title: Fluorescence kinetics of PSII crystals containing Ca(2+) or Sr(2+) in the oxygen evolving complex.
abstract: Photosystem II (PSII) is the pigment-protein complex which converts sunlight energy into chemical energy by catalysing the process of light-driven oxidation of water into reducing equivalents in the form of protons and electrons. Three-dimensional structures from x-ray crystallography have been used extensively to model these processes. However, the crystal structures are not necessarily identical to those of the solubilised complexes. Here we compared picosecond fluorescence of solubilised and crystallised PSII core particles isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The fluorescence of the crystals is sensitive to the presence of artificial electron acceptors (K3Fe(CN)3) and electron transport inhibitors (DCMU). In PSII with reaction centres in the open state, the picosecond fluorescence of PSII crystals and solubilised PSII is indistinguishable. Additionally we compared picosecond fluorescence of native PSII with PSII in which Ca(2) in the oxygen evolving complex (OEC) is biosynthetically replaced by Sr(2+). With the Sr(2+) replaced OEC the average fluorescence decay slows down slightly (81ps to 85ps), and reaction centres are less readily closed, indicating that both energy transfer/trapping and electron transfer are affected by the replacement.
authors.affiliation: Department of Physics and Astronomy, Faculty of Sciences, VU University Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands. Electronic address: b.f.van.oort@vu.nl.
authors.forename: Bart
authors.initials: B
authors.lastname: van Oort
authors.affiliation: Department of Plant Molecular Physiology, Faculty of Biology, University of Warsaw, Miecznikowa 1, 02-096 Warsaw, Poland.
authors.forename: Joanna
authors.initials: J
authors.lastname: Kargul
authors.forename: Karim
authors.initials: K
authors.lastname: Maghlaoui
authors.affiliation: Division of Molecular Biosciences, Faculty of Natural Sciences, Imperial College London, London SW7 2AZ, UK.
authors.forename: James
authors.initials: J
authors.lastname: Barber
authors.affiliation: Laboratory of Biophysics, Wageningen University, P. O. Box 8128, 6700 ET Wageningen, The Netherlands.
authors.forename: Herbert
authors.initials: H
authors.lastname: van Amerongen
mesh_terms: D002118:Calcium / Q000378:metabolism*; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000378:metabolism*; D005453:Fluorescence; D007700:Kinetics; D010100:Oxygen / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D012995:Solubility; D013324:Strontium / Q000378:metabolism*
chemical_list: D045332:Photosystem II Protein Complex; D010100:Oxygen; D002118:Calcium; D013324:Strontium
keywords: Ca(2+)/Sr(2+) exchange; Crystallography; FLIM; Fluorescence lifetime imaging; Photosystem II; Picosecond fluorescence