MOSSBAUER AND MAGNETIC-SUSCEPTIBILITY STUDIES ON IRON(II) METALLOTHIONEIN FROM RABBIT LIVER - EVIDENCE FOR THE EXISTENCE OF AN UNUSUAL TYPEOF [M(3)(CYSS)(9)](3-) CLUSTER

The magnetic properties of the Fe(II)-binding sites in Fe(II)(7)-metal lothionein (MT) have been studied using Mossbauer spectroscopy and mag netic-susceptibility measurements. In agreement with our previous resu lts, simulation of the Mossbauer spectra showed the presence of parama gnetic and diamagnetic subspectra in the ratio 3:4. By comparison with Mossbauer spectra of the inorganic adamantane-like (Et(4)N)(2)[Fe-4(S Et)(10)] model compound, the diamagnetic component in Fe(II)(7)-MT has been assigned to a four-metal cluster in which there is antiferromagn etic coupling between the high-spin Fe(II) ions. It is suggested that the organization of this cluster is similar to that determined in the three-dimensional structure of the protein, containing diamagnetic Zn( II) and/or Cd(II) ions. From magnetic-susceptibility studies, an avera ge magnetic moment of approximately 8.5 mu(B) was obtained for the thr ee remaining bound Fe(II) ions, responsible for the paramagnetic compo nent observed in the Mossbauer studies. This value is slightly lower t han that for three completely uncoupled Fe(II) ions, suggesting the ex istence of a three-metal cluster within which there is weak exchange c oupling between adjacent Fe(II) ions. The spin-Hamiltonian formalism i ncluding, besides zero-field and Zeeman interaction, also exchange int eraction among the three Fe(II) ions in the three-metal cluster, H = - J(12) (S-1.S-2)-J(23) (S-2.S-3)-J(13) (S-1.S-3), was applied to simula te both magnetic-Mossbauer and magnetic-susceptibility data. Reasonabl e fits were achieved only with values \J(12)\ = \J(23)\ = \J(13)\ = \J \ < 1 cm(-1). Such a situation could not be reconciled with the chair- like geometry of the [M(3)(CysS)(9)](3-) cluster determined with param agnetic metal ions, where significantly stronger coupling would be ant icipated (\J\ = 50-70 cm(-1)). However, modest exchange-coupling prope rties have been reported for a number of crystallographically characte rized trinuclear [Fe-3(SR)(3)X(6)](3-) clusters (X = Cl, Br; R = Phe, p-tolyl, 2,6-Me(2)C(6)H(3)) distinguished by the preferential formatio n of a planar Fe-3(mu(2)-SR)(3) ring [Whitener, M. A., Bashkin, J. A., Hagen, K. S., Girerd, J.-J., Gamp, E. Edelstein, N. and Helm, R. H. ( 1986) J. Amer. Chem. Sec, 108, 5607-5620]. It is therefore more likely that a pseudo-planar geometry rather than a chair-like geometry is pr esent in the Fe-3 cluster of Fe(II)(7)-MT. This would represent the fi rst example of structural differences on binding divalent metal ions t o this protein.