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Spectroscopie RPE

Spectroscopie RPE. Résonance Paramagnetique Electronique. Introduction and principles. Paramagnetic species (in biology). Spin S = 1/2 systems. hyperfine coupling anisotropy. Spin S > 1/2 systems. half-integer integer polynuclear systems. Bio-EPR spectroscopy.

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Spectroscopie RPE

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  1. Spectroscopie RPE RésonanceParamagnetiqueElectronique

  2. Introduction and principles • Paramagnetic species (in biology) • Spin S = 1/2 systems hyperfine coupling anisotropy • Spin S > 1/2 systems half-integer integer polynuclear systems • Bio-EPR spectroscopy

  3. Introduction and principle

  4. History of (bio)EPR 1944- discovery of the EPR effect E.K. Zavoisky concomitantly: solid-state physicists: quantum theoretical bases 1952- first detection of an EPR spectrum (radical) 1956- J.E. Bennett and D.J.E. Ingram : single crystal EPR studies of Hemoglobin 1959- R.H. Sands H. Beinert : EPR studies of frozen solutions of heart tissue 1959- B.G. Malmström and T. Vänngard : EPR studies of frozen solutions of multicentre metalloenzymes

  5. History Two and a half decades after the first observation of EPR signal : EPR Spectroscopy became a practical research tool for non specialists Powerful tool to study paramagnetic systems unpaired electrons « EPR signal intensity and Powder Shapes: A Reexamination » R. Aasa, T. Vänngård, J. Magn. Res. 1975, 19, 308-315

  6. Electronspin electron: magnetic moment and spin quantum number S = 1/2 with magnetic components ms = +1/2 and ms = –1/2 Stern and Gerlach experiment: Otto Stern and Walther Gerlach - 1922

  7. Zero field field B0 B0 Electron Spin - classical view Electron spin: angular momentum S = 1/2 Classical view: E = - . B0

  8. Some definitions Electron spin: angular momentum S = 1/2 Magnetic moment • ge = 2,002319304386 •  = Bohr magneton • e : gyromagnetic constant of the electron

  9. Electron Spin - quantum mechanical view Applied magnetic field along z. Projection of S along z : -S  ms S i.e. ms = -1/2 ; + 1/2 ms = +1/2 E = 1/2 geB0 Energy ms = -1/2 E = -1/2 geB0 B0 B0 = 0

  10. Bohr magneton9,2740154 10-24 J T-1 Free electron ge2,002319304386 Magnetic fieldTesla T perpendicular mode: B1 B0 Planck constant6,6260755 10-34 J s Frequency of the radiation EPRtransitions E = 1/2 geB0 Energy E = - 1/2 geB0

  11. EPRtransitions h Energy B0 Br

  12. EPRspectrometers Energy: 0.3 cm-1 Units: 1 T = 10000 G 336 mT = 3360 G

  13. EPRspectrometers Diode gun -waves source) -waves bridge Magnet CPU

  14. EPRspectrometers

  15. Intensity of EPR signal Depends on: • Sample concentration (spin concentration) • Power of the microwaves • Temperature • (Modulation) Experimental key parameters !

  16. EPR SAMPLES -powder -solution typical biological samples • frozen solutions: • cylindrical quartz tube • 100-200 L • 3-4 mm inner diameter room temperature experiments: water - high dielectric constant flat cells or capillary

  17. Paramagnetic species

  18. Organic radicals • Inorganic radicals: NO, O2 • metal ions and metal complexes (metalloproteins) • defect centers in crystals or semiconductors • metallic conductors

  19. 3d7 3d8 TRANSITION METAL IONS

  20. TRANSITION METAL five d orbitals:

  21. low spin (S = 1/2) high spin (S = 5/2) dz2 dx2-y2 dx2-y2 dz2 0 0 dyz dxz dxy dxy dxz dyz CASE of IRON FeII (3d6) FeIII (3d5) FeIV (3d4) FeV (3d3)

  22. dx2-y2 dxy dz2 dxz dyz CASE of COPPER CuI (3d10) CuII (3d9) CuIII (3d8)

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