Copper(II) complexes of Neobelliera Bullata Trypsin Modulating Oostatic Factor and its analogues

The stoichiometry, stability constants and solution structure of the complexes formed in the reaction of copper(II) with hexapeptide NPTNLH, i.e. the Neobelliera Bullata Trypsin Modulating Oostatic Factor (Neb-TMOF), and its analogues DPTNLH, Ac-NPTNLH and Ac-DPTNLH have been determined by potentiometric, UV-visible, CD and EPR spectroscopic methods. Upon raising pH for Ac-NPTNLH and Ac-DPTNLH peptides, copper(II) coordination starts from the imidazole nitrogen of the His⁶; afterwards three deprotonated amide nitrogens are progressively involved in metal ions coordination. In a wide pH range of 4.5-8.5 for the NPTNLH and DPTNLH ligands the CuL complex dominates with the imidazole nitrogen of His⁶ coordinated to form a macrochelate. The N-terminal amino group of the NPTNLH and DPTNLH peptides takes part in the coordination of the metal ion in the CuL, CuH₋₁L and CuH₋₂L complexes. However, at pH above 9 the CuH₋₃L complex with the {N_Im, 3N⁻} coordination mode is formed. For the CuH₋₂L complex the spectroscopic data clearly indicate the 4N {NH₂, CO or COO⁻, 2N⁻, N_Im} bonding mode with the axial coordination of the N-terminal amine group to the metal ion.     

Coordination abilities of neurokinin A and its derivative and products of metal-catalyzed oxidation

The classical tachykinins, substance P, neurokinin A and neurokinin B are predominantly found in the nervous system where they act as neurotransmitters and neuromodulators. Significantly reduced levels of these peptides were observed in neurodegenerative diseases and it may be suggested that this reduction may also result from the copper(II)-catalyzed oxidation. The studies of the interaction of copper(II) with neurokinin A and the copper(II)-catalyzed oxidation were performed. Copper(II) complexes of the neurokinin A (His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH₂) and acetyl-neurokinin A (Ac-His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH₂) were studied by potentiometric, UV-Vis (UV-visible), CD (circular dichroism) and EPR spectroscopic methods to determine the stoichiometry, stability constants and coordination modes in the complexes formed. The histidine residue in first position of the peptide chain of neurokinin A coordinates strongly to Cu(II) ion with histamine-like {NH₂, N_Im} coordination mode. With increasing of pH, the formation of a dimeric complex Cu₂H₂L₂ was found but this dimeric species does not prevent the deprotonation and coordination of the amide nitrogens. In the Ac-neurokinin A case copper(II) coordination starts from the imidazole nitrogen of the His; afterwards three deprotonated amide nitrogens are progressively involved in copper coordination. To elucidate the products of the copper(II)-catalyzed oxidation of the neurokinin A and Ac-neurokinin A, liquid chromatography-mass spectrometry (LC-MS) method and Cu(II)/hydrogen peroxide as a model oxidizing system were employed.Oxidation target for both studied peptides is the histidine residue coordinated to the metal ions. Both peptides contain Met and His residues and are very susceptible on the copper(II)-catalyzed oxidation.     

The role of terminal amino group and histidine at the fourth position in the metal ion binding of oligopeptides revisited: Copper(II) and nickel(II) complexes of glycyl-glycyl-glycyl-histamine and its N-Boc protected derivative

Copper(II) and nickel(II) binding properties of two pseudo tetrapeptides, N-Boc-Gly-Gly-Gly-Histamine (BGGGHa) and Gly-Gly-Gly-Histamine (GGGHa) have been investigated by pH-potentiometric titrations, UV-visible-, EPR-, NMR- and ESI-HRMS (electrospray ionization high resolution MS) spectroscopies, in order to compare the role of N-terminal amino group and imidazole moiety at the fourth position in the complex formation processes. Substantially higher stabilities were determined for the ML complexes of GGGHa, compared to those of BGGGHa, supporting the coordination of the terminal amino group and the histamine imidazole of the non-protected ligand. A dimeric Cu₂H_− 2L₂ species, formed through the deprotonation of peptide groups of the ligands, was found in the GGGHa-copper(II) system. Deprotonation and coordination of further amide nitrogens led to CuH_− 2L and, above pH ~ 10, CuH_− 3L. Experimental data supports a {NH₂,2 × N_amide,N_im} macrochelate structure in CuH_− 2L whereas a {NH₂,3 × N_amide} coordination environment in CuH_− 3L. The first two amide deprotonation processes were found to be strongly cooperative with nickel(II) and spectroscopic studies proved the transformation of the octahedral parent complexes to square planar, yellow, diamagnetic species, NiH_− 2L and above pH ~ 9, NiH_− 3L. In the basic pH-range deprotonation and coordination of the amide groups also took place in the BGGGHa containing systems, leading to complexes with a {3 × N_amide,N_im} donor set, and in parallel the re-dissolving of precipitate. Above pH ~ 11, a further proton release from the pyrrolic NH group of the imidazole ring of BGGGHa occurred providing an additional proof for the different binding modes of the two ligands.     

Copper effective binding with 32-62 and 94-125 peptide fragments of histone H2B

In an attempt to investigate the role of histone H2B in Cu(II) induced toxicity and carcinogenesis, we synthesized the terminally blocked peptides H2B_32-62 (SRKESYSVYVYKVLKQVH₄₈PDTGISSKAMGIM) and Η2Β_94-125 (IQTAVRLLLPGELAKH₁₁₀AVSEGTKAVTKYTSS), mimicking the N-terminal histone-fold domain and C-terminal tail of histone H2B, respectively and studied their interaction with Cu(II) ions by means of potentiometric titrations and spectroscopic techniques (UV-visible, CD and EPR). Both peptides, H2B_32-62 and H2B_94-125, interacted efficiently with Cu(II) ions, forming several species from pH 4 to 11, with His₄₈ and His₁₁₀ serving as anchors for metal binding. In H2B_32-62, the effective Cu(II) binding is emphasized by the formation of a soluble Cu(II)-H2B_32-62 complex, unlike the unbound peptide that precipitated over pH 7.9. At physiological pH, both peptides form tetragonal 3N species with a {N_Im, 2N⁻} coordination mode. At this pH, H2B_32-62 presented the formation of coordination isomers, differentiated by the presence in one of them, of an axial coordination of the carboxylate group of Asp₅₀. Copper binding with both H2B_32-62 and H2B_94-125 may induce a conformational change in the peptides' original structure. At physiological conditions, this effect may interfere with nucleosome's structure and dynamics, including the ubiquitination of Lys₁₂₀ which is linked to gene silencing.     

Nickel(II) complexes of the multihistidine peptide fragments of human prion protein

Nickel(II) complexes of the peptide fragments of human prion protein containing histidyl residues both inside and outside the octarepeat domain have been studied by the combined application of potentiometric, UV-visible and circular dichroism spectroscopic methods. The imidazole-N donor atoms of histidyl residues are the exclusive metal binding sites below pH 7.5, but the formation of stable macrochelates was characteristic only for the peptide HuPrP(76-114) containing four histidyl residues. Yellow colored square planar complexes were obtained above pH 7.5-8 with the cooperative deprotonation of three amide nitrogens in the [N_im,N⁻,N⁻,N⁻] coordination mode. It was found that the peptides can bind as many nickel(II) ions as the number of independent histidyl residues. All data supported that the complex formation processes of nickel(II) are very similar to those of copper(II), but with a significantly reduced stability for nickel(II), which shifts the complex formation reactions into the slightly alkaline pH range. The formation of coordination isomers was characteristic of the mononuclear complexes with a significant preference for the nickel(II) binding at the histidyl sites outside the octarepeat domain. The results obtained for the two-histidine fragments of the protein, HuPrP(91-115), HuPrP(76-114)H85A and HuPrP(84-114)H96A, made it possible to compare the binding ability of the His96 and His111 sites. These data reveal a significant difference in the nickel(II) and copper(II) binding sites of the peptides: His96 was found to predominate almost completely for nickel(II) ions, while the opposite order, but with comparable concentrations, was reported for copper(II).     

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

A histidine-rich peptide HSHRDFQPVLHL-NH₂ (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 {NH₂, 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 {NH₂, 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.     

Coordination abilities of substituted β-aminophosphonates towards Cu and Zn ions

The formation constants of equimolar and bis-chelate copper(II) and zinc(II) complexes with three aliphatic and four aromatic-substituted β-aminophosphonates have been determined in water solution by potentiometric studies. Spectroscopic parameters clearly indicate involvement of {NH₃, PO₃ ²⁻} in both metal ions coordination. The comparison of the stability constants reveals slightly higher coordination power of the aliphatic-substituted β-aminophosphonic acids, which may be due to the higher basicity of their amino groups. All studied ligands are more effective in Cu²⁺ and Zn²⁺ coordination than phosphonic analogue of simple β-amino acid.     

Thermodynamic and structural characterization of the macrochelates formed in the reactions of copper(II) and zinc(II) ions with peptides of histidine

Copper(II) complexes of the peptides Ac-HisSarHis-NH₂, Ac-HisSarHisSarHis-NH₂ and Ac-HisSarHisSarHisSarHis-NH₂ have been studied by potentiometric, UV-Vis, CD and EPR spectroscopic methods. Stability constants for the corresponding zinc(II) complexes have also been reported. The formation of M(II)-2N_im, M(II)-3N_im and M(II)-4N_im bonded macrochelates was suggested in the pH range 5-7. The macrochelates were, however, not stable enough to prevent metal ion hydrolysis in slightly alkaline solutions. In the case of copper(II) complexes, the metal ion promoted deprotonation and coordination of the amide groups of histidyl residues were also suggested. The stability constants of macrochelate complexes were compared to the literature data reported for the macrochelates of the other peptides of histidine. It was found that the thermodynamic stability of macrochelate species is largely influenced by the number and location of histidyl residues in the peptide backbone. The highest stability was obtained for the HXHYH-type sequences, while the distant arrangement of histidyl residues resulted in a significant reduction of the stability constants.     

Impact of ring size on the copper(II) coordination abilities of cyclic tetrapeptides

A new, 14-membered, tetraza cyclic tetrapeptide containing histidine and lysine side-chains, c(β³homoLysdHisβ-AlaHis), was designed, synthesized and characterized; its copper(II) binding properties were investigated in dependence of pH by potentiometric and spectroscopic methods. In line with previous studies of similar systems, the progressive involvement of amide nitrogens in copper(II) coordination was evidenced for pH values greater than 6. At physiological pH the dominant species consists of a copper(II) center coordinated by two amide nitrogens, an imidazole nitrogen and a water molecule. In contrast, at pH values higher than 8.7, a copper(II) coordination environment consisting of four amide nitrogens in the equatorial plane and the axial imidazole ligands is formed as clearly indicated by spectroscopic data and theoretical calculations. The behavior of this 14-membered cyclic tetrapeptide is compared to that of its 12-membered cyclic analog, particular attention being paid to the effects of ring size on the respective copper(II) binding abilities.     

Inhibitory Effect of Alloferons in Combination with Human Lymphocytes on Human Herpesvirus 1 (HHV-1) Replication In Vitro

Over the past two decades there has been intense study of compounds from vertebrates, microorganisms, plants, mushrooms, marine sponges, worms, etc. as well as insects in terms of their antiviral activity. Insects produce a variety of biologically active peptides. One of them is alloferon. The in vitro and in vivo experiments demonstrate that synthetic alloferon has an immunomodulatory properties. It was reported that alloferon and its analogues (alloferon I and II) have antimicrobial properties, as well. The aim of this study was to evaluate in vitro the effect of alloferon I and II, either alone or in combination with human lymphocytes, on human herpesvirus type 1 (HHV-1) McIntyre strain replication. On the base of results we can conclude that alloferon I and II inhibit the replication of HHV-1 McIntyre strain in HEp-2 cells. Enhanced antiviral activity was observed when infected cells were treated with alloferons and unstimulated or phytohemagglutinin PHA-stimulated lymphocytes simultaneously. After application of alloferons and PHA-stimulated lymphocytes to the HHV-1 infected HEp-2 culture, the mean HHV-1 titer reduction for alloferon and II, when used at the highest dose—400 µg/mL, were 3.69 and 3.27 log₁₀/TCID_50/mL, respectively.     

Impact of histidine residue on chelating ability of 2'-deoxyriboadenosine

Copper(II) complexes with a new chelator-type nucleoside-histidine modified 2′-deoxyriboadenosine (N-[(9-β-D-2′-deoxyribofuranosylpurin-6-yl)-carbamoyl]histidine) were studied by potentiometric and spectroscopic (UV-visible, CD, EPR) techniques, in conjunction with computer modeling optimization. The ligand can act as bidentate or tridentate depending on pH range. In acidic pH a very stable dimeric complex Cu₂L₂ predominates with coordination spheres of both metal ions composed of oxygen atoms from carboxylic groups, one oxygen atom from ureido group and two nitrogen atoms derived from purine base and histidine ring. Above pH 5, deprotonation of carbamoyl nitrogens leads to the formation of CuL₂, Cu₂L₂H_− 1 and Cu₂L₂H_− 2 species. The CuL₂H_− 1 and CuL₂H_− 2 complexes with three or four nitrogens in Cu(II) coordination sphere have been detected in alkaline medium.Our findings suggest that N-[(9-beta-D-2′-deoxyribofuranosylpurin-6-yl)-carbamoyl]histidine chelates copper(II) ions very efficiently. The resulting complex might be used as an alternative base-pairing mode in which hydrogen-bonded base pairs present in natural DNA are replaced by metal-mediated ones.     

The cyclopeptides with the multi-His motif as ligands for copper(II)

The coordination properties of cyclic octapeptides with multi-His motif: c(His-Gly-His-Xaa-His-Gly-His-Xaa) where Xaa = Asp or Lys, were investigated. The binding abilities of this peptides towards Cu(II) ions were studied by using different analytic methods as: potentiometry, spectroscopy and mass spectrometry. The obtained results show that the studied peptides in physiological related pH prefer formation of the species with the {4N_Im} binding mode. The efficiency of Cu(II) binding depends on additional side chain groups Asp or Lys. Additionally the analysis of results for His containing cyclopeptides with different numbers of amino acid residues in cyclopeptide ring e.g. four, eight shows that in higher pH in both cases the binding by four amide nitrogens is not observed in the case of α-amino acid peptides.     

Copper and nickel binding in multi-histidinic peptide fragments

Multi-histidinic peptides have been investigated for Cu(II) and Ni(II) binding. We present spectroscopic evidence that, at low pH and from sub-stoichiometric to stoichiometric amounts of metals, macrochelate and multi-histidinic Cu(II) and Ni(II) complexes form; but, from neutral pH and above, both copper and nickel bind to individual histidine residues. NMR, EPR, UV–Visible (UV–Vis) and UV–Visible CD spectroscopy were used to understand about the variety of complexes obtained at low pHs, where amide deprotonation and coordination is unfavoured. A structural transition between two coordination geometries, as the pH is raised, was observed. Metal binds to N_δ of histidine imidazole when main-chain coordination is involved and coordinates via N_ε under mildly acidic conditions and sub-stoichiometric amounts of metals. From EPR results a distortion from planarity has been evidenced for the Cu(II) multi-histidinic macrochelate systems, which may be relevant to biological activity. The behaviour of our peptides was comparable to the pH dependent effect on Cu(II) coordination observed in octapeptide repeat domain in prion proteins and in amyloid precursor peptides involved in Alzheimer’s disease. Changes in pH and levels of metal affect coordination mode and can have implications for the affinity, folding and redox properties of proteins and peptide fragments.     

Hydrothermal synthesis and structure of a Cu(I) bromide coordination polymer, [(tpyprz)3Cu10Br10] (tpyprz = tetra-2-pyridylpyrazine)

The hydrothermal reaction of CuBr₂ and tpyprz in the presence of NH₄VO₃ and HF for 72 h at 170 °C provided [(tpyprz)₃Cu₁₀Br₁₀] (1) in 20% yield. The two-dimensional structure of 1 may be described as Cu(I)-tpyprz chains, linked through {Cu₄Br₅}⁻ clusters in the ac-plane and decorated with {Cu₃Br₅}²⁻ clusters projecting from one face of the layer in the b-direction. The Cu(I) sites exhibit distorted trigonal coordination {CuBr₃} and distorted tetrahedral geometries, {CuBr₂N₂} and {CuN₄}. Crystal data for 1: monoclinic space group C2, a = 12.7561(8) Å, b = 19.359(1) Å, c = 15.860(1) Å, β = 97.178(1)°, V = 3885.8(4) Å³, Z = 2, D_calc = 2.222 g cm⁻³, μ(Mo Kα) = 78.75 cm⁻¹.     

Decomposition of reactive oxygen species by copper(II) bis(1-pyrazolyl)methane complexes

Two bis(1-pyrazolyl)alkane ligands, bis(3,5-dimethyl-1-pyrazolyl)methane and bis(4-iodo-3,5-dimethyl-1-pyrazolyl)methane, and their copper(II) complexes, bis(3,5-dimethyl-1-pyrazolyl)methanedinitratocopper(II) [CuL₁(NO₃)₂] and bis(4-iodo-3,5-dimethyl-1-pyrazolyl)methanedinitratocopper(II) [CuL₂(NO₃)₂]·2H₂O, were prepared. Physiochemical properties of the copper(II) complexes were studied by spectroscopic (UV–vis, IR, EPR) techniques and cyclic voltammetry. Spectroscopic analysis revealed a 1:1 stoichiometry of ligand:copper(II) ion and a bindentate coordination mode for the nitrate ions in both of the complexes. According to experimental and theoretical ab initio data, the copper(II) ion is located in an octahedral hexacoordinated environment. Both complexes were able to catalyze the dismutation of superoxide anion () (pH 7.5) and decomposition of H₂O₂ (pH 7.5) and peroxynitrite (pH 10.9). In addition, both complexes exhibited superoxide dismutase (SOD) like activity toward extracellular and intracellular reactive oxygen species produced by activated human neutrophils in whole blood. Thus, these complexes represent useful SOD mimetics with a broad range of antioxidant activity toward a variety of reactive oxidants.     

Interaction of Cu(II)with His-Val-Gly-Asp and of Zn(II) with His-Val-His,Two Peptides at the Active Site of Cu,Zn-Superoxide Dismutase

His-Val-His and His-Val-Gly-Asp are two naturally occurring peptide sequences, present at the active site of Cu,Zn-superoxide dismutase (Cu,Zn-SOD). We have already studied the interaction of His-Val-His=A (copper binding site) with Cu(II) and of His-Val-Gly-Asp=B (zinc binding site) with Zn(II). As a continuation of this work and for comparison purposes we have also studied the interaction of Zn(II) with His-Val-His and Cu(II) with His-Val-Gly-Asp using both potentiometric and spectroscopic methods (visible, EPR, NMR). The stoichiometry, stability constants and solution structure of the complexes formed have been determined. Histamine type of coordination is observed for/ZnAH/²⁺, /ZnA/⁺, /ZnA²H/⁺ and/ZnA²/ in acidic pH while deprotonation of coordinated water molecules is observed at higher pH. /CUB/ species is characterized by the formation of a macrochelate and histamine type coordination. Its stability results in the suppression of amide deprotonation which occurs at high pH resulting in the formation of the highly distorted from square planar geometry 4N complex/CuBH_-3/³.     

Copper(II) complexes with peptide fragments encompassing the sequence 122-130 of human doppel protein

Copper(II) complexes of the peptide fragment (Dpl122-130) encompassing the sequence 122-130 of human doppel protein were characterized by potentiometric, UV-Visible, CD and EPR spectroscopic methods. An analogous peptide, in which the aspartate residue was substituted by an asparagine amino acid, was synthesized in order to provide evidence on the possible role of carboxylate group in copper(II) coordination. It was found that the carboxylic group is directly involved in copper(II) coordination at acidic pH, forming the CuLH₂ species with Dpl122-130. This copper(II) complex displayed EPR parameters very similar to those of the analogous complex with the whole doppel protein. At pH higher than 7, the complexes showed magnetic parameters similar to those of the major species of protein formed in the pH range 7-8, with the metal coordination environment consisting of one imidazole and three amide nitrogen atoms. The comparison of Cu-Dpl122-130 binding constant values with those of the prion peptide fragments (PrP106-114), showed that doppel peptide had a higher metal binding affinity at acidic pH whereas the prion peptide fragment binds the metal tightly at physiological pH.     

Coordination of Ni2+ and Cu2+ to metal ion binding domains of E. coli SlyD protein

The C-terminal region of Escherichia coli SlyD is unstructured and extremely rich in potential metal-binding amino acids, especially in histidine residues. SlyD is able to bind two to seven nickel ions per molecule, in a variety of coordination geometries and coordination numbers. This protein contributes to the insertion of nickel into the hydrogenase precursor protein and it has a peptidyl-prolyl cis/trans-isomerase activity which can be regulated through nickel ions. This inspired us to undertake systematic studies on the coordination ability of two histidine-rich peptides from the C-terminus of the SlyD protein with nickel. Also, it is known that histidine-rich regions are part of a Cu^2 + binding domain involved in copper uptake under conditions of metal starvation in vivo in other bacteria. For this reason we decided to examine the complex formation of Ac-AHGHVHGAHDHHHD-NH₂ and Ac-GHGHDHGHEHG-NH₂ fragments with copper ions, which are also reference metal ions in this study. Experiments were performed in a DMSO/water 30:70 solvent. The Ac-AHGHVHGAHDHHHD-NH₂ and Ac-GHGHDHGHEHG-NH₂ fragments were synthesized and their interactions with Ni^2 + and Cu^2 + ions were studied by potentiometric, mass spectrometric, UV-vis, CD, EPR, and NMR spectroscopic techniques in solution. The results show that the Ac-GHGHDHGHEHG-NH₂ fragment forms equimolar complexes with both nickel and copper ions. At physiological pH, the metal ion is bound only through nitrogens from imidazole sidechain of histidine residues. On the contrary, Ac-AHGHVHGAHDHHHD-NH₂ binds 2 metal ions per molecule, at pH range 5 to 7, even if the 1:2 metal:peptide ratios were used. NMR studies indicate the involvement of all His residues in this pH-range in metal binding of the latter peptide. At higher pH, the stoichiometry changes to 1:1 and the His residues are displaced by amide nitrogens.     

The first monoribbed-functionalized tris-dioximate iron(II) clathrochelate with two inherent NH2-substituents, its reactivity, acid-base and coordination-chemical properties

The diamine bis-α-benzildioximate iron(II) clathrochelate with two inherent NH₂-groups at one of the three ribbed fragments was obtained in a high yield by nucleophilic substitution of the dichlorine-containing macrobicyclic precursor with liquid ammonia. These amino-groups have an essential amide character and undergo deprotonation in the presence of strong bases. The resulted clathrochelate dianion behaves as an acido-ligand and coordinates to copper(II) ion giving the heteronuclear copper(II)-iron(II) complexes with the Cu(N⁻)₄ coordination polyhedron. The reaction of this dianion with benzil afforded the clathrochelate product with annulated heterocyclic ribbed piperazinone fragment as a result of benzilic-type rearrangement with 1,2-shift of the phenyl substituent.The complexes obtained have been characterized using elemental analysis, MALDI-TOF, IR, UV-Vis, multinuclear NMR and EPR spectra, and X-ray crystallography. N₆-coordination polyhedra of their encapsulated iron(II) ions possess a distorted trigonal-prismatic geometry.     

The copper(II) binding properties of the cyclic peptide c(HGHK)

The new cyclic tetrapeptide c(HGHK) was synthesised in the solid phase and its complexes with copper(II) were studied in aqueous solution at various pH values by means of potentiometric and spectroscopic methods (UV, EPR, CD). Six mononuclear coordination species were clearly identified within the pH range 3-11. Spectroscopic data strongly suggest sequential formation of N, 2N, 3N and 4N equatorial donor sets around the copper(II) centre from the lowest to the highest pH, involving both imidazole nitrogens and amide nitrogens. A detailed comparison with the copper(II) binding properties of HGHG and Ac-HGHG ligands is also reported.