The complexation of aqueous metal ions relevant to biological applications: 2. Evaluation of simultaneous equilibria of poorly soluble zinc salts with select amino acids

Abstract

Simultaneous equilibria calculations were completed for seven aqueous zinc-ligand systems: zinc citrate plus either glycine, alanine, or serine, and zinc succinate plus either glycine, alanine, or serine, and zinc oxalate plus glycine. Mixed-ligand complexes were predicted for all but the zinc citrate-glycine system, and the proportion tends to peak around 5 molar equivalents of amino acid. Potential bioavailability of zinc appears to be increased by the inclusion of amino acids in solution, roughly in parallel with the increase in solubility of the zinc salt. Therefore, measurement of the change in solubility caused by addition of amino acids to aqueous solution gives qualitative insight to the potential increase in bioavailability of the metal ion, and mixed-ligand complexes are a significant proportion of the complexes present in solution.     

The complexation of aqueous metal ions relevant to biological applications. 1. Poorly soluble zinc salts and enhanced solubility with added amino acid

Abstract

The effect of added amino acid (glycine, alanine and serine) on the solubility of zinc citrate, zinc succinate and zinc oxalate was investigated. Equilibrated solutions were prepared and titrated to endpoint at pH 5.5 with Na₂EDTA using Xylenol Orange indicator. In every case, a significant enhancement in solubility was seen as the amino acid increased from 1 to 50 molar excess. The increase was non-linear, and, with zinc succinate, showed saturation effects. In these cases, solution composition may be more complicated than initially thought, including mixed-ligand complexes and increasing percent free metal ion as the proportion of amino acid increases.     

The computed distribution of copper(II) and zinc(II) ions among seventeen amino acids present in human blood plasma

The equilibrium distribution of copper(II) and zinc(II) ions among a mixture of 17 amino acids has been computed from stability-constant and blood-plasma-composition data. At pH7.4, 98% of the copper(II) in the simulated plasma solution is co-ordinated to histidine and cystine, predominantly as the mixed-ligand complexes [Cu.His.Cystine](-) and [Cu.H.His.Cystine]. Approximately half of the zinc(II) is co-ordinated to cysteine and histidine, but appreciable complex-formation occurs with most of the other amino acids. Stability constants are given for copper(II) and zinc(II) amino acid complexes, including some mixed-ligand species, at 37 degrees C and I=0.15m.     

Chiral discrimination phenomena in mixed-ligand copper(II) complexes with amino acids and 1,3-dicarbonyl compounds

Circular dichroism (CD) spectra of individual mixed-ligand copper(II) complexes of 1,3-dicarbonyl compounds, (1S)- or (1R)-3-hydroxymethylene camphor, (1S)-3-trifluoroacetyl camphor, or (1R)-2-hydroxymethylene menthone, and α-amino acids, alanine, valine, proline, or their N-alkyl derivatives, were calculated from CD spectra of equilibrium solutions containing the above constituents in methanol or ethylene dichloride. Diastereomeric mixed-ligand complexes incorporating identical dicarbonyl but enantiomeric N-alkyl-α-amino acid ligands exhibit quasi-enantiomeric CD spectra. Unsubstituted amino acids, on the contrary, will make no decisive contributions to the net optical activity spectrum of the mixed-ligand complexes. Formation constants of diastereomeric mixed-ligand complexes have been calculated from data on disproportionation of the latter into corresponding equally paired complexes. Enantioselectivity was demonstrated to amount to up to 700 cal/mol. Possible steric structures of mixed-ligand complexes are discussed. © 1993 Wiley-Liss, Inc.     

Application of saccharose as copper(II) ligand for electroless copper plating solutions

Saccharose, forming sufficiently stable complexes with copper(II) ions in alkaline solutions, was found to be a suitable ligand for copper(II) chelating in alkaline (pH>12) electroless copper deposition solutions. Reduction of copper(II)-saccharose complexes by hydrated formaldehyde was investigated and the copper deposits formed were characterized. The thickness of the compact copper coatings obtained under optimal operating conditions in 1h reaches ca. 2 microm at ambient temperature. The plating solutions were stable and no signs of Cu(II) reduction in the bulk solution were observed. Results were compared with those systems operating with other copper(II) ligands.     

Chiral recognition by the copper(II) complex of 6-deoxy-6-N-(2-methylaminopyridine)-β-cyclodextrin

A modified β-cyclodextrin bearing a 2-aminomethylpyridine binding site for copper(II) (6-deoxy-6-[N-(2-methylamino)pyridine)]-β-cyclodextrin, CDampy was synthesized by C₆-monofunctionalization. The acid-base properties of the new ligand in aqueous solution were investigated by potentiometry and calorimetry, and its conformations as a function of pH were studied by NMR and circular dichroism (c.d.). The formation of binary copper(II) complexes was studied by potentiometry, EPR, and c.d. The copper(II) complex was used as chiral selector for the HPLC enantiomeric separation of underivatized aromatic amino acids. Enantioselectivity in the overall stability constants of the ternary complexes with D- or L-Trp was detected by potentiometry, whereas the complexes of the Ala enantiomers did not show any difference in stability. These results were consistent with a preferred cis coordination of the amino group of the ligand and of the amino acid in the ternary complexes (“cis effect”), which leads to the inclusion of the aromatic side chain of D-Trp, but not of that of L-Trp. In Trp-containing ternary complexes, the two enantiomers showed differences in the fluorescence lifetime distribution, consistent with only one conformer of D-Trp and two conformers of L-Trp, and the latter were found to be more accessible to fluorescence quenching by acrylamide and KI. Chirality 9:341–349, 1997. © 1997 Wiley-Liss, Inc.     

Characteristics of the mechanism of action of complex copper (II) compounds with alpha-amino acids

There was no direct inhibition of DNA synthesis in ascites hepatoma 22A cells after intraperitoneal injection of single doses of copper (II) complexes with amino acids into tumor-bearing C3HA mice. Meanwhile cis-dichlorodiamine platinum (II) (DDP) as well as sarcolysine showed such inhibition. Copper (II) complexes with alpha-amino acids displayed as significant superoxide dismutase-like activity at concentrations corresponding to therapeutic doses of these compounds. The complexes of copper (II) combined with DDP give an additive antitumor effect in solid tumors of mice.     

The complex formation of copper(II) with GHL and HSA

The formation constants for complexes of copper(II) with GHL have been determined by means of pH titrations and ESR spectroscopy in aqueous solutions. GHL has an extremely high affinity for copper(II) and forms very stable 1:1 complexes and a comparatively weak 1:2 complex. The ? amino group of GHL seems not to be involved in complex formation as can be deducted from both equilibrium constants and ESR spectroscopy. The ternary system copper(II)-GHL-HSA was investigated by ESR spectroscopy and optical absorption spectroscopy in aqueous solution at physiological pH (7.4). At equimolar concentrations, copper(II), HSA and GHL form a ternary complex.     

Copper(II), cobalt(II) and nickel(II) complexes of alkyl-substituted tris(2-benzimidazolylmethyl)amines and their ternary copper(II) systems containing several thiolates

Copper(II), cobalt and nickel(II) complexes of tris(benzimidazolylmethyl)amine(1) and of its methyl(2), isobutyl(3) or isopropyl(4)-substituted derivatives of one of the backbone methylene groups were prepared and characterized. The ligands (1)–(3) afforded trigonal bipyramidal copper(II) complexes, whereas ligand (4) gave a tetrahedrally distorted tetragonal one because of the steric hindrance arising from the isopropyl group. All the cobalt(II) complexes prepared were supposed to be tetrahedral or pseudotrigonal bipyramidal, and all the nickel(II) complexes to be slightly tetrahedrally distorted octahedral. Ternary copper(II) systems containing several thiolates as the third component exhibited intense blue, brown or green color under a reduced temperature by virtue of the charge transfer bands, S^? → Cu.     

Z-DNA is Formed by poly(dC-dG) and poly (dm5C-dG) at Micro or Nanomolar Concentrations of some Zinc(II) and Copper(II) Complexes

Abstract

We report studies on the interaction of some zinc(II) and copper(II) complexes of amines and amino acids with poly(dC-dG) and poly(dm⁵C-dG). Of the zinc complexes the species zinc-tris(2-aminoethyl) amine is found to be the most efficient for inducing Z-DNA giving a mid point at low ionic strength of 1.4μM (poly(dC-dG)) and 44μM (poly(dm⁵C-dG). While an antagonistic effect on raising the ionic strength is observed, the transition occurs at only 2μM for poly(dm⁵C-dG) at 150mM NaCl. The most efficient copper(II) complex is that of diethylene triamine, though copper(II) complexes are generally less efficient than zinc(II) complexes. We also report kinetic and thermodynamic studies upon the B-Z transition induced by these complexes. A model is proposed for the interaction of one of the zinc complexes which involves not only direct zinc-DNA binding but also the formation of hydrogen bonds between the metal bond amine groups and the residues adjacent to the coordination site.     

Spectroscopic studies on the copper(II)—Inosine system

Interactions of inosine derivatives with copper(II) were studied in the pH range 1.4–13 in 50% H₂O-50% DMSO solution. The distinct pH dependence of the optical spectra observed in copper(II)-inosine complexes are correlated to their respective EPR changes as a function of pH. It was concluded that a simple 1:1 complex of copper(II)-inosine is formed in the pH range 1.4–5.0 and bis complexes are present in the pH 5.0–6.2 region solutions of inosine and Cu(II). From pH 6.2 to 7.8 a diamagnetic, hydroxybridged complex dominates. At pH 7.8–9.2 an insoluble, oxybridged species is formed in addition to the soluble paramagnetic Cu(NI)₄ complex. Starting from pH 9.1 the N-polymeric complex is formed which is stable up to pH 12.5, and above pH 12.5 the only species is the Cu(ribose)₂ complex.     

Ternary copper(II) complexes involving 2-(aminomethyl)-benzimidazole and some bio-relevant ligands. Equilibrium studies and kinetics of hydrolysis for glycine methyl ester under complex formation

Formation equilibria of copper(II) complexes of 2-(aminomethyl)-benzimidazole (AMBI) and the ternary complexes Cu(AMBI)L (L = amino acid, amide, dicarboxylic acid or DNA constituents) have been investigated. Ternary complexes of amino acids or amides are formed by a simultaneous mechanism. Amino acids form the complex Cu(AMBI)L, whereas amides form two complex species Cu(AMBI)L and Cu(AMBI)(LH₋₁). The ternary complexes of copper(II) with AMBI and dicarboxylic acids or DNA units are formed by a stepwise mechanism, whereby binding of copper(II) to AMBI is followed by ligation of the dicarboxylic acids or DNA components. The values of Δ log K indicate that the ternary complexes containing aromatic amino acids are significantly more stable than the complexes containing alkyl- and hydroxyalkyl-substituted amino acids. This may be taken as an evidence for a stacking interaction between the aromatic moiety of AMBI and the aromatic side chains of the bio-active ligands. The solid complexes Cu(AMBI)L where L = 1,1-cyclobutanedicarboxylic acid (CBDCA) and malonic acid were separated and identified by elemental analysis and infrared spectroscopy and magnetic moment. The decomposition course and steps for the isolated complexes were analyzed and the kinetic parameters of the non-isothermal decomposition were calculated. The hydrolysis of glycine methyl ester (MeGly) is catalyzed by the Cu(AMBI)²⁺ complex. The kinetic data is fitted assuming that the hydrolysis reaction proceeds in two steps. The first step, involving coordination of the amino acid ester by the amino and carbonyl groups, is followed by rate-determining attack by OH⁻ ion. The second step involves the equilibrium formation of the hydroxo-complex Cu(AMBI)(MeGly)(OH) followed by intramolecular OH⁻ attack.     

Binuclear schiff base complexes of nickel(II) or copper(II) linked by dithiols as a bridging group

Binuclear Schiff base complexes of nickel(II) or copper(II) were synthesized by bridging unsymmetrical Schiff base complexes of nickel(II) and copper(II) with 2,5-dimercapto-1,3,4-thiadiazole or 2-mercaptoethyl ether.Although the magnetic susceptibility and ESR measurements showed that copper(II)copper(II) interaction of the binuclear copper(II) complexes is very weak in the solid state, they catalyzed chemiluminescence of luminol by hydrogen peroxide in DMF solution, suggesting that small amounts of the complexes might be present in the form of the appropriate copper(II)copper(II) distance to activate hydrogen peroxide in solution.     

Effect of zinc ion on the interaction of some amino acid compounds of copper(II) with hydrogen peroxide.

Reaction of elemental copper and zinc powder mixtures with glycine (NH2.CH2COOH; HA) or aspartic acid (NH2CHCOOHCH2COOH; H2B) (in 1:1:2 ratio, respectively) in the presence of excess hydrogen peroxide (H2O2) at 50 degrees C, results in the formation of a new mixed metal peroxy carbonate compound corresponding to formula [Cu(Zn)2(O2(2-) (CO3)2(H2O)4], while the same reaction with elemental copper powder alone yields merely peroxy amino acid compounds having the formula [Cu(O2(2-)) (HA)2(H2O)] and [Cu(O2(2-)) (H2B) (H2O)2] for glycine and aspartic acid, respectively. These compounds have been characterized by elemental analysis, ESR, and electronic and IR spectra. It is interesting to note that both amino acids are converted to carbonate in the presence of zinc alone. A method analogous to that described above, for the reaction of elemental copper, zinc powder mixtures with succinic acid [(CH2COOH)2] or acetic acid (CH3COOH) in excess H2O2, on the other hand, gave a product essentially comprising copper succinate or acetate, respectively. These observations suggest an interesting and perhaps important phenomenon by which only the simple amino acids such as glycine and aspartic acid are converted to carbonates while their corresponding carboxylic acids form only their respective salts.     

Z-DNA is formed by poly (dC-dG) and poly (dm5C-dG) at micro or nanomolar concentrations of some zinc(II) and copper(II) complexes

We report studies on the interaction of some zinc(II) and copper(II) complexes of amines and amino acids with poly(dC-dG) and poly(dm5C-dG). Of the zinc complexes the species zinc-tris(2-aminoethyl) amine is found to be the most efficient for inducing Z-DNA giving a mid point at low ionic strength of 1.4 microM (poly(dC-dG] and 44nM (poly(dm5C-dG). While an antagonistic effect on raising the ionic strength is observed, the transition occurs at only 2 microM for poly(dm5C-dG) at 150mM NaCl. The most efficient copper(II) complex is that of diethylene triamine, though copper(II) complexes are generally less efficient than zinc(II) complexes. We also report kinetic and thermodynamic studies upon the B-Z transition induced by these complexes. A model is proposed for the interaction of one of the zinc complexes which involves not only direct zinc-DNA binding but also the formation of hydrogen bonds between the metal bond amine groups and the residues adjacent to the coordination site.     

Cobalt(II), nickel(II), copper(II) and zinc(II) complexes with an open-chain pentadentate nitrogen ligand

The open-chain, potentially, pentadentate, ligan 1,11-bis(dimethylamino)-3,6,9-trimethyl-3,6,9,-triazaundecane (Me₇tetren) forms a series of metal complexes having the general formula [M(Me₇tetren)]Y₂ (Y = 1, M = Co, Ni; Y = ClO₄, M = Co, Ni, Cu, Zn). On the basis of their physical properties, it is suggested that all these compounds contains isostructural five-coordinate [M(Me₇tetren)]²⁺ cations, the ligand acting as pentadentate. These complexes react in solution with thiocyanate ion to give mono- and, with exception of copper(II), di-thiocyanato five- and six-co-ordinate derivatives. Mono-thiocyanato derivatives of cobalt(II), nickel(II) and zinc(II) have been isolated as tetraphenylborate salts. Cobalt(II) and nickel (II) di-thiocyanato derivatives have been also isolated. Results are discussed in terms of the steric requirements of the ligand and electronic properties of the metal ions.     

EPR characterization of mono(thiosemicarbazones) copper(II) complexes. Note II

Copper(II) complexes with thiosemicarbazones have been shown to be more active in cell destruction, in the inhibition of DNA synthesis than the uncomplexed ligand. Several derivatives of thiosemicarbazones and their iron and copper complexes have been studied for their cytotoxicity and inhibiting activity against DNA synthesis. In the present work complexes formed in H2O-DMSO solution between copper(II) and the acetophenone thiosemicarbazone (ATSC) and the o-aminobenzaldehyde thiosemicarbazone (o-NH2TSC) have been studied. EPR studies have been performed at different pH values and metal-to-ligand ratios. The spectra have been recorded at both room (298 K) and low temperatures (120 K). A possible relationship between structure and activity is attempted on the basis of the EPR data.     

The stability constants of copper(II) complexes with some alpha-amino acids in dioxan-water mixtures

In this study, the overall stability constants of copper(II) complexes with some alpha-amino acids (glycine, dl-alanine, dl-valine, l-leucine, l-asparagine, l-glutamine) were determined by potentiometric titration in water, 25% dioxan-75% water, 35% dioxan-65% water, 50% dioxan-50% water, and 60% dioxan-40% water. The titrations were performed at 25 degrees C, under nitrogen atmosphere, and the ionic strength of the medium was maintained at 0.10 M by using sodium perchlorate. The formation curves of their complexes (n-p[L]) were obtained by means of the titration data. Then the stability constants were determined in relation to these curves. The mol ratio of copper(II) to alpha-amino acid was also determined and it was found that the complexes were CuL(2) type. Another important result obtained was that the tendency of amino acids to form complexes with copper(II) was greater in dioxan-water mixtures compared to water.     

New dinuclear copper(II) and zinc(II) complexes for the investigation of sugar–metal ion interactions

We have studied the binding interactions of biologically important carbohydrates (d-glucose, d-xylose and d-mannose) with the newly synthesized five-coordinate dinuclear copper(II) complex, [Cu₂(hpnbpda)(μ-OAc)] (1) and zinc(II) complex, [Zn₂(hpnbpda)(μ-OAc)] (2) [H₃hpnbpda = N,N′-bis(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine-N,N′-diacetic acid] in aqueous alkaline solution. The complexes 1 and 2 are fully characterized both in solid and solution using different analytical techniques. A geometrical optimization was made of the ligand H₃hpnbpda and the complexes 1 and 2 by molecular mechanics (MM+) method in order to establish the stable conformations. All carbohydrates bind to the metal complexes in a 1:1 molar ratio. The binding events have been investigated by a combined approach of FTIR, UV–vis and ¹³C NMR spectroscopic techniques. UV–vis spectra indicate a significant blue shift of the absorption maximum of complex 1 during carbohydrate coordination highlighting the sugar binding ability of complex 1. The apparent binding constants of the substrate-bound copper(II) complexes have been determined from the UV–vis titration experiments. The binding ability and mode of binding of these sugar substrates with complex 2 are indicated by their characteristic coordination induced shift (CIS) values in ¹³C NMR spectra for carbon atoms C1, C2, and C3 of sugar substrates.     

Comparative solvolytic stabilities of copper(II) nanoballs and dinuclear Cu(II) paddle wheel units

The self-assembly of copper(II) ions and 5-(2-(2-hydroxyethoxy)ethoxy)benzene-1,3-dicarboxylic acid (2) leads to hollow nanoballs in which 12 dinuclear copper(II) paddle wheel units are interconnected via 24 ligands, as determined by single crystal X-ray structure analysis. The nanoball dissociates in aqueous solutions, and in the presence of an excess of ligand it transforms into a three-dimensional network, but is stable in organic solvents. The thermodynamic stability of the nanoball against dissociation in aqueous solution is studied and compared to simple copper(II) paddle wheel complexes. The results reveal enhanced thermodynamic stability of the nanoball as compared to discrete copper(II) paddle wheel complexes due to chelate effects and positive cooperativity.