Quantitative electrospray ionization mass spectrometry of zinc finger oxidation: the reaction of XPA zinc finger with H(2)O(2)

Oxidation plays an important role in the functioning of zinc fingers (ZFs). Electrospray ionization mass spectrometry (ESI-MS) is a very useful technique to study products of ZF oxidation, but its application has been limited largely to qualitative analysis of reaction products. On the other hand, ESI-MS has been applied successfully on several occasions to determine binding constants in metalloproteins. We used a synthetic 37-residue peptide acetyl-DYVICEECGKEFMDSYLMNHFDLPTCDNCRDADDKHK-amide (XPAzf), which corresponds to the Cys4 ZF sequence of human nucleotide excision repair protein XPA, to find out whether ESI-MS might be used quantitatively to study ZF reaction kinetics. For this purpose, we studied oxidation of the Zn(II) complex of XPAzf (ZnXPAzf) by H(2)O(2) using three techniques in parallel: high-performance liquid chromatography (HPLC) of covalent reaction products, 4-(2-pyridylazo)-resorcinol monosodium salt (PAR)-based spectrophotometric zinc release assay, and ESI-MS. Single and double intrapeptide disulfides were detected by ESI-MS to be the sole reaction products. All three techniques yielded independently the same reaction rate, thereby demonstrating that ESI-MS may indeed be used in quantitative kinetic studies of ZF reactions. The comparison of experimental information demonstrated that the formation of the Cys5-Cys8 single disulfide was responsible for zinc release.     

The Salmonella enterica ZinT structure,zinc affinity and interaction with the high-affinity uptake protein ZnuA provide insight into the management of periplasmic zinc

Background

In Gram-negative bacteria the ZnuABC transporter ensures adequate zinc import in Zn(II)-poor environments, like those encountered by pathogens within the infected host. Recently, the metal-binding protein ZinT was suggested to operate as an accessory component of ZnuABC in periplasmic zinc recruitment. Since ZinT is known to form a ZinT–ZnuA complex in the presence of Zn(II) it was proposed to transfer Zn(II) to ZnuA. The present work was undertaken to test this claim.

Methods

ZinT and its structural relationship with ZnuA have been characterized by multiple biophysical techniques (X-ray crystallography, SAXS, analytical ultracentrifugation, fluorescence spectroscopy).

Results

The metal-free and metal-bound crystal structures of Salmonella enterica ZinT show one Zn(II) binding site and limited structural changes upon metal removal. Spectroscopic titrations with Zn(II) yield a K_D value of 22 ± 2 nM for ZinT, while those with ZnuA point to one high affinity (K_D < 20 nM) and one low affinity Zn(II) binding site (K_D in the micromolar range). Sedimentation velocity experiments established that Zn(II)-bound ZinT interacts with ZnuA, whereas apo-ZinT does not. The model of the ZinT–ZnuA complex derived from small angle X-ray scattering experiments points to a disposition that favors metal transfer as the metal binding cavities of the two proteins face each other.

Conclusions

ZinT acts as a Zn(II)-buffering protein that delivers Zn(II) to ZnuA.

General significance

Knowledge of the ZinT–ZnuA relationship is crucial for understanding bacterial Zn(II) uptake.     

On the possible roles of N-terminal His-rich domains of Cu,Zn SODs of some Gram-negative bacteria

The Cu,Zn superoxide dismutases (Cu,Zn SOD) isolated from some Gram-negative bacteria possess a His-rich N-terminal metal binding extension. The N-terminal domain of Haemophilus ducreyi Cu,Zn SOD has been previously proposed to play a copper(II)-, and may be a zinc(II)-chaperoning role under metal ion starvation, and to behave as a temporary (low activity) superoxide dismutating center if copper(II) is available. The N-terminal extension of Cu,Zn SOD from Actinobacillus pleuropneumoniae starts with an analogous sequence (HxDHxH), but contains considerably fewer metal binding sites. In order to study the possibility of the generalization of the above mentioned functions over all Gram-negative bacteria possessing His-rich N-terminal extension, here we report thermodynamic and solution structural analysis of the copper(II) and zinc(II) complexes of a peptide corresponding to the first eight amino acids (HADHDHKK-NH₂, L) of the enzyme isolated from A. pleuropneumoniae. In equimolar solutions of Cu(II)/Zn(II) and the peptide the MH₂L complexes are dominant in the neutral pH-range. L has extraordinary copper(II) sequestering capacity (K_D,Cu = 7.4 × 10^− 13 M at pH 7.4), which is provided only by non-amide (side chain) donors. The central ion in CuH₂L is coordinated by four nitrogens {NH₂,3N_im} in the equatorial plane. In ZnH₂L the peptide binds to zinc(II) through a {NH₂,2N_im,COO⁻} donor set, and its zinc binding affinity is relatively modest (K_D,Zn = 4.8 × 10^− 7 M at pH 7.4). Consequently, the presented data do support a general chaperoning role of the N-terminal His-rich region of Gram-negative bacteria in copper(II) uptake, but do not confirm similar function for zinc(II). Interestingly, the complex CuH₂L has very high SOD-like activity, which may further support the multifunctional role of the copper(II)-bound N-terminal His-rich domain of Cu,Zn SODs of Gram-negative bacteria. The proposed structure for the MH₂L complexes has been verified by semiempirical quantum chemical calculations (PM6), too.     

Zinc finger proteins as templates for metal ion exchange: Substitution effects on the C-finger of HIV nucleocapsid NCp7 using M(chelate) species (M = Pt, Pd, Au)

The interactions of monofunctional [MCl(chelate)] compounds (M = Pt(II), Pd(II) or Au(III) and chelate = diethylenetriamine, dien or 2,2′,2″-terpyridine, terpy) with the C-terminal finger of the HIV nucleocapsid NCp7 zinc finger (ZF) were studied by mass spectrometry and circular dichroism spectroscopy. In the case of [M(dien)] species, Pt(II) and Pd(II) behaved in a similar fashion with evidence of adducts caused by displacement of Pt-Cl or Pd-Cl by zinc-bound thiolate. Labilization, presumably under the influence of the strong trans influence of thiolate, resulted in loss of ligand (dien) as well as zinc ejection and formation of species with only Pd(II) or Pt(II) bound to the finger. For both Au(III) compounds the reactions were very fast and only “gold fingers” with no ancillary ligands were observed. For all terpyridine compounds ligand scrambling and metal exchange occurred with formation of [Zn(terpy)]²⁺. The results conform well to those proposed from the study of model Zn compounds such as N,N′-bis(2-mercapto-ethyl)-1,4-diazacycloheptanezinc(II), [Zn(bme-dach)]₂. The possible structures of the adducts formed are discussed and, for Pt(II) and Pd(II), the evidence for possible expansion of the zinc coordination sphere from four- to five-coordinate is discussed. This observation reinforces the possibility of change in geometry for zinc in biology, even in common “structural” sites in metalloenzymes. The results further show that the extent and rate of zinc displacement by inorganic compounds can be modulated by the nature (metal, ligands) of the reacting compound.     

Synthesis, crystal structure and electroluminescent properties of a Schiff base zinc complex

A new complex of zinc with a Schiff base, zinc(N,N′-bis(salicylidene)-3, 6-dioxa-1, 8-diaminooctane monohydrate) (ZnBSO · H₂O), was synthesized and characterized by means of elemental analyses, IR spectra and DTA-TG. Its structure was determined by X-ray single crystal analysis. It was demonstrated that the zinc atom is coordinated by the two oxygen atoms in phenolate and two nitrogen atoms in imine of the ligand in a slightly distorted tetrahedral geometry, while the two oxygen atoms from the oxa-alkyl chain are not coordinated to Zn(II) atom. The energy levels of the HOMO, LUMO and the electrochemical band gap were determined by cyclic voltammeter. The electroluminescent devices with the complex as the emitter showed bright blue emission with a peak at 450 nm, which is same as the fluorescence of the complex in both solution and solid states.     

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 (H₂N-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 ε-amino group. As a consequence of its multidentate nature, L binds zinc and copper with extraordinary high affinity (K_D,Zn = 1.6 × 10⁻⁹ M and K_D,Cu = 5.0 × 10⁻¹² 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.     

Interaction of arsenite with a zinc finger CCHC peptide: evidence for formation of an As-Zn-peptide mixed complex

The interaction of arsenite with a Cys₃His (CCHC) zinc finger model (34-51) HIV-1 nucleocapsid protein p7 (NCp7) peptide in the absence and presence of Zn^II was studied using fluorescence spectroscopy, CD (circular dichroism) and ESI-MS (Electrospray Ionization Mass Spectrometry). We found that arsenic forms different complexes with the free peptide and the zinc finger peptide. In the former case the peptide conformation differed greatly from that of the zinc finger, whereas in the second case a mixed As-Zn-peptide complex was formed with partial preservation of zinc finger conformation. An apparent stability constant was estimated for the mixed As-Zn-peptide complex (K = 2083 M^− 1 and 442 M^− 1 at 25 °C and pHs 6 and 7, respectively). Our study also shows that the interaction of arsenic with the CCHC motif is facilitated by glutathione (GSH), through formation of a GS-As-peptide conjugate.     

A fluorescent ligand rationally designed to be selective for zinc(II) over larger metal ions. The structures of the zinc(II) and cadmium(II) complexes of N,N-bis(2-methylquinoline)-2-(2-aminoethyl)pyridine

The metal ion coordinating properties of the ligands N,N-bis(2-methylquinoline)-2-(2-aminoethyl)pyridine (DQPEA) and N,N-bis(2-methylquinoline)-2-(2-aminomethyl)pyridine (DQPMA) are presented. DQPEA and DQPMA differ only in that DQPEA forms six-membered chelate rings that involve the pyridyl group, whereas DQPMA forms analogous five-membered chelate rings.These two ligands illustrate the application of a ligand design principle, which states that increase of chelate ring size in a ligand will result in increase in selectivity for smaller relative to larger metal ions. The formation constants (log K₁) of DQPEA and DQPMA with Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) are reported. As expected from the applied ligand design principle, small metal ions such as Ni(II) and Zn(II) show increases in log K₁ with DQPEA (six-membered chelate ring) relative to DQPMA (five-membered chelate ring), while large metal ions such as Cd(II) and Pb(II) show decreases in log K₁ when the chelate ring increases in size. In order to further understand the steric origin of the destabilization of complexes of metal ions of differing sizes by the six-membered chelate ring of DQPEA, the structures of [Zn(DQPEA)H₂O](ClO₄)₂ (1) [triclinic, , a = 9.2906(10), b = 10.3943(10), c = 17.3880(18) Å, α = 82.748(7)°, β = 88.519(7)°, γ = 66.957(6)°, Z = 4, R = 0.073] and [Cd(DQPEA)(NO₃)₂] (2) [monoclinic, C2/c, a = 22.160(3), b = 15.9444(18), c = 16.6962(18) Å, β = 119.780(3)°, Z = 8, R = 0.0425] are reported. The Zn in (1) is five-coordinate, with a water molecule completing the coordination sphere. The Cd(II) in (2) is six-coordinate, with two unidentate nitrates coordinated to the Cd. It is found that the bonds to the quinaldine nitrogens in the DQPEA complexes are considerably stretched as compared to those of analogous TPyA (tri(pyridylmethyl)amine) complexes, which effect is attributed to the greater steric crowding in the DQPEA complexes. The structures are analyzed for indications of the origins of the destabilization of the complex of the large Cd(II) ion relative to the smaller Zn(II) ion. A possible cause is the greater distortion of the six-membered chelate ring in (2) than in (1), as evidenced by torsion angles that are further away from the ideal values in (2) than in (1). Fluorescence properties of the DQPMA and DQPEA complexes of Zn(II) and Cd(II) are reported. It is found that the DQPEA complex of Zn(II) has increased fluorescence intensity compared to the DQPMA complex, while for the Cd(II) complex the opposite is found. This is related to the greater strain in the six-membered chelate ring of the Cd(II) DQPEA complex as compared to the Zn(II) complex, with resulting poorer overlap in the Cd-N bond, and hence greater ability to quench the fluorescence in the Cd(II) complex.     

Adsorption of copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution

Biochars produced by pyrolysis of hardwood at 450 °C (HW450) and corn straw at 600 °C (CS600) were characterized and investigated as adsorbents for the removal of Cu(II) and Zn(II) from aqueous solution. The adsorption data were well described by a Langmuir isotherm, with maximum Cu(II) and Zn(II) adsorption capacities of 12.52 and 11.0 mg/g for CS600, 6.79 and 4.54 mg/g for HW450, respectively. Thermodynamic analysis suggested that the adsorption was an endothermic process and did not occur spontaneously. Although Cu(II) adsorption was only marginally affected by Zn(II), Cu(II) competed with Zn(II) for binding sites at Cu(II) and Zn(II) concentrations ?1.0 mM. Results from this study indicated that plant-residue or agricultural waste derived biochar can act as effective surface sorbent, but their ability to treat mixed waste streams needs to be carefully evaluated on an individual basis.     

Structure, speciation, DNA binding and nuclease activity of two bipyridyl-zinc complexes bearing trimethylaminomethyl groups

The Zn(II) complexes of two bipyridyl derivatives with trimethylaminomethyl groups, [Zn(L¹)₃](ZnBr₄)(ClO₄)₂Br₄ · 12H₂O (1) and [Zn(L²)₃](ClO₄)₈ · H₂O (2) (L¹ = [4,4′-(Me₃NCH₂)₂-bpy]²⁺, L² = [5,5′-(Me₃NCH₂)₂-bpy]²⁺ and bpy = 2,2′-bipyridyl), were synthesized and characterized. The structure of complex 1 has been determined by X-ray diffraction, and the configuration of ligand-coordinated Zn(II) ion can be well described as distorted octahedral. Species distribution of both complexes in aqueous solution was investigated by potentiometeric titration. DNA-binding ability of both complexes was studied by UV spectroscopy titration. Under hydrolytic condition, both complexes (150 μM) can effectively cleave plasmid pBR322 DNA from supercoiled to nicked form, giving hydrolysis rate constants of 2.63 × 10⁻⁵ and 9.92 × 10⁻⁵ s⁻¹, respectively (pH 7.75, 37 °C). The result shows that complex 2 is one of the most active artificial nucleases based on zinc.     

The chemical cell biology of zinc: structure and intracellular fluorescence of a zinc-quinolinesulfonamide complex

p -toluenesulfonamido-quinoline, TSQ, are potentially powerful probes of intracellular zinc chemistry; however, the structure, thermodynamics, and stoichiometry of the metal complexes, and the molecular basis of Zn(II) recognition, remain open issues. To address these, we report the first structural characterization of a Zn(II) complex of a TSQ derivative, namely 2-methyl-6-methoxy-8-p-toluenesulfonamido-quinoline (3) and describe its unusual coordination chemistry. The crystal structure of the fluorescent complex of 3 with zinc reveals a 2 : 1 stoichiometry wherein bidentate coordination of two nitrogens from each ligand gives rise to a highly distorted tetrahedral Zn(II) center. Both sulfonamido groups in the zinc complex are tilted away from zinc to make room for coordination of the amide nitrogens. Zn-O(2) and Zn-O(4) distances are essentially nonbonding (3.06 and 3.10 Å, respectively). The bond angles [N(1)-Zn-N(2) 83.5° and N(3)-Zn-N(4) 83.0°] are quite small relative to the 109° angle of an ideal tetrahedral center. This result provides an insight into the zinc-binding mode of the TSQ derivative zinquin, in which a methyl group replaces the hydrogen in the 2-position of the quinoline ring. The methyl group and sulfonamide oxygen atoms clearly hinder formation of both square planar and octahedral complexes. We also show here that the Zn(II) complex of 3 in DMSO-water (80/20 w/w) exhibits an overall binding stability (logβ ₂ = 18.24 ± 0.02) similar to zinquin. Fluorescence microscopy suggests that each of these members of this family demarks a similar set of Zn(II)-enriched compartments that are common to all eukaryotic cells examined to date, and further shows that the ester function is not required for observation of these ubiquitous Zn-loaded compartments. The combined structural, thermodynamic, and physiological results provide a basis for design of other Zn(II)-specific membrane permeant probes with a range of Zn(II) affinities and photophysical properties. Received: 8 May 1999 / Accepted: 15 September 1999     

Pb-207 NMR spectroscopy reveals that Pb(II) coordinates with glutathione (GSH) and tris cysteine zinc finger proteins in a PbS3 coordination environment

²⁰⁷Pb NMR spectroscopy can be used to monitor the binding of Pb(II) to thiol rich biological small molecules such as glutathione and to zinc finger proteins. The UV/visible (UV/Vis) absorption band centered at 334 nM and the observed ²⁰⁷Pb signal in ²⁰⁷Pb NMR (δ ~ 5750 ppm) indicate that glutathione binds Pb(II) in a trigonal pyramidal geometry (PbS₃) at pH 7.5 or higher with a 1:3 molar ratio of Pb(II) to GSH. While previous studies using UV/Vis and extended X-ray absorption fine structure (EXAFS) spectroscopy were interpreted to show that the zinc binding domain from HIV nucleocapsid protein (HIV-CCHC) binds Pb(II) in a single PbS₃ environment, the more sensitive ²⁰⁷Pb NMR spectra (at pH 7.0, 1:1 molar ratio) provide compelling evidence for the presence of two PbS₃ structures (δ - 5790 and 5744 ppm), one of which is more stable at high temperatures. It has previously been proposed that the HIV-CCHH peptide does not fold properly to afford a PbS₂N motif, because histidine does not bind to Pb(II). These predictions are confirmed by the present studies. These results demonstrate the applicability of ²⁰⁷Pb NMR to biomolecular structure determination in proteins with cysteine binding sites for the first time.     

Substituted pyridylmethylamide ligands and their zinc complexes

Mononuclear zinc complexes of a family of pyridylmethylamide ligands abbreviated as HL, HL^Ph, HL^Me3, HL^Ph3, and MeL^SMe [HL = N-(2-pyridylmethyl)acetamide; HL^Ph = 2-phenyl-N-(2-pyridylmethyl)acetamide; HL^Me3 = 2,2-dimethyl-N-(2-pyridylmethyl)propionamide; HL^Ph3 = 2,2,2-triphenyl-N-(2-pyridylmethyl)acetamide; MeL^SMe = N-methyl-2-methylsulfanyl-N-pyridin-2-ylmethyl-acetamide] were synthesized and characterized spectroscopically and by single crystal X-ray structural analysis. The reaction of zinc(II) salts with the HL ligands yielded complexes [Zn(HL)₂(OTf)₂] (1), [Zn(HL)₂(H₂O)](ClO₄)₂ (2), [Zn(HL^Ph3)₂(H₂O)](ClO₄)₂ (3), [Zn(HL^Ph)Cl₂] (4), [Zn(HL^Me3)Cl₂] (5), and [Zn(MeL^SMe)Cl₂] (6). The complexes are either four-, five- or six-coordinate, encompassing a variety of geometries including tetrahedral, square-pyramidal, trigonal-bipyramidal, and octahedral.     

The Metalloregulatory Zinc Site in Streptococcus pneumoniae AdcR, a Zinc-activated MarR Family Repressor

Streptococcus pneumoniae D39 AdcR (adhesin competence repressor) is the first metal-sensing member of the MarR (multiple antibiotic resistance repressor) family to be characterized. Expression profiling with a ΔadcR strain grown in liquid culture (brain-heart infusion) under microaerobic conditions revealed upregulation of 13 genes, including adcR and adcCBA, encoding a high-affinity ABC uptake system for zinc, and genes encoding cell-surface zinc-binding pneumococcal histidine triad (Pht) proteins and AdcAII (Lmb, laminin binding). The ΔadcR, H108Q and H112Q adcR mutant allelic strains grown in 0.2 mM Zn(II) exhibit a slow-growth phenotype and an approximately twofold increase in cell-associated Zn(II). Apo- and Zn(II)-bound AdcR are homodimers in solution and binding to a 28-mer DNA containing an adc operator is strongly stimulated by Zn(II) with K_DNA-Zn = 2.4 × 10⁸ M^- 1 (pH 6.0, 0.2 M NaCl, 25 °C). AdcR binds two Zn(II) per dimer, with stepwise Zn(II) affinities K_Zn1 and K_Zn2 of ≥ 10⁹ M^- 1 at pH 6.0 and ≥ 10¹² M^- 1 at pH 8.0, and one to three lower affinity Zn(II) depending on the pH. X-ray absorption spectroscopy of the high-affinity site reveals a pentacoordinate N/O complex and no cysteine coordination, the latter finding corroborated by wild type-like functional properties of C30A AdcR. Alanine substitution of conserved residues His42 in the DNA-binding domain, and His108 and His112 in the C-terminal regulatory domain, abolish high-affinity Zn(II) binding and greatly reduce Zn(II)-activated binding to DNA. NMR studies reveal that these mutants adopt the same folded conformation as dimeric wild type apo-AdcR, but fail to conformationally switch upon Zn(II) binding. These studies implicate His42, His108 and H112 as metalloregulatory zinc ligands in S. pneumoniae AdcR.     

Synthesis, characterization and structure of a new zinc(II) complex containing the hexadentate N,N′,N,N′-bis[(2-hydroxy-3,5-di-tert-butylbenzyl)(2-pyridylmethyl)]-ethylenediamine ligand: Generation of phenoxyl radical species

This work summarizes the results of our studies on the structural, spectral and redox properties of a mononuclear zinc(II) complex with the new H₂L ligand (H₂L = N,N′,N,N′-bis[(2-hydroxy-3,5-di-tert-butylbenzyl)(2-pyridylmethyl)]-ethylene diamine). The crystal structure of the complex [Zn^II(HL)] · ClO₄ (1) was determined by X-ray crystallographic analysis. The structure of this complex consists of a discrete mononuclear cation [Zn^II(HL)]⁺, in a strongly distorted geometry with a slight tendency toward a distorted square pyramidal geometry, as reflected by the structural index parameter τ of 0.44. The zinc(II) cation is coordinated to one oxygen and four nitrogen atoms: the pyridine nitrogen atoms (N22 and N32), tertiary amine nitrogen atoms (N1 and N4) and phenolate oxygen atom (O10). ¹H and ¹³C NMR spectral data show a rigid solution structure for 1 in agreement with X-ray structure. Potentiometric studies of complex 1 were also performed and revealed three titratable protons which are attributed to the protonation/deprotonation of two phenol groups (p[K]_a1 = 4.04 and p[K]_a3 = 11.34) and dissociation of a metal-bound water molecule (p[K]_a2 = 7.8). The phenolate groups in complex 1 are suitably protected by bulky substituents (tert-butyl) in the ortho- and para-positions, which through electrochemical oxidation generate a one-electron oxidized phenoxyl species in solution. This radical species was characterized by UV-Vis, EPR and electrochemical studies. The Zn(II)-phenoxyl radical species is of bioinorganic relevance, since its spectroscopic, redox and reactivity properties can be used to establish the role of phenoxyl radicals in biological and catalytical systems.     

Zinc(II) chlorido complexes of protonated kinetin and its derivatives: Synthesis, properties and X-ray structure of [Zn(Hkinetin)Cl3]·kinetin

The syntheses and characterization of five novel zinc(II) complexes with protonated kinetin (6-furfurylaminopurine) and its derivatives are described. Based on the results following from elemental analyses (C, H, N), FTIR, Raman, ¹H and ¹³C NMR spectroscopy, conductivity measurements, thermogravimetric (TG) and differential thermal analyses (DTA), and single crystal X-ray analysis, the complexes of the general composition [Zn(HLⁿ)Cl₃]·xLⁿ (1-5) have been prepared, where L¹ = kinetin (6-furfurylaminopurine), L² = 6-(5-methylfurfurylamino)purine, L³ = 2-chloro-6-furfurylaminopurine, L⁴ = 2-chloro-6-(5-methylfurfurylamino)purine and L⁵ = 2-chloro-6-furfurylamino-9-isopropylpurine, and x = 1/2-2. The structure of [Zn(HL¹)Cl₃]·L¹ (1) has been determined by single crystal X-ray analysis. The Zn(II) atom is tetrahedrally coordinated by three chlorido ligands and one N3-protonated organic molecule forming a ZnCl₃N donor set. The organic ligand L¹ is coordinated to the Zn(II) centre through the N7 atom of the purine moiety. NMR spectroscopic study confirmed the N3 and N7 atom to be the protonation, and coordination site, respectively.     

Synthesis and structures of pyridinecarboxylate-containing zinc(II) complexes in dien and tren system

Four new zinc(II) complexes [Zn(dien)(μ-nic)]₂(BPh₄)₂·2CH₃OH (1), {[Zn(dien)(isonic)]BPh₄}_n (2), [Zn(tren)(nic)]BPh₄ (3) and [Zn(tren)(isonic)]BPh₄ (4) (dien/tren = diethylenetriamine/triethylenetriamine, nic/isonic = nicotinate/isonicotinate anion) were synthesized and structurally characterized by IR, ¹H NMR and single crystal X-ray diffraction. In the zinc(II) complexes of dien, both nicotinate and isonicotinate connect the zinc(II) ions via N,O-bis-monodentate mode. Complex 1 contains a centrosymmetric dinuclear unit bridged by two nicotinate anions in anti-parallel way. Complex 2 is characterized by an infinite one-dimensional zigzag chain bridged by isonicotinate anion in an end-to-end mode. The Zn···Zn distance is 6.782 for 1 and 8.805 Å for 2. While in the complexes of tren, both 3 and 4 are mononuclear complexes with nicotinate and isonicotinate coordinated to zinc(II) ion through only one oxygen atom of their carboxylate groups. The zinc(II) ions in all of the four complexes are in a distorted trigonal bipyramidal geometry. Complex 3 forms a dinuclear unit and complex 4 forms an infinite 2D sheet structure through intermolecular H-bonds. In all of the crystal lattices, the counterions act to balance the electronic charge at the same time to construct different 3D structures through noncovalent interactions such as C-H···π, N-H···π and van der Waals interactions.     

Coordination properties of ethyl bis(pyridin-2-ylmethyl)phosphate ligand with copper and zinc chloride. X-ray crystal structure of Cu(II) complex

A new ethyl bis(pyridin-2-ylmethyl)phosphate (2-bis(pm)Ope) ligand has been synthesized and used for synthesis of copper(II) and zinc(II) complexes of the formula [MCl₂(2-bis(pm)Ope)] [M = Cu(II), Zn(II)]. Despite having the same general formula, Cu(II) and Zn(II) complexes are not isostructural. The Zn(II) complex is four coordinated (MCl₂N₂) forming probably tetrahedral structure whereas the Cu(II) complex of distorted square pyramidal geometry is five coordinated (MCl₂ON₂). The later compound not only coordinates by two nitrogen atoms of pyridine rings but also by the oxygen atom of pyridin-2-ylmethoxyl residue. The compound (2-bis(pm)Ope) has been obtained as the product of diethyl (pyridin-2-ylmethyl)phosphate’s (2-pmOpe) transestrification. The compounds have been identified and characterized by IR, far-IR, ¹H NMR, ³¹P NMR and elemental analyses. The crystal structure of copper(II) complex i.e. [CuCl₂(2-bis(pm)Ope)] has been determined by the X-ray diffraction method. The low temperature magnetic study reveals significant antiferromagnetic interaction between copper centers through the H-bond system.     

DNA binding, cleavage, and cytotoxic activity of the preorganized dinuclear zinc(II) complex of triazacyclononane derivatives

A preorganized cleft dinuclear zinc(II) complex of 2,6-bis(1-methyl-1,4,7-triazacyclonon-1-yl)pyridine 1 as an artificial nuclease was prepared via an improved method. The interactions of 1, 2 [1,4,7-triazacyclononane (TACN)], and their zinc(II) complexes with calf thymus DNA were studied by spectroscopic techniques, including fluorescence and CD spectroscopy. The results indicate that the DNA binding affinities of these compounds are in the following order: Zn(II)2 -1 > Zn(II) -2 > 1 > 2. The binding constants of the Zn (II)2 -1 and Zn(II)-2 complexes are 3.57 x 10(6) and 1.43 x 10(5) M(-1), respectively. Agarose gel electrophoresis was used to assess the plasmid pUC 19 DNA cleavage activities in the presence of the dinuclear Zn (II)2 -1 complex, which exhibits powerful DNA cleavage efficiency. Kinetic data for DNA cleavage promoted by the Zn(II)2 -1 complex under physiological conditions give the observed rate constant ( k obs) of 0.136 h(-1), which shows an 10(7)-fold rate acceleration over uncatalyzed supercoiled DNA. The comparison of the dinuclear Zn(II)2 -1 complex with the mononuclear zinc(II) complex of 1,4,7-triazacyclononane indicates that the DNA cleavage acceleration promoted by the Zn(II)2 -1 complex is due to the efficient cooperative catalysis of the two proximate zinc(II) cation centers. A hydrolytic mechanism of the cleavage process was suggested, and a preliminary study of the antitumor activity was also conducted.     

Syntheses, spectroscopies and structures of zinc complexes with malate

Reactions of zinc(II) ion with racemic malic acid (C₄H₆O₅ = H₃mal) result in the isolation of four new zinc(II) malato complexes: (NH₄)[Zn(R-H₂mal)₃] · H₂O (1), trans-[Zn(R-H₂mal)(S-H₂mal)(H₂O)₂] · 2H₂O (2), (NH₄)₂[Zn(R-Hmal)(S-Hmal)] · 2H₂O (3), and [Zn₂(R-Hmal)(S-Hmal)(H₂O)₄]_n · 2nH₂O (4). Three R-malic acids in 1 act as bidentate ligands via their alcoholic and the central carboxy groups with Zn(II) ion, leaving the terminal carboxylic acid groups free. The R- and S-malates of 2 coordinate in a bidentate manner with zinc ion in trans-form. In 3, Zn(II) ion is coordinated by R- and S-malates in a tridentate fashion via their alcoholic and two carboxy groups. Complex 4 forms a two-dimensional layered structure through the links of a new dimeric unit [Zn₂(R-Hmal)(S-Hmal)(H₂O)₄] with one of the oxygen atoms from the terminal carboxy group of malate ligand. The coordination of malates depends on pH variation, on Zn:malate ratio, and also on temperature. Tridentate chelation of malate in 3 is found between pH 4.5-9.0. The soluble monomeric species 1-3 have been investigated using ¹³C NMR spectra by long-time acquisition. The solution NMR spectra indicate that zinc malate complexes dissociate in H₂O (D₂O). Obvious downfield shifts of the central carboxy carbon atoms in 1-3 are observed compared with those of free malate, which indicate that these zinc malate complexes dissociate in aqueous solution.