Tamás Gajda

Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg2+– Cl–, OH–, CO32–, SO42–, and PO43– aqueous systems (IUPAC Technical Report)

This document presents a critical evaluation of the equilibrium constants and reaction enthalpies for the complex formation reactions between aqueous Hg(II) and the common environmental inorganic ligands Cl–, OH–, CO32–, SO42–, and PO43–. The analysis used data from the IUPAC Stability Constants database, SC-Database, focusing particularly on values for 25 °C and perchlorate media. Specific ion interaction theory (SIT) was applied to reliable data available for the ionic strength range Ic < 3.0 mol dm–3. Recommended values of log10βp,q,r° and the associated reaction enthalpies, ∆rHm°, valid at Im = 0 mol kg–1 and 25 °C, were obtained by weighted linear regression using the SIT equations. Also reported are the equations and specific ion interaction coefficients required to calculate log10βp,q,r° values at higher ionic strengths and other temperatures. A similar analysis is reported for the reactions of H+ with CO32– and PO43–. Diagrams are presented to show the calculated distribution of Hg(II) amongst these inorganic ligands in model natural waters. Under typical environmental conditions, Hg(II) speciation is dominated by the formation of HgCl2(aq), Hg(OH)Cl(aq), and Hg(OH)2(aq).

Chemical speciation of environmentally significant metals with inorganic ligands Part 2: The Cu2+-OH-, Cl-, CO32-, SO42-, and PO43- systems (IUPAC Technical Report)

The numerical modeling of CdII speciation amongst the environmental inorganic ligands Cl–, OH–, CO32–, SO42–, and PO43– requires reliable values for the relevant stability (formation) constants. This paper compiles and provides a critical review of these constants and related thermodynamic data. It recommends values of log10βp,q,r° valid at Im = 0 mol kg–1 and 25 °C (298.15 K), along with the equations and empirical reaction ion interaction coefficients, ∆ε , required to calculate log10βp,q,r values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies, ∆rH, are reported where available. Unfortunately, with the exception of the CdII-chlorido system and (at low ionic strengths) the CdII-sulfato system, the equilibrium reactions for the title systems are relatively poorly characterized. In weakly acidic fresh water systems (–log10 {[H+]/c°} < 6), in the absence of organic ligands (e.g., humic substances), CdII speciation is dominated by Cd2+(aq), with CdSO4(aq) as a minor species. In this respect, CdII is similar to CuII [2007PBa] and PbII [2009PBa]. However, in weakly alkaline fresh water solutions, 7.5 < –log10 {[H+]/c°} < 8.6, the speciation of CdII is still dominated by Cd2+(aq), whereas for CuII [2007PBa] and PbII [2009PBa] the carbonato- species MCO3(aq) dominates. In weakly acidic saline systems (–log10 {[H+]/cϒ} < 6; –log10 {[Cl–]/c°} < 2.0) the speciation is dominated by CdCln(2–n)+ complexes, (n = 1–3), with Cd2+(aq) as a minor species. This is qualitatively similar to the situation for CuII and PbII. However, in weakly alkaline saline solutions, including seawater, the chlorido- complexes still dominate the speciation of CdII because of the relatively low stability of CdCO3(aq). In contrast, the speciation of CuII [2007PBa] and PbII [2009PBa] in seawater is dominated by the respective species MCO3(aq). There is scope for additional high-quality measurements in the Cd2+ + H+ + CO32– system as the large uncertainties in the stability constants for the Cd2+-carbonato complexes significantly affect the speciation calculations.

Nickel(II)-, copper(II)- and zinc(II)-complexes of some substituted imidazole ligands

Nickel(II)-, copper(II)- and zinc(II)-complexes of four imidazole derivatives (4(5)-aminoimidazole-5(4)-carboxamide (aic), 2,2′-biimidazole (biim), bis(1,1′-imidazol-2-yl)(4-imidazol-4(5)-yl)-2-aza-butane (biib) and imidazole-4-acetic acid (iaa)), having different coordination environments around the imidazole ring(s), have been studied by potentiometric, UV–VIS and EPR spectroscopic methods. The data revealed very strong bidentate coordination of biim, in spite of the low basicity of the donor sites. Three dominant species (CuLH, Cu2L2, Cu2LH−1) are formed in the Cu(II)-biib system. Since the biib offers extremely stable tri- and tetradentate coordination to copper(II) (in CuLH and Cu2L2, respectively), the bis-imidazolyl like coordination, which is predominant in metal complexes of any other bis-imidazolyl like ligand, does not appear in its original form in the copper(II)-biib system. The EPR spectra of above dimer species show coupling between the copper(II) centers. In the M(II)-iaa systems only parent complexes are formed up to pH 9, while at higher pH further deprotonations were observed, which resulted in a formation of dimer complexes in case of copper(II), having antiferromagnetically coupled metal centers.

Unusual coordination behavior of D-fructose towards dimethyltin(IV): metal-promoted deprotonation of alcoholic OH groups in aqueous solutions of low pH

Abstract To study the effect of the conformation of sugar hydroxy groups on metal complexation processes, complex formation of eight saccharides (D-fructose, L-sorbose, L-arabinose, D-arabinose, D-glucose, D-sorbitol, 2-deoxy-D-glucose and D-saccharose) with dimethyltin(IV) cations was investigated in aqueous solution by potentiometric equilibrium measurements, 13 C NMR, polarimetric and Mossbauer spectroscopic methods. The experimental results proved that deprotonation of D-fructose and L-sorbose is caused by the coordination of dimethyltin(IV) in the unusual low pH interval 4–6 in contrast to the other saccharides deprotonated in analogous way at pH > 8. Increasing the pH of the solution resulted in the formation of further complexes. Stability and composition of the species was determined by potentiometric studies. 13 C NMR measurements led to the assignment of the sugar OH groups participating in the processes. Mossbauer investigations in the quick-frozen solutions permitted the determination of the stereochemistry of tin(IV) in the complexes.

Heterodinuclear Zinc(II)−Iron(III) Complexes and Dinuclear Zinc Complexes as Models for Zinc-Containing Phosphatases

Five new dinuclear model complexes for zinc-containing phosphatases with dinucleating ligands have been prepared and characterized by single-crystal X-ray crystallography. The heterodinuclear, μ-alkoxo-bridged zinc(II)−iron(III) complexes 1−3 contain the symmetric ligands N,N,N′,N′-tetrakis[2-(5,6-dimethyl)benzimidazolylmethyl]-1,3-diamino-2propanol (Htdmbpo) and N,N,N′,N′-tetrakis{2-[N′′-(2hydroxyethyl)]benzimidazolylmethyl}-1,3-diamino-2-propanol (Hthebpo), and the asymmetric ligand N,N-bis[2-(4,5-dimethyl)benzimidazolylmethyl]-N′,N′-bis(2-pyridylmethyl)-1,3-diamino-2-propanol (Hbdmbbppo), respectively. X-ray crystallography revealed that the zinc center exhibits a trigonal-bipyramidal coordination, while the octahedral coordination sphere of the iron center is completed by a solvent molecule. In contrast, the zinc complexes 4 and 5, which also have (alkoxo)(cacodylato)dimetal cores with the dinucleating ligands used in 1 and 3, exhibit both metal centers in a trigonal-bipyramidal environment. Additionally, the solution speciation of the zinc(II) complexes formed with Htdmbpo and Hbdmbbppo were determined and the activity of the in situ prepared zinc complexes towards the transesterification of the RNA model substrate 2-(hydroxypropyl)-4-nitrophenyl phosphate (hpnp) was investigated. The dinuclear [Zn2LH−1(OH)]2+ complex of both ligands efficiently promotes the transesterification. The kinetic data indicated a higher activity for the complex of the asymmetric ligand Hbdmbbppo, as a result of its stronger substrate binding ability. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Copper and zinc binding properties of the N-terminal histidine-rich sequence of Haemophilus ducreyi Cu,Zn superoxide dismutase.

The Cu,Zn superoxide dismutase (Cu,ZnSOD) isolated from Haemophilus ducreyi possesses a His-rich N-terminal metal binding domain, which has been previously proposed to play a copper(II) chaperoning role. To analyze the metal binding ability and selectivity of the histidine-rich domain we have carried out thermodynamic and solution structural analysis of the copper(II) and zinc(II) complexes of a peptide corresponding to the first 11 amino acids of the enzyme (H(2)N-HGDHMHNHDTK-OH, L). This peptide has highly versatile metal binding ability and provides one and three high affinity binding sites for zinc(II) and copper(II), respectively. In equimolar solutions the MHL complexes are dominant in the neutral pH-range with protonated lysine epsilon-amino group. As a consequence of its multidentate nature, L binds zinc and copper with extraordinary high affinity (K(D,Zn)=1.6x10(-9)M and K(D,Cu)=5.0x10(-12)M at pH 7.4) and appears as the strongest zinc(II) and copper(II) chelator between the His-rich peptides so far investigated. These K(D) values support the already proposed role of the N-terminal His-rich region of H. ducreyi Cu,ZnSOD in copper recruitment under metal starvation, and indicate a similar function in the zinc(II) uptake, too. The kinetics of copper(II) transfer from L to the active site of Cu-free N-deleted H. ducreyi Cu,ZnSOD showed significant pH and copper-to-peptide ratio dependence, indicating specific structural requirements during the metal ion transfer to the active site. Interestingly, the complex CuHL has significant superoxide dismutase like activity, which may suggest multifunctional role of the copper(II)-bound N-terminal His-rich domain of H. ducreyi Cu,ZnSOD.

Oxovanadium(IV) complexes of imidazole-4-acetic, imidazole-4,5-dicarboxylic and pyrazole-3,5-dicarboxylic acids

Abstract Complex formation between oxovanadium(IV) and several diazole derivatives (imidazole-4-acetic, imidazole-4,5-dicarboxylic and pyrazole-3,5-dicarboxylic acid) was studied in aqueous solution by pH-potentiometric and spectroscopic (electron paramagnetic resonance and electronic absorption) techniques. The results show that the imidazole-4-acetate ligand, being suitable for bidentate coordination, forms monomeric or mixed hydroxo-bridged species. In the case of the dicarboxylic acids both having two bidentate sites for metal binding the formation of tetrameric or dimeric complexes is strongly favored. Pyrazole-3,5-dicarboxylic acid forms dinuclear, while imidazole-4,5-dicarboxylic acid forms cyclic tetrameric species, both types containing diazolate bridges. There is evidence for the VO2+-supported deprotonation of the imidazole N1 (pyrrole-type) group at pH as low as 5 in aqueous solution.

N-terminal fragment of the anti-angiogenic human endostatin binds copper(II) with very high affinity

A histidine-rich peptide HSHRDFQPVLHL-NH(2) (L), identical with the N-terminal fragment of the anti-angiogenic human endostatin has been synthesized. Endostatin is a recently identified broad spectrum angiogenesis inhibitor, which inhibits 65 different tumor types. The N-terminal 25-mer peptide fragment of human endostatin has the same antitumor effect as the entire protein. The zinc(II) binding is crucial for the antitumor effect in both cases. Our peptide may provide all critical interactions for zinc(II) binding present in the N-terminal 25-mer peptide fragment. In addition, the N-terminus of human endostatin has a supposedly high affinity binding site for copper(II), similar to human serum albumin. Since copper(II) is intimately involved in angiogenesis, this may have biological relevance. In order to determine the metal binding properties of the N-terminal fragment of endostatin, we performed equilibrium, UV-visible (UV-vis), CD, EPR and NMR studies on the zinc(II) and copper(II) complexes of L. In the presence of zinc(II) the formation of a stable [NH(2),3N(im),COO(-)] coordinated complex was detected in the neutral pH-range. This coordination mode is probably identical to that present in the zinc(II) complex of the above mentioned N-terminal 25-mer peptide fragment of human endostatin. Moreover, L has extremely high copper(II) binding affinity, close to those of copper-containing metalloenzymes, and forms albumin-like [NH(2),N(-),N(-),N(im)] coordinated copper(II) complex in the neutral pH-range, which may suggest that copper(II) binding is involved in the biological activity of endostatin.

Copper(II), nickel(II) and zinc(II) complexes of N-acetyl-His-Pro-His-His-NH2: equilibria, solution structure and enzyme mimicking.

The copper(II), nickel(II) and zinc(II) binding ability of the multi-histidine peptide N-acetyl-His-Pro-His-His-NH(2) has been studied by combined pH-potentiometry and visible, CD and EPR spectroscopies. The internal proline residue, preventing the metal ion induced successive amide deprotonations, resulted in the shift of this process toward higher pH values as compared to other peptides. The metal ions in the parent [ML](2+) complexes are exclusively bound by the three imidazole side chains. In [CuH(-1)L](+), formed between pH 6-8, the side chains of the two adjacent histidines and the peptide nitrogen between them are involved in metal ion binding. The next deprotonation results in the proton loss of the coordinated water molecule (CuH(-1)L(OH)). The latter two species exert polyfunctional catalytic activity, since they possess superoxide dismutase-, catecholase- (the oxidation of 3,5-di-tert-butylcatechol) and phosphatase-like (transesterification of the activated phosphoester 2-hydroxypropyl-4-nitrophenyl phosphate) properties. On further increase of the pH rearrangement of the coordination sphere takes place leading to the [CuH(-3)L](-) species, the deprotonated amide nitrogen displaces a coordinated imidazole nitrogen from the equatorial position of the metal ion. The shapes of the visible and CD spectra reflect a distorted arrangement of the donor atoms around the metal ion. In presence of zinc(II) the species [ZnL](2+) forms only above pH 6, which is shortly followed by precipitation. On the other hand, the [NiL](2+) complex is stable over a wide pH range, its deprotonation takes place only above pH 8. At pH 10 an octahedral NiH(-2)L species is present at first, which transforms slowly to a yellow square planar complex.

Coordination chemistry of polyhydroxy acids: role of the hydroxy groups

The coordination properties of five aldonic acids (d-gluconic, d- and l-gulonic, d-galactonic and d-ribonic acids) and of 2-amino-2-deoxy-d-gluconic acid towards copper(II) were studied in aqueous media by combined pH-metric, circular dichroism (CD) and electron paramagnetic resonance (EPR) spectroscopic and 13C NMR relaxation measurements. The equilibrium study (298 K, I = 0.1 M (NaClO4)) revealed the formation of several monomeric and dimeric complexes deprotonated to an extent depending on the pH and the metal-to-ligand ratio. The EPR spectra confirmed the formation of binuclear complexes involving strong antiferromagnetic coupling between the metal centres. For the dimeric species Cu2L2H−4 of different aldonic acids, the shape of the CD spectrum was determined by the configurations of C(2), C(3) and C(4), suggesting the involvement in the coordination of the hydroxy groups on these carbons. At low metal-to-ligand ratios, exclusively the monomeric complex CuL2H−2 was formed. The EPR study of this species indicated the existence of cis and trans isomers. This was also proved by 13C NMR relaxation measurements. The 13C relaxation enhancements induced by the paramagnetic metal centre allowed us to extract carbon-to-metal distances. The results also provided evidence of metal-promoted deprotonation and coordination of the C(2)OH group besides the carboxylate oxygen. In this way, the C(4)OH hydroxy group is at an appropriate distance for hydrogen bonding to an axially bound water molecule, which is in fact shown by the temperature dependence of the NMR parameters. Concerning the copper(II) complexes of 2-amino-2-deoxy-d-gluconic acid, the bis amino acid type coordination diminishes significantly the coordination ability (possibility) of the hydroxy groups compared with that in the aldonic acids, but it is still higher than that in the copper(II)-serine complexes.