title: High-field ELDOR-detected NMR study of a nitroxide radical in disordered solids: towards characterization of heterogeneity of microenvironments in spin-labeled systems.
abstract: The combination of high-field EPR with site-directed spin-labeling (SDSL) techniques employing nitroxide radicals has turned out to be particularly powerful in probing the polarity and proticity characteristics of protein/matrix systems. This information is concluded from the principal components of the nitroxide Zeeman (g), nitrogen hyperfine (A) and quadrupole (P) tensors of the spin labels attached to specific sites. Recent multi-frequency high-field EPR studies underlined the complexity of the problem to treat the nitroxide microenvironment in proteins adequately due to inherent heterogeneities which result in several principal x-components of the nitroxide g-tensor. Concomitant, but distinctly different nitrogen hyperfine components could, however, not be determined from high-field cw EPR experiments owing to the large intrinsic EPR linewidth in fully protonated guest/host systems. It is shown in this work that, using the W-band (95GHz) ELDOR- (electron-electron double resonance) detected NMR (EDNMR) method, different principal nitrogen hyperfine, Azz, and quadrupole, Pzz, tensor values of a nitroxide radical in glassy 2-propanol matrix can be measured with high accuracy. They belong to nitroxides with different hydrogen-bond situations. The satisfactory resolution and superior sensitivity of EDNMR as compared to the standard ENDOR (electron-nuclear double resonance) method are demonstrated.
authors.affiliation: Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany.
authors.forename: Anna
authors.initials: A
authors.lastname: Nalepa
authors.affiliation: Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany; Department of Physics, Free University Berlin, Arnimallee 14, D-14195 Berlin, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Möbius
authors.affiliation: Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lubitz
authors.affiliation: Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany. Electronic address: anton.savitsky@cec.mpg.de.
authors.forename: Anton
authors.initials: A
authors.lastname: Savitsky
mesh_terms: D000465:Algorithms; D017753:Ecosystem; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D009589:Nitrogen Oxides / Q000032:analysis* / Q000737:chemistry*; D013113:Spin Labels
chemical_list: D009589:Nitrogen Oxides; D013113:Spin Labels; C039900:nitroxyl
keywords: ELDOR-detected NMR; ENDOR; High-field EPR; Hyperfine interaction; Matrix effects; Nitroxide radical