Examples of high-frequency EPR studies in bioinorganic chemistry

• Published in: Journal of biological inorganic chemistry 2003-02, Vol.8 (3), p.235-247
• Main Authors: Andersson, K Kristoffer, Schmidt, Peter P, Katterle, Bettina, Strand, Kari R, Palmer, Amy E, Lee, Sang-Kyu, Solomon, Edward I, Gräslund, Astrid, Barra, Anne-Laure
• Format: Article
• Language: English
• Subjects: Animals; Anisotropy; Binding Sites; Chemistry, Bioinorganic - methods; Electromagnetic Fields; Electron Spin Resonance Spectroscopy - methods; Free Radicals - chemistry; Hydrogen Bonding; Laccase; Metalloproteins - chemistry; Metalloproteins - metabolism; Metals, Heavy - chemistry; Metals, Heavy - metabolism; Oxidoreductases - chemistry; Oxidoreductases - metabolism; Ribonucleotide Reductases - chemistry; Ribonucleotide Reductases - metabolism
• ISSN: 0949-8257; 1432-1327
• DOI: 10.1007/s00775-002-0429-0

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.