An electron paramagnetic resonance study of native and modified freeze-dried cupric insulin hexamer.

Cupric insulin was modified by the addition of cross-linking disulphide bridges between hexamers. The electron paramagnetic resonance (EPR) spectrum of this freeze-dried material was compared with that of freeze-dried unmodified cupric insulin containing various amounts of copper and added water. The modified insulin was found to have cupric ion sites magnetically very similar to that of native insulin containing two cupric ions per hexamer. Native hexamer produced in the presence of 2 Cu(II) ions per hexamer gave, after freeze-drying, an EPR spectrum with A_∥^Cu=16.5 mT, g_∥=2.285 and g_⊥=2.059 (site 1). The use of 4 or 6 Cu(II) ions per hexamer resulted in spectra with two components-a major component with the same A_∥^Cu and g_∥ values as the sample containing 2 Cu(II) ions (site 1) and an additional minor component (site 2). These sites have been identified with the analogous zinc binding site within the hexamer formed by three B-10 histidine residues (site 1) [1, 2] and the site formed by the B-1 α-amino and A-17 glutamyl-γ-barboxylic acid functions where excess zinc is bound (site 2) [3, 4]. The addition of water to native hexamer containing 2, 4, or 6 Cu(II) ions resulted in the appearance of three distinct EPR absorptions, one of which had the same parameters as the freeze-dried native insulin containing 2 Cu(II) ions per hexamer (site 1). Two further sites appeared (3 and 4) with the following parameters: A_∥^Cu=15.0 mT, g_∥=2.353, and g_⊥=2.07; A_∥^Cu=16.5 mT, g_∥=2.315, and g_⊥=2.07, respectively.     

The binding of copper ions to daunomycin and adriamycin

Using visible absorption, CD, ¹H nmr, and epr spectroscopy, the Cu(II) binging properties of daunomycin, adriamycin, and N-trifluoroacetyl daunomycin in water and ethanol have been explored. The drugs form two water soluble complexes having Cu-drug stoichiometries of 1:1 and 1:2, and with apparent pK_as of formation of 5.6 and 6.5, respectively. At pH values above ～8, the drugs form insoluble polymeric complexes with Cu(II). Similar species are also observed in ethanol. The structure of the compounds have been interpreted in terms of binding of the deprotonated hydroxyquinone portion of the drug to the copper ion. No evidence for the binding of the amino group on daunosamine was found.     

Crystal structure and magnetic behavior of a copper(II)-(pyrazine 2,3-dicarboxylate) coordination polymer: 3D architecture stabilized by H-bonding

Reaction of Cu(ClO₄)₂ · 6H₂O and pyrazine 2,3-dicarboxylate (pzdc) in aqueous ammonia medium results [Cu(pyrazine 2,3-dicarboxylate)(H₂O)₂] · H₂O (1). The X-ray single crystal structure reveals that the compound is a 1D polymeric sinusoidal infinite chain which through intra- and inter-molecular hydrogen bonding interactions, involving lattice and coordinated water molecules with dicarboxylate oxygens and pyrazine nitrogens, gives rise to a 3D architecture. The variable temperature magnetic measurements show weak antiferromagnetic interactions between the Cu(II) centers. The best fit parameters through the typical equation for a uniform copper (II) chain are: J=−0.25 cm⁻¹, g=2.17, R=1.3×10−6. The EPR spectrum does not alter with temperature (from r.t. to 4 K). The spectra are typical for square-pyramidal geometry of copper(II) ions, g_∥=2.24 and g_⊥=2.10 (average g=2.15, in good agreement to the value obtained by susceptibility fit).     

A spectral investigation of the copper(II) complex of the antitumor compound bleomycin

A thorough spectral investigation of the copper(II) complex of the antitumor compound, bleomycin, has been carried out in solution employing optical, difference optical, electron spin resonance, and circular dichroism techniques. The optical spectrum of a pH = 7 solution of the 1:1 complex between copper(II) and bleomycin is characterized by a broad weak band in the visible region (λ_max = 610 nm) that cannot be resolved and intense ultraviolet bands at 317 (? = 2800), 327 (shoulder), 250 (? = 4700), and 257 nm (shoulder). The circular dichroism spectrum in the visible region shows the broad and weak visible absorption band contains at least three components (558, 675, and 880 nm) that are likely to be “d-d” in origin. The electron spin resonance spectrum is characteristic of a tetragonal d⁹ copper(II) system showing no rhombic distoritions at X-band frequencies (g_x = g_y ± 0.002). The spin Hamiltonian parameters for the pH = 7.0 solution corrected for second order effects are A_∥ = 177 × 10^?4 cm^?1, A_⊥ ? 15 × 10^?4 cm^?1, g_∥ = 2.214, g_⊥ = 2.039. Most interesting was the observation of extra hyperfine splitting due to endogenous nitrogen coordination in a 30% glycerol glass (A^N = 12.0 × 10^?4 cm^?1). That pattern is best interpreted as a seven-line sequence associated with three liganded nitrogens. A dramatic change in all spectral properties occurs when the pH of the copper(II)-bleomycin complex is lowered to 2.5. All these data taken together suggest a CuN₃O coordination complex in solution. Details and justifications as well as a discussion of the limitations of the interpretations are presented.     

Square-planar copper(II) halide complexes of tridentate ligands with π-π stacking interactions and alternating short and long Cu ? Cu distances

The preparation of a new tridentate N₂O-donor ligand N-(2-pyridylmethyl)-3-methoxysalicylaldiminato (HL) is described, together with the corresponding copper(II) complexes [Cu(L)X] (X = Cl⁻, Br⁻). The compounds were characterized by elemental analysis, spectral, magnetic and crystallographic studies. In both compounds, the local molecular structure of the Cu(II) ion involves a square-planar CuN₂OX chromophore, consisting of a deprotonated phenolate oxygen, an imine nitrogen, the pyridine nitrogen and X. In the solid state, π-π stacking interactions are dominantly present, involving the pyridine and phenolate rings of neighboring molecules, which lead to a one-dimensional arrangement with alternating short and long Cu ? Cu distances of [3.720, 4.599 Å] for the bromo complex and of [3.698, 4.696 Å] for the chloro complex. The temperature-dependent magnetic measurements and EPR data of polycrystalline samples, as well as of frozen solutions in CHCl₃ show that there is no observable exchange interaction between the Cu ions. The EPR parameters (g_∥, A_∥) agree with a perfect planar geometry, just as found in the X-ray analysis.     

Formation of a supramolecular ladder using dinuclear dicyanamide bridged Cu(II) species: Synthesis, crystal structure and magnetic property

The reaction of Cu(ClO₄)₂ · 6H₂O with bis(3-aminopropyl)methylamine and sodium dicyanamide in aqueous medium results in the formation of a dimeric dicyanamide complex of Cu(II), [Cu₂(medpt)₂(dca)₂](ClO₄)₂. The single crystal X-ray structure reveals that the dinuclear entities are extended to form a supramolecular 1D ladder by H-bonding. Each dinuclear entity is joined to the adjacent unit via the perchlorate anion. Variable temperature magnetic study was performed and the best-fit parameters are J = −1.20 ± 0.02 cm⁻¹, g = 2.08 ± 0.01 with R = 2 × 10⁻⁵. These clearly indicate the antiferromagnetic interaction between the Cu(II) center.     

Studies of the copper and heme cofactors of pseudomonad L-tryptophan-2,3-dioxygenase by electron paramagnetic resonance spectroscopy

l-Tryptophan-2,3-dioxygenase, (EC 1.13.1.12) purified from Pseudomonas acidovorans, is inactivated on aerobic aging or on treatment with K₃Fe(CN)₆, but regains activity in the presence of reducing agents such as sodium ascorbate. Examination of oxidized, inactive enzyme by electron paramagnetic resonance (epr) spectroscopy has revealed the presence of high spin ferriheme (g = 6.2) and of Cu(II) (g_⊥ = 2.065, g_∥ = 2.265) in the enzyme.The epr signal of Cu(II) in inactive tryptophan oxygenase is attenuated on the addition of ascorbate, whereas the high spin ferriheme signal is unaffected, indicating that the site of action of reducing agents in activating the enzyme is the enzymic copper. Quantitation of the Cu(II) signal in inactive tryptophan oxygenase by double integration accounts for 45% of the total copper.Addition of l-tryptophan to either inactive or active enzyme produces a decrease of 44 ± 5% of the epr signal of high spin ferriheme and the emergence of the epr signal of a low spin ferriheme (g_{1, 2, 3} = 2.66, 2.20, 1.81). Disappearance of the high spin ferriheme is hyperbolic (Hill coefficient, n = 1.02) with respect to l-tryptophan concentration, while the appearance of the low spin ferriheme is sigmoidal (Hill coefficient, n = 1.33) with respect to l-tryptophan concentration. The characteristics of the epr signal of this low spin ferriheme are intermediate between those of the signals of the hydroxides of hemoglobin and myoglobin and those in which two histidines are ligated to the ferriheme of hemoglobin. This may be the first example of the observation by epr of an allosteric parameter of an enzyme.     

The binuclear cupric cluster of Cancer magister methemocyanin

Cancer magister hemocyanin oxidized by H₂O₂ (Felsenfeld, G.F. and Printz, M.P. (1959) J. Am. Chem. Soc. 81, 6259–6264) contained 80–95% cupric copper, small amounts of EPR-detectable Cu(II), and native hemocyanin. The small amounts of EPR-detectable Cu(II) showed a signal characteristic of mononuclear Cu(II) in the region of g = 2 between 233°K and 10°K, with normal Curie behavior, and no Δm = 2 signal. Magnetic susceptibility measurements show the methemocyanin to be diamagnetic over the temperature range 1.5–77°K. It had an optical absorption maximum at 680 nm, ϵ = 60 ± 7 M_Cu⁻¹ · cm⁻¹ at 25°C, and at 327 nm (ϵ = 3 · 10² M_Cu⁻¹ · cm⁻¹), 360 nm, 420 nm, and 680 nm at 77°K. CD bands were observed at 340, 400–450, and 650 nm (very broad). Methemocyanin was not reduced to O₂-binding cuprous hemocyanin by dithionite, hydroxylamine, ascorbate, ferrocyanide, H₂O₂, or superoxide. Based upon the normal Curie behavior of the small EPR-detectable signal, the absence of paramagnetism, and some similarities of optical spectra between methemocyanin and oxyhemocyanin, we conclude that the diagmagnetic Cu(II) of this methemocyanin occurs in spin-coupled binuclear Cu(II) clusters having a configuration related to the binuclear Cu(II) cluster in oxyhemocyanin; but that inability of methemocyanin to undergo reductive reactivation, and the low molecular extinction coefficients of the optical absorption bands indicate that some chemical or steric alteration, perhaps peroxidatic in nature, takes place during its formation.     

A family of polyamino phenolic macrocyclic ligands. Acid-base and coordination properties towards Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) ions

The acid-base and coordination properties towards Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) of four polyamino-phenol macrocycles 15-hydroxy-3,6,9-triazabicyclo[9.3.1]pentadeca-11,13,1¹⁵-triene L1, 18-hydroxy-3,6,9,12-tetraazabicyclo[12.3.1]octadeca-14,16,1¹⁸-triene L2, 21-hydroxy-3,6,9,12,15-pentaazabicyclo[15.3.1]enaicosa-17,19,1²¹-triene L3 and 24-hydroxy-3,6,9,12,15,18-hexaazabicyclo[18.3.1]tetraicosa-20,22,1²⁴-triene L4 are reported. The protonation and stability constants were determined by means of potentiometric measurements in 0.15 mol dm⁻³ NMe₄Cl aqueous solution at 298.1 K. L1 forms highly unsaturated Co(II), Cu(II), Zn(II) and Cd(II) mononuclear complexes that are prone to give dimeric dinuclear species with [(MH₋₁L1)₂]²⁺ stoichiometry, in solution. L2 forms stable Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) mononuclear complexes that can coordinate external species as OH⁻ anion, giving hydroxylated complexes at alkaline pH. L3 forms stable Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) mononuclear complexes and Co(II), Ni(II), Cu(II) and Zn(II) dinuclear [M₂H₋₁L3]³⁺ species. L4 forms stable mono- and dinuclear Co(II), Cu(II), Zn(II) and Cd(II) complexes, but only mononuclear species with Pb(II). The effect of macrocyclic size is considered in the discussion of results.     

Methyl-substituted 4-nitropyridine N-oxides as ligands: structural, spectroscopic, magnetic and cytotoxic characteristics of copper(II) complexes

Seven new mono- and dinuclear Cu(II) complexes containing various methyl substituted 4-nitropyridine N-oxides as ligands were isolated and characterized physicochemically and biologically. The characterization included elemental analysis, magnetic and spectroscopic methods (diffuse reflectance and UV-visible absorption, IR, FIR). A single crystal X-ray diffraction analysis was performed for the complex with 2,5-dimethyl-4-nitropyridine N-oxide. Trans- and cis-square planar configuration around Cu ion was established for mono- and dinuclear species, respectively. In methanolic solutions the dinuclear species decompose into mononuclear ones with increasing 4 → 6 coordination number with attachment of two solvent molecules.The IR spectra showed that the strength of the Cu-ligand bond gauged by the degree of N-O elongation changed irregularly with position and number of methyl groups. Cytotoxic studies on the MCF-7 human breast cancer line revealed a structure-activity relationship: double blocking of the NO₂ group with two CH₃ groups rendered the complex completely inactive.     

Decomposition and superoxide dismutase activity of the copper complex of D-penicillamine (Cu(II)6Cu(I)8 (D-penicillamine)12 Cl)5−

The possible time- and/or light-dependent decomposition of the purple Cu(I), Cu(II)-complex of D-penicillamine (Cu(II)₆Cu(I)₈(D-penicillamine)₁₂Cl)^5? was examined. Superoxide dismutase activity of the freshly prepared complex was assayed using the nitroblue tetrazolium assay. The formazan colour formation was inhibited by 50% in the presence of approximately 500 μM copper. Ageing of the copper complex, especially in the light, resulted in a marked increase of EDTA-sensitive activity. Upon gel chromatography of the aged samples the original low inhibitory activity was restored. All EDTA-sensitive inhibitory activity was found in a clearly separated low M_r copper-containing fraction. Aerobic irradiation with a tungsten lamp at 30 °C accelerated the decomposition of (Cu(II)₆Cu(I)₈(D-penicillamine)₁₂Cl)^5?. ?_Cu518 = 1800 M^?1 cm^?1 dropped to ?_Cu640 = 60 M^?1 cm^?1. The photochemical conversion of (Cu(II)_6? Cu(I)₈(D-penicillamine)₁₂Cl)^5? was complete within 48 h. Due to the identical electronic absorption profile of both, the decomposition product and Cu(II) D-penicillamine disulphide the latter complex was assigned to be the unknown low M_r copper-compound. Circular dichroism and electron paramagnetic resonance measurements support this conclusion.     

The formation of a Cu(II)-amikacin complex and its biological activity and uses. Indirect estimation of amikacin by atomic absorption spectroscopy (AAS)

The [Cu(amikacin)(H₂O)₃] complex was prepared and characterized by elemental analysis, TGA and spectroscopic techniques (viz. IR, electronic and EPR spectra). The spectral results obtained indicate distorted octahedral geometry around the Cu(II) ion. The orbital reduction factors K∥ and K⊥ have been calculated. Amikacin can be estimated indirectly by atomic absorption spectroscopy using carbonate as an auxiliary ligand. The complex shows biological activity towards six organisms. Amikacin is biologically active towards Sarcina lutea, whereas the Cu(II)-amikacin complex is biologically inactive.     

Synthesis, crystal structure, spectral and magnetic studies and catecholase activity of copper(II) complexes with di- and tri-podal ligands

Five complexes of copper(II) acetate with Schiff base ligands based on salicylaldehyde and N,N-dimethylamino)ethyl/propyl amine and their reduced products, have been synthesized and characterized by various spectroscopic methods. The solid state structures of 1, 2 and 3 have been determined using single crystal X-ray diffraction method. The structures of the other two compounds have been proposed on the basis of spectroscopic and physical methods. The compounds 1, 3 and 4 are dinuclear complexes of the tridentate ligands, where the two Cu(II) centers have square pyramidal geometry with bridging acetate or phenoxo groups. Each arm of the tripodand ligand forms a mononuclear, magnetically dilute complex 5 having five coordinated Cu(II) ions. Complex 2 is mononuclear with a square pyramidal stereochemistry. The catalytic performance of the oxidation of 3,5-di-tert-butylcatechol to quinone was studied using UV-Vis absorption spectral methods. Complex 4 exhibits the highest activity with a turnover number of 41 h⁻¹ while other showed lower rates of oxidation. A kinetic treatment on the basis of Michaelis-Menten model was applied. Ease of removal of the exogenous acetate ligands and easy access to the Cu(II) ions have been seen to affect the activity in the complexes. At the same time presence of two endogenous phenoxo bridges in the dinuclear complexes reduces the activity.     

Synthesis and characterization of new unsaturated macrobicyclic and bis(macrocyclic) copper(II) complexes containing N-CH2-N linkages

New copper(II) complexes [CuL²]²⁺ (L²=7,7,9-trimethyl-1,3,6,10,13-pentaazabicyclo[11,2,1^1.13]hexadec-9-ene) and [Cu₂(L³)(H₂O)₂]⁴⁺ have been prepared by the reaction of [CuL¹]²⁺ (L¹=5,5,7-trimethyl-1,4,8,11,14-pentaazatetradce-7-ene) and formaldehyde. The mononuclear complex [CuL²]²⁺ has a square-planar coordination geometry with a 5-6-5-6 chelate ring sequence and is relatively stable even in low pH at room temperature. The dinuclear complex [Cu₂(L³)(H₂O)₂]⁴⁺ consists of two unsaturated 15-membered pentaaza macrocyclic units (7,7,9-trimethyl-1,3,6,10,13-pentaazacyclopentadec-9-ene) that are linked together by a methylene group in a tilted face-to-face arrangement [Cu?Cu distance: 7.413(2) Å ]. Each macrocyclic unit of [Cu₂(L³)(H₂O)₂]⁴⁺ contains one four-membered chelate ring and has a severely distorted octahedral coordination polyhedron. The dinuclear complex is quite stable in aqueous solutions containing an excess of formaldehyde or in dry acetonitrile but is decomposed to [CuL¹]²⁺ and [CuL²]²⁺ in pure water.     

Chlorido-bridged polymeric and dinuclear copper(II) complexes with tridentate Schiff base: Synthesis, crystal structure and magnetic properties

Two copper(II) complexes, [Cu(qsal)Cl](DMF) (1) and [Cu₂(qsalBr)₂Cl₂](DMF) (2), with tridentate Schiff base ligands, 8-(salicylideneamino)quinoline (Hqsal) and 8-(5-bromo-salicylideneamino)quinoline (HqsalBr), respectively, were synthesised and structurally characterized. Each copper(II) ion in the two complexes is in a distorted square pyramidal N₂OCl₂ environment. Complex 1 exists as a polymeric species via equatorial-apical chloride bridges, whereas 2 is a di-chlorido-bridged dinuclear complex, where each bridging chloride simultaneously occupies an in-plane coordination site on one copper(II) ion and an apical site on the other copper(II) ion. Variable-temperature magnetical susceptibility measurements on the two complexes in the temperature range 2-300 K indicate the occurrence of intrachain ferromagnetic (J = +6.58 cm⁻¹) and intramolecular antiferromagnetical (J = −6.91 cm⁻¹) interactions.     

Relaxation dynamics in three polypyridyl iron(II)-based complexes probed by nanosecond and sub-picosecond transient absorption spectroscopy

The results of a sub-picosecond transient absorption spectroscopy study on a mononuclear and two dinuclear low-spin iron(II) complexes is reported. The dinuclear derivatives are homonuclear (i.e. Fe–Fe) and heterodinuclear (Fe–Zn) in nature. The ligands we used were 2-pyridylmethyl-ketazine and 2-pyridylmethyl-hydrazone. Irradiation was made on the metal-to-ligand CT band occurring around 500 nm. The observed pattern of the relaxation decays is consistent with the population of the metastable ⁵T₂ ligand field state within the first 100 fs after the photon absorption from the three different chromophores. The suggested implication of triplet intermediate states was not detected. The ground state recovery was observed to occur with a time constant of 350 ps for the mononuclear complex and 1600–1800 ps for the two dinuclear complexes.     

Synthetic,structural and molecular mechanics investigation of some mono- and dinuclear copper(I) and copper(II) complexes of macrocyclic and related acyclic ligands

A number of di-Cu(II) complexes of the new tetraimine macrocyclic ligand derived from the Schiff base [2 + 2] condensation of 2,5-diformylfuran with 3-oxa-pentane-1,5-diamine have been prepared by methods which employ the heavier alkaline earth metal ions as templates followed by transmetallation. The complexes have been characterised by spectroscopic and other physical methods. Several of the di-Cu(I) complexes react reversibly with CO in solution and irreversibly with O₂ in a 4:1 Cu:O₂ stoicheiometry. Depending on conditions the oxidation product may be a dinuclear Cu(II) complex of the macrocycle or a mononuclear Cu(II) complex of a new ring-opened ligand. The single crystal X-ray structure of the latter complex has been determined.[CuL](BPh₄)₂ is monoclinic, space group C2/c with a=20.12(1), b=14.48(1), c=22.37(2) Å, β=110.1(1)°, Z=4. 1389 Independent reflections above background were measured on a diffractometer and the structure refined to R=0.108. The cation has imposed C₂ symmetry. The copper atom is bonded to four nitrogen atoms in the ‘outer’ compartment of the ligand with unique CuN distances of 2.050(17) and 1.977(17) Å. The geometry of the copper atom is intermediate between square planar and tetrahedral with an angle of 39.7° between two CuN₂ planes. Molecular mechanics calculations show that this distortion is due to steric effects.     

The coordination chemistry of mono(di-2-pyridylamine) copper(II) complexes with monovalent and divalent oxoanions: crystal structure, spectroscopic and magnetic properties of dinuclear [Cu(L)(μ-H2PO4)(H2PO4)]2 and polynuclear [Cu(L)(μ3-HPO4)]n

The crystal structures of two copper(II) complexes containing the ligand di-2-pyridylamine (dpyam) with monovalent H₂PO₄ ⁻ and divalent HPO₄ ²⁻ oxoanions, [Cu(dpyam)(μ-H₂PO₄-O,O^′)(H₂PO₄)]₂ (1) and [Cu(dpyam)(μ₃-HPO₄-O,O^′,O″)]_n (2), are reported and determined by X-ray crystallography. The dinuclear Cu(II) complex 1 was obtained by the reaction of dpyam with Cu(NO₃)₂ · 3H₂O and KH₂PO₄ in a water-ethanol (45/55) mixture. The molecules are linked into dinuclear units by two bridging didentate dihydrogenphosphate groups (endo/exo) in an equatorial-equatorial configuration giving a slightly distorted square pyramidal stereochemistry. The Cu-Cu contact distance of 5.136(2) Å is unusually large due to the exo/endo binding of the phosphate bridges. Complex 2 is a polymeric copper(II) derivative with helical [Cu(HPO₄)]₃ units surrounded by dpyam ligands and stabilized by intermolecular hydrogen bonds. Two nearest Cu(II) ions are bridged by a tridentate hydrogenphosphate group which is didentately coordinated to one copper(II) ion, and monodentately coordinated to another in an equatorial-equatorial configuration in an unusual bridging coordination mode. Each copper(II) ion in 2 exhibits a tetrahedrally distorted square-based geometry with the third oxygen atom (Cu-O=2.719(3) Å), from the hydrogenphosphate group weakly bound in an approximately axial position giving an extremely tetrahedrally distorted square-based pyramidal CuN₂O₂O^′ chromophore. The magnetic susceptibility measurements (5-300 K) reveals an antiferromagnetic interaction with J values of −2.85(1) and −26.20(2) cm⁻¹ for complexes 1 and 2, respectively. Some magneto-structural trends are discussed, along with their EPR and electronic reflectance spectra and compared with those of related complexes.     

Structural implications derived from the analysis of electron paramagnetic resonance spectra of natural and artificial copper proteins

An extension of a method relating chemical structure to the EPR parameters A_∥ and g_∥ is presented. For complexes having the same atoms of ligation, a decrease in charge of the metal-ligand complex decreases g_∥ and increases A_∥. From this analysis, one concludes that in artificial copper proteins as well as in the naturally occurring nonblue copper proteins copper is ligated to oxygen and nitrogen but not to sulfur. A method is presented for the interpretation of EPR changes that occur with ligand exchange reactions at the Type 2 (nonblue) copper sites such as occur in laccase.     

Novel tetranuclear copper(II) complex of dicarboxyamine ligand: Syntheses, crystal structures and redox properties

The reaction between a new amino dicarboxylic ligand N-(2-carboxybenzomethyl)-β-alanine (H₂cbal) obtained by reducing the Schiff base N-(2-carboxybenzoimine)-β-alanine and copper(II) perchlorate afforded a novel tetracopper(II) complex. This tetracopper(II) complex shows unusual structure and novel core topology. The electrochemical study of the complex using cyclic voltammetrry in acetonitrile indicated the presence of a reversible one-electron reduction and two irreversible reductions at higher potentials. The EPR studies of the complex and one electron reduce form of the complex in acetonitrile at 115 K showed an axial signal with g_∥ > g_⊥ > 2.0 and an isotropic signal, respectively.