Spectroscopic studies on metal distribution in Co(II)/Zn(II) mixed‐metal clusters in rabbit liver metallothionein 2

Abstract

Metal selectivity of metal-thiolate clusters in rabbit liver metallothionein (MT) 2 has been studied by examining the metal distribution of two similarly sized divalent metal ions, cobalt and zinc, which have different thiolate affinity. The forms of mixed-metal cluster species in (Co/Zn),-MT generated with different ratios of both metal ions offered to the metal-free protein were investigated using EPR, ultraviolet/visible absorption and MCD spectroscopy. The results demonstrated that the distribution of these metals between the two metal-thiolate clusters is not randlom. Thus, the EPR absorption intensities of the bound Co(I1) ions in the Zn-cluster matrix increased linearly up to a ratio of Co(II)/ Zn(I1) equivalents of 3 : 4, with the final EPR intensity of three non-interacting Co(I1)-binding sites. This EPR behaviour is consistent with a binding scheme in which one Co(I1) ion occupies a metalbinding site within the three-metal cluster and the remaining two Co(I1) ions occupy two distinctly separate sites in the four-metal cluster. With four or more Co(I1) ions in the cluster matrix, magnetic coupling between adjacent, sulphur-bridged Co(I1) ions was observed. In previous studies on mixedmetal clusters in MT formed with Co(II)/Cd(II), Zn(II)/Cd(II) and Cd(II)/Fe(II), changes in the respective cluster volumes were shown to be a significant factor dictating the widely differring metal distributions in thesc systems. Based on the results of the current study, it is suggested that both the sizes of the two metal ions and their relative affinities towards the cysteine-thiolate ligands are important in the formation of mixed-metal clusters in MT.Metallothionein(s) (MT) comprise a class of low-molecular-mass (6 -7 kDa), cysteine-rich metal-binding proteins found in a variety of plant and animal species [l, 21. All mammalian MT isolated thus Tar are monomeric proteins which contain 61 or 62 amino acids, including 20 cysteines at invariant positions and characteristically lack aromatic amino acids and histidine [2]. The MT usually bind 7 mol equivalents of divalent zinc and/or cadmium ions and up to 12mol equivalents of monovalent copper ions [3 -51. Early spectroscopic and chemical studies on the divalent metal derivatives led to the proposal of two separate adamantane-type metalthiolate clusters containing three and four metal ions [6 -91. In this spectroscopic cluster model, the metals were coordinated exclusively by cysteines in a tetrahedral tetrathiolate coordination, requiring that cysteine thiolates act as both terminal and bridging ligands. The determination of the three-dimensional structure of MT in solution by two-dimensional NMR methods [lo -121, confirmed the spectroscopic cluster model. Moreover, there is a good agreement between the solution structure of rat liver Cd,-MT and the recently reexamined crystal structure of Znz/Cd5-MT from the same species [13]. Evidence from the assignment ofisotropically shifted 'H resonances [14] and low-temperature MCD studies I151 of Co7-MT, and from extended X-ray a...

Cu2ZnSnS4 Nanorods Doped with Tetrahedral, High Spin Transition Metal Ions: Mn2+, Co2+, and Ni2+

“…This high g value agrees well with related literature reports (see Supporting Information). − Based on the g value of 4.3, we can estimate the value of E/D to be ∼0.02 (unitless). ,− A signal at ∼3400 G is due to Cu 2+ defects, with g z = g ∥ = 2.26 and g x , g y = g ⊥ = 2.025, A x , A y = A ⊥ = 16 × 10 –4 cm –1 , and A z = A ∥ = 133 × 10 –4 cm –1 (see Supporting Information). These values are similar to those in previous reports for Cu 2+ spectra. − We believe that the resolution differences are due to changes in concentration of the Cu 2+ impurity in each sample.…”

Thermal stability of Clostridium pasteurianum rubredoxin: Deconvoluting the contributions of the metal site and the protein

“…Direct quantitative displacement of Zn 2ϩ by Cd 2ϩ was also shown for ZnRd upon incubation with a modest excess of cadmium salts. This displacement order is consistent with the expected trend in thiophilicity of these divalent metal ions, i.e., Cd 2ϩ Ͼ Zn 2ϩ Ͼ Fe 2ϩ~W erth & Johnson, 1990;Pountney & Vasak, 1992;Brouwer, 1996;Bonomi et al, 1998!. The crystal structure of Cp ZnRd is nearly superimposable on that of the Cp FeRd~Dauter et al, 1996!. X-ray crystallographic~Dauter et al, 1996!…”

Synthesis, Solid‐State Structures, and EPR Spectroscopic Studies on Polycrystalline and Single‐Crystal Samples of α‐Diimine Cobalt(II) Complexes

“…to be substantially populated. [28] The high anisotropy observed is also expected for Co II compounds with a distorted tetrahedral coordination. [29,30] Since the structural data do not show relevant chemical paths connecting the metal sites that could efficiently transmit exchange interactions, the principal g values obtained from the powder spectra (Table 3) can be considered as corresponding to isolated Co II ions, in which the resonance lines may be broadened by dipolar interactions.…”