The copper(II) coordination abilities of three novel cyclic tetrapeptides with -His-Xaa-His- motif

The Role of the Disulfide Bridge in the Copper (II) Binding by the Cyclic His4-Peptide

In this paper, we present studies on the influence of the disulfide bridge on the copper (II) ions’ binding abilities by the cyclic His₄-peptide. The studied ligand HKHPHRHC_-S-S-C consists of nine amino acids. The cyclic structure was obtained through a disulfide bridge between two cysteinyl groups. Moreover, this peptide is characterized by the presence of four His residues in the sequence, which makes it an interesting ligand for transition metal ions. The potentiometric and spectroscopic (UV-Vis spectroscopy and circular dichroism spectroscopy (CD)) studies were carried out in various molar ligand to metal ratios: 2:1, 1:1, and 1:2, in the pH range of 2.5–11 at 25 °C. The results showed that the cyclic His₄-peptide promotes dinuclear complexes in each of these systems and forms the final dinuclear species with the {N_Im, 3N^-_amide}{N_Im, 3N^-_amide} coordination mode. The obtained data shows that cyclization by the formation of the disulfide bond has an impact on the peptide chain flexibility and appearance of additional potential donors for metal ions and influences the copper (II) ions’ coordination.

Peptidomimetics - An infinite reservoir of metal binding motifs in metabolically stable and biologically active molecules

The involvement of metal ions in interactions with therapeutic peptides is inevitable. They are one of the factors able to fine-tune the biological properties of antimicrobial peptides, a promising group of drugs with one large drawback - a problematic metabolic stability. Appropriately chosen, proteolytically stable peptidomimetics seem to be a reasonable solution of the problem, and the use of D-, β-, γ-amino acids, unnatural amino acids, azapeptides, peptoids, cyclopeptides and dehydropeptides is an infinite reservoir of metal binding motifs in metabolically stable, well-designed, biologically active molecules. Below, their specific structural features, metal-chelating abilities and antimicrobial potential are discussed.

Stability of Cu(ii) complexes with FomA protein fragments containing two His residues in the peptide chain

The coordination process of Cu(ii) ions with FomA protein fragments ofFusobacterium nucleatumcontaining two histydyl residues was determined.

The Coordination Abilities of New Cyclic Analogs of Somatostatin

Two new somatostatin analogs with a characteristic part of the sequence -c(Cys-Phe-Trp-Lys-Thr-Cys)- and with two histidine and two aspartic acid moieties in their structures were synthesized and analyzed in terms of their coordination abilities with copper (II) ions. Both peptides bind Cu(II) effectively. Ligands form 4N complexes with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left\{ {{\text{N}}_{\text{Im}} ,{ 3} {\text{N}}_{\text{amide}}^{ - } } \right\}$$\end{document}NIm,3Namide- binding mode in a basic range of pH. But in spite of very similar sequences of the two peptides a significant difference in the effectiveness of the binding of copper (II) ions was observed.

The impact of structural variation in simple lanthanide binding peptides

A series of di-, tri- and tetra-peptides were synthesised using l- and d-glutamic acid in order to determine the effects of peptide length and stereochemistry on lanthanide binding affinity.

Metal-binding and redox properties of substituted linear and cyclic ATCUN motifs

The amino-terminal copper and nickel binding (ATCUN) motif is a short peptide sequence found in human serum albumin and other proteins. Synthetic ATCUN-metal complexes have been used to oxidatively cleave proteins and DNA, cross-link proteins, and damage cancer cells. The ATCUN motif consists of a tripeptide that coordinates Cu(II) and Ni(II) ions in a square planar geometry, anchored by chelation sites at the N-terminal amine, histidine imidazole and two backbone amides. Many studies have shown that the histidine is required for tight binding and square planar geometry. Previously, we showed that macrocyclization of the ATCUN motif can lead to high-affinity binding with altered metal ion selectivity and enhanced Cu(II)/Cu(III) redox cycling (Inorg. Chem. 2013, 52, 2729-2735). In this work, we synthesize and characterize several linear and cyclic ATCUN variants to explore how substitutions at the histidine alter the metal-binding and catalytic properties. UV-visible spectroscopy, EPR spectroscopy and mass spectrometry indicate that cyclization can promote the formation of ATCUN-like complexes even in the absence of imidazole. We also report several novel ATCUN-like complexes and quantify their redox properties. These findings further demonstrate the effects of conformational constraints on short, metal-binding peptides, and also provide novel redox-active metallopeptides suitable for testing as catalysts for stereoselective or regioselective oxidation reactions.

Copper(II) coordination properties of decapeptides containing three His residues: the impact of cyclization and Asp residue coordination

Two decapeptides containing three His and two Pro-Gly β-turn inducer units (C-Asn, cyclic) and three His and a single Pro-Gly unit (O-Asn, open) have been synthesized. A detailed potentiometric study showed that while O-Asn binds up to 3 equiv. of Cu(2+) ions, C-Asn only coordinates two before precipitation occurred. Nonetheless, at a 1 : 1 Cu(2+)/peptide ratio both peptides form a major [CuHL](3+) species and spectroscopic studies (UV-Vis, CD and EPR) revealed a very similar copper(ii) complex where the metal ion is coordinated solely by the imidazole rings of the His residues adopting a square planar or square pyramidal geometry. The corrected stability constants of the protonated species (log K(CuH(O-Asn)) = 8.17 and log K(CuH(C-Asn)) = 9.11) indicate that the cyclic peptide binds Cu(2+) with higher affinity and this value represents the highest value reported so far for this type of coordination. Additionally, the calculated value of the effective stability constant, K(eff), showed that C-Asn has a higher affinity for Cu(2+) at all pH values not only at a 1 : 1 ratio but even at a 2 : 1 ratio. The replacement of the asparagine residue by an aspartic amino acid increases the Cu(2+) affinity of the aspartic counterparts, C-Asp and O-Asp, which at a 1 : 1 Cu(2+)/peptide ratio also form a major species, [CuHL](2+) in these cases, with Cu(2+) coordinated to the three histidine residues and one aspartic residue. These data show how cyclization and coordination to the aspartic residue increase the binding strength and preclude the coordination of the amide nitrogen up to higher pH values, stabilizing therefore, the species where Cu(2+) is solely coordinated by the side chain functionalities.

Macrocyclization of the ATCUN motif controls metal binding and catalysis

We report the design, synthesis, and characterization of macrocyclic analogues of the amino-terminal copper and nickel binding (ATCUN) motif. These macrocycles have altered pH transitions for metal binding, and unlike linear ATCUN motifs, the optimal cyclic peptide 1 binds Cu(II) selectively over Ni(II) at physiological pH. UV-vis and EPR spectroscopy showed that cyclic peptide 1 can coordinate Cu(II) or Ni(II) in a square planar geometry. Metal binding titration and ESI-MS data revealed a 1:1 binding stoichiometry. Macrocyclization allows for coordination of Cu(II) or Ni(II) as in linear ATCUN motifs, but with enhanced DNA cleavage by the Cu(II)-1 complex relative to linear analogues. The Cu(II)-1 complex was also capable of producing diffusible hydroxyl radicals, which is unique among ATCUN motifs and most other common copper(II) chelators.

The coordination abilities of the multiHis-cyclopeptide with two metal-binding centers--potentiometric and spectroscopic investigation

In this paper we present the formation of mono- and binuclear complexes by the octapeptide (c(HKHPHKHP)) with copper(II) ions. The ligand was synthesized manually by the solid-phase method. Its characteristic cyclic structure significantly influences the coordination abilities. The studied peptide has two Pro amino acid residues in the sequence. This causes the formation of two independent centres able to undertake metal ion binding. The potentiometric and spectroscopic (UV-vis, CD and EPR) studies were carried out in aqueous solutions in the pH range 2.5-11.0 at 298 K, HNO(3) was used as the solvent with KNO(3), where the ionic strength was 0.1 M. The analysis of the potentiometric together with spectroscopic studies have shown that the investigated peptide forms only mono-nuclear complexes when the metal-to-ligand molar ratio is 1 : 1. When the concentration of Cu(ii) ions increases and the ligand-to-metal molar ratio is 1 : 2 the formation of binuclear complexes is preferred in the system.

Interactions of disulfide-constrained cyclic tetrapeptides with Cu(2+)

The purpose of this work is to characterize the interactions of two disulfide-constrained cyclic tetrapeptides [c(Ac-Cys-Pro-Phe-Cys-NH(2)), SS1; c(Ac-Cys-Pro-Gly-Cys-NH(2)), SS2] with Cu(2+) ions in order to facilitate the design of cyclic peptides as sensors for metal ions. The Cu(2+)-peptide complex cations at m/z 569.1315 for Cu(2+)-SS1 and m/z 479.0815 for Cu(2+)-SS2 were detected by mass spectrometry. The gas-phase fragmentation of the Cu(2+)-peptide complexes was studied by collision-induced dissociation and suggests the atoms involved in the coordination. Cu(2+) ion binds to a single SS1 or SS2 with K (d(app)) of 0.57 ± 0.02 and 0.55 ± 0.01 μM, respectively. Isothermal titration calorimetry data indicate both enthalpic and entropic contributions for the binding of Cu(2+) ion to SS1 and SS2. The characteristic wavenumber of 947 cm(-1) and the changes at 1,664 and 1,530 cm(-1) in the infrared spectrum suggest that the sulfydryl of cysteine, the carbonyl group, and amide II are involved in the coordination of Cu(2+). The X-ray absorption near-edge structure signal from the Cu(2+)-peptide complex corresponds to the four-coordination structure. The extended X-ray absorption fine structure and electron paramagnetic resonance results demonstrate the Cu(2+) ion is in an S/N/2O coordination environment, and is a distinct type II copper center. Theoretical calculations further demonstrate that Cu(2+) ion binds to SS1 or SS2 in a slightly distorted tetragonal geometry with an S/N/2O environment and the minimum potential energy.

Harnessing the flexibility of peptidic scaffolds to control their copper(II)-coordination properties: a potentiometric and spectroscopic study

Designing small peptides that are capable of binding Cu(2+) ions mainly through the side-chain functionalities is a hard task because the amide nitrogen atoms strongly compete for Cu(2+) ion coordination. However, the design of such peptides is important for obtaining biomimetic small systems of metalloenyzmes as well as for the development of artificial systems. With this in mind, a cyclic decapeptide, C-Asp, which contained three His residues and one Asp residue, and its linear derivative, O-Asp, were synthesized. The C-Asp peptide has two Pro-Gly β-turn-inducer units and, as a result of cyclization, and as shown by CD spectroscopy, its backbone is constrained into a more defined conformation than O-Asp, which is linear and contains a single Pro-Gly unit. A detailed potentiometric, mass spectrometric, and spectroscopic study (UV/Vis, CD, and EPR spectroscopy) showed that at a 1:1 Cu(2+)/peptide ratio, both peptides formed a major [CuHL](2+) species in the pH range 5.0-7.5 (C-Asp) and 5.5-7.0 (O-Asp). The corrected stability constants of the protonated species (log K*(CuH(O-Asp))=9.28 and log K*(CuH(C-Asp))=10.79) indicate that the cyclic peptide binds Cu(2+) ions with higher affinity. In addition, the calculated value of K(eff) shows that this higher affinity for Cu(2+) ions prevails at all pH values, not only for a 1:1 ratio but even for a 2:1 ratio. The spectroscopic data of both [CuHL](2+) species are consistent with the exclusive coordination of Cu(2+) ions by the side-chain functionalities of the three His residues and the Asp residue in a square-planar or square-pyramidal geometry. Nonetheless, although these data show that, upon metal coordination, both peptides adopt a similar fold, the larger conformational constraints that are present in the cyclic scaffold results in different behaviour for both [CuHL](2+) species. CD and NMR analysis revealed the formation of a more rigid structure and a slower Cu(2+)-exchange rate for [CuH(C-Asp)](2+) compared to [CuH(O-Asp](2+). This detailed comparative study shows that cyclization has a remarkable effect on the Cu(2+)-coordination properties of the C-Asp peptide, which binds Cu(2+) ions with higher affinity at all pH values, stabilizes the [CuHL](2+) species in a wider pH range, and has a slower Cu(2+)-exchange rate compared to O-Asp.

Dioxygen oxidation Cu(II) → Cu(III) in the copper complex of cyclo(Lys-dHis-βAla-His): a case study by EXAFS and XANES approach

A former spectroscopic study of Cu(II) coordination by the 13-membered ring cyclic tetrapeptide c(Lys-dHis-βAla-His) (DK13), revealed the presence, at alkaline pH, of a stable peptide/Cu(III) complex formed in solution by atmospheric dioxygen oxidation. To understand the nature of this coordination compound and to investigate the role of the His residues in the Cu(III) species formation, Cu K-edge XANES, and EXAFS spectra have been collected for DK13 and two other 13-membered cyclo-peptides: the diastereoisomer c(Lys-His-βAla-His) (LK13), and c(Gly-βAla-Gly-Lys) (GK13), devoid of His residues. Comparison of pre-edge peak features with those of Cu model compounds, allowed us to get information on copper oxidation state in two of the three peptides, DK13 and GK13: DK13 contains only Cu(III) ions in the experimental conditions, while GK13 binds only with Cu(II). For LK13/Cu complex, EXAFS spectrum suggested and UV-vis analysis confirmed the presence of a mixture of Cu(II) and Cu(III) coordinated species. Theoretical XANES spectra have been calculated by means of the MXAN code. The good agreement between theoretical and experimental XANES data collected for DK13, suggests that the refined structure, at least in the first coordination shell around Cu, is a good approximation of the DK13/Cu(III) coordination species present at strongly alkaline pH. All the data are consistent with a slightly distorted pyramidal CuN(4) unit, coming from the peptide bonds. Surprisingly, the His side-chains seemed not involved in the final, stable, Cu(III) scaffold.

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.

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(beta(3)homoLysdHisbeta-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.

Short-chain oligopeptides with copper(II) binding properties: The impact of specific structural modifications on the copper(II) coordination abilities

A series of linear tetrapeptides containing two histidyl residues in position 2 and 4, namely DHGH, DHGdH, KHGH, KHGdH, Ac-DHGH-NH(2), Ac-DHGdH-NH(2), Ac-KHGH-NH(2), and Ac-KHGdH-NH(2), were synthesized and characterised. Their copper(II) binding properties were investigated in depth through a variety of physicochemical methods. Potentiometric titrations were first carried out to establish the stoichiometry and the stability of the resulting copper(II)-peptide complexes. The copper(II) chromophores that are formed in the various cases in dependence of pH were subsequently characterised by extensive spectroscopic analysis (UV-Vis, EPR, CD) in strict correlation with potentiometric data. The effects of the nature of the first amino acid (Lys versus Asp) and of N-terminal amino group protection on copper(II) binding were specifically addressed. On turn, the careful comparison of the copper(II) coordination abilities of the linear peptides with those of their cyclic analogs provided insight into the effects of cyclization on the overall metal binding properties.