Resolved Hyperfine at L-band for High-Spin CoEDTA, A Model for Co Sites in Proteins

Low-frequency electron paramagnetic resonance (EPR) spectra were obtained for the Co complex of ethylene diamine tetraacetic acid (CoEDTA). It was found that the cobalt hyperfine at g(eff)-mid is better resolved at a low frequency, L-band (1.37 GHz), and not resolved at X-band (9.631 GHz), which is the conventional frequency used for most spectra for metal complexes. Resolved cobalt hyperfine lines lead to additional EPR parameters like A-mid for cobalt and a more-accurate determination of g-mid. Resolved hyperfine lines in the L-band, but not the S-band, spectra were obtained at a concentration of 1 mM. Knowing these additional EPR parameters provides a means to better determine the electron density in the ground state orbital for each cobalt complex, as well as to determine differences upon a change of ligation. If zinc sites can be replaced by cobalt, the cobalt spectra for these sites will enhance the characterization of the zinc sites.