Structural and spectroscopic comparison of five-coordinate cobalt(II) and nickel(II) thiolato complexes with the related four-coordinate complexes

Coordination properties of dithiobutylamine (DTBA), a newly introduced protein disulfide reducing agent

The acid-base properties and metal-binding abilities of (2S)-2-amino-1,4-dimercaptobutane, otherwise termed dithiobutylamine (DTBA), which is a newly introduced reagent useful for reducing protein and peptide disulfides, were studied in solution using potentiometry, (1)H NMR spectroscopy, spectropolarimetry, and UV-vis spectroscopy. The list of metal ions studied here includes Zn(II), Cd(II), Ni(II), Co(II), and Cu(I). We found that DTBA forms specific and very stable polynuclear and mononuclear complexes with all of these metal ions using both of its sulfur donors. DTBA forms complexes more stable than those of the commonly used disulfide reducing agent DTT, giving it more interference capacity in studies of metal binding in thiol-containing biomolecules. The ability of DTBA to strongly bind metal ions is reflected in its limited properties as a thiol protectant in their presence, which is manifested through slower disulfide reduction kinetics. We found that this effect correlated with the stabilities of the complexes. Additionally, the reducing properties of DTBA toward MMTS-modified papain (MMTS = S-methylmethanethiosulfonate) were also significantly affected by the investigated metal ions. In this case, however, electrostatic interactions and stereospecific effects, rather than metal-binding abilities, were found to be responsible for the reduced protective properties of DTBA. Despite its limitations, a high affinity toward metal ions makes DTBA an attractive agent in competition studies with metalloproteins.

Structure and spectroscopic studies of homo-and heterometallic complexes of adipic acid dihydrazide

A single crystal of adipic acid dihydrazide, ADH, has been analyzed. Its reaction with Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), Hg(2+), Ag(+), Pd(2+) and/or Pt(2+) gave homometallic and heterometallic complexes which are characterized by partial elemental analysis, spectra (MS, ESR, (1)H NMR, electronic; IR), thermal analysis and magnetic measurements. Some complexes: Zn(0.73)Cu(ADH)Cl4·H2O; Zn(0.71)Hg(0.36)(ADH)Cl4·H2O; Zn(0.65)Cd(0.46)(ADH)Cl4·½H2O; Zn(0.75)Co(0.41)(ADH-2H)Cl2·3H2O; Cd0.85Co0.43(ADH)Cl4·½EtOH were isolated having nonstiochiometric metal ratios. The ligand behaves as a neutral (bidentate or tetradentate) and/or binegative tetradentate. A square-pyramid, square-planar and tetrahedral structures were proposed for the homo Co(II), Cu(II) and Ni(II) complexes, respectively. A similar and different stereochemistry around each metal ion (tetrahedral+tetrahedral; tetrahedral+square-planar; tetrahedral+tetrahedral and/or tetrahedral+octahedral) was suggested for the heterometallic complexes. Some complexes were found highly stable with stability point >240 °C; the most stable is [HgNi(ADH-2H)Cl2]. The presence of diamagnetic atom (Zn, Cd or Hg) reduces the magnetic moments and gave anomalous moments. The degradation steps and the hydrated complexes are confirmed through the TGA study. The order of covalency of [Zn(0.73)Cu(ADH)Cl4]·H2O, [CdCu(ADH)Cl4]·H2O and [HgCu(ADH-2H)Cl2] matches with the size of the second metal (Zn complex>Cd complex>Hg complex). Some heterometallic complexes were found nonstoichiometric through the analysis of their metal content and supported by TGA.

Harnessing scorpionate ligand equilibria for modeling reduced nickel superoxide dismutase intermediates

Hydrotris(3-phenyl-5-methylpyrazoyl)boratonickel(II) complexes with organoxanthate or dithiocarbamate coligands equilibrate between kappa(2)- and kappa(3)-chelation modes of the scorpionate ligand in solution, connecting N2S2 square-planar and N3S2 pyramidal ligand fields and a spin crossover. The complexes also exhibit quasi-reversible oxidations at low anodic potentials, thus modeling the structure, dynamics, and redox reactivity of the reduced NiSOD active site.