title: Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band.
abstract: Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties. Spin lattice relaxation times, spin packet line widths, nuclear hyperfine splitting, and overall lineshapes were characterized for six low molecular weight nitroxides in dilute deoxygenated aqueous solution at X-band. The nitroxides included 6-member, unsaturated 5-member, or saturated 5-member rings, most of which were isotopically labeled. The spectra are near the fast tumbling limit with T(1)∼T(2) in the range of 0.50-1.1 μs at ambient temperature. Both spin-lattice relaxation T(1) and spin-spin relaxation T(2) are longer for (15)N- than for (14)N-nitroxides. The dominant contributions to T(1) are modulation of nitrogen hyperfine anisotropy and spin rotation. Dependence of T(1) on nitrogen nuclear spin state m(I) was observed for both (14)N and (15)N. Unresolved hydrogen/deuterium hyperfine couplings dominate overall line widths. Lineshapes were simulated by including all nuclear hyperfine couplings and spin packet line widths that agreed with values obtained by electron spin echo. Line widths and relaxation times are predicted to be about the same at 250 MHz as at X-band.
authors.affiliation: Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208, USA.
authors.forename: Joshua R
authors.initials: JR
authors.lastname: Biller
authors.forename: Virginia
authors.initials: V
authors.lastname: Meyer
authors.forename: Hanan
authors.initials: H
authors.lastname: Elajaili
authors.forename: Gerald M
authors.initials: GM
authors.lastname: Rosen
authors.forename: Joseph P Y
authors.initials: JP
authors.lastname: Kao
authors.forename: Sandra S
authors.initials: SS
authors.lastname: Eaton
authors.forename: Gareth R
authors.initials: GR
authors.lastname: Eaton
mesh_terms: D016880:Anisotropy; D003903:Deuterium; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D007202:Indicators and Reagents; D007553:Isotope Labeling; D009587:Nitrogen Isotopes; D009589:Nitrogen Oxides / Q000737:chemistry*; D009590:Nitrogen Radioisotopes; D012996:Solutions; D013113:Spin Labels; D014867:Water / Q000737:chemistry
chemical_list: D007202:Indicators and Reagents; D009587:Nitrogen Isotopes; D009589:Nitrogen Oxides; D009590:Nitrogen Radioisotopes; D012996:Solutions; D013113:Spin Labels; D014867:Water; D003903:Deuterium