title: Xanthophyll-induced aggregation of LHCII as a switch between light-harvesting and energy dissipation systems.
abstract: The xanthophyll cycle pigments, violaxanthin and zeaxanthin, present outside the light-harvesting pigment-protein complexes of Photosystem II (LHCII) considerably enhance specific aggregation of proteins as revealed by analysis of the 77 K chlorophyll a fluorescence emission spectra. Analysis of the infrared absorption spectra in the Amide I region shows that the aggregation is associated with formation of intermolecular hydrogen bonding between the alpha helices of neighboring complexes. The aggregation gives rise to new electronic energy levels, in the Soret region (530 nm) and corresponding to the Q spectral region (691 nm), as revealed by analysis of the resonance light scattering spectra. New electronic energy levels are interpreted in terms of exciton coupling of protein-bound photosynthetic pigments. The energy of the Q excitonic level of chlorophyll is not high enough to drive the light reactions of Photosystem II but better suited to transfer excitation energy to Photosystem I, which creates favourable energetic conditions for the state I-state II transition. The lack of fluorescence emission from this energy level, at physiological temperatures, is indicative of either very high thermal energy conversion rate or efficient excitation quenching by carotenoids. Chlorophyll a fluorescence was quenched up to 61% and 34% in the zeaxanthin- and violaxanthin-containing samples, respectively, as compared to pure LHCII. Enhanced aggregation of LHCII, observed in the presence of the xanthophyll cycle pigments, is discussed in terms of the switch between light-harvesting and energy dissipation systems.
authors.affiliation: Department of Biophysics, Institute of Physics, Maria Curie-Sklodowska University, 20-601 Lublin, Poland. wieslaw@tytan.umcs.lublin.pl
authors.forename: Wieslaw I
authors.initials: WI
authors.lastname: Gruszecki
authors.forename: Wojciech
authors.initials: W
authors.lastname: Grudzinski
authors.forename: Małgorzata
authors.initials: M
authors.lastname: Gospodarek
authors.forename: Magdalena
authors.initials: M
authors.lastname: Patyra
authors.forename: Waldemar
authors.initials: W
authors.lastname: Maksymiec
mesh_terms: D002338:Carotenoids / Q000737:chemistry; D002736:Chloroplasts / Q000378:metabolism; D004735:Energy Transfer; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry*; D032305:Lycium / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D012542:Scattering, Radiation; D013050:Spectrometry, Fluorescence; D013055:Spectrophotometry, Infrared; D032413:Viola / Q000378:metabolism; D024341:Xanthophylls / Q000737:chemistry / Q000378:metabolism*; D003313:Zea mays / Q000378:metabolism; D065146:Zeaxanthins
chemical_list: D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; D024341:Xanthophylls; D065146:Zeaxanthins; D002338:Carotenoids; C005613:violaxanthin