Copper(II) complexes of N-terminal protected tri- and tetra-peptides containing histidine residues

Copper(II) complexes of peptides containing two or three histidyl residues (Ac-HisGlyHis-OH, Ac-HisGlyHis-NHMe, Ac-HisHisGlyHis-OH and Ac-HisHisGlyHis-NHMe) have been studied by potentiometric, UV-Vis, EPR and CD spectroscopic measurements. The imidazole nitrogen atoms are described as the primary metal binding sites of all ligands resulting in the formation of various macrochelates in the pH range 4 to 7. The (Nim, N-, Nim)-co-ordinated [CuH-1L]0+ complexes were mainly detected in samples containing free carboxylates at the C-termini, whilst the [CuH-2L]-(0) complexes were the predominant species in slightly alkaline solution and their binding modes were described via 4N-co-ordination (Nim, N-, N-, Nim) in (7,5,6)-membered fused chelate rings. Deprotonation and co-ordination of the third amide nitrogens were detected above pH approximately 9 in all cases.     

Palladium(II) complexes of oligopeptides containing aspartyl and glutamyl residues

Potentiometric and ¹H NMR spectroscopic measurements have been performed on palladium(II) complexes of di-, tri- and tetra-peptides containing aspartyl and glutamyl residues including AspAla, AlaAsp, AspAsp, AspAspAsp, GluGluGlu, GlyAspGly, AspAspAspAsp and GlyGlyAspGly. In the case of dipeptides the coordination modes are basically determined by the peptide backbone. The presence of the extra β-carboxylate residues does not result in new binding modes but these functions may slightly affect the thermodynamic stability of several species. For tripeptides the most important findings are connected to the governing role of the β-carboxylate group of internal aspartyl residues but similar effects were not observed for the peptides containing glutamic acid. Aspartic acid in the second position of a tripeptide (Xaa-Asp-Yaa sequence) promotes the binding of the preceding and prevents deprotonation and coordination of subsequent amide functions. In the case of tetrapeptides, the aspartyl residues present in the third position from the N-terminus (Xaa-Yaa-Asp-Zaa sequence) have the most pronounced effect on complex formation. In this case, the (NH₂, N⁻, N⁻, β-COO⁻)-coordination is the major binding mode and the species [PdH₋₂L] can exist in a wide pH range. The enhanced stability of these complexes was explained by the conformational changes of the coordinated ligands.     

Copper(II) complexes of terminally protected pentapeptides containing three histidyl residues in alternating positions, Ac-His-Xaa-His-Yaa-His-NH2

Copper(II) complexes of the pentapeptides Ac-HisAlaHisValHis-NH2, Ac-HisValHisAlaHis-NH2, Ac-HisProHisAlaHis-NH2, Ac-HisAlaHisProHis-NH2, Ac-HisGlyHisValHis-NH2 and Ac-HisValHisGlyHis-NH2 have been studied by potentiometric, UV-Vis, CD and EPR spectroscopic methods. It has been found that the pentapeptides are efficient ligands for the complexation with copper(II) and exhibit an outstanding versatility in the co-ordination geometry of complexes. The presence of three histidyl residues provides a high possibility for the formation of macrochelates via the exclusive binding of imidazole-N donor atoms. The macrochelation suppresses, but cannot preclude the deprotonation and metal ion co-ordination of amide functions and the species [CuH(-2)L] and [Cu2H(-4)L] predominate at physiological pH in equimolar solutions and in the presence of excess metal ions, respectively. It is also clear from the data that both C-terminal and internal histidyl residues can work as the anchoring sites for metal binding and subsequent amide deprotonation resulting in the formation of co-ordination isomers and dinuclear species in equimolar solutions and in the presence of excess metal ions, respectively. In more alkaline solutions (pH approximately 10) a third amide function can be deprotonated and co-ordinated in the species [CuH(-3)L]- with (N-,N-,N-,N(im)) co-ordination. The dinuclear species [Cu2H(-5)L]- and [Cu2H(-6)L](2-) containing hydroxide ions and/or imidazolato bridges are formed at high pH in the presence of excess of metal ions. The insertion of one proline into the sequence preceding histidyl residues hinders the deprotonation of amide functions at that site and the formation of only mononuclear complexes was observed with these peptides.     

Copper(II) complexes containing hemilabile tridentate ligand as chromotropic probes

Two copper(II) complexes with the general formula [Cu(L)(H₂O)](ClO₄)₂ (1) and [Cu(L)₂](ClO₄)₂ (2), where L=3-((pyridin-2-ylmethyl)amino)propanamide, were synthesized and characterized by elemental analyses, IR, UV–vis spectroscopy techniques and molar conductance measurements. The crystal structures of the complexes were identified by single crystal X-ray diffraction analysis. The tridentate ligand L acts as an N₂O-donor through the nitrogen atoms of the pyridine and amine moieties as well the oxygen atom of the amide group. The copper(II) ions in both complexes have distorted octahedron structures so that the Cu(II) ion in 1 is coordinated by an aqua ligand and a tridentate ligand defining the basal plane, and by two oxygen atoms of the perchlorate ions occupying the axial positions. However, two ligands L are coordinated to the copper(II) ion in 2, where four nitrogen atoms of pyridine and amine groups occupy the equatorial positions and two oxygen atoms of the amide moieties exist in the apices. The chromotropism (halo-, solvato- and ionochromism) of both complexes were studied using visible absorption spectroscopy. The complexes are soluble in water and organic solvents and display reversible halochromism. The solvatochromism property is due to structural change followed by solvation of the vacant sites of the complexes. The complexes demonstrated obvious ionochromism and are highly sensitive and selective towards CN^? and N₃^? anions in the presence of other halide and pseudo-halide ions.     

Copper(II) oxalate and oxamate complexes

Reaction of Cu^II salts with phenanthroline and oxalate (ox) or oxamate (oxm) gives [Cu(phen)(ox)(H₂O)] · H₂O or [Cu(phen)(oxm)(H₂O)] · H₂O complexes while direct treatment of Cu^II salts with oxalate or oxamate gives [NH₄]₂[Cu(ox)₂] and [Cu(oxm)₂(H₂O)₂] respectively. The X-ray structures of one example of each system, aquo-oxamato-phenanthroline-copper(II)-dihydrate and the polymeric ammonium-bis(aquo)-tetraoxalato-dicopper(II)-dihydrate, are reported.     

Copper(II) complexes with aroylhydrazones

The coordination chemistry of copper(II) with tridentate aroylhydrazones is briefly discussed in this article. Two types of aroylhydrazones derived from aroylhydrazines and ortho-hydroxy aldehydes or 2-pyridine-carboxaldehyde have been used. The characterization of the complexes has been performed with the help of various physico-chemical techniques. Solid state structural patterns have been established by X-ray crystallography. In the solid state, structural varieties of these complexes are seen to range from monomeric, dimeric, polymeric and one-dimensional self-assembly via hydrogen bonds and π-π interactions. EPR spectroscopy and variable temperature magnetic susceptibility measurements have been used to reveal the nature of the coordination geometry and magnetic characteristics of these complexes.     

Copper(II) and nickel(II) complexes of schiff bases

Summary The isolation of complexes of some nickel(II) and copper(II) salts with the Schiff base derived from 2-aminobenzimidazole and 4-methylbenzaldehyde (abimbz) is reported. They are of the general type M(abimbz)₂X₂ (M=Ni or Cu; X=Cl, Br, or ClO₄). The compounds have been characterized by elemental analyses, conductivity measurements, i.r., electronic and e.p.r. spectral studies and magnetic measurements. The i.r. spectra show that the ligand is bidentate through the tertiary nitrogen of the imidazole ring and the exocyclic imine nitrogen. Possible structures for the complexes are suggested.     

Nickel(II) and Copper(II) complexes of sulphamide and ethylenediamine

Summary The following ethylenediamine(sulphamide)nickel(II) and ethylenediamine(sulphamide)copper(II) complexes were prepared: Ni(en)₃(H₂Su)Cl₂, in which H₂Su is not coordinated; Ni(en)₂(H₂Su)Cl₂ , Ni(en)₂(HSu)Cl · EtOH, Ni(en)₂(HSu)₂,Ni(en)₂(HSu)₂(H₂Su)₂ · EtOH, Cu(en)₂(HSu)₂(H₂Su) · O.5EtOH, in which the sulphamide molecule, H₂Su, or the sulphamidato anion [HSu] are totally or partially coordinated. The comparison of the i.r. spectra of these complexes with those of H₂ Su and NaHSu and with those of the M(en)₂Cl₂ complexes permits the isolation of the sulphamide or sulphamidato (SO₂) and (SN₂) bands which show oxygen-coordination of these ligands to the metal. Some (MO) bands were identified in the far i.r. region.     

Copper(II) complexes of pyrrolidine dithiocarbamate

Copper(II)-pyrrolidine dithiocarbamate (PDTC) complexes having the general formula, [Cu(PDTC)₂], [Cu(PDTC)X₂]⁻ (where X = Cl⁻, I⁻, CN⁻, SCN⁻) and [Cu(PDTC)(en)]⁺ (en = ethylenediamine) have been prepared and characterized by IR spectroscopy and by thermogravimetric analysis (TGA and DTA). The IR data suggests that coordination of pyrrolidine dithiocarbamate (PDTC) takes place through the two sulphur atoms in a symmetrical bidentate fashion. The results of thermal analysis are consistent with the proposed composition of the complexes.     

Copper(II) complexes of sulfanilamide derivates

Summary Nine different Cu^II(sulf)₂ and three mixed Cu^II(sulf)₂X_2–3 (X=NH₃ or pyridine) derivatives were prepared from Cu^II and sulfanilamides (sulfH=sulfadiazine, sulfadimethoxine, sulfadimidine, sulfamerazine, sulfamethoxydiazine, sulfamethoxypyridazine, sulfapyridine, sulfathiazole and sulfisomidine) in alkaline solution and their e.s.r., i.r. and ligandfield spectroscopic properties were evaluated. Two types of Cu(sulf)₂ complexes were formed: dimeric and monomeric/ polymeric species.     

Polychlorophenyl-platinum(II) complexes containing triphenylphosphine

A new method to prepare compounds of the type trans-[PtCl(R)(PPh₃)₂] (R = C₆H₅; 2,5-C₆H₃Cl₂; 2,3,4-, and 2,4,6-C₆H₂Cl₃; 2,3,4,5-, 2,3,4,6- and 2,3,5,6-C₆HCl₄ and C₆Cl₅) by reaction of cis-[PtCl₂(PPh₃)₂] and HgR₂ in the molten state is described.The reactions of the complexes with HCl, Cl₂ and I₂ have been examined in order to give information about the relative ease of cleavage of the various Ptaryl bonds. The replacement of Cl by NCS suggests an associative mechanism even for the complexes in which the polychlorophenyl ligand has chlorine atoms in both ortho positions.     

Nickel(II) complexes of oligopeptides containing aspartyl and glutamyl residues. Potentiometric and spectroscopic studies

Nickel(II) complexes of di-, tri- and tetra-peptides built up from Asp and/or Glu residues have been studied by potentiometric, UV–Vis and circular dichroism spectroscopic methods. The stoichiometry of the complexes are the same as in the case of common oligopeptides, but the presence of the side chain carboxylate groups results in differences in their stabilities and coordination modes. The presence of the β-carboxylate groups increases the metal binding affinity of the peptides in all cases. This is due to the coordination of the first, second and third aspartic acid residue in the case of the NiL, NiH₋₁L and NiH₋₂L complexes, respectively. The high negative charge of Asp₄ suppresses the metal ion coordination of the third amide function, therefore the NiH₋₃L complex does not form with this tetra-peptide. In the case of peptides containing glutamic acid, no stability enhancement appears because there is only a weak interaction between the nickel(II) ion and the γ-carboxylate group, which is not able to compensate the disfavoured effect of the increasing negative charge of the complexes.     

Copper(II) and copper(I) complexes of benzenecarbothioamides

Summary Copper(II) salts react with benzenecarbothioamide(BCTA);N,N-dimethylbencenecarbothioamide (DMBCTA) andN,N-diethylbenzenecarbothioamide (DEBCTA) to give complexes with 11, 21, 31 ligand/metal stoichiometric ratios, and a diamagnetic complex [Cu₂(DEBCTA)Br₄]₂ which appears to contains copper(I). These compounds were characterized by elemental analyses, conductivities measurements, i.r., electronic and e.p.r. studies and magnetic measurements.The results suggest tetrahedral geometry for the copper(II) complexes, a dimeric structure for bromide-DMBCTA and chloride and bromide-BCTA derivatives, and a square pyramid geometry for the CuBr₂-DEBCTA complex. No information is yet available on the coordination geometry of the copper(I) complex.     

Copper(II) and ruthenium(II)/(III) Schiff base complexes

Metal complexes of general formula [Cu(L)](ClO4)2, [Ru(L)(PPh3)2]Cl2 and [Ru(L)(PPh3)Cl]Cl2[L = 1,4-di- (o-benzylidiminophenoxy/benzylidiminophenylthio)butane] containing N2O2 or N2S2 donor atoms have been prepared and characterised by spectral, magnetic and cyclic voltammetric studies. The rhombic nature of the e.s.r. spectra of the RuIII complexes indicates an asymmetry in the electronic environment around the Ru atom. e.s.r. spectra of the CuII complexes show a typical four-line spectrum with approximate tetrahedral distortion. The observed low A values in the CuII complexes, of the order of 132–160 × 10–4cm–1, indicates a tetrahedrally distorted square planar structure.The influence of modified ligands is reflected in the metal-centered redox potentials. CuII complexes having the N2S2 chromophore, in MeCN on a glassy carbon electrode, undergo quasi-reversible reduction in the 540–680 mV range. A depression in E1/2 values for the open chain N2S2 chromophoric macrocyclic CuII complexes, compared to electronically similar cyclic tetradentate CuII analogues, is due to the increased stabilization of the CuI state by added flexibility provided through the open chain donor sites.     

Copper(II) and zinc(II) complexes with 2-formylpyridine-derived hydrazones

In the present work 2-formylpyridine-para-chloro-phenyl hydrazone (H2FopClPh) and 2-formylpyridine-para-nitro-phenyl hydrazone (H2FopNO₂Ph) were obtained, as well as their copper(II) and zinc(II) complexes [Cu(H2FopClPh)Cl₂] (1), [Cu(2FopNO₂Ph)Cl] (2), [Zn(H2FopClPh)Cl₂] (3) and [Zn(H2FopNO₂Ph)Cl₂] (4). Upon re-crystallization in DMSO:acetone conversion of 2 into [Cu(2FopNO₂Ph)Cl(DMSO)] (2a) and of 4 into [Zn(2FopNO₂Ph)Cl(DMSO)] (4a) occurred. The crystal structures of 1, 2a, 3 and 4a were determined.     

Copper(II) complexes of rat amylin fragments

The fragments of rat amylin rIAPP(17-29) (Ac-VRSSNNLGPVLPP-NH(2)), rIAPP(17-22) (Ac-VRSSNN-NH(2)), rIAPP(19-22) (Ac-SSNN-NH(2)) and rIAPP(17-20) (Ac-VRSS-NH(2)) together with the related mutant peptides (Ac-VASS-NH(2) and Ac-VRAA-NH(2)) have been synthesized and their copper(II) complexes studied by potentiometric, UV-Vis, CD and EPR spectroscopic methods. Despite the lack of any common strongly coordinating donor functions some of these fragments are able to bind copper(II) ions in the physiological pH range. The longest fragment rat amylin(17-29) keeps one equivalent copper(II) ion in solution in the whole pH range, while two other peptides Ac-VRSSNN-NH(2) and Ac-SSNN-NH(2) are also able to interact with copper(II) ions in the slightly alkaline pH range. According to the spectral parameters of the complexes, the peptides can be classified into two different categories: (i) the tetrapeptides Ac-VRSS-NH(2), Ac-VASS-NH(2) and Ac-VRAA-NH(2) can interact with copper(II) only under strongly alkaline conditions (pH > 10.0) and the formation of only one species with four amide nitrogen coordination can be detected; (ii) the peptides Ac-VRSSNNLGPVLPP-NH(2), Ac-VRSSNN-NH(2) and Ac-SSNN-NH(2) can form complexes above pH 6.0 with the major stoichiometries [CuH(-2)L], [CuH(-3)L](-) and [CuH(-4)L](2-). These data support that rIAPP(17-29) can interact with copper(II) ions under physiological conditions and the SSNN tetrapeptide fragment can be considered as the shortest sequence responsible for metal binding. Density functional theory (DFT) calculations provide some information on the possible coordination modes of Ac-SSNN-NH(2) towards the copper(II) ion and suggest that for [CuH(-2)L], [CuH(-3)L](-) and [CuH(-4)L](2-), the binding of two, three and four deprotonated amide nitrogens, with NH(-) of the side chain of asparagine as anchoring group, is probable. Moreover, these data reveal that peptides can be effective metal binding ligands even in the absence of anchoring groups, if more polar side chains are present in a specific sequence.     

Copper(I) and copper(II) complexes of diamino-dithioether ligands

Summary Copper(II) salts were reacted with two diamino-dithioether ligands, i.e. 1,3-di(o-aminophenylthio)propane (abbreviated H₂L¹) and 1,2-di(o-aminophenylthio)xylene (abbreviated H₂L²). Mixtures of copper(I) and copper(II) complexes were obtained with Cl^– and ClO ₄ ^– counterions. The major products were the copper(I) complexes, which were obtained pure after recrystallisation from MeCN-MeOH. The ligands lose two protons from the amine functions to form copper(I) complexes of general formula [CuL]X, where X = ClO ₄ ^– or Cl^–. The complexes were oxidised to [CuL]X₂ with H₂O₂ in DMF. Cu(NO₃)₂ on the other hand gave [CuH₂LNO₃]NO₃.