Metal thiolate clusters in cobalt(II)-metallothionein

Rabbit liver metallothionein-1 in which all seven metal-binding sites are occupied by cobalt(II) exhibits spectral features typical of tetrathiolate coordination with approximate Td microsymmetry [Vasák, M. (1980) J. Am. Chem. Soc. 102, 3953-3955]. With a total of 20 cysteine residues per molecule, this mode of metal binding implies that some of the thiolate ligands are shared by neighboring Co(II) ions, resulting in clustered structures. In this study, evidence for the existence of thiolate-linked Co(II) clusters is presented and their mode of formation is explored by comparing the optical and magnetic properties of forms of Co(II)-metallothionein containing 1-7 equivalents of Co(II). Preparations with up to 4 Co(II) equivalents display electronic spectra in the d-d and charge-transfer regions that resemble those of isolated tetrahedral Co(II)-tetrathiolate complexes. Upon binding of more than four Co(II) ions, however, the spectrum changes progressively and approaches in the fully saturated Co(II)-metallothionein an absorption profile similar to that of crystallographically defined model (Co)II-tetrathiolate clusters [Dance, I. G. (1979) J. Am. Chem. Soc. 101, 6264-6273]. These effects are closely paralleled by changes in the ESR spectrum. Above 4 Co(II) equivalents per thionein, the ESR signal at gx approximately 5.9 measured at 4 K decreases progressively in intensity, until in the fully occupied protein the complex is nearly diamagnetic. These changes, which were confirmed by measurements of paramagnetic susceptibility, establish the existence of Co(II) thiolate clusters in Co(II)-metallothionein. The loss of paramagnetism reflects most likely antiferromagnetic coupling of neighboring Co(II) ions brought about by a superexchange mechanism via the thiolate bridging ligands.