Copper(II) ion binding to cellular prion protein

Prion diseases are fatal neurodegenerative diseases thought to arise from the post-translational conversion of normal cellular prion protein to a scrapie isoform. Experimental data suggest a role for copper(II) ions in the process. An ab initio QM/MM approach and available experimental data were combined in order to identify and evaluate three potential copper(II) ion binding sites in the C-terminal portion of the normal cellular prion protein. Our results suggest that copper(II) ion binds to His 187 but not to His 140 and His 177 of the binding site in the cellular prion protein.     

On the structure of copper(II) coordination compounds with L-histidine

Quantum chemical calculations are performed for the spatial and electron structure of complex compounds of L-histidine and its ionized forms with copper(II) for a variety of compositions within the density functional theory (DFT) using the B3LYP functional and 6-311G(d) basis. The solvent (water) is considered within the PCM approximation. EPR spectroscopy is used to study the equilibrium in the copper(II)–L-histidine system in an aqueous solution at рН 2–11. A comparison between the theoretical calculations and the EPR spectra suggests the following geometry for the coordination environment of the copper(II) ion in the complex compounds: CuHLL–square-planar coordination; CuL₂, CuHLL′, and CuLL′–distorted square pyramid; and CuL₂′–octahedral environment.     

On the complexation of copper (II) ion with 2-hydroxybenzamide

The complexation of the Cu2+ ion with 2-Hydroxybenzamide (salicylamide, HL) has been studied, at 25 degrees C, by potentiometric measurements with a glass electrode in NaCIO4 media for ionic strength ranging from 0.5 to 3 mol/dm3. The data are consistent with the formation of the complexes CuH(-1)(HL)+, CuH(-2)(HL)2, Cu2H(-2)(HL)2(2+) and CuH(-2)(HL). The minor species, Cu2H(-2)(HL)2(2+) and CuH(-2)(HL), amount to at least 20% of the total copper. Elaboration of the data according to the Specific Interaction Theory yields the constants valid in the infinite dilution reference state: [formulas: see text] and the interaction coefficients (kg/mol) of complex species with medium ions: b(L-,Na+) = 0.11 +/- 0.03; b(CuH(-1)(HL)+,NaClO4) = 0.17 +/- 0.05; b(CuH(-2)(HL)2,NaClO4) = 0.11 +/- 0.05; b(Cu2H(-2)(HL)2(2+),NaClO4) = 0.2(7) +/- 0.1; b(CuH(-2)(HL),NaClO4) = -0.0(3) +/- 0.1.     

A re-investigation of copper coordination in the octa-repeats region of the prion protein

An aqueous solution spectroscopic (Vis and EPR) study of the copper(II) complexes with the Ac-HGGG-NH2 and Ac-PHGGGWGQ-NH2 polypeptides (generically designated as L) suggests square base pyramids ascribable to [Cu(L)H(-2)] complex species, which contain three nitrogen donor atoms, arising from imidazole and peptide groups, in the equatorial plane and for a pseudo-octahedral geometry in the case of [CuLH-3]- and [Cu(L)H-4]2- which have four nitrogen donor atoms in their equatorial plane. The coordination sphere of the copper complex in the [Cu(L)H(-2)] species, which is present at neutral pH values, is completed by two oxygen donor atoms. ESI-MS spectra ascertained that water molecules are not present in the coordination equatorial plane of this latter species, in comparison with other copper(II) complexes with ligands bearing nitrogen and oxygen donor atoms and surely having equatorial water molecules. This indicates the coordination of a carbonyl oxygen atom in the equatorial plane has to be invoked. However, no direct proof about the involvement of a carbonyl group oxygen donor atom apically linked to copper was obtained, due to the flexibility of these structures at room temperature. Additionally, the low A(ll) value leads one to consider another oxygen atom of a carbonyl group being involved in the apical bond to copper in a fast exchange fashion. This apical interaction, which may also involve a water molecule, is more pronounced in the Cu-Ac-HGGG-NH2 than in the analogous Cu-Ac-PHGGGWGQ-NH2 system, probably because of the presence of tryptophan and proline in the polypeptide sequence.     

Luminescence quenching of the excited tris-bipyridyl ruthenium(II) ion by copper(II)-cyclodextrin complexes

The quenching of the luminescence intensity and lifetime of the electronically excited species Ru(bipy) ₃ ²⁺ by a series of copper (II) cyclodextrin complexes is studied. It is found that conventional Stern-Volmer behaviour is not followed. A modified version of the Stern-Volmer equation, one which assumes purely static quenching, is in good agreement with experimental data. Inclusion of the Ru(bipy) ₃ ²⁺ by the metallo-cyclodextrin complex is observed to play a key role in the quenching mechanism.     

Structural and dynamic characterization of copper(II) binding of the human prion protein outside the octarepeat region

Human prion protein (hPrP) fragments encompassing the 91-120 region, namely hPrP92-100 (SP1), hPrP106-113 (SP2), hPrP91-120 (LP1), and hPrP91-114 (LP2), were considered for delineation of the Cu(II)-binding site(s). NMR and EPR spectroscopy results obtained from LP1 or LP2 were compared with those obtained from SP1 and SP2. The coexistence of two binding sites, one centered at His96 and the other at His111, was evidenced and ratified by ESI mass spectrometry at low and high metal:peptide ratios. While room-temperature NMR spectroscopy data were consistent with the binding site centered on His111 being approximately fourfold stronger than that centered on His96, low-temperature EPR spectroscopy results yielded evidence for the opposite trend. This disagreement, which has also occurred in the literature, was clarified by temperature-dependent molecular dynamics runs that demonstrated Met112 approaching the metal at room temperature, a process that is expected to stabilize the His111-centered binding site through hydrophobic shielding of the metal coordination sphere.     

Tetrazole peptides as copper(II) ion chelators

The 1,5-disubstituted tetrazole ring a mimetic of the cis-amide bond is an unique element modifying the ability of peptides to chelate copper(II) ions. The position of the tetrazole ring system in the peptide backbone plays a critical role in the stabilization of the metallopeptide molecule. The insertion of a tetrazole between amide groups leads to enhancing the stability of the complex and to obtaining a very effective peptide chelating agent. These findings can provide important information for modeling biologically relevant peptide–metal binding sites. Some aspects of biological activity of tetrazole modified exogenous opioid peptides in the presence of copper(II) ions are also presented in this review.     

The octarepeat domain of the prion protein binds Cu(II) with three distinct coordination modes at pH 7.4

The prion protein (PrP) binds Cu2+ in its N-terminal octarepeat domain. This unusual domain is comprised of four or more tandem repeats of the fundamental sequence PHGGGWGQ. Previous work from our laboratories demonstrates that at full copper occupancy, each HGGGW segment binds a single Cu2+. However, several recent studies suggest that low copper occupancy favors different coordination modes, possibly involving imidazoles from histidines in adjacent octapeptide segments. This is investigated here using a combination of X-band EPR, S-band EPR, and ESEEM, along with a library of modified peptides designed to favor different coordination interactions. At pH 7.4, three distinct coordination modes are identified. Each mode is fully characterized to reveal a series of copper-dependent octarepeat domain structures. Multiple His coordination is clearly identified at low copper stoichiometry. In addition, EPR detected copper-copper interactions at full occupancy suggest that the octarepeat domain partially collapses, perhaps stabilizing this specific binding mode and facilitating cooperative copper uptake. This work provides the first complete characterization of all dominant copper coordination modes at pH 7.4.     

Determination of cobalt(II), copper(II) and iron(II) by ion chromatography with chemiluminescence detection

An optimized flow-injection manifold for the chemiluminescence determination of cobalt(II), copper(II), iron(II) and chromium(III) by their catalytic effect on the luminol reaction is described. Detection limits are 0.0006, 0.08, 0.3 and 0.1 ng ml^?1, respectively. The suppression effect of several carboxylic acids on the emission intensity is discussed. A procedure for the separation of cobalt(II), copper(II) and iron(II) on a low-capacity, silica-based cation-exchange column, using 5 mM oxalic acid at pH 4.2 as the mobile phase and post-column detection via the luminol reaction, is also described. Detection limits for cobalt(II) and copper(II) are 0.01 and 5 ng ml^?1, respectively.     

Isomerism of copper(II) coordination compounds with hydroxamic acids

The structure of Cu^II complexes with hydroxamic acids Cu[R¹N(O)−(O)CR²]₂, where R¹=Ph, R²=Me; R¹=Me, R²=Ph, was studied by ESR spectroscopy. In toluene solutions and low-temperature glasses, the complexes exist as two forms, which were identified ascis-andtrans-isomers. The proportions of the isomers were determined. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 726–729, April, 1999.     

Thermodynamics of copper(II) dicarboxylate coordination in aqueous solution

Summary The association of H⁺ and Cu²⁺ with malonate, maleate, succinate and phthalate ions has been investigated both potentiometrically and calorimetrically at 25° and I = 0.1 mol dm^–3 (NaClO₄).The structures of the complexes existing in aqueous solution are proposed on the basis of thermodynamic data.     

Synthesis of copper(II) coordination compounds with benzoxazine derivatives

Copper(II) complexes with 1,2-dihydro-4H-3,1-benzoxazine (HL) derivatives, CuLOH, were prepared by anodic electrosynthesis. The use of an amalgamated copper anode resulted in an increase in the electrosynthesis rate. It follows from the IR spectra of the products that the benzoxazine ring in the ligand molecule is opened upon complexation, while a six-membered chelate ring involving the oxygen atom of the deprotonated phenol group and the imine nitrogen atom is formed.     

Chemical modulation of the luminescence of a DNA-bound diruthenium(II) complex by copper(II) ion and EDTA

A method for the chemical modulation of the photoluminescence of a DNA-bound diruthenium(II) complex, [(bipy)₂Ru(bpib)Ru(bipy)₂]⁴⁺ (bipy = 2,2′-bipyridine, bpib = 1,4-bis([1, 10]phenanthroline [5,6-d]imidazol-2-yl) benzene) by the introduction of Cu²⁺ ion and EDTA has been developed. The diruthenium(II) complex showed strong photoluminescence both in buffer solutions and on an indium-tin oxide (ITO) surface, which was not modulated by Cu²⁺ or EDTA. The DNA-bound [(bipy)₂Ru(bpib)Ru(bipy)₂]⁴⁺ with a binding constant of 3.8 × 10⁴ M⁻¹ showed an enhancement in the luminescence based on the electrostatic interaction between the complex and DNA. The presence of Cu²⁺ was found to quench the luminescence of DNA-bound [(bipy)₂Ru(bpib)Ru(bipy)₂]⁴⁺, but the quenched luminescence was recovered by addition of an equimolar concentration of EDTA. Hence, the photoluminescence of DNA-bound [(bipy)₂Ru(bpib)Ru(bipy)₂]⁴⁺ depends strongly on the introduction of Cu²⁺ and EDTA.     

Studies of nitric oxide interaction with mono- and dinuclear copper(II) complexes of prion protein bis-octarepeat fragments

The interaction of nitric oxide with copper(ii) complexes of two octarepeat sequences belonging to the prion protein was studied, considering both mononuclear and dinuclear systems, i.e. Cu-Ac-(PHGGGWGQ)(2)-NH(2) and Cu(2)-Ac-(PHGGGWGQ)(2)-NH(2), respectively. The NO interaction with both systems was followed in aqueous solutions at physiological pH value, by using UV-Vis and EPR spectroscopic techniques as well as cyclic voltammetry. The mechanism of NO interaction with the mononuclear copper complex can be considered similar to that previously observed for the analogous copper systems with Ac-HGGG-NH(2) and Ac-PHGGGWGQ-NH(2). A more complicated behaviour was found with the copper dinuclear system, in which the involvement of two different intermediate complex species was evidenced. A positive cooperativity between the two copper ions, in the reduction process was inferred. When working with a large excess of the Ac-(PHGGGWGQ)(2)-NH(2) ligand, the frozen-solution EPR parameters pertain to the well characterized [Cu(N(im))(4)](2+) unit, which did not exhibit any interaction with NO. The presence of a free coordination site is the necessary requirement for the NO interaction to occur, as found only in the square-pyramidal geometry of [Cu(L)H(-2)] or [Cu(2)(L)H(-4)] complex species, which form when copper and ligand concentrations are similar.     

Pharmacological activities of a propylthiouracil compound structurally modified by coordination with copper(II)

For the first time, pharmacological activities for propylthiouracil (actually used as antithyroid drug) were determined. In addition, a new propylthiouracil copper(II) complex ([Cu(PTU)₂]₂) was synthesized and characterized by FTIR, EPR, UV–visible, and diffuse reflectance spectroscopies including elemental analysis, dissolution profiles, and stability studies. Taking into account the correlation between Graves’ disease and the formation of reactive oxygen species (ROS) and other free radicals, the ligand and the complex were tested for their antioxidant effects on O₂^·? and OH^· radicals. A significant increase in the disruption of OH^· radical was observed for PTU and its copper(II) complex, but neither of them have the ability to dismutate the O₂^·? radical. Antimicrobial activities were also determined observing that the complex is very active against Gram-positive bacteria. In addition, the ability of PTU and its complex to inhibit acid and alkaline phosphatases were analyzed. Results showed that PTU had no effect, while the complex behaved as a potent ALP (alkaline phosphatases) inhibitor. Finally, albumin interaction experiments denoted high affinity towards the complex in contrast with PTU with a constant binding value two hundred times higher than the ligand and bearing two binding sites. Based on this study, it has been hypothesized that ([Cu(PTU)₂]₂ would be a promising candidate for further evaluation as an antioxidant, antimicrobial and phosphatase alkaline inhibitor agent.     

Simultaneous determination of copper(II) and cobalt(II) by ion chromatography coupled with chemiluminescent detection.

In the absence of any special luminescent reagents, a weakly chemiluminescent emission was observed during the decomposition of hydrogen peroxide, catalyzed by transition-metal ions, such as Cu(II) and Co(II), in basic aqueous solution. The chemiluminescent intensity was significantly enhanced by the addition of ethyldimethylcetylammonium bromide and uranine. The signal-to-noise ratio (S/N) was proportional to the concentrations of Cu(II) and Co(II). Based on these phenomena, a flow-injection chemiluminescent method for the simultaneous separation and determination of Cu(II) and Co(II) was developed. The detection limits of the present chemiluminescent method for Cu(II) and Co(II) were 7.5 and 0.01 ng/ml, respectively. After ion chromatographic separation of Cu(II) and Co(II) by an IonPac CS5A column with oxalic acid and lithium hydroxide monohydrate as the eluent, the present chemiluminescent system was used as a post-column detector for these two transition metal ions in natural water samples.     

Synthesis of new oxamide-based ligand and its coordination behavior towards copper(II) ion: spectral and electrochemical studies

A new ligand N,N'-bis{3-(2-formyl-4-methyl-phenol)-6-iminopropyl}oxamide (L) and its mono- and binuclear copper(II) complexes have been synthesized and characterized. The ligand shows absorption maxima at 249 and 360 with a weak transition at 455 nm. The ligand was found to be fluorescent and shows an emission maximum at 516 nm on excitation at 360 nm. The electronic spectra of the mono- and binuclear Cu(II) complexes exhibited a d-d transition in the region 520-560 nm characteristic of square planar geometry around Cu(II) ion. The ESR spectrum of the mononuclear complex showed four lines with nuclear hyperfine splitting. The binuclear complex showed a broad ESR spectrum with g=2.10 due to antiferromagnetic interaction between the two Cu(II) ions. The room-temperature magnetic moment values (micro(eff)) for the mono- and binuclear Cu(II) complexes are found to be 1.70 micro(B) and 1.45 micro(B), respectively. The electrochemical studies of the mononuclear Cu(II) complex showed a single irreversible one-electron wave at -0.70 V (E(pc)) and the binuclear Cu(II) complex showed two irreversible one-electron reduction waves at -0.75 V (E(pc)(1)) and -1.27 V (E(pc)(2)) in the cathodic region.     

Rare configuration of tautomeric benzimidazolecarboxylate ligands in cadmium(II) and copper(II) coordination polymers

Two Cd(HBimc)-based isomers, [Cd(HBimc^N)(HBimc^T)(H₂O)]·3.5H₂O·EtOH (1a·3.5H₂O·EtOH, H₂Bimc=1H-benzimidazole-5-carboxylic acid) and [Cd(HBimc^N)(HBimc^T)(H₂O)] (1b), and two Cu(HMBimc)-based coordination polymers, [Cu(HMBimc^N)₂(H₂O)]·1/2H₂O (2·1/2H₂O, H₂MBimc=2-methyl-1H-benzimidazole-5-carboxylic acid) and [Cu(HMBimc^T)₂]·2THF·H₂O (3·2THF·H₂O), were self-assembled from Cd(ClO₄)₂·6H₂O/H₂Bimc and Cu(ClO₄)₂·6H₂O/H₂MBimc systems, respectively. Compound 1a adopts a ladder-like chain structure, comprised of a hydrogen-bond-stabilized Cd₂(HBimc^N)₂-metallocyclic stair and a 1D straight -(Cd-HBimc^T)_n- edge, whereas compound 1b exhibits a 2D (4,4)-rhombus layered structure, intercrossed by 1D -(Cd-HBimc^N)_n- chains and -(Cd-HBimc^T)_n- chains. Compound 2 shows a 1D double-stranded wave-like chain from two single-stranded wave-like -(Cu-HMBimc^N)_n- chains and compound 3 adopts a 2D (4,4)-topological layer structure, intercrossed by subunits of 1D -(Cu-HMBimc^T)_n- chains. Interestingly, a pair of tautomeric HBimc building blocks—normal (N or HBimc^N) and tautomer (T or HBimc^T)—is simultaneously included in the structures of 1a and 1b, whilst the N- and T-configured HMBimc building blocks are present as separate entities in Cu species, 2 and 3, respectively. The existence of only a tautomer (T) mode of the benzimidazolecarboxylate-based ligand in a Cu(II) network is observed for the first time.     

Phosphorescent sensor for robust quantification of copper(II) ion

A phosphorescent sensor based on a multichromophoric iridium(III) complex was synthesized and characterized. The construct exhibits concomitant changes in its phosphorescence intensity ratio and phosphorescence lifetime in response to copper(II) ion. The sensor, which is reversible and selective, is able to quantify copper(II) ions in aqueous media, and it detects intracellular copper ratiometrically.