title: Examples of high-frequency EPR studies in bioinorganic chemistry.
abstract: Low-temperature EPR spectroscopy with frequencies between 95 and 345 GHz and magnetic fields up to 12 T has been used to study metal sites in proteins or inorganic complexes and free radicals. The high-field EPR method was used to resolve g-value anisotropy by separating it from overlapping hyperfine couplings. The presence of hydrogen bonding interactions to the tyrosyl radical oxygens in ribonucleotide reductases were detected. At 285 GHz the g-value anisotropy from the rhombic type 2 Cu(II) signal in the enzyme laccase has its g-value anisotropy clearly resolved from slightly different overlapping axial species. Simple metal site systems with S>1/2 undergo a zero-field splitting, which can be described by the spin Hamiltonian. From high-frequency EPR, the D values that are small compared to the frequency (high-field limit) can be determined directly by measuring the distance of the outermost signal to the center of the spectrum, which corresponds to (2 S-1)* mid R: Dmid R: For example, D values of 0.8 and 0.3 cm(-1) are observed for S=5/2 Fe(III)-EDTA and transferrin, respectively. When D values are larger compared to the frequency and in the case of half-integer spin systems, they can be obtained from the frequency dependence of the shifts of g(eff), as observed for myoglobin in the presence ( D=5 cm(-1)) or absence ( D=9.5 cm(-1)) of fluoride. The 285 and 345 GHz spectra of the Fe(II)-NO-EDTA complex show that it is best described as a S=3/2 system with D=11.5 cm(-1), E=0.1 cm(-1), and g(x)= g(y)= g(z)=2.0. Finally, the effects of HF-EPR on X-band EPR silent states and weak magnetic interactions are demonstrated.
authors.affiliation: Department of Biochemistry, University of Oslo, Blindern, PO Box 1041, 0316, Oslo, Norway. k.k.andersson@biokjemi.uio.no
authors.forename: K Kristoffer
authors.initials: KK
authors.lastname: Andersson
authors.forename: Peter P
authors.initials: PP
authors.lastname: Schmidt
authors.forename: Bettina
authors.initials: B
authors.lastname: Katterle
authors.forename: Kari R
authors.initials: KR
authors.lastname: Strand
authors.forename: Amy E
authors.initials: AE
authors.lastname: Palmer
authors.forename: Sang-Kyu
authors.initials: SK
authors.lastname: Lee
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
authors.forename: Astrid
authors.initials: A
authors.lastname: Gräslund
authors.forename: Anne-Laure
authors.initials: AL
authors.lastname: Barra
mesh_terms: D000818:Animals; D016880:Anisotropy; D001665:Binding Sites; D015393:Chemistry, Bioinorganic / Q000379:methods*; D004574:Electromagnetic Fields; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005609:Free Radicals / Q000737:chemistry; D006860:Hydrogen Bonding; D042845:Laccase; D008667:Metalloproteins / Q000737:chemistry* / Q000378:metabolism; D019216:Metals, Heavy / Q000737:chemistry / Q000378:metabolism; D010088:Oxidoreductases / Q000737:chemistry / Q000378:metabolism; D012264:Ribonucleotide Reductases / Q000737:chemistry / Q000378:metabolism
chemical_list: D005609:Free Radicals; D008667:Metalloproteins; D019216:Metals, Heavy; D010088:Oxidoreductases; D042845:Laccase; D012264:Ribonucleotide Reductases