title: Triplet states in photosystem I of spinach chloroplasts and subchloroplast particles.
abstract: We report light-induced electron paramagnetic resonance triplet spectra from samples of chloroplasts or digitonin photosystem I particles that depend upon the dark redox state of the bound acceptors of photosystem I. If the reaction centers are prepared in the redox state P-700 A X- FdB-FdA-, then upon illumination at 11K we observe a polarized chlorophyll triplet species which we interpret as arising from radical pair recombination between P-700+ and A-. This chlorophyll triplet is apparently the analog of the PR state of photosynthetic bacteria [Parson, W.W. & Cogdell, R.J. (1975) Biochim. Biophys. Acta 416, 105-149]. If the reaction centers are prepared in the dark redox state P-700 A X FdB-FdA-, then upon illumination at 11K we observe a different triplet species of uncertain origin, possibly pheophytin or carotenoid. This species is closely associated with the photosystem I reaction center and it traps excitation when P-700 is oxidized.
mesh_terms: D002736:Chloroplasts / Q000378:metabolism*; D003624:Darkness; D004227:Dithionite; D004578:Electron Spin Resonance Spectroscopy; D005292:Ferricyanides; D008027:Light; D010084:Oxidation-Reduction; D010788:Photosynthesis; D010944:Plants
chemical_list: D005292:Ferricyanides; D004227:Dithionite