Single-crystal EPR studies of a spin-frustrated imidazolate-bridged trinuclear copper(II) complex

The single-crystal and polycrystalline powder EPR spectra of the trinuclear compound [L₃Cu₃(Im)₃](ClO₄)₃, L = 1,4,7-trimethyl-1,4,7-triazacyclononane, and Im = imidazolate, have been measured in the temperature range 4.2–300 K. The spectra are explained based on the spin-frustration, the low symmetry, and the intercluster exchange interactions.     

Interactions of nitric oxide with copper(II) dithiocarbamates in aqueous solution

This is the first report on the formation of air-stable copper nitrosyl complexes. The interaction of nitric oxide, NO, with Cu(DTC)(2).3H(2)O (DTC: dithiocarbamate) and was studied in aqueous solution at pH 7.4 and 293 K. The stability constants were determined from UV-Vis data, using LETAGROP program. The high values obtained, log beta(1)=9.743(5) and log beta(2)=15.44(2) for Cu(ProDTC)(2)-NO, (ProDTC=L-prolinedithiocarbamate) and log beta(1)=8.723(5) and log beta(2)=11.45(2) for Cu(MorDTC)(2)-NO system, (MorDTC=morpholyldithiocarbamate), indicate the formation of two stable nitrosyl complexes, Cu(DTC)(2)NO and Cu(DTC)(2)(NO)(2). Coordinated NO is neither affected by the presence of air nor when the solution is purged with Ar. Cu(MorDTC)(2)NO.3H(2)O was isolated in the solid state and its nuNO (IR) band at 1682 cm(-1), but affected by temperature variations over 333 K.     

Mercury(II) complex formation with glutathione in alkaline aqueous solution

The structure and speciation of the complexes formed between mercury(II) ions and glutathione (GSH = L-glutamyl-L-cysteinyl-glycine) have been studied for a series of alkaline aqueous solutions (\( C_{{{\text{Hg}}^{{2 + }}}}\,{\sim18\,{\rm{mmol}}\,{\rm{{dm^{-3}}}}}\) and C _GSH = 40–200 mmol dm^?3 at pH ～10.5) by means of extended X-ray absorption fine structure (EXAFS) and ¹⁹⁹Hg NMR spectroscopy at ambient temperature. The dominant complexes are [Hg(GS)₂]^4? and [Hg(GS)₃]^7?, with mean Hg–S bond distances of 2.32(1) and 2.42(2) Å observed in digonal and trigonal Hg–S coordination, respectively. The proportions of the Hg²⁺–glutathione complexes were evaluated by fitting linear combinations of model EXAFS oscillations representing each species to the experimental EXAFS spectra. The [Hg(GS)₄]^10? complex, with four sulfur atoms coordinated at a mean Hg–S bond distance of 2.52(2) Å, is present in minor amounts (<30%) in solutions containing a large excess of glutathione (C _GSH ≥ 160 mmol dm^?3). Comparable alkaline mercury(II) cysteine (H₂Cys) solutions were also investigated and a reduced tendency to form higher complexes was observed, because the deprotonated amino group of Cys^2? allows the stable [Hg(S,N-Cys)₂]^2? chelate to form. The effect of temperature on the distribution of the Hg²⁺–glutathione complexes was studied by comparing the EXAFS spectra at ambient temperature and at 25 K of a series of glycerol/water (33/67, v/v) frozen glasses with \( C_{{{\text{Hg}}^{{2 + }} }} \,{\sim7\,{\rm{mmol}}\,{\rm{{dm^{-3}}}}} \) and C _GSH = 16–81 mmol dm^?3. Complexes with high Hg–S coordination numbers, [Hg(GS)₃]^7? and [Hg(GS)₄]^10?, became strongly favored when just a moderate excess of glutathione (C _GSH ≥28 mmol dm^?3) was used in the glassy samples, as expected for a stepwise exothermic bond formation. Addition of glycerol had no effect on the Hg(II)–glutathione speciation, as shown by the similarity of the EXAFS spectra obtained at room temperature for two parallel series of Hg(II)-glutathione solutions with \( C_{{{\text{Hg}}^{{2 + }} }} \,{\sim7\,{\rm{mmol}}\,{\rm{{dm^{-3}}}}},\) with and without 33% glycerol. Also, the ¹⁹⁹Hg NMR chemical shifts of a series of ～18 mmol dm^?3 mercury(II) glutathione solutions with 33% glycerol were not significantly different from those of the corresponding series in aqueous solution.     

Nature of Cu(II)-poly(glutamic acid) complex in aqueous solution

Complex formation of poly(glutumic acid) with cupric ions in aqueous solution was investigated by three different methods: optical spectroscopy, optical rotatory dispersion, and electron spin resonance. Formation of a characteristic complex was found to occur in a pH region suitable for the helix-coil transition. An analysis of the ESH spectrum of the complex is given, and the results of calculation of bonding parameters suggest that the bond between copper and nitrogen atoms had an appreciably covalent character. The change in the secondary structure of the polymer as a result of complex formation is discussed.     

Hydrolysis of dinucleoside phosphates - mRNA 5' cap analogues - promoted by a binuclear copper(II)-zinc(II) complex

The hydrolysis of a 5' cap analogue, diadenosinyl-5',5'-triphosphate (ApppA), and two dinucleoside monophosphates: adenylyl(3',5')adenosine (ApA) and uridylyl(3',5')uridine (UpU) promoted by an imidazolate-bridged heterobinuclear copper(II)-zinc(II) complex, Cu(II)-diethylenetriamino-micro-imidazolato-Zn(II)- tris(aminoethyl)amine trisperchlorate (denoted as Cu,Zn-complex in the followings) has been investigated. Kinetic measurements were performed in order to explore the effects of pH, the total concentration of the Cu,Zn-complex and temperature on the cleavage rate. The catalytic activity of the Cu,Zn-complex was quantified by pseudo-first-order rate constants obtained in the excess of the cleaving agent. The results show that the Cu,Zn-complex and its deprotonated forms have phosphoesterase activity and with ApppA the metal complex promoted cleavage takes place selectively within the triphosphate bridge.     

NMR investigation of the copper(II)-carnosine complex in water solution

The predominant species in water solution of excess carnosine and Cu2+ ions was characterized by measuring 13C and 1H spin-lattice and spin-spin relaxation rates. Four peptide molecules were calculated to be coordinated to the metal through the imidazole nitrogen. Evaluation of ion-proton distances demonstrated that metal complexation does not involve severe changes in conformation of the peptide.     

Speciation of phytate ion in aqueous solution. Protonation constants and copper(II) interactions in NaNO3aq at different ionic strengths

The acid base behavior of phytate has been studied (at t=25 degrees C by potentiometry, ISE-H+ glass electrode) in NaNO3aq at different ionic strengths (0.1 < or = I/mol L(-1) < or = 1.0). The interactions with copper(II) were investigated in the same experimental conditions in different metal to ligand (Phy) ratios (1:1 < or = Cu2+ :Phy < or = 4:1), by using both ISE-H+ and ISE-Cu2+ electrodes. Phytate acid base behavior in sodium nitrate is very similar to that in sodium chloride, previously investigated. In the experimental conditions adopted, the formation of three CuiHjPhy(12-2i-j)- species is observed: the mononuclear CuH4Phy6- and CuH5Phy5-, and the dinuclear Cu2H5Phy3-. Analysis of complex formation constants at different ionic strengths reveals that both ISE-H+ and ISE-Cu2+ electrodes gave, within the experimental error, analogous values. Dependence of complex formation constants on ionic strength was modeled by EDH (Extended Debye-Hückel) and SIT (Specific ion Interaction Theory) equations. The sequestering ability of phytate toward copper(II) has been evaluated by the calculation of pL50 (the total ligand concentration, as -log CL, able to bind 50% of metal cation), an empirical parameter already proposed for an objective "quantification" of this ability. A thorough analysis of literature data on phytate-copper(II) complexes has been performed.     

Metal--glutathione interaction in aqueous solution. Nickel(II), cobalt(II) and copper(II) complexes with oxidized glutathione.

The interaction of copper(II), nickel(II) and cobalt(II) ions with oxidized glutathione in aqueous solutions have been examined by spectroscopic methods. Cu(II) is the only ion which interacts with disulphide bridge and forms dimeric species containing the Cu(II)-S-S-Cu(II) unit. Ni(II) and Co(II) bind mainly with the terminal NH2 and COO- groups of glutamic acid, and the complexes formed are of nearly octahedral symmetry. At high pH, in the Co(II)-GSSG solution Co(II) is oxidized to Co(III) with the concomitant reduction of GSSG to GSH. Considerable differences were observed between the oxidized and reduced form of glutathione in the coordination ability towards metal ions.     

The complex formation of Cu(II) with mono- and di-ethanolamine in aqueous solution

The complex formation of Cu(II) with mono- and di-ethanolamine is studied by means of ESR spectroscopy. For both ligands, four complex species with the stoichiometries 110, 120, 12-1 and 12-2 are detected, and their relative stabilities are calculated. From the results obtained, deprotonation of the hydroxyl groups and formation of stable chelate rings are proposed. The similiarities and differences between the complex behaviour of the related compounds tri-ethanolamine, ammonia and β-methoxy- ethylamine (also studied by ESR spectroscopy) are further analysed.     

Study of copper(II) ternary complexes with phosphocreatine and some polyamines in aqueous solution

Ternary systems of Cu(II) with phosphocreatine (PCr) and the polyamines (PAs), ethylenediamine (en), 1,3-diaminopropane (tn), putrescine (Put), spermidine (Spd), and spermine (Spm), were investigated in aqueous solution through potentiometry, ultraviolet-visible, EPR and Raman spectroscopy. The binary complex CuPCr was also studied by Raman spectroscopy, and the calculation of the minimum stabilization energy was done assuming this molecule in aqueous solution. The stability constants of the CuPCrPA ternary complexes were determined by potentiometry (T = 25 °C, I = 0.1 mol L^− 1, KNO₃). The stability order determined was CuPCrSpm > CuPCrSpd > CuPCren > CuPCrtn > CuPCrPut, the same order of the corresponding binary complexes of Cu(II) with these polyamines. The evaluation of intramolecular PA-PCr interactions in protonated and deprotonated species of ternary complexes was carried out using the equation Δlog K = log β_{CuPCrPAHq + p} − (log β_CuPAHq + log β_CuPCrHp). All of the CuPCrPA ternary complexes have a square planar structure and are bonded to PCr through the nitrogen atom of the guanidine group and the oxygen atom of the phosphate group, and to the PAs through two nitrogen atoms of the amine groups. The structure of the complex CuPCrSpm is planar with distortion towards tetrahedral. Calculation of the minimum stabilization energy for the CuPCr and CuPCrenH complexes confirmed the proposed coordination mode.     

Mixed-ligand complex formation of mercury (II) ethylenediaminetetraacetate with cysteine and methionine in aqueous solution

The mixed-ligand complex formation in the systems Hg²⁺-Edta⁴⁻-L (L = Cys²⁻, Met⁻) has been studied by means of calorimetry, pH-potentiometry and NMR spectroscopy in aqueous solution at 298.15 K and the ionic strength of I = 0.5 (KNO₃). The thermodynamic parameters of formation of the HgEdtaL, HgEdtaHL and (HgEdta)₂L complexes have been determined. The most probable coordination mode for the complexone and the amino acid in the mixed-ligand complexes is discussed.     

A proton NMR study of the glycine-mercury(II) system in aqueous solution

The proton NMR spectrum of glycine was monitored in D2O solution as a function of added Hg(II) concentration and pD. Reliable values were established for formation constants for the Hg(II):glycine 1:1 and 1:2 complexes and also for the mixed glycine/deuteroxy and glycine/chloride complexes. Ligand exchange kinetics are relatively slow, and it is possible to observe coupling to 199Hg through the coordinating nitrogen. The formation constants were used to calculate speciation over a range of ligand concentrations for the Hg(II)/glycine and Hg(II)/glycine/chloride systems.     

Interactions in solution of calcium(II) and copper(II) with nucleoside monophosphates: a calorimetric study

The thermodynamic parameters enthalpy and entropy of the interaction between calcium(II) or copper(II) with 5′-UMP, 5′-CMP, 5′-AMP, 5′-GMP or 5′-IMP in aqueous solution were determined calorimetrically (ionic strength adjusted to 0.1 with tetramethylammonium bromide) at 25 °C and pH 7 for Ca(II) or pH 3–5 for Cu(II). The experimental conditions were carefully selected to avoid polynuclear complex formation and nucleotide self-stacking. The calorimetric data confirm the tendency toward macrochelation which was indicated by Sigel after very precise potentiometric studies, and which follows the order Cu(II)>Ca(II) for the metal ions and GMP>IMP>AMP>CMP=UMP for the nucleotides. Macrochelate formation for these metal-nucleoside monophosphate complexes is energetically favorable and entropically unfavorable. Received: 13 August 1999 / Accepted: 1 February 2000     

The copper(II) catalyzed reaction of L-ascorbinc acid with tris(oxalato)cobaltate(III) ion in aqueous solution

The kinetics of the reaction between tris(oxalato)cobaltate(III) ion and L-ascorbic acid were studied in aqueous solution over a range of copper(II) ion concentrations, ascorbic acid concentrations, pH values and temperatures. A rate law of the type rate = k[HA^?][Cu²⁺][Co³⁺], where HA^? = ascorbate, is suggested by the experimental results. A mechanism involving a copper-ascorbate complex is proposed.     

Catalytic activity of a copper(II)-oxidized glutathione complex on aqueous superoxide ion dismutation

The study of the catalytic activity of a Cu(II)-oxidized glutathione system upon the disproportionation of superoxide radicals shows that the mononuclear complex MA catalyzes dismutation in the pH range 7-9. The corresponding first-order rate constant of value kcat congruent to 6 X 10(6) M-1 sec-1 is pH independent, whereas the second-order rate constant ks for the reference solutions is pH dependent. The kcat constant is about 10-, 100-, and 300-fold higher than the ks constant at pH 7, 8, and 9, respectively. The measured effect is explained in terms of free axial sites in the square-planar arrangement around the copper ion.