Vasák M, Kägi J H
Proc Natl Acad Sci U S A. 1981 Nov;78(11):6709-13. doi: 10.1073/pnas.78.11.6709.
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.
兔肝脏金属硫蛋白-1中所有七个金属结合位点均被钴（II）占据，呈现出具有近似Td微观对称性的四硫醇盐配位的典型光谱特征[瓦萨克，M.（1980年）《美国化学会志》102，3953 - 3955]。每个分子共有20个半胱氨酸残基，这种金属结合模式意味着一些硫醇盐配体由相邻的钴（II）离子共享，从而形成簇状结构。在本研究中，给出了硫醇盐连接的钴（II）簇存在的证据，并通过比较含有1 - 7当量钴（II）的钴（II）-金属硫蛋白形式的光学和磁性性质来探索其形成模式。含有至多4当量钴（II）的制剂在d - d和电荷转移区域显示出类似于孤立四面体钴（II）-四硫醇盐配合物的电子光谱。然而，当结合超过四个钴（II）离子时，光谱逐渐变化，在完全饱和的钴（II）-金属硫蛋白中接近与晶体学定义的模型（钴）II -四硫醇盐簇相似的吸收谱[丹斯，I. G.（1979年）《美国化学会志》101，6264 - 6273]。这些效应与电子顺磁共振光谱的变化密切平行。每分子硫蛋白超过4当量钴（II）时，在4 K下测量的gx约为5.9处的电子顺磁共振信号强度逐渐降低，直到在完全占据的蛋白质中该配合物几乎呈抗磁性。通过顺磁磁化率测量证实的这些变化，确立了钴（II）-金属硫蛋白中钴（II）硫醇盐簇的存在。顺磁性的丧失最有可能反映了通过硫醇盐桥连配体的超交换机制导致的相邻钴（II）离子的反铁磁耦合。
