title: The lowest-energy chlorophyll of photosystem II is adjacent to the peripheral antenna: Emitting states of CP47 assigned via circularly polarized luminescence.
abstract: The identification of low-energy chlorophyll pigments in photosystem II (PSII) is critical to our understanding of the kinetics and mechanism of this important enzyme. We report parallel circular dichroism (CD) and circularly polarized luminescence (CPL) measurements at liquid helium temperatures of the proximal antenna protein CP47. This assembly hosts the lowest-energy chlorophylls in PSII, responsible for the well-known "F695" fluorescence band of thylakoids and PSII core complexes. Our new spectra enable a clear identification of the lowest-energy exciton state of CP47. This state exhibits a small but measurable excitonic delocalization, as predicated by its CD and CPL. Using structure-based simulations incorporating the new spectra, we propose a revised set of site energies for the 16 chlorophylls of CP47. The significant difference from previous analyses is that the lowest-energy pigment is assigned as Chl 612 (alternately numbered Chl 11). The new assignment is readily reconciled with the large number of experimental observations in the literature, while the most common previous assignment for the lowest energy pigment, Chl 627(29), is shown to be inconsistent with CD and CPL results. Chl 612(11) is near the peripheral light-harvesting system in higher plants, in a lumen-exposed region of the thylakoid membrane. The low-energy pigment is also near a recently proposed binding site of the PsbS protein. This result consequently has significant implications for our understanding of the kinetics and regulation of energy transfer in PSII.
authors.affiliation: Research School of Chemistry, The Australian National University, Canberra, Australia. Electronic address: jeremy.hall@anu.edu.au.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Hall
authors.affiliation: Institut für Theoretische Physik, Johannes Kepler Universität, Linz, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Renger
authors.affiliation: Institut für Theoretische Physik, Johannes Kepler Universität, Linz, Austria.
authors.forename: Frank
authors.initials: F
authors.lastname: Müh
authors.affiliation: Estacion Experimental de Aula Dei (CSIC), Avda. Montañana 1005, 50059- Zaragoza, Spain.
authors.forename: Rafael
authors.initials: R
authors.lastname: Picorel
authors.affiliation: Research School of Chemistry, The Australian National University, Canberra, Australia.
authors.forename: Elmars
authors.initials: E
authors.lastname: Krausz
mesh_terms: D002734:Chlorophyll / Q000737:chemistry*; D002942:Circular Dichroism; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry*; D049449:Luminescence; D045332:Photosystem II Protein Complex / Q000737:chemistry*
chemical_list: D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; C070970:photosystem II, chlorophyll-binding protein, CP-47; D002734:Chlorophyll
keywords: CP47; Circularly polarized luminescence; Fluorescence; Lowest excited states; Photosystem II; PsbS