title: Molecular adaptation of photoprotection: triplet states in light-harvesting proteins.
abstract: The photosynthetic light-harvesting systems of purple bacteria and plants both utilize specific carotenoids as quenchers of the harmful (bacterio)chlorophyll triplet states via triplet-triplet energy transfer. Here, we explore how the binding of carotenoids to the different types of light-harvesting proteins found in plants and purple bacteria provides adaptation in this vital photoprotective function. We show that the creation of the carotenoid triplet states in the light-harvesting complexes may occur without detectable conformational changes, in contrast to that found for carotenoids in solution. However, in plant light-harvesting complexes, the triplet wavefunction is shared between the carotenoids and their adjacent chlorophylls. This is not observed for the antenna proteins of purple bacteria, where the triplet is virtually fully located on the carotenoid molecule. These results explain the faster triplet-triplet transfer times in plant light-harvesting complexes. We show that this molecular mechanism, which spreads the location of the triplet wavefunction through the pigments of plant light-harvesting complexes, results in the absence of any detectable chlorophyll triplet in these complexes upon excitation, and we propose that it emerged as a photoprotective adaptation during the evolution of oxygenic photosynthesis.
authors.affiliation: CEA, Institute of Biology and Technology of Saclay, Gif sur Yvette, France.
authors.forename: Andrew
authors.initials: A
authors.lastname: Gall
authors.forename: Rudi
authors.initials: R
authors.lastname: Berera
authors.forename: Maxime T A
authors.initials: MT
authors.lastname: Alexandre
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Pascal
authors.forename: Luc
authors.initials: L
authors.lastname: Bordes
authors.forename: Maria M
authors.initials: MM
authors.lastname: Mendes-Pinto
authors.forename: Sandra
authors.initials: S
authors.lastname: Andrianambinintsoa
authors.forename: Katerina V
authors.initials: KV
authors.lastname: Stoitchkova
authors.forename: Alessandro
authors.initials: A
authors.lastname: Marin
authors.forename: Leonas
authors.initials: L
authors.lastname: Valkunas
authors.forename: Peter
authors.initials: P
authors.lastname: Horton
authors.forename: John T M
authors.initials: JT
authors.lastname: Kennis
authors.forename: Rienk
authors.initials: R
authors.lastname: van Grondelle
authors.forename: Alexander
authors.initials: A
authors.lastname: Ruban
authors.forename: Bruno
authors.initials: B
authors.lastname: Robert
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry; D002338:Carotenoids / Q000737:chemistry / Q000378:metabolism; D004583:Electrons; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D020560:Proteobacteria / Q000378:metabolism; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman; D018724:Spinacia oleracea / Q000378:metabolism; D014732:Vibration
chemical_list: D001429:Bacteriochlorophylls; D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; D002338:Carotenoids