Coordination Of Amide Nitrogens Research Articles

Coordination, redox properties and SOD activity of Cu(II) complexes of multihistidine peptides

The results of electrochemical and SOD activity measurements of such copper(II) complexes of terminally protected multihistidine peptides that may mimic the active site of CuZnSOD enzyme are submitted and completed with solution equilibrium studies of some copper(II)-ligand systems. The equilibrium data confirm that the thermodynamic stabilities increase with the increasing number of histidyl residues in the amino acid sequence, the stability order, however, can be finely tuned by the number and quality of amino acids between histidine residues. Based on the cyclic voltammetric studies we concluded that the formal reduction potential values of imidazole nitrogen coordinated complexes decrease with the increasing number of imidazole donor atoms in the coordination sphere. However, the redox parameters of [CuH−1L]+ and [CuH−2L] complexes containing amide nitrogen coordination can be determined as well. All formal potential values of [CuL]2+, [CuH−1L]+ and [CuH−2L] complexes fall in the middle potential range of SOD activity. Finally, after the detailed analysis of species distribution curves based upon the equilibrium data SOD activity of copper(II) containing systems at two pH (pH=6.8 and 7.4) were determined. The imidazole coordinated [CuL]2+ complexes of the multihistidine peptide containing the HXH sequence exhibit the most significant activity, but the presence of amide nitrogen coordinated species with slightly distorted geometry could considerably contribute to the SOD activity.

Transition metal complexes of short multihistidine peptides

Nickel(ii), cobalt(ii) and cadmium(ii) complexes of terminally protected multihistidine peptides including Ac-HGH-OH, Ac-HGH-NHMe, Ac-HHGH-OH, Ac-HAHVH-NH(2), Ac-HVHGH-NH(2), Ac-HGHVH-NH(2) and Ac-(His-Sar)(n)-His-NH(2) (n = 1, 2 or 3) were studied by potentiometric, UV-Vis, CD and (1)H NMR spectroscopic techniques. It was found that the complexes in which the histidine imidazole nitrogens coordinate with ML stoichiometry are the main species in the physiological pH-range in all cases. The stability of these complexes is determined by the number of bound imidazole rings, the presence of the carboxylate group and the quality of the metal ion centre. The larger the number of coordinated imidazole-N donor atoms, the higher the stability of the complex. The stability constants of the ML complexes follow the Ni(ii) > Co(ii) approximately Cd(ii) order. Cobalt(ii) and cadmium(ii) are not, but nickel(ii) is able to promote the deprotonation and the coordination of amide nitrogens and NiH(-2)L and NiH(-3)L (Ni(2)H(-4)L) species predominate in basic solutions. For the pentapeptides with the exception of the sarcosine containing ligand the presence of coordination isomers is supported by spectroscopic methods. These data reveal that the favoured isomers are coordinated on the C-termini, but the ratio of isomers depends on the sequence of peptides.

Approaching the minimal metal ion binding peptide for structural and functional metalloenzyme mimicking

The peptides Ac-His-Pro-His-Pro-His-NH(2) (L1) and Ac-Lys-His-Pro-His-Pro-His-Gln-NH(2) (L2) have been prepared and the equilibria of their proton, copper(II) and zinc(II) complexes in aqueous solution have been studied by the combination of pH-potentiometric titrations, UV/visible and circular dichroism (CD) spectroscopy. The latter methods also provided information on solution structure of the complexes formed under different conditions. Both ligands formed complexes with three imidazole nitrogens around the metal ion at pH ~7. In the L1 containing system precipitation of either copper(II) or zinc(II) complexes occurred upon slight increase of the pH. The re-titration of the filtered and acidified precipitates revealed that the insoluble materials were neutral complexes rather than metal-hydroxides. Indeed, by attaching amino acids with polar side-chains to the His-Pro-His-Pro-His template in L2 we could prevent any precipitation, and the soluble complexes around pH ~7 exerted three imidazole nitrogens and a (deprotonated) water molecule around the metal ions. To our knowledge L2 provides the first example of a short peptide preventing both the amide nitrogen coordination in copper(II) and the formation of copper(II) and zinc(II) hydroxides. The zinc(II) and copper(II) complexes at pH ~7 having similar structure to the natural hydrolytic and redox enzymes, respectively, showed considerable activity in hydrolytic cleavage assays with a model substrate (2-hydroxypropyl-4-nitrophenyl phosphate), as well as with native plasmid DNA, and in a superoxide dismutase-like reaction.

Potentiometric and spectroscopic studies on the copper(II) and zinc(II) complexes of bis(imidazol-2-yl) derivatives of tripeptides

Copper(II) and zinc(II) complexes of tripeptides containing chelating bis(imidazol-2-yl)methyl agents (BIM) at the C-termini (GlyIleGly-BIMA, AlePheGly-BIMA and GlyGlyHis-BIMA) were studied by potentiometric, UV–Vis, EPR, MALDI-MS and 1H NMR techniques. The imidazole nitrogen donor atoms were described as the primary metal binding sites forming protonated mono- and bis(ligand) complexes in the acidic pH range. The formation of stable mononuclear ML and ML 2 complexes was detected following the deprotonation of the terminal amino group. In these complexes the ligands are bound to the metal ions at least tridentately, resulting in macrochelates. In the copper(II) containing system the deprotonation and coordination of amide nitrogens take place at higher pH. For GlyIleGly-BIMA, polynuclear species exist in basic solution with participation of both binding sites of the molecules. For GlyGlyHis-BIMA the “GlyGlyHis” like coordination was able to replace the bis(imidazol-2-yl)methyl residue in the coordination sphere of the copper(II) ion, and the [CuH −2L] complex was predominant above pH 9. The formation of stable ML and ML 2 complexes in the zinc(II)-GlyGlyHis-BIMA system hinders the hydrolysis of the metal ion, but the deprotonation and coordination of the amide nitrogen was not detected.

The reactivity of N-coordinated amides in metallopeptide frameworks: molecular events in metal-induced pathogenic pathways?

The amino acid derived tertiary amide ligand tert-butoxycarbonyl-(S)-alanine-N,N-bis(picolyl)amide (Boc-(S)-Ala-bpa, 1) has been synthesized as a model for metal-coordinating peptide frameworks. Its reactions with copper(II) and cadmium(II) salts have been studied. Binding of Cu2+ results in amide bond cleavage and formation of [(bpa)(solvent)Cu]2+ complexes. In contrast, the stable, eight-coordinate complex [(Boc-(S)-Ala-bpa)Cd(NO3)2] (5) has been isolated and characterized by X-ray crystallography. An unusual tertiary amide nitrogen coordination is observed in 5; this gives rise to significantly reduced cis-trans isomerization barriers. Possible implications for metal-induced conformational changes in proteins are discussed.

SYNTHETIC AND SPECTROSCOPIC ASPECTS OF TETRAAZAMACROCYCLIC COMPLEXES OF PALLADIUM(II)

Palladium(II) complexes of the general composition [PdMacL]Cl2 (MacL = macrocyclic ligand) containing 14–18 membered rings have been synthesized by the template condensation of the aliphatic diamines H2N(CH2)nNH2 (n = 2 or 3) with the dicarboxylic acids HOOC-(CH2)m-COOH (m = 1 or 2) in methanol. A square-planar environment of the ligand around the metal ion has been suggested on the basis of chemical analyses, infrared and 1H NMR spectral studies, electronic spectra, molecular weight determinations and conductivity measurements. Spectroscopic studies confirm coordination of amide nitrogens to the central metal ion. The molar conductance values of the palladium(II) complexes show that they are 1:2 electrolytes.

The effect of histidyl residues on the complexation of bis(imidazolyl) containing tripeptides with copper(II) ion

Copper(II) complexes of tripeptide derivatives of bis(imidazol-2-yl) group have been studied by potentiometric, UV–visible and EPR spectroscopic methods. The peptide molecules correspond to the amino acid sequence of collagen containing histidyl residues in different locations and were connected to the bis(imidazol-2-yl) group either on the C-termini (BOC–Pro–Leu–His–BIMA, BOC–His–Leu–Gly–BIMA) or on the N-termini (BIP–His–Ala–Gly–OEt, BIP–Ile–Ala–His–OMe). It was concluded that the imidazole nitrogen donor atoms of the bis(imidazol-2-yl) moiety are the primary metal binding sites, but the histidyl imidazole nitrogens in the side chains have also some effect on the stability and the coordination mode of the complexes. All ligands can coordinate tridentately to copper(II) ion forming a six-membered chelate and a macrochelate in the [CuL] 2+ complexes, which results in a slight distortion in the coordination geometry of [CuL 2] 2+ complexes. The deprotonation and coordination of amide nitrogens, however, were not observed in any cases.

Amide nitrogen co-ordination of CoIIand NiIIin ternary 2,2′-bipyridine-containing systems. A solution and solid-state study

The ternary systems 2,2′-bipyridine (bipy)–M–amino acid [M = CoII or NiII; amino acid (H2L)=N-p-tolyl-sulfonylglycine (tsgly), -β-alanine (ts-β-ala) or N-benzoylglycine (H2hip)] were investigated in aqueous solution by means of polarography, electronic spectroscopy and potentiometry, in order to identify the type, number and stability of complex species, as a function of pH and bipy : M : amino acid ratio. With tsgly the prevailing species are [MII(bipy)n(HL)]+ and [MII(bipy)nL](n= 1 or 2), with pKNH for the deprotonation of sulfonamide nitrogen 7.8(2) and 7.4(2), for CoII and NiII, respectively and the crystal and molecular structure of [Co(bipy)2(tsglyNO)]·2H2O (NO indicates binding as a N,O-chelating dianion) was determined. The CoII is six-co-ordinated by four nitrogen atoms of the bipy moieties, the deprotonated sulfonamide nitrogen and one carboxylic oxygen of the tsgly dianion. On standing in air, an alkaline solution of bipy–CoII–tsgly turned from orange to wine-red in a few days, indicating the oxidation of CoII to CoIII. With ts-β-ala and H2hip metal hydrolysis prevents the formation of deprotonated complexes.

Unusual binding ability of α,β-dehydrodipeptides towards metal ions

Ni II, Zn II and Co II complexes of α,β-dehydro-dipeptides (containing Gly, Leu, Ala, Val or Phe residues) were studied by potentiometric and spectroscopic methods. Deprotonation and coordination of amide nitrogens occurred in all cases around the physiological pH range. The dipeptides with composition of Xaa-Δ-Ala formed octahedral species, while Gly-Δ-Xaa (Xaa = Leu or Phe) formed square planar bis complexes with Ni II.

Deprotonated amide nitrogen co-ordination to the cadmium(II) ion in ternary 2,2′-bipyridine complexes with N-sulfonyl amino acids

A solid-state investigation of cadmium(II) and zinc(II) ternary 2,2′-bipyridine (bipy), complexes of N-(benzenesulfonyl)glycine (bsgly) and N-(toluene-p-sulfonyl)glycine (tsgly) is presented. The crystal structure of [{Cd(bipy)(tsglyO)2}2]1, separated from preparations at pH 7, and of [{Cd(bipy)(bsglyNO)}4]·8H2O 2 separated at pH 11 are described. Complex 1 crystallizes in the triclinic space group P, Z= 2, a= 10.474(10), b= 15.902(3), c= 18.202(9)A, α= 98.49(2), β= 91.17(5) and γ= 98.31(3)°. The structure consists of dimeric units in which the cadmium atoms are bridged by two carboxylic oxygens. Metal co-ordination is via the two nitrogens of a bipy molecule and four carboxylate oxygens from the amino acid monoanion. A long Cd ⋯ O contact completes seven co-ordination. The final R index was 0.0509. Complex 2 crystallizes in the monoclinic space group P21/c, Z= 4, a= 12.525(5), b= 15.251(4), c= 21.717(3)A and β= 92.36(3)°. The structure is built up of tetrameric units in which the cadmium atoms are joined by monoatomic bridging carboxylic oxygens. Each cadmium is six-co-ordinated through bipy nitrogens and sulfonamide nitrogen and carboxylate oxygen of the amino acid molecules. The final R index was 0.0398. Infrared data for the homologous zinc(II) complexes, very similar to those of the above derivatives, indicate that zinc is also able to promote sulfonamide nitrogen deprotonation in these amino acid ligands in the solid state.

Formation and spectral characterization of Cu(II)-Poly(L-Ornithine) complexes

Cu(II)-Poly-(l-ornithine) complexes in aqueous solution have been studied using potentiometric titration and absorption and circular dichroism spectra. As in the case of Cu(II)-poly(L-arginine) complexes studied previously, two types of compounds have been detected, labeled complexes I and II. Complex I contains two amine nitrogens and two water molecules coordinated to the copper. Complex II, two amine and two amide nitrogens. Amide nitrogen coordination confers optical activity to the copper d-d transitions. Furthermore, amine and amide nitrogen coordination to the copper are characterized by charge transfer transitions at 250 and 320 nm respectively which were already identified in Cu(II)-poly(L-arginine) systems.