BDNF- and VEGF-Responsive Stimulus to an NGF Mimic Cyclic Peptide with Copper Ionophore Capability and Ctr1/CCS-Driven Signaling

Neurotrophins are a family of growth factors that play a key role in the development and regulation of the functioning of the central nervous system. Their use as drugs is made difficult by their poor stability, cellular permeability, and side effects. Continuing our effort to use peptides that mimic the neurotrophic growth factor (NGF), the family model protein, and specifically the N-terminus of the protein, here we report on the spectroscopic characterization and resistance to hydrolysis of the 14-membered cyclic peptide reproducing the N-terminus sequence (SSSHPIFHRGEFSV (c-NGF(1-14)). Far-UV CD spectra and a computational study show that this peptide has a rigid conformation and left-handed chirality typical of polyproline II that favors its interaction with the D5 domain of the NGF receptor TrkA. c-NGF(1-14) is able to bind Cu2+ with good affinity; the resulting complexes have been characterized by potentiometric and spectroscopic measurements. Experiments on PC12 cells show that c-NGF(1-14) acts as an ionophore, influencing the degree and the localization of both the membrane transporter (Ctr1) and the copper intracellular transporter (CCS). c-NGF(1-14) induces PC12 differentiation, mimics the protein in TrkA phosphorylation, and activates the kinase cascade, inducing Erk1/2 phosphorylation. c-NGF(1-14) biological activities are enhanced when the peptide interacts with Cu2+ even with the submicromolar quantities present in the culture media as demonstrated by ICP-OES measurements. Finally, c-NGF(1-14) and Cu2+ concur to activate the cAMP response element-binding protein CREB that, in turn, induces the brain-derived neurotrophic factor (BDNF) and the vascular endothelial growth factor (VEGF) release.

Copper(II) Complexes with Carnosine Conjugates of Hyaluronic Acids at Different Dipeptide Loading Percentages Behave as Multiple SOD Mimics and Stimulate Nrf2 Translocation and Antioxidant Response in In Vitro Inflammatory Model

A series of copper(II) complexes with the formula [Cu2+Hy(x)Car%] varying the molecular weight (MW) of Hyaluronic acid (Hy, x = 200 or 700 kDa) conjugated with carnosine (Car) present at different loading were synthesized and characterized via different spectroscopic techniques. The metal complexes behaved as Cu, Zn-superoxide dismutase (SOD1) mimics and showed some of the most efficient reaction rate values produced using a synthetic and water-soluble copper(II)-based SOD mimic reported to date. The increase in the percentage of Car moieties parallels the enhancement of the I50 value determined via the indirect method of Fridovich. The presence of the non-functionalized Hy OH groups favors the scavenger activity of the copper(II) complexes with HyCar, recalling similar behavior previously found for the copper(II) complexes with Car conjugated using β-cyclodextrin or trehalose. In keeping with the new abilities of SOD1 to activate protective agents against oxidative stress in rheumatoid arthritis and osteoarthritis diseases, Cu2+ interaction with HyCar promotes the nuclear translocation of erythroid 2-related factor that regulates the expressions of target genes, including Heme-Oxigenase-1, thus stimulating an antioxidant response in osteoblasts subjected to an inflammatory/oxidative insult.

A Deeper Insight in Metal Binding to the hCtr1 N-terminus Fragment: Affinity, Speciation and Binding Mode of Binuclear Cu2+ and Mononuclear Ag+ Complex Species

Ctr1 regulates copper uptake and its intracellular distribution. The first 14 amino acid sequence of the Ctr1 ectodomain Ctr1_(1-14) encompasses the characteristic Amino Terminal Cu²⁺ and Ni²⁺ binding motif (ATCUN) as well as the bis-His binding motif (His5 and His6). We report a combined thermodynamic and spectroscopic (UV-vis, CD, EPR) study dealing with the formation of Cu²⁺ homobinuclear complexes with Ctr1_(1-14), the percentage of which is not negligible even in the presence of a small Cu²⁺ excess and clearly prevails at a M/L ratio of 1.9. Ascorbate fails to reduce Cu²⁺ when bound to the ATCUN motif, while it reduces Cu²⁺ when bound to the His5-His6 motif involved in the formation of binuclear species. The histidine diade characterizes the second binding site and is thought to be responsible for ascorbate oxidation. Binding constants and speciation of Ag⁺ complexes with Ctr1_(1-14), which are assumed to mimic Cu⁺ interaction with N-terminus of Ctr1_(1-14), were also determined. A preliminary immunoblot assay evidences that the anti-Ctr1 extracellular antibody recognizes Ctr1_(1-14) in a different way from the longer Ctr1_(1-25) that encompasses a second His and Met rich domain.

Peptides Derived from Angiogenin Regulate Cellular Copper Uptake

The angiogenin protein (ANG) is one of the most potent endogenous angiogenic factors. In this work we characterized by means of potentiometric, spectroscopic and voltammetric techniques, the copper complex species formed with peptide fragments derived from the N-terminal domain of the protein, encompassing the sequence 1-17 and having free amino, Ang1-17, or acetylated N-terminus group, AcAng1-17, so to explore the role of amino group in metal binding and cellular copper uptake. The obtained data show that amino group is the main copper anchoring site for Ang1-17. The affinity constant values, metal coordination geometry and complexes redox-potentials strongly depend, for both peptides, on the number of copper equivalents added. Confocal laser scanning microscope analysis on neuroblastoma cells showed that in the presence of one equivalent of copper ion, the free amino Ang1-17 increases cellular copper uptake while the acetylated AcAng1-17 strongly decreases the intracellular metal level. The activity of peptides was also compared to that of the protein normally present in the plasma (wtANG) as well as to the recombinant form (rANG) most commonly used in literature experiments. The two protein isoforms bind copper ions but with a different coordination environment. Confocal laser scanning microscope data showed that the wtANG induces a strong increase in intracellular copper compared to control while the rANG decreases the copper signal inside cells. These data demonstrate the relevance of copper complexes' geometry to modulate peptides' activity and show that wtANG, normally present in the plasma, can affect cellular copper uptake.

Copper (II) binding properties of an octapeptide fragment from the R3 region of tau protein: A combined potentiometric, spectroscopic and mass spectrometric study

The copper(II) complexes of a peptide fragment of the R3 domain of tau protein (tau(326-333) Ac-GNIHHKPG-NH₂) and its mutants (Ac-GNGHHKPG-NH₂, Ac-GNIHHKAG-NH₂, Ac-GNGAHKPG-NH₂ and Ac-GNGHAKPG-NH₂) have been studied by potentiometric and spectroscopic (UV-Vis, CD) methods. ESR spectroscopy and mass spectrometry were also used to prove the coordination mode of the mononuclear complexes and the formation of dinuclear species, respectively. It has been demonstrated that the (326-333) fragment of tau protein is a versatile and effective ligand for copper(II) coordination. The versatility of copper(II) binding is related to the presence of two adjacent histidyl residues in the sequence, which results in the coexistence of mononuclear, bis(ligand) and dinuclear complexes at different metal to ligand ratios. The 1:1 mononuclear complexes are, however, the dominant species with all peptides and the imidazole-N and one to three deprotonated amide nitrogen atoms towards the N-terminal side of the histidyl residue have been suggested as metal binding sites. This binding mode allows the formation of coordination isomers because any of the two histidine moieties can be the primary anchoring site. It is evident from the CD spectroscopic measurements that the isomers are present in almost equal concentration. The copper(II) binding affinity of the native fragment of tau protein is comparable to that of a similar 2-histidine fragment of amyloid-β mutant, Ac-SGAEGHHQK-NH₂ but the comparison with an independent histidyl residue (H32) from the N-terminal region of the protein reveals the predominance of H32 over the histidines in the R3 domain.

Graphene Oxide Nanosheets Tailored With Aromatic Dipeptide Nanoassemblies for a Tuneable Interaction With Cell Membranes

Engineered graphene-based derivatives are attractive and promising candidates for nanomedicine applications because of their versatility as 2D nanomaterials. However, the safe application of these materials needs to solve the still unanswered issue of graphene nanotoxicity. In this work, we investigated the self-assembly of dityrosine peptides driven by graphene oxide (GO) and/or copper ions in the comparison with the more hydrophobic diphenylalanine dipeptide. To scrutinize the peptide aggregation process, in the absence or presence of GO and/or Cu²⁺, we used atomic force microscopy, circular dichroism, UV-visible, fluorescence and electron paramagnetic resonance spectroscopies. The perturbative effect by the hybrid nanomaterials made of peptide-decorated GO nanosheets on model cell membranes of supported lipid bilayers was investigated. In particular, quartz crystal microbalance with dissipation monitoring and fluorescence recovery after photobleaching techniques were used to track the changes in the viscoelastic properties and fluidity of the cell membrane, respectively. Also, cellular experiments with two model tumour cell lines at a short time of incubation, evidenced the high potential of this approach to set up versatile nanoplatforms for nanomedicine and theranostic applications.

The curious case of opossum prion: a physicochemical study on copper(ii) binding to the bis-decarepeat fragment from the protein N-terminal domain

The opossum is a peculiar model of immunity to prion diseases. Here we scrutinised the bis-decarepeat peptide sequence of the opossum prion (Op_bis-deca) protein by a multitechnique approach, with a combined experimental (potentiometry, UV-visible, circular dichroism, NMR and EPR spectroscopy, quartz crystal microbalance with dissipation monitoring and confocal microscopy) and simulation (DFT calculations) approach. Results showed that the macrochelate structures formed upon the binding to Cu(ii) by the analogous bis-octarepeat peptide sequence of human prion (Hu_bis-octa) are not found in the case of Op_bis-deca. At physiological pH and equimolar amount of copper ions, the [CuLH-2] is the major species formed by Op_bis-deca. In this species one imidazole and two amide nitrogen atoms are involved in metal coordination and its stability constant value is lower than that of the analogous species formed by Hu_bis-octa, due to the presence of an extra proline residue. Moreover, the study on the interaction of the peptides or the peptide/Cu(ii) complexes with the model cell membranes made of supported lipid bilayers disclosed different levels of interaction, monitored by the viscoelastic changes of the membranes, which exhibited a similar viscoelastic response at the interface of the two complexes, while in the absence of Cu(ii), the Hu_bis-octa/SLB interface was more viscoelastic than the Op_bis-deca one.

The copper(II) binding centres of carbonic anhydrase are differently affected by reductants that ensure the redox intracellular environment

Copper is involved in several biological processes. The static and labile copper pools are controlled by means of a network of influx and efflux transporters, storage proteins, chaperones, transcription factors and small molecules as glutathione (GSH), which contributes to the cell reducing environment. To follow the fate of intracellular copper labile pool, a variant of human apocarbonic anhydrase has been proposed as fluorescent probe to monitor cytoplasmic Cu²⁺. Aware that in this cellular compartment copper ion is present as Cu⁺, electron spin resonance technique (ESR) was used to ascertain whether (bovine or human) carbonic anhydrase (CA) was able to accommodate Cu⁺ in the same sites occupied by Cu²⁺, in the presence of naturally occurring reducing agents such as ascorbate and GSH. Our ESR results on Cu²⁺ complexes with CA allow for a complete characterization of the two metal binding sites of the protein in solution. The use of the reported affinity constants of zinc in the catalytic site and of Cu²⁺ in the peripheral and catalytic site, allow us to obtain the speciation of copper species mimicking the spectroscopic study conditions. The different Cu²⁺ coordination features in the catalytic and the peripheral (the N-terminus cleft mouth) binding sites influence the chemical reduction effect of the two main naturally occurring reductants. Ascorbate reversibly reduces the Cu²⁺ complex with CA, while glutathione irreversibly induces the formation of Cu²⁺ complex with its oxidized form (GSSG). Our results questioned the use of CA as intracellular Cu²⁺ sensor. Furthermore, translating these findings to intracellular environment, the conversion of GSH in GSSG can significantly alter the metallostasis.

Copper ion interaction with the RNase catalytic site fragment of the angiogenin protein: an experimental and theoretical investigation

The angiogenin protein (Ang) is a member of the vertebrate-specific secreted ribonucleases and one of the most potent angiogenic factors known. Ang is a normal constituent of human plasma and its concentration increases under some physiological and pathological conditions to promote neovascularization. Ang was originally identified as an angiogenic tumour factor, but its biological activity has been found to extend from inducing angiogenesis to promoting cell survival in different neurodegenerative diseases. Ang exhibits weak ribonucleolytic activity, which is critical for its biological functions. The RNase catalytic sites are two histidine residues, His-13 and His-114, and the lysine Lys-40. Copper is also an essential cofactor in angiogenesis and influences angiogenin's biological properties. The main Cu(ii) anchoring site of Ang is His-114, where metal binding inhibits RNase activity of the protein. To reveal the Cu(ii) coordination environment in the C-terminal domain of the Ang protein, we report on the characterization, by means of potentiometric, voltammetric, and spectroscopic (CD, UV-Vis and EPR) methods and DFT calculations, of Cu(ii) complexes formed with a peptide fragment including the Ang sequence 112-117 (PVHLDQ). Potentiometric titrations indicated that [CuLH_-2] is the predominant species at physiological pH. EPR, voltammetric data and DFT calculations are consistent with a CuN₃O₂ coordination mode in which a distorted square pyramidal arrangement of the peptide was observed with the equatorial positions occupied by the nitrogen atoms of the deprotonated amides of the Asp and Leu residues, the δ-N atom of histidine and the oxygen atom of the aspartic carboxylic group. Moreover, two analogous peptides encompassing the PVHLNQ and LVHLDQ sequences were also characterized by using thermodynamic, spectroscopic and DFT studies to reveal the role they play in Cu(ii) complex formation by the carboxylate side chain of the Asp and Pro residues, a known breaking-point in metal coordination.

From Peptide Fragments to Whole Protein: Copper(II) Load and Coordination Features of IAPP

The copper-binding features of rat islet amyloid polypeptide (r-IAPP) are herein disclosed through the determination of the stability constants and spectroscopic properties of its copper complex species. To mimic the metal binding sites of the human IAPP (h-IAPP), a soluble, single-point mutated variant of r-IAPP, having a histidine residue in place of Arg18, was synthesized, that is, r-IAPP(1-37; R18H). The peptide IAPP(1-8) was also characterized to have deeper insight into the N-terminus copper(II)-binding features of r-IAPP as well as of its mutated form. A combined experimental (thermodynamic and spectroscopic) and computational approach allowed us to assess the metal loading and the coordination features of the whole IAPP. At physiological pH, the N-terminal amino group is the Cu²⁺ main binding site both of entire r-IAPP and of its mutated form that mimics h-IAPP. The histidine residue present in this mutated polypeptide accounts for the second Cu²⁺ binding. We can speculate that the copper driven toxicity of h-IAPP in comparison to that of r-IAPP can be attributed to the different metal loading and the presence of a second metal anchoring site, the His₁₈ , whose role is usually invoked in the process of h-IAPP aggregation.

Influence of the N-terminus acetylation of Semax, a synthetic analog of ACTH(4-10), on copper(II) and zinc(II) coordination and biological properties

Semax is a heptapeptide (Met-Glu-His-Phe-Pro-Gly-Pro) that encompasses the sequence 4-7 of N-terminal domain of the adrenocorticotropic hormone and a C-terminal Pro-Gly-Pro tripeptide. N-terminal amino group acetylation (Ac-Semax) modulates the chemical and biological properties of parental peptide, modifying the ability of Semax to form complex species with Cu(II) ion. At physiological pH, the main complex species formed by Ac-Semax, [CuLH_-2]^2-, consists in a distorted CuN₃O chromophore with a weak apical interaction of the methionine sulphur. Such a complex differs from the Cu(II)-Semax complex system, which exhibits a CuN₄ chromophore. The reduced ligand field affects the [CuLH_-2]^2- formal redox potential, which is more positive than that of Cu(II)-Semax corresponding species. In the amino-free form, the resulting complex species is redox-stable and unreactive against ascorbic acid, unlike the acetylated form. Semax acetylation did not protect from Cu(II) induced toxicity on a SH-SY5Y neuroblastoma cell line, thus demonstrating the crucial role played by the free NH₂ terminus in the cell protection. Since several brain diseases are associated either to Cu(II) or Zn(II) dyshomeostasis, here we characterized also the complex species formed by Zn(II) with Semax and Ac-Semax. Both peptides were able to form Zn(II) complex species with comparable strength. Confocal microscopy imaging confirmed that peptide group acetylation does not affect the Zn(II) influx in neuroblastoma cells. Moreover, a punctuate distribution of Zn(II) within the cells suggests a preferred subcellular localization that might explain the zinc toxic effect. A future perspective can be the use of Ac-Semax as ionophore in antibody drug conjugates to produce a dysmetallostasis in tumor cells.

Liposome antibody–ionophore conjugate antiproliferative activity increases by cellular metallostasis alteration

Carnosine derivative containing liposomes functionalized with the Fab' fragment of Trastuzumab were synthesized.

Electrochemical characterization of ordered arrays of metallo-porphyrins in aqueous solution

Metallated meso-tetrakis(N-methyl-4-pyridyl)porphyrin (MTMPyP) and 5,11,17,23-tetrasulfonato-25,26,27,28-tetrakis-(hydroxylcarbonylmethoxy)-calix[4]arene (C(4)TsTc) were used as key components for building up discrete supramolecular entities starting from the formation of the template species MTMPyP:C(4)TsTc (1 : 4, M = Cu, Zn). The stepwise addition of further amount of porphyrin allows the facile non-covalent synthesis of discrete supramolecular entities (2 : 4 and 3 : 4) which can be built up just by programming the right stoichiometric addition of the proper porphyrin. The redox potentials of these supramolecular complexes in aqueous media, as well as those of the parent metalloporphyrins, have been characterized by using square wave voltammetry technique. The use of the simulation procedure leads us to establish the electrochemical steps involved in the redox processes for each supramolecular species, evidencing multistep electron reductions which were not experimentally resolved clearly because of their closeness. The most striking result is that the electrochemistry of each of these supramolecular complexes is different from that of the parent components. This "anomalous" behavior can be explained only considering each of these supramolecular complexes as a unique entity, in which such an internal electronic communication might occur. The formation of the 1 : 4 supramolecular complex produces a negative shift as to the metallated porphyrin redox potentials of about 30 mV. In the case of 2 : 4 and 3 : 4 species, the redox potentials progressively shifts towards more positive values by about 10-15 mV for each complexation step.

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.

Cellular and molecular effects of protons: apoptosis induction and potential implications for cancer therapy

Due to their ballistic precision, apoptosis induction by protons could be a strategy to specifically eliminate neoplastic cells. To characterize the cellular and molecular effects of these hadrons, we performed dose-response and time-course experiments by exposing different cell lines (PC3, Ca301D, MCF7) to increasing doses of protons and examining them with FACS, RT-PCR, and electron spin resonance (ESR). Irradiation with a dose of 10 Gy of a 26,7 Mev proton beam altered cell structures such as membranes, caused DNA double strand breaks, and significantly increased intracellular levels of hydroxyl ions, are active oxygen species (ROS). This modified the transcriptome of irradiated cells, activated the mitochondrial (intrinsic) pathway of apoptosis, and resulted in cycle arrest at the G2/M boundary. The number of necrotic cells within the irradiated cell population did not significantly increase with respect to the controls. The effects of irradiation with 20 Gy were qualitatively as well as quantitatively similar, but exposure to 40 Gy caused massive necrosis. Similar experiments with photons demonstrated that they induce apoptosis in a significantly lower number of cells and in a temporally delayed manner. These data advance our knowledge on the cellular and molecular effects of proton irradiation and could be useful for improving current hadrontherapy protocols.

Nitrogen oxide interaction with copper complexes formed by small peptides belonging to the prion protein octa-repeat region

The interaction between NO and copper(II) complexes formed by peptides coming from the N-terminal prion protein octa-repeat region was studied. Aqueous solutions of the Cu-Ac-HGGG-NH(2) and the Cu-Ac-PHGGGWGQ-NH(2) systems around pH 7.5 were tested after the addition of NONOates as a source of NO. UV-Vis, room temperature and frozen solution EPR spectra showed the occurrence of copper(ii) reduction in all these complexes. The reduction of these complexes is probably mediated by the formation of a labile NO adduct, which, after re-oxidation, leads to a relatively stable NO(2)(-) adduct through the apical coordination along the void site of their square pyramidal structure. In fact, the most significant shifts in EPR magnetic parameters (g(||) and A(||) or g(iso) and A(iso)) as well as in the optical parameters (lambda(max) and epsilon(max)) gave a reason for geometrical changes of the copper coordination polyhedron from a distorted square pyramid to a pseudo-octahedron. The presence of oxygen in the aqueous solution hindered the reduction ability of NO towards copper, but it made it easier to return to the original species. In order to elucidate the possible mechanism of this interaction, the reduction of copper complexed by these ligands was followed by means of zinc powder addition. The further addition of nitrite to the solution containing reduced copper led to the conclusion that nitrite could easily form an adduct, which after re-oxidation presented the same spectral features of the species obtained when the NO interaction was followed. The complexity of this interaction could involve both an inner or an outer-sphere electron transfer mechanism.

Bioconjugates of cyclodextrins of manganese salen-type ligand with superoxide dismutase activity

6A,6B-Dideoxy-6A,6B-di[(N-salicylidene)amino]-beta-cyclodextrin was synthesized and characterized by NMR, UV and CD spectroscopy in order to prepare a N,N(')-bis-(salicylidene)ethane-1,2-diamine (SalenH(2)) type ligand. The manganese(III) complex was synthesized and characterized by UV and cyclic voltammetry and compared to EUK-8. The superoxide dismutase (SOD)-like and catalase-like activities were tested by indirect assay. The cyclodextrin complex shows a larger solubility than EUK-8 and good SOD-like activity. Catalase activity is also shown.

Water stability and cytotoxic activity relationship of a series of ferrocenium derivatives. ESR insights on the radical production during the degradation process

The cytotoxicity of some ferrocenium salts and the lack of activity of the corresponding ferrocenes has been already demonstrated. The cytotoxic activity in different conditions of decamethylferrocenium tetrafluoroborate (DEMFc(+)) in comparison with four other ferrocenium derivatives on MCF-7 cell line is reported. The relative stability in aqueous solutions with different buffering agents is investigated by means of UV-vis spectroscopy and correlated to the cytotoxic properties of the compounds. DEMFc(+), the most stable compound, shows the highest efficiency in inhibiting cell growth (IC(50) 35 microM, for 48 h treatment). Relaxation time measurements point out the involvement of water molecules in the degradation process. ESR results confirm the ability of ferrocenium cations to produce oxygen radical species as a consequence of their degradation in water. Oxygen-dependent formation of both hydroxyl and superoxide radicals is established by the spin-trapping technique. A direct evidence of the DEMFc(+) radical production into the viable cells is obtained by means of fluorescence-activated cell sorter (FACS) analysis that reveals a dose-dependent growth of 8-oxoguanine, the initial product of the guanine oxidation. This DNA oxidative stress justifies the cytotoxic effect of DEMFc(+). Furthermore, the cytotoxic cooperative effect of bleomycin, an iron-dependent antitumor drug, and DEMFc(+) has been tested. We have demonstrated the synergic effect between the two drugs, that is explained by the complementary oxidative damage inflicted to DNA as well as by the increasing of bleomycin activation by the iron(II/III) species available in the cell compartment from ferrocenium degradation.

ESR study of the non-enzymic scission of xyloglucan by an ascorbate–H2O2–copper system: the involvement of the hydroxyl radical and the degradation of ascorbate

Xyloglucan is degraded by a mixture of copper(II), hydrogen peroxide and ascorbate. In the presence of ascorbate and/or hydrogen peroxide, copper(II) species were rapidly reduced to copper(I), which react with hydrogen peroxide. Spin-trapping experiments showed that hydroxyl radicals formed and attacked xyloglucan causing its degradation. The formation of a carbon-centred ascorbyl (C-ascorbyl) radical and its degradation with the formation of oxalate, was also caused by hydroxyl radicals. As a consequence, the features of the bis(oxalate) copper(II) complex clearly appeared in the frozen solution ESR spectra. The formation of carbon-centred radicals on the xyloglucan is the trigger for a series of possible molecular rearrangements which led to its oxidative scission.

Copper(II) binding modes in the prion octapeptide PHGGGWGQ: a spectroscopic and voltammetric study

The N-terminal octapeptide repeat region of human prion protein (PrPc) is known to bind Cu(II). To investigate the binding modes of copper in PrPc, an octapeptide Ac-PHGGGWGQ-NH2 (1), which corresponds to an octa-repeat sequence, and a tetrapeptide Ac-HGGG-NH2 (2) have been synthesised. The copper(II) complexes formed with 1 and 2 have been studied by circular dichroism (CD) and electron spin resonance (ESR) spectroscopy. Both peptides form 1:1 complexes with Cu(II) at neutral and basic pH. CD, ESR and visible absorption spectra suggest a similar co-ordination sphere of the metal ion in both peptides, which at neutral pH consists of a square pyramidal geometry with three peptidic nitrogens and the imidazole nitrogen as donor atoms. Cyclic voltammetric measurements were used to confirm the geometrical features of these copper(II) complexes: the observation of negative redox potentials are in good agreement with the inferred geometry. All these results taken together suggest that peptide 1 provides a single metal binding site to which copper(II) binds strongly at neutral and basic pH and that the binding of the metal induces the formation of a stiffened structure in the HGGG peptide fragment.

Tyrosinase Models. Synthesis, Structure, Catechol Oxidase Activity, and Phenol Monooxygenase Activity of a Dinuclear Copper Complex Derived from a Triamino Pentabenzimidazole Ligand

The dicopper(II) complex with the ligand N,N,N',N',N"-pentakis[(1-methyl-2-benzimidazolyl)methyl]dipropylenetriamine (LB5) has been synthesized and structurally characterized. The small size and the quality of the single crystal required that data be collected using synchrotron radiation at 276 K. [Cu(2)(LB5)(H(2)O)(2)][ClO(4)](4): platelet shaped, P&onemacr;, a = 11.028 Å, b = 17.915 Å, c = 20.745 Å, alpha = 107.44 degrees, beta = 101.56 degrees, gamma = 104.89 degrees, V = 3603.7 Å(3), Z = 2; number of unique data, I >/= 2sigma(I) = 3447; number of refined parameters = 428; R = 0.12. The ligand binds the two coppers nonsymmetrically; Cu1 is coordinated through five N donors and Cu2 through the remaining three N donors, while two water molecules complete the coordination sphere. Cu1 has distorted TBP geometry, while Cu2 has distorted SP geometry. Voltammetric experiments show quasireversible reductions at the two copper centers, with redox potential higher for the CuN(3) center (0.40 V) and lower for the CuN(5) center (0.17 V). The complex binds azide in the terminal mode at the CuN(3) center with affinity lower than that exhibited by related dinuclear polyaminobenzimidazole complexes where this ligand is bound in the bridging mode. The catechol oxidase activity of [Cu(2)(LB5)](4+) has been examined in comparison with that exhibited by [Cu(2)(L-55)](4+) (L-55 = alpha,alpha'-bis{bis[(1-methyl-2-benzimidazolyl)methyl]amino}-m-xylene) and [Cu(2)(L-66)](4+) (L-66 = alpha,alpha'-bis{bis[2-(1-methyl-2-benzimidazolyl)ethyl]amino}-m-xylene) by studying the catalytic oxidation of 3,5-di-tert-butylcatechol in methanol/aqueous buffer pH 5.1. Kinetic experiments show that [Cu(2)(L-55)](4+) is the most efficient catalyst (rate constant 140 M(-1) s(-1)), followed by [Cu(2)(LB5)](4+) (60 M(-1) s(-1)), in this oxidation, while [Cu(2)(L-66)](4+) undergoes an extremely fast stoichiometric phase followed by a slow and substrate-concentration-independent catalytic phase. The catalytic activity of [Cu(2)(L-66)](4+), however, is strongly promoted by hydrogen peroxide, because this oxidant allows a fast reoxidation of the dicopper(I) complex during turnover. The activity of [Cu(2)(LB5)](4+) is also promoted by hydrogen peroxide, while that of [Cu(2)(L-55)](4+) is little affected. The phenol monooxygenase activity of [Cu(2)(LB5)](2+) has been compared with that of [Cu(2)(L-55)](2+) and [Cu(2)(L-66)](2+) by studying the ortho hydroxylation of methyl 4-hydroxybenzoate to give methyl 3,4-dihydroxybenzoate. The LB5 complex is much more selective than the other complexes since its reaction produces only catechol, while the main product obtained with the other complexes is an addition product containing a phenol residue condensed at ring position 2 of the catechol.

High Superoxide Dismutase Activity of a Novel, Intramolecularly Imidazolato-Bridged Asymmetric Dicopper(II) Species. Design, Synthesis, Structure, and Magnetism of Copper(II) Complexes with a Mixed Pyrazole-Imidazole Donor Set

A new dinucleating ligand, 1,5-bis(1-pyrazolyl)-3-[bis(2-imidazolyl)methyl] azapentane (Hbpzbiap), containing pyrazoles and imidazoles has been designed and synthesized. The synthesis and characterization of the copper complexes with the ligand Hbpzbiap and its dehydronated form are described. This study is aimed at modeling the active site of copper-zinc superoxide dismutase (SOD). Single crystals of the imidazolato-bridged complex [Cu(2)(bpzbiap)Cl(3)] (1) and non-imidazolato-bridged complex [Cu(2)(Hbpzbiap)Cl(4)] (2) were obtained and their structures determined by X-ray diffraction. Both structures show two copper centers in two different coordination environments: a distorted square pyramid and a distorted tetrahedron. The Cu-nitrogen bond lengths range from 1.919(4) to 2.039(3) Å and are as expected. The copper-copper distances from 5.566(1) to 6.104(1) Å being only slightly shorter than that found in bovine erythrocyte SOD. Temperature-dependent magnetic susceptibility study of 1 shows antiferromagnetic behavior with -2J = 96 cm(-)(1). From pH-dependent electron paramagnetic resonance and electronic spectra, [Cu(2)(bpzbiap)Cl(3)] has been demonstrated to be stable over a quite wide pH range including the physiological pH values. A low concentration of this complex (1) catalyzes the dismutation of superoxide at biological pH. Voltammetric studies indicate a quasi-reversible redox behavior in aqueous solution at pH 7. These results clearly indicate that complex 1 is a good model for superoxide dismutase.