Magnetic circular dichroism and electron paramagnetic resonance studies of iron(II)-metallothionein

The electronic and magnetic properties of the Fe(II)-thiolate centers in Fe(II)-metallothionein have been investigated by low-temperature magnetic circular dichroism and electron paramagnetic resonance spectroscopies at various levels of Fe(II) incorporation. In agreement with previous results [Good, M., & Vasák, M. (1986) Biochemistry 25, 8353-8356], rabbit liver metallothionein was found to bind a maximum of seven Fe(II) ions, with cluster formation occurring when more than four Fe(II) ions are bound at pH 8.5. The results indicate that all the iron in fully loaded Fe(II)-metallothionein is accommodated in Fe(II)-thiolate clusters that have either S = 0 or S = 2 ground states as a result of antiferromagnetic coupling between high-spin Fe(II) ions. By analogy with the cluster composition and mechanism of assembly that have been established for other divalent metal ions, the clusters with S = 0 and S = 2 ground states are attributed to tetranuclear and trinuclear centers, respectively. EPR signals indicative of S = 2 species were observed for samples containing monomeric tetrathiolate-Fe(II) centers and trinuclear Fe(II)-thiolate clusters. However, the nature of the zero-field splitting of the S = 2 ground states that is indicated by the EPR signals is not consistent with that deduced from Mössbauer and magnetic circular dichroism studies, suggesting heterogeneity in both types of center.