title: Laser-flash-activated electron paramagnetic resonance studies of primary photochemical reactions in chloroplasts.
abstract: Electron paramagnetic resonance studies of the primary reactants of Photosystems I and II have been conducted at cryogenic temperatures after laser-flash activation with monochromatic light.P-700 photooxidation occurs irreversibly in chloroplasts and in Photosystem I fragments after activation with a 730 nm laser flash at a temperature of 35 degrees K. Flash activation of chloroplasts or Photosystem II chloroplast fragments with 660 nm light results in the production of a free-radical signal (g = 2.002, linewidth approximately 8 gauss) which decays with a half-time of 5.0 ms at 35 degrees K. The half-time of decay is independent of temperature in the range of 10-77 degrees K. This reversible signal can be eliminated by preillumination of the sample at 35 degrees K with 660 nm light (but not by 730 nm light), by preillumination with 660 nm light at room temperature in the presence of 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea (DCMU) plus hydroxylamine, or by adjustment of the oxidation-reduction potential of the chloroplasts to - 150 mV prior to freezing. In the presence of ferricyanide (20-50 mM), two free-radical signals are photoinduced during a 660 nm flash at 35 degrees K. One signal decays with a half-time of 5 ms, whereas the second signal is formed irreversibly. These results are discussed in terms of a current model for the Photosystem II primary reaction at low temperature which postulates a back-reaction between P-680+ and the primary electron acceptor.
mesh_terms: D001205:Ascorbic Acid / Q000494:pharmacology; D002736:Chloroplasts / Q000528:radiation effects*; D004237:Diuron / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D005292:Ferricyanides / Q000494:pharmacology; D005615:Freezing; D006898:Hydroxylamines / Q000494:pharmacology; D007700:Kinetics; D007834:Lasers; D010777:Photochemistry; D010944:Plants / Q000378:metabolism; D011830:Radiation Effects
chemical_list: D005292:Ferricyanides; D006898:Hydroxylamines; D004237:Diuron; D001205:Ascorbic Acid