title: Carotenoid triplet states in photosystem II: coupling with low-energy states of the core complex.
abstract: The photo-excited triplet states of carotenoids, sensitised by triplet-triplet energy transfer from the chlorophyll triplet states, have been investigated in the isolated Photosystem II (PSII) core complex and PSII-LHCII (Light Harvesting Complex II) supercomplex by Optically Detected Magnetic Resonance techniques, using both fluorescence (FDMR) and absorption (ADMR) detection. The absence of Photosystem I allows us to reach the full assignment of the carotenoid triplet states populated in PSII under steady state illumination at low temperature. Five carotenoid triplet ((3)Car) populations were identified in PSII-LHCII, and four in the PSII core complex. Thus, four (3)Car populations are attributed to β-carotene molecules bound to the core complex. All of them show associated fluorescence emission maxima which are relatively red-shifted with respect to the bulk emission of both the PSII-LHCII and the isolated core complexes. In particular the two populations characterised by Zero Field Splitting parameters |D|=0.0370-0.0373 cm(-1)/|E|=0.00373-0.00375 cm(-1) and |D|=0.0381-0.0385 cm(-1)/|E|=0.00393-0.00389 cm(-1), are coupled by singlet energy transfer with chlorophylls which have a red-shifted emission peaking at 705 nm. This observation supports previous suggestions that pointed towards the presence of long-wavelength chlorophyll spectral forms in the PSII core complex. The fifth (3)Car component is observed only in the PSII-LHCII supercomplex and is then assigned to the peripheral light harvesting system.
authors.affiliation: Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Via Celoria 26, 20133 Milan, Italy. Electronic address: stefano.santabarbara@cnr.it.
authors.forename: Stefano
authors.initials: S
authors.lastname: Santabarbara
authors.affiliation: Department of Chemical Sciences, Università di Padova, Via Marzolo 1, 35131 Padova, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Agostini
authors.affiliation: Istituto di Biologia e Biotecnologia Agraria, Consiglio Nazionale delle Ricerche, Via Bassini 15a, 20133 Milano, Italy.
authors.forename: Anna Paola
authors.initials: AP
authors.lastname: Casazza
authors.affiliation: Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Via Celoria 26, 20133 Milan, Italy.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Zucchelli
authors.affiliation: Department of Chemical Sciences, Università di Padova, Via Marzolo 1, 35131 Padova, Italy. Electronic address: donatella.carbonera@unipd.it.
authors.forename: Donatella
authors.initials: D
authors.lastname: Carbonera
mesh_terms: D002338:Carotenoids / Q000737:chemistry*; D004735:Energy Transfer; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
chemical_list: D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; D002338:Carotenoids
keywords: Carotenoid; Optically Detected Magnetic Resonance; Photosystem II; Photosystem II core complex; Triplet state; Triplet state quenching