In ferritin, iron is stored by oxidative deposition of the ferrous ion to form a hydrous ferric oxide mineral core. Two intermediates, formed during the initial stages of iron accumulation in apoferritin, have been observed previously in our laboratory and have been identified as a mononuclear Fe3(+)-protein complex and a mixed-valence Fe2(+)-Fe3(+)-protein complex. The physical characteristics of the mixed-valence Fe2(+)-Fe3+ complex and its relationship to the mononuclear Fe3+ complex in horse spleen apoferritin samples to which 0-240 iron atoms were added was examined by EPR spectroscopy. The results indicate that the mononuclear complex is not a precursor to the formation of the mixed-valence complex. Competitive binding studies with Cd2+, Zn2+, Tb3+, and UO2+(2) suggest that the mixed-valence complex is formed on the interior of the protein in the vicinity of the 2-fold axis of the subunit dimer. The mixed-valence complex could be generated by the partial oxidation of Fe2+ in apoferritin containing 120 Fe2+ or by the addition of up to 120 Fe2+ to ferritin already containing 18 Fe3+/protein molecule. The fact that the complex is generated during early Fe2+ oxidation suggests that it may be a key intermediate during the initial oxidative deposition of iron in the protein. The unusual EPR powder lineshape at 9.3 GHz of the mixed-valence complex was simulated with a rhombic g-tensor (gx = 1.95, gy = 1.88, gz = 1.77) and large linewidths and g-strain parameters. The presence of significant g-strain in the complex probably accounts for the failure to observe an EPR signal at 35 GHz and likely reflect considerable flexibility in the structure of the metal site. The temperature dependence of the EPR intensity in the range 8-38 K was modeled successfully by an effective spin Hamiltonian including exchange coupling (-2JS1.S2) and zero-field terms, from which an antiferromagnetic coupling of J = -4.0 +/- 0.5 cm-1 was obtained. This low value for J may reflect the presence of a mu-oxo bridge(s) in the dimer.