Examples of high-frequency EPR studies in bioinorganic chemistry

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 H-s = betaSgB + D[S-z(2) - S(S + 1)/3 + (E/D)(S-x(2) - S-y(2))]. 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 (2S-1)* \D\. 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.