title: Electron paramagnetic resonance kinetic studies of the S states in spinach thylakoids.
abstract: The Tyrz+ decay kinetics have been analyzed by using time-resolved EPR to determine the half-time of each Si-->S(i + 1) transition in the O2-evolving complex of spinach thylakoids under physiological conditions. Using dark-adapted thylakoids and appropriate single-turnover flash sequences, we were able to detect the signal IIvf kinetics of the Tyrz+ S0-->Tyrz S1, Tyrz+ S1-->Tyrz S2, Tyrz+ S2-->Tyrz S3, and Tyrz+ S3-->(S4)-->Tyrz S0 transitions. To correct for damping of the S state synchronization during the flash sequence, the Kok parameters were estimated by measuring the oxygen flash pattern in situ using nitroxide-based EPR oximetry. Following deconvolution of the individual S state contributions, the signal IIvf decay kinetics yield the following half-times for the S state transitions: S0-->S1 in 40-60 microseconds, S1-->S2 in 85 microseconds, S2-->S3 in 140 microseconds, and S3-->(S4)-->S0 in 750 microseconds. Preliminary results with detergent-solubilized PSII membranes suggest that the S3-->S0 transition at least is slowed by a factor of approximately 2 in this system. Ramifications of these half-times in terms of electron transfer events on the donor site of PSII are discussed.
authors.affiliation: Department of Chemistry, Faculty of Science, Australian National University, Canberra, Australia.
authors.forename: M R
authors.initials: MR
authors.lastname: Razeghifard
authors.forename: C
authors.initials: C
authors.lastname: Klughammer
authors.forename: R J
authors.initials: RJ
authors.lastname: Pace
mesh_terms: D001665:Binding Sites; D060365:Chloroplast Proteins; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D007700:Kinetics; D008565:Membrane Proteins / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism*; D010940:Plant Proteins / Q000737:chemistry*; D018724:Spinacia oleracea / Q000737:chemistry; D014443:Tyrosine / Q000737:chemistry / Q000378:metabolism
chemical_list: D060365:Chloroplast Proteins; D008565:Membrane Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D010940:Plant Proteins; D014443:Tyrosine; D010100:Oxygen