Linear electric field effect and electron spin-echo studies of uteroferrin: Evidence for coordination by a nitrogen-containing ligand.

Uteroferrin and semimethemerythrin, proteins possessing spin- coupled binuclear iron centers, exhibit large linear electric field effects (LEFE) in their mixed-valence, EPR-active states. This indicates that the paramagnetic center of each protein is noncentrosymmetric and suggests that charge may be localized on one of the iron atoms. The magnetic field dependence of the LEFE for both proteins demonstrates that the direction of most facile polarization of the binuclear iron centers is near the orientation giving rise to g(min). Electron spin echo studies of uteroferrin reveal that its magnetic electron interacts with at least one and possibly two classes of nitrogen nuclei. Furthermore, comparison of echo envelope spectra for uteroferrin with that of ferric bleomycin suggests that one of these nuclei is from a histidine ligand.