title: Disentangling the low-energy states of the major light-harvesting complex of plants and their role in photoprotection.
abstract: The ability to dissipate large fractions of their absorbed light energy as heat is a vital photoprotective function of the peripheral light-harvesting pigment-protein complexes in photosystem II of plants. The major component of this process, known as qE, is characterised by the appearance of low-energy (red-shifted) absorption and fluorescence bands. Although the appearance of these red states has been established, the molecular mechanism, their site and particularly their involvement in qE are strongly debated. Here, room-temperature single-molecule fluorescence spectroscopy was used to study the red emission states of the major plant light-harvesting complex (LHCII) in different environments, in particular conditions mimicking qE. It was found that most states correspond to peak emission at around 700nm and are unrelated to energy dissipative states, though their frequency of occurrence increased under conditions that mimicked qE. Longer-wavelength emission appeared to be directly related to energy dissipative states, in particular emission beyond 770nm. The ensemble average of the red emission bands shares many properties with those obtained from previous bulk in vitro and in vivo studies. We propose the existence of at least three excitation energy dissipating mechanisms in LHCII, each of which is associated with a different spectral signature and whose contribution to qE is determined by environmental control of protein conformational disorder. Emission at 700nm is attributed to a conformational change in the Lut 2 domain, which is facilitated by the conformational change associated with the primary quenching mechanism involving Lut 1.
authors.affiliation: Department of Physics, Faculty of Natural and Agricultural Sciences, University of Pretoria, Private bag X20, Hatfield 0028, South Africa; Department of Physics and Astronomy, Faculty of Sciences, VU University, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands. Electronic address: tjaart.kruger@up.ac.za.
authors.forename: Tjaart P J
authors.initials: TP
authors.lastname: Krüger
authors.affiliation: Department of Physics and Astronomy, Faculty of Sciences, VU University, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands; Institute of Biology and Technology of Saclay, CEA, UMR 8221 CNRS, University Paris Sud, CEA Saclay, 91191 Gif-sur-Yvette, France. Electronic address: cristian.ilioaia@cea.fr.
authors.forename: Cristian
authors.initials: C
authors.lastname: Ilioaia
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, UK. Electronic address: matt.johnson@sheffield.ac.uk.
authors.forename: Matthew P
authors.initials: MP
authors.lastname: Johnson
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK. Electronic address: a.ruban@qmul.ac.uk.
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Ruban
authors.affiliation: Department of Physics and Astronomy, Faculty of Sciences, VU University, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands. Electronic address: r.van.grondelle@vu.nl.
authors.forename: Rienk
authors.initials: R
authors.lastname: van Grondelle
mesh_terms: D000595:Amino Acid Sequence; D005453:Fluorescence; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D055668:Photochemical Processes; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D018724:Spinacia oleracea / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D045342:Light-Harvesting Protein Complexes; D010940:Plant Proteins
keywords: Light-harvesting complex; NPQ; Photoprotection; Photosystem II; Protein dynamics; Single-molecule spectroscopy (SMS)