title: Carotenoid-chlorophyll coupling and fluorescence quenching correlate with protein packing density in grana-thylakoids.
abstract: The regulation of light-harvesting in photosynthesis under conditions of varying solar light irradiation is essential for the survival and fitness of plants and algae. It has been proposed that rearrangements of protein distribution in the stacked grana region of thylakoid membranes connected to changes in the electronic pigment-interaction play a key role for this regulation. In particular, carotenoid-chlorophyll interactions seem to be crucial for the down-regulation of photosynthetic light-harvesting. So far, it has been difficult to determine the influence of the dense protein packing found in native photosynthetic membrane on these interactions. We investigated the changes of the electronic couplings between carotenoids and chlorophylls and the quenching in grana thylakoids of varying protein packing density by two-photon spectroscopy, conventional chlorophyll fluorometry, low-temperature fluorescence spectroscopy, and electron micrographs of freeze-fracture membranes. We observed an increasing carotenoid-chlorophyll coupling and fluorescence quenching with increasing packing density. Simultaneously, the antennas size and excitonic connectivity of Photosystem II increased with increasing quenching and carotenoid-chlorophyll coupling whereas isolated, decoupled LHCII trimers decreased. Two distinct quenching data regimes could be identified that show up at different protein packing densities. In the regime corresponding to higher protein packing densities, quenching is strongly correlated to carotenoid-chlorophyll interactions whereas in the second regime, a weak correlation is apparent with low protein packing densities. Native membranes are in the strong-coupling data regime. Consequently, PSII and LHCII in grana membranes of plants are already quenched by protein crowding. We concluded that this ensures efficient electronic connection of all pigment-protein complexes for intermolecular energy transfer to the reaction centers and allows simultaneously sensitive regulation of light harvesting by only small changes in the protein packaging.
authors: Institute of Physical and Theoretical Chemistry, Department of Biophysical Chemistry, Technische Universität Braunschweig , Hans-Sommer-Straße 10, 38106 Braunschweig, Germany.
authors: Christoph-Peter
authors: CP
authors: Holleboom
authors: Sunny
authors: S
authors: Yoo
authors: Pen-Nan
authors: PN
authors: Liao
authors: Ian
authors: I
authors: Compton
authors: Winfried
authors: W
authors: Haase
authors: Helmut
authors: H
authors: Kirchhoff
authors: Peter Jomo
authors: PJ
authors: Walla
mesh_terms: D017360:Arabidopsis / Q000378:metabolism; D002338:Carotenoids / Q000737:chemistry*; D002734:Chlorophyll / Q000737:chemistry*; D015536:Down-Regulation; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D013696:Temperature; D020524:Thylakoids / Q000378:metabolism*
chemical_list: D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D002734:Chlorophyll; D002338:Carotenoids