold
authors.forename: Heiko
authors.initials: H
authors.lastname: Lokstein
mesh_terms: D055592:Biophysical Phenomena; D001703:Biophysics; D002734:Chlorophyll / Q000737:chemistry* / Q000378:metabolism; D008027:Light; D045342:Light-Harvesting Protein Complexes; D015233:Models, Statistical; D008962:Models, Theoretical; D018532:Pisum sativum / Q000737:chemistry; D017785:Photons; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D011485:Protein Binding; D013053:Spectrophotometry
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D002734:Chlorophyll