Imidazole Nitrogen Donor Research Articles

Histidine tracts in human transcription factors: insight into metal ion coordination ability

Consecutive histidine repeats are chosen both by nature and by molecular biologists due to their high affinity towards metal ions. Screening of the human genome showed that transcription factors are extremely rich in His tracts. In this work, we examine two of such His-rich regions from forkhead box and MAFA proteins—MB3 (contains 18 His) and MB6 (with 21 His residues), focusing on the affinity and binding modes of Cu2+ and Zn2+ towards the two His-rich regions. In the case of Zn2+ species, the availability of imidazole nitrogen donors enhances metal complex stability. Interestingly, an opposite tendency is observed for Cu2+ complexes at above physiological pH, in which amide nitrogens participate in binding.

Syntheses and crystal structures of thorium(IV) and uranium(IV) tripodal metalloligands

Two tetravalent actinide tripodal metalloligands, [Th(H3L)(NO3)(MeO)]·2NO3 (1) and [U(H3L)Cl(MeO)]2[(UO2)Cl4]0.5·3Cl·2CH3OH·2H2O (2) (H3L=tris{[2-{(imidazole)methylidene} amino]ethyl}amine; CH3O=methoxide), were synthesized and structurally characterized. Both 1 and 2 contain the expected tripodal shape with the heptadentate Schiff base wrapping around Th(IV) or U(IV) ion. Further coordination of nitrato/chlorine and methoxide makes a ten-fold and a nine-fold coordination environments for Th(IV) ion (1) and U(IV) ion (2), respectively. Metalloligand 2 is a mixed valent uranium complex due to partial oxidation of U(IV) to U(VI). These tripodal metalloligands will have the potential to link secondary metal ions via deprotonated imidazole nitrogen donors to form either discrete heterometallic cages or supramolecular assemblies containing tetravalent actinide ions.

Copper(II) complex formation processes of alloferon I with point mutation H1K; combined spectroscopic and potentiometric studies

Mononuclear and polynuclear complexes of the alloferon I with point mutation (H1K) Lys-Gly-Val-Ser-Gly-His6-Gly-Gln-His9-Gly-Val-His12-Gly (AlloK) and its acetylated derivative Ac-Lys-Gly-Val-Ser-Gly-His6-Gly-Gln-His9-Gly-Val-His12-Gly (Ac-AlloK) have been studied by potentiometric, UV–visible, CD, EPR spectroscopic and mass spectrometry (MS) methods. The high water solubility of the resulting metal complexes allowed us to obtain a complete complex speciation at different metal-to-ligand ratios ranging from 1:1 to 4:1 for AlloK while to 3:1 for Ac-AlloK. At physiological pH 7.4 and the metal-to-ligand 1:1molar ratio the AlloK peptide forms the CuL complex with the 4N {NH2, N−, 2NIm} binding mode. In the Cu(II)–AlloK 4:1 system in wide pH 6.5–10 range the Cu4H−7L complex dominates with the 3N {NH2,2N−} 3×{NIm,2N−} coordination mode. Imidazole nitrogen donor atoms are the primary and exclusive metal binding sites of Ac-AlloK. For Ac-AlloK and 1:1 metal-to-ligand molar ratio the CuHL complex with the 3N {3NIm} binding sites in pH 4.5–7.5 range is present in solution.The amine nitrogen donor and all of the histidine residues can be considered to be independent metal-binding sites in the species formed in the systems studied. As a consequence, tri- (for the Ac-AlloK) and tetra-nuclear (for the AlloK peptide) complexes for the metal-to-ligand 3:1 and 4:1molar ratios, respectively, are present in the solution.

Interaction of Copper(II) with the Prion Peptide Fragment HuPrP(76−114) Encompassing Four Histidyl Residues within and outside the Octarepeat Domain

Complex formation processes between the 39-mer residue peptide fragment of human prion protein, HuPrP(76-114), and copper(II) ions have been studied by potentiometric, UV-vis, circular dichroism (CD), electron paramagnetic resonance, and electrospray ionization mass spectrometry methods. This peptide consists of 39 amino acid residues and contains two histidines (His77 and His85) belonging to the octarepeat domain and two histidines (His96 and His111) outside this domain. It was found that HuPrP(76-114) is able to bind 4 equiv of metal ions and all histidyl residues are independent, except nonequivalent metal binding sites in the oligonuclear species. Imidazole nitrogen donor atoms are the primary and exclusive metal binding sites below pH 5.5 in the form of various macrochelates. The macrochelation slightly suppresses, but cannot prevent, the deprotonation and metal ion coordination of amide functions, resulting in the formation of (N(im),N(-)), (N(im),N(-),N(-)), and (N(im),N(-),N(-),N(-))-coordinated copper(II) complexes in the pH range from 5.5 to 9. CD spectroscopy results gave clear evidence for the differences in the metal binding affinity of the histidyl sites according to the following order: His111 > His96 >> His77 approximately His85. Among the oligonuclear complexes, the formation of di- and tetranuclear species seems to be favored over the trinuclear ones, at pH values beyond the physiological ones. This phenomenon was not observed in the complex formation reactions of HuPrP(84-114), a peptide fragment containing only one histidyl residue from the octarepeat. As a consequence, the data support the existence of cooperativity in the metal binding ability of this peptide probably due to the presence of two octarepeat sequences of the dimeric octarepeat domain of HuPrP(76-114) at basic pH values.

Synthesis, characterization and antimicrobial activity of transition metal complexes with the Schiff base derived from imidazole-2-carboxaldehyde and glycylglycine

The Co(II), Ni(II) and Cu(II) complexes of the Schiff base derived from imidazole-2-carboxaldehyde and glycylglycine were synthesized and characterized by elemental analysis, PMR, molar conductance, IR, electronic, magnetic measurements, ESR, redox properties, thermal studies, XRD and SEM. Conductance measurements indicate that the complexes are 1 : 1 electrolytes. IR data show that the ligand is tetradentate with imidazole nitrogen, azomethine nitrogen, amide nitrogen and carboxylato oxygen donor groups. Electronic spectral measurements indicate tetrahedral geometry for Co(II) and Ni(II) complexes and square-planar geometry for Cu(II). Magnetic measurements show weak ferromagnetic behavior for Co(II) and Ni(II) complexes, and paramagnetic for Cu(II) complex. The X-band ESR spectral data indicate a covalent link between metal and ligand. The complexes were found to be stabilized in unusual oxidation states of metal during electrolysis. Thermal analysis of the complexes indicate that decomposition takes place in three steps. XRD shows that Ni(II) and Cu(II) complexes have the respective crystallite size of 42 and 34 nm, while the Co(II) complex is amorphous. The surface morphology of the complexes was studied by SEM. The antimicrobial activities of the ligand and its complexes indicate that the metal complexes possess high antibacterial and antifungal activities towards the bacterial species S. aureus, E. coli, K. pneumaniae, P. vulgaris and P. aeruginosa and the fungal species A. niger, R. stolonifer, A. flavus, R. bataicola and C. albicans.

Copper(II) Interaction with Prion Peptide Fragments Encompassing Histidine Residues Within and Outside the Octarepeat Domain: Speciation, Stability Constants and Binding Details

A 31-mer polypeptide, which encompasses residues 84-114 of human prion protein HuPrP(84-114) and contains three histidyl residues, namely one from the octarepeat (His85) and two histidyl residues from outside the octarepeat region (His96 and His111), and its mutants with two histidyl residues HuPrP(84-114)His85Ala, HuPrP(84-114) His96Ala, HuPrP(84-114)His111Ala and HuPrP(91-115) have been synthesised and their Cu2+ complexes studied by potentiometric and spectroscopic (UV/Vis, CD, EPR, ESI-MS) techniques. The results revealed a high Cu2+-binding affinity of all peptides, and the spectroscopic studies made it possible to clarify the coordination mode of the peptides in the different complex species. The imidazole nitrogen donor atoms of histidyl residues are the exclusive metal-binding sites below pH 5.5, and they have a preference for macrochelate structure formation. The deprotonation and metal-ion coordination of amide functions take place by increasing the pH; all of the histidines can be considered to be independent metal-binding sites in these species. As a consequence, di- and trinuclear complexes can be present even in equimolar samples of the metal ion and peptides, but the ratios of polynuclear species do not exceed the statistically expected ones; this excludes the possibility of cooperative Cu2+ binding. The species with a (N(im),N,N)-binding mode are favoured around pH 7, and their stability is enhanced by the macrochelation from another histidyl residue in the mononuclear complexes. The independence of the histidyl sites results in the existence of coordination isomers and the preference for metal binding follows the order of: His111>His96>His85. Deprotonation and metal-ion coordination of the third amide functions were detected in slightly alkaline solutions at each of the metal-binding sites; all had a (N(im),N,N,N)-coordination mode. Spectroscopic measurements also made it clear that the four lysyl amino groups of the peptides are not metal-binding sites in any cases.

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.

De Novo Design and Spectroscopic Characterization of a Dinucleating Copper-Binding Pentadecapeptide

A spectroscopic study of aqueous solutions of Ac-WGHGHGHGPGHGHGH-NH(2) (HGP) indicates that copper(II) binds to the peptide to form a 2:1 Cu(2+)/HGP complex with four nitrogen atoms in the copper coordination environment. Electron paramagnetic resonance (EPR) and UV-visible data suggest copper binding through the peptide backbone and imidazole nitrogen donors. Circular dichroism data show that HGP is unbound below pH 5.5 and is copper-saturated at pH 9 and above. The apo form of the peptide is unstructured in solution and is organized into a turn conformation in the presence of 2 mol equiv of Cu(2+) at basic pH. EPR measurements for 2:1 Cu(2+)/HGP solutions in the g = 2 region and within the pH range 7-11 exhibit axial spectra. A molecular-mechanics-minimized model of the Cu(2+)/HGP complex gave a Cu...Cu separation of 8 A.

Structure and Stability of the CuII Complexes with Tandem Repeats of the Chicken Prion

Prion protein (PrP) misfolding is one of the pivotal issues in understanding the rudiments of neurodegenerative disorders. The conformational change of mammalian cellular PrP to scrapie PrP is caused by an unknown agent, but there is reasonable evidence supporting the key role of copper ions in this process. The structure of the avian PrP was found to be very similar to the mammalian protein, although there is only 30% homology in the secondary structure. This work shows that copper ions are very effectively bound by hexarepeat fragments of chicken prion protein, although not as effectively as it was found in the case of mammalian protein. By means of potentiometric and spectroscopic techniques (nuclear magnetic resonance, circular dichroism, UV-vis, and electronic paramagnetic resonance), it was shown that Cu(II) ions coordinate to the chicken PrP hexapeptide domain in physiological pH via imidazole nitrogen donors of His residue(s). The binding pattern changes the structure of peptide involved, indicating a possible impact of Cu(II) ions in the biology and pathology of nonmammalian PrP, which could be similar to that found for mammalian PrP. The present study shows that, similar to the human prion octapeptide repeats, chicken prion hexapeptide repeats might bind copper ions in two different ways, depending on the number of repeats and metal/ligand molar ratio: (i) an intra-repeat coordination mode in which copper ion is chelated by His imidazole and deprotonated amide nitrogen in monomeric peptide and (ii) an inter-repeat coordination mode in which a polymeric peptide ligand (dimer and trimer) forms polyimidazole complexes that are very stable at physiological pH. Two proline residues inserted into the hexapeptide unit have a critical impact on the metal-binding ability.

Acid–base properties and copper(II) complexes of dipeptides containing histidine and additional chelating bis(imidazol-2-yl) residues

Copper(II) complexes of dipeptides of histidine containing additional chelating bis(imidazol-2-yl) agent at the C-termini (PheHis-BIMA [N-phenylalanyl-histidyl-bis(imidazol-2-yl)methylamine] and HisPhe-BIMA [N-histidyl-phenylalanyl-bis(imidazol-2-yl)methylamine]) were studied by potentiometric, UV–Visible and Electron Paramagnetic Resonance (EPR) techniques. The imidazole nitrogen donor atoms of the bis(imidazol-2-yl)methyl group are described as the primary metal binding sites forming stable mono- and bis(ligand) complexes at acidic pH. The formation of a ligand-bridged dinuclear complex [Cu2L2]4+ is detected in equimolar solutions of copper(II) and HisPhe-BIMA. The coordination isomers of the dinuclear complex are described via the metal binding of the bis(imidazol-2-yl)methyl, amino-carbonyl and amino-imidazole(His) functions. In the case of the copper(II)–PheHis-BIMA system the [NH2, N−(amide), N(Im)] tridentate coordination of the ligand is favoured and results in the formation of di- and trinuclear complexes [Cu2H−1L]3+ and [Cu3H−2L2]4+ in equimolar solutions. The presence of these coordination modes shifts the formation of “tripeptide-like” ([NH2, N−, N−, N(Im)]-coordinated) [CuH−2L] complexes into alkaline pH range as compared to other dipeptide derivatives of bis(imidazol-2-yl) ligands. Although there are different types of imidazoles in these ligands, the deprotonation and coordination of the pyrrole-type N(1)H groups does not occur below pH 10.

Transition metal complexes of bis(imidazol-2-yl) derivatives of dipeptides

Copper(II), nickel(II) and zinc(II) complexes of the dipeptides containing chelating bis(imidazolyl) agents at the C-termini (GlyLeu-BIMA, LeuGly-BIMA, PheGly-BIMA and AlaPro-BIMA; BIMA = bis(imidazol-2-yl)methylamine) were studied by potentiometric, UV-VIS, EPR and MALDI-MS techniques. The imidazole nitrogen donor atoms were described as the primary metal binding sites forming stable mono- and bis-(ligand) complexes in the acidic pH range. The formation of dinuclear complexes was detected in equimolar solutions of copper(II) and dipeptides and various isomeric forms of these species ([Cu2L2]4+ and [Cu2H−2L2]2+) were indentified by EPR measurements. Deprotonation and co-ordination of the amide groups of dipeptides took place in slightly alkaline solution resulting in the formation of [MH−2L] complexes of copper(II) and nickel(II). Metal ion co-ordination of one of the imidazole nitrogen donor atoms promoted the ionization of the pyrrole type N(1)H group of imidazole in copper(II) complexes and resulted in the formation of a trinuclear complex ([Cu3H−6L2]). The latter species contains negatively charged bridging imidazolato residues.

Metal complexes of peptides containing monodentate or chelating imidazole nitrogen donors: Factors influencing the coordination of amide groups and imidazole side chains

Transition metal complexes of peptides containing monodentate or chelating imidazole side chains have been studied by the combined application of potentiometric and spectroscopic techniques. The results obtained on the complexes of peptides containing C-terminal histidyl residues (Gly 3 His, Gly 4 His and Gly 5 His) provided clear evidence that both amino and imidazole functions are effective metal binding sites. The formation of various macrochelates were described via the coordination of both termini, but the major species were characterised by 4N-coordination starting from the N-termini. The coordination chemistry of a series of peptide molecules containing bis(imidazolyl) agents revealed that the donor functions of the peptide backbone cannot compete with chelation of the bis(imidazolyl) residue. However, the presence of terminal amino group promotes amide coordination, while imidazole residues act as additional donor sites or bridging ligands.

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.

Metal binding selectivity of oligopeptides

Oligopeptides provide one of the major groups of organic substances that binds metal ions. As a consequence, a huge number of papers has already been published on the transition metal peptide complexes and the most important observations were reviewed by several authors [1–3]. Complex formation processes between transition elements and peptide molecules have been studied by a number of different techniques and it has been well-identified that terminal amino and subsequent amide nitrogens are the main metal binding sites. Among metal ions palladium(II), copper(II) and nickel(II) were reported to form especially stable complexes even with simple dipeptides. In these, the nitrogen of the deprotonated amide group act as the strongest donor, but terminal carboxylic and amino groups, as well as the carbonyl oxygen being also implicated in metal binding. In case of oligoglycines the complex formation involves the succesive formation of IN-, 2N-, 3N- or 4N-coordinated species with increasing pH. The values pK = 2, 4 and 8 were reported for the metal ion induced deprotonation and coordination of peptide nitrogens in the palladium(II), copper(II) and nickel(II) complexes, respectively. The deprotonation and coordination of the amide groups, however, depend on the additional donor groups present in the side chains of peptide molecules. The effects of imidazole nitrogen, carboxylate oxygen and various sulfur donor atoms are discussed in this study.

Potentiometric and spectroscopic studies on the ternary complexes of copper(II) with dipeptides and nucleobases

Ternary complexes of copper(II) with dipeptides (GlyGly, GlyAsp, and MetVal) and nucleobases (MeC, MeA, MeUH, MeTH, EtGH, uridine, and thymidine) or derivatives of amino acids (Ac-Lys and Ac-Hm) were studied by potentiometric, UV-visible, and EPR spectroscopic methods. It was found that all ligands form stable adducts with the CuAH −1 species of dipeptides, and the ternary complexes formed by the coordination of imidazole nitrogen donor atom are the most favored in the physiological pH range. The formation of a dimeric species Cu 2A 2H −2B was detected in the case of EtGH supporting that both N1 and N7 nitrogens of purine take part in metal ion coordination. The data of spectroscopic measurements supported the equatorial coordination of nitrogen donors in CuABH −1 ternary complexes.

Competition between the terminal amino and imidazole nitrogen donors for coordination to Ni(II) ions in oligopeptides

Spectroscopic and potentiometric studies on Ni(II) complexes with oligopeptides containing a histidine (His) residue in the fourth position of the peptide sequence have shown that imidazole nitrogen can act as the major binding site for metal ions. As a result, coordination of Ni(II) to an imidazole nitrogen of a His residue leads to more stable species than those obtained for linear peptides containing non-coordinating side chains, such as tetraalanine.

Thermodynamic stereoselectivity assisted by weak interactions in metal complexes. Copper(II) ternary complexes of cyclo-L-histidyl-L-histidine and L- or D-amino acids in aqueous solution

The thermodynamic stereoselectivity and spectroscopic characteristics of copper(II) ternary complexes with L- or D-amino acids (Xaa) and cyclo-L-histidyl-L-histidine, cyclo(-His-His-), have been investigated. Ternary complex formation with L-amino acids is more enthalpy and less entropy favoured than that of the analogous D-amino acids. For aromatic amino acids these differnces increase going from phenylalanine to tryptophan, suggesting involvement of aromatic ring-stacking interactions between one of the dipeptide imidazoles and the Xaa aromatic side chain in the mixed-ligand complexes. Enthalpy changes are the driving force for the thermodynamic stereoselectivity. A detailed analysis of the CD spectra of these ternary systems resulted in a correlation between Δ(Δε), the difference between the molar CD coefficients at the wavelength where it is maximized, and the ΔG° and ΔH° values. On the basis of EPR spectra, it is excluded that the differences between the mixed-ligand complexes with L- or D-amino acids are due to a different degree of solvation. It is proposed that hydrophobic and ring-stacking interactions, responsible for stereoselectivity, are favoured by a certain degree of tetrahedral distortion of the tetragonal copper(II) chromophore formed by the two imidazole nitrogen donors of cyclo(-His-His-) and the nitrogen and carboxylate oxygen donors of the L- or D-amino acids.

Pentacoordinate zinc complexes of imidazole nitrogen donors as structural models for the active site in enzymes: preparation and crystal structures of (.mu.-2,2'-biimidazole)tetrakis(2,2'-biimidazole)dizinc(II) tetraperchlorate trihydrate and bis(2,2'-biimidazole)(formato)zinc(II) perchlorate

Le premier compose du titre cristallise dans le systeme orthorhombique avec le groupe d'espace P2 1 2 1 2 1 . Le second cristallise dans le systeme monoclinique avec le groupe d'espace P2 1 /c. Affinement de la structure jusqu'a 4,34% et 4,78% respectivement

Preparation and complexation of polydentate and macrocyclic ligands incorporating benzimidazole. X-Ray crystal structure of 6,7,9,10,12,13,15,16-octahydro-23H,25H-bis(benzimidazo[1,2-j:2′,1′-o])[1,4,7,13,10,16]tetraoxadiazacyclo-octadecine

A number of ligands, including some macrocycles, all with benzimidazole units incorporated, have been synthesised. These co-ordinate CuII and other transition metals with the imidazole nitrogen and other donor atoms included in the ligand design. The crystal structure data for a representative macrocycle, 6,7,9,10,12,13,15,16-octahydr-23H,25H-bis(benzimidazo[1,2-j:2′,1′-o])[1,4,7,13,10,16]tetraoxadiazacyclo-octadecine, are presented, together with spectroscopic and other details of representative complexes.

Extensive studies of the heme coordination structure of indoleamine 2,3-dioxygenase and of tryptophan binding with magnetic and natural circular dichroism and electron paramagnetic resonance spectroscopy

In order to probe the active site of the heme protein indoleamine 2,3-dioxygenase, magnetic and natural circular dichroism (MCD and CD) and electron paramagnetic resonance (EPR) studies of the substrate ( l-tryptophan)-free and substrate-bound enzyme with and without various exogenous ligands have been carried out. The MCD spectra of the ferric and ferrous derivatives are similar to those of the analogous myoglobin and horseradish peroxidase species. This provides strong support for histidine imidazole as the fifth ligand to the heme iron of indoleamine 2,3-dioxygenase. The substrate-free native ferric enzyme exhibits predominantly high-spin EPR signals ( g ⊥ = 6, g | = 2) along with weak low-spin signals ( g = 2.86, 2.28, 1.60); similar EPR, spin-state and MCD features are found for the benzimidazole adduct of ferric myoglobin. This suggests that the substrate-free ferric enzyme has a sterically hindered histidine imidazole nitrogen donor sixth ligand. Upon substrate binding, noticeable MCD and EPR spectral changes are detected that are indicative of an increased low spin content (from 30 to over 70% at ambient temperature). Concomitantly, new low spin EPR signals ( g = 2.53, 2.18,, 1.86) and MCD features characteristic of hydroxide complexes of histidine-ligated heme proteins appear. For almost all of the other ferric and ferrous derivatives, only small substrate effects are observed with MCD spectroscopy, while substantial substrate effects are seen with CD spectroscopy. Thus, changes in the heme coordination structure of the ferric enzyme and in the protein conformation at the active site of the ferric and ferrous enzyme are induced by substrate binding. The observed substrate effects on the ferric enzyme may correlate with the previously observed kinetic substrate inhibition of indoleamine 2,3-dioxygenase activity, while such effects on the ferrous enzyme suggest the possibility that the substrate is activated during turnover.