title: Evidence for water-mediated triplet-triplet energy transfer in the photoprotective site of the peridinin-chlorophyll a-protein.
abstract: Experimental and theoretical studies indicate that water molecules between redox partners can significantly affect their electron-transfer and possibly also the triplet-triplet energy transfer (TTET) properties when in the vicinity of chromophores. In the present work, the interaction of an intervening water molecule with the peridinin triplet state in the peridinin-chlorophyll a-protein (PCP) from Amphidinium carterae is studied by using orientation selective (2)H electron spin echo envelope modulation (ESEEM) spectroscopy, in conjunction with quantum mechanical calculations. This water molecule is located at the interface between the chlorophyll and peridinin pigments involved in the photoprotection mechanism (Chl601(602)-Per614(624), for nomenclature see reference [1]), based on TTET. The characteristic deuterium modulation pattern is observed in the electron spin-echo envelopes for the PCP complex exchanged against (2)H2O. Simulations of the time- and frequency-domain two-pulse and three-pulse ESEEM require two types of coupled (2)H. The more strongly coupled (2)H has an isotropic coupling constant (aiso) of -0.4MHz. This Fermi contact contribution for one of the two water protons and the precise geometry of the water molecule at the interface between the chlorophyll and peridinin pigments, resulting from the analysis, provide experimental evidence for direct involvement of this structured water molecule in the mechanism of TTET. The PCP antenna, characterised by a unity efficiency of the process, represents a model for future investigations on protein- and solvent-mediated TTET in the field of natural/artificial photosynthesis.
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy. Electronic address: marilena.divalentin@unipd.it.
authors.forename: Marilena
authors.initials: M
authors.lastname: Di Valentin
authors.forename: Claudia E
authors.initials: CE
authors.lastname: Tait
authors.forename: Enrico
authors.initials: E
authors.lastname: Salvadori
authors.forename: Laura
authors.initials: L
authors.lastname: Orian
authors.forename: Antonino
authors.initials: A
authors.lastname: Polimeno
authors.forename: Donatella
authors.initials: D
authors.lastname: Carbonera
mesh_terms: D002338:Carotenoids / Q000737:chemistry* / Q000378:metabolism; D061146:Chlorophyll Binding Proteins / Q000737:chemistry* / Q000378:metabolism; D004141:Dinoflagellida / Q000737:chemistry; D004583:Electrons; D004735:Energy Transfer; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry; D008958:Models, Molecular; D010788:Photosynthesis; D014867:Water / Q000737:chemistry*
chemical_list: D061146:Chlorophyll Binding Proteins; D045342:Light-Harvesting Protein Complexes; D014867:Water; C016040:peridinin; D002338:Carotenoids
keywords: ESEEM; Hybrid DFT calculation; Light-harvesting; Photoprotection; Pulsed EPR; QM–MM calculation