Oxidation of copper(II)-coordinated diethylenetriamine by hexacyanoferrate(III) ion. A kinetic investigation

The stoichiometry and kinetics of the reaction between [Cu(dien)(OH)]⁺ and [Fe(CN)₆]³⁻ in aqueous alkaline medium are described. The rate equation − (d[Fe(III)]/dt = {k₁[OH⁻]²[[Cu(dien)(OH)]⁺] + k₂[OH⁻] × [[Cu(dien)(OH)]⁺]²}([Fe(III)]/[Fe(II)]) (Fe(III) = [Fe(CN)₆]³⁻; Fe(II) = [Fe(CN)₆]⁴⁻, the 4:4:1 OH⁻/Fe(III)/[Cu(dien)(OH)]⁺ stoichiometric ratio and the nature of the ultimate products identified in the reaction solution suggest the fast formation of a doubly deprotonated Cu(III)-diamido complex which slowly undergoes an internal redox process where the ligand is oxidised to the Schiff base H₂NCH₂CH₂NCHCHNH.The [[Cu(dien)(OH)]⁺]² term in the rate equation is explained with the formation of a transient μ-hydroxo mixed-valence Cu dimer. A two-electron internal reduction of the Cu(III) complex yielding a Cu(I) intermediate is suggested to account for the presence of monovalent copper in a precipitate which forms at relatively high reactant concentrations and in the absence of dioxygen.     

Heterometallic complex formation on p-sulfonatothiacalix[4]arene platform resulting in pH- and redox-modification of [Ru(bpy)3] luminescence

The pH-dependent heterometallic complex formation with p-sulfonatothiacalix[4]arene (TCAS) as bridging ligand in aqueous solutions was revealed by the use of spectrophotometry, nuclear magnetic relaxation and fluorimetry methods. The novelty of the structural motif presented is that the appendance of emission metal center ([Ru(bpy)₃]²⁺) is achieved through the cooperative non-covalent interactions with the upper rim of TCAS. The second metal block (Fe(III), Fe(II) and Mn(II)), bound with the lower rim of TCAS in the inner sphere coordination mode is serving as quencher of [Ru(bpy)₃]²⁺ emission. The difference between the complex ability of Fe(III) and Fe(II) ions provides pH conditions for redox-dependent emission of [Ru(bpy)₃]²⁺.     

Copper(I)-incorporation into spinach apoplastocyanin

Spinach plastocyanin was converted into the apoprotein. CuSO₄ and oxidized Cu(II)- thionein reacted with the apoprotein to Cu(II) plastocyanin. Cu(I) transfer from Cu(I)0-thionein was only 15%. The structural analogue of the copper thiolate chromophore [Cu(I)(thiourea)₃]Cl as well as [Cu(CH₃CN)₄]ClO₄ successfully formed the Cu(I)- holoprotein. Characteristic circular dichroism bands at θ₂₈₄ (?5300 deg·cm²·dmol^?1 and θ₃₁₀ (+3300 deg·cm²·dmol^?1) were seen. Upon oxidation with ferricyanide and dialysis against phosphate buffer the correct Cu(II) binding into the active centre of Cu(II) plastocyanin was confirmed by EPR-measurements. The use of [Cu(I)(thiourea)₃] Cl as a convenient Cu(I) source for reconstitution studies on copper proteins is highly recommended.     

Biological assays and noncovalent interactions of pyridine-2-carbaldehyde thiosemicarbazonecopper(II) drugs with [poly(dA–dT)]2, [poly(dG–dC)]2, and calf thymus DNA

The interaction of the Cu(II) drugs CuL(NO₃) and CuL′(NO₃) (HL is pyridine-2-carbaldehyde thiosemicarbazone and HL′ is pyridine-2-carbaldehyde 4N-methylthiosemicarbazone, in water named [CuL]⁺ and [CuL′]⁺) with [poly(dA–dT)]₂, [poly(dG–dC)]₂, and calf thymus (CT) DNA has been probed in aqueous solution at pH 6.0, I = 0.1 M, and T = 25 °C by absorbance, fluorescence, circular dichroism, and viscosity measurements. The results reveal that these drugs act as groove binders with [poly(dA–dT)]₂, with a site size n = 6–7, whereas they act as external binders with [poly(dG–dC)]₂ and/or CT-DNA, thus establishing overall electrostatic interaction with n = 1. The binding constants with [CuL′]⁺ were slightly larger than with [CuL]⁺. The title compounds display some cleavage activity in the presence of thiols, bringing about the rupture of the DNA strands by the reactive oxygen species formed by reoxidation of Cu(I) to Cu(II); this feature was not observed in the absence of thiols. Mutagenic assays performed both in the presence and in the absence of S9 mix, probed by the Ames test on TA 98, TA 100, and TA 102, were negative. Weak genotoxic activity was detected for [CuL]⁺ and [CuL′]⁺, with a significative dose–response effect for [CuL′]⁺, which was shown to be more cytotoxic in the Ames test and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell proliferation assays. Methylation of the terminal NH₂ group enhances the antiproliferative activity of the pyridine-2-carbaldehyde thiosemicarbazones.     

Myometrial LH/hCG receptors during the estrous cycle and pregnancy in pigs

Receptors for luteinizing hormone/human chorionic gonadotropin (LH/hCG) have been identified in porcine, rabbit, rat, and human myometrium. To determine the estrous cycle and pregnancy related changes in the receptor capacity and affinity, radioreceptor assays were performed with membrane homogenates of porcine uterine tissues. Cycling gilts were divided into four experimental groups: I (n=6), day 1–2; II (n=5), day 6–7; III (n=5), day 11–12; and IV (n=6), day 18–20 of the estrous cycle. Pregnant pigs were divided into three experimental groups: I (n=5), day 35–40; II (n=5), day 65–70; and III (n=4), day 95–105 of pregnancy. The concentrations [femtomoles/mg protein (fmol/mg protein)] and affinities of unoccupied LH/hCG binding sites were characterized in all samples of myometrium. Receptor concentrations were highest (P<0.01) in groups II and III (19.3±2.5 and 35.8±2.1 fmol/mg protein, respectively), and was lowest in groups I and IV (5.3±1.4 and 7.5±0.7 fmol/mg protein, respectively). Receptor affinity constants (K_a) were consistent (P>0.05) throughout the estrous cycle [I, (5.1±1.5)×10⁹; II, (3.0±0.8)×10⁹; III, (3.2±0.9)×10⁹; IV, 5.5±0.7×10⁹ lm⁻¹]. Plasma hormone concentrations of progesterone, estrogen and LH were typical of values noted at these times. During pregnancy, receptor concentrations were greatest (P<0.05) in group II (85.4±18.5 fmol/mg protein). In groups I and III receptor numbers were 10.8±2.3 and 26.7±6.6 fmol/mg protein, respectively. The K_a in group I was 10 times greater (P<0.05) than K_a in groups II and III, (I, 3.1±0.9×10¹⁰ lm⁻¹; II, 3.4±0.3×10⁹ lm⁻¹; III, 3.3±1.1×10⁹ lm⁻¹). Plasma hormone concentrations typically found during pregnancy were noted. The function of these LH/hCG binding sites remains unknown; however, changes in receptor capacity during the estrous cycle and pregnancy support a role for modulation of the receptor by hormonal factors.     

Rational enhancement of the coordination capability of Ru(III)(salen)-nitronyl nitroxide building block: A step towards 2p–3d–4d magnetic edifices

The reaction of [Ru(salen)(PPh₃)Cl] and the 5-imidazol-substituted nitronyl nitroxide radical (NIT-(5)ImH) yields the [Ru(salen)(PPh₃)(NIT-(5)ImH)](ClO₄) (1) complex which has been characterized by single crystal X-ray diffraction. This analysis reveals that the Ru(III) ion is coordinated to a tetradentate salen^2? ligand in equatorial positions while one PPh₃ ligand and one NIT-(5)ImH radical are coordinated in axial positions. This led to Ru^III ions in tetragonally elongated octahedral geometry. From the magnetic point of view ferromagnetic intramolecular interaction (J₁ = +2.47 cm^?1) have been found between the Ru(III) ion and the coordinated NIT-(5)ImH while no significant intermolecular antiferromagnetic interactions are observed at low temperature leading to a ground spin state S = 1. The absence of intermolecular magnetic interaction is explained by considering the crystal packing of (1) where the [Ru(salen)(PPh₃)(NIT-(5)ImH)]⁺ moieties are relatively well isolated. This has to be compared with the situation observed in the previously reported [Ru(salen)(PPh₃)-(NIT)]⁺ compound (2) where ferromagnetic Ru^III–NIT interaction were identified and the crystal packing generate intermolecular antiferromagnetic interactions that complicated the study. The analysis of this compound confirms the rather isotropic g values that were found of (2) and of [Ru(salen)(PPh₃)(N₃)], (3) a radical-free analogue. Moreover it is also a step towards extended structures based on Ru^III–NIT moieties since this compound possesses a free bischelating site likely to coordinate additional metallic ions.     

The effects of structural variations of thiophene-containing Ru(II) complexes on the acid–base and DNA binding properties

A phenylthiophenyl-bearing Ru(II) complex of [Ru(bpy)₂(Hbptip)](PF₆)₂ {bpy?=?2,2′-bipyridine, Hbptip?=?2-(4-phenylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline} was synthesized and characterized by elemental analysis, ¹H NMR spectroscopy, and electrospray ionization mass spectrometry. The ground- and excited-state acid–base properties of the complex were studied by UV–visible absorption and photoluminescence spectrophotometric pH titrations and the negative logarithm values of the ground-state acid ionization constants were derived to be pK _a1?=?1.31?±?0.09 and pK _a2?=?5.71?±?0.11 with the pK _a2 associated deprotonation/protonation process occurring over 3 pK _a units more acidic than thiophenyl-free parent complex of [Ru(bpy)₂(Hpip)]²⁺ {Hpip?=?2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline}. The calf thymus DNA-binding properties of [Ru(bpy)₂(Hbptip)]²⁺ in Tris–HCl buffer (pH 7.1 and 50?mM NaCl) were investigated by DNA viscosities and density functional theoretical calculations as well as UV–visible and emission spectroscopy techniques of UV–visible and luminescence titrations, steady-state emission quenching by [Fe(CN)₆]^4?, DNA competitive binding with ethidium bromide, DNA melting experiments, and reverse salt effects. The complex was evidenced to bind to the DNA intercalatively with binding affinity being greater than those for previously reported analogs of [Ru(bpy)₂(Hip)]²⁺, [Ru(bpy)₂(Htip)]²⁺, and [Ru(bpy)₂(Haptip)]²⁺ {Hip?=?1H-imidazo[4,5-f][1,10]phenanthroline, Htip?=?2-thiophenimidazo[4,5-f][1,10]phenanthroline, Haptip?=?2-(5-phenylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline}.     

Synthesis,characterization and DNA-binding properties of novel dipyridophenazine complex of ruthenium (II) : [Ru(IP)2(DPPZ)]2+

A novel ruthenium(II) complex of dipyridophenazine (DPPZ) with the ancillary ligand imidazole[4,5-f] [1,10]phenanthroline (IP), [Ru(IP)₂(DPPZ)] (PF₆)₂, has been synthesized and characterized by elemental analysis, 1D and 2D ¹H NMR, fast-atom bombardment mass spectra (FABMS), electronic spectroscopy and cyclic voltammetry. The DNA-binding properties of the complex were studied by spectroscopic methods. The intrinsic binding constant, K =2.1 × 10⁷M⁻¹, of the complex to calf thymus DNA has been determined by absorption titration in 5 mmol dm⁻³ Tris-HCl, 50 mmol dm⁻³ NaCl buffer (pH 7.0). The excited state lifetimes and luminescence quenching with [Fe(CN)₆]⁴⁻ as the quencher in the presence of DNA were also tested and mono-exponentiality was observed for the emission decay curves. Viscosity measurements together with the optical titrations unambiguously proved that the complex bound with DNA intercalatively and that the binding affinity to DNA was several times larger than that of the parent complex [Ru(bpy)₂(DPPZ)]²⁺.     

Spectroscopic and Structural Characterization,Enzyme Inhibitions,and Antioxidant Effects of New Ru(II) and Ni(II) Complexes of Schiff Base

The new complex compounds [RuLCl(p‐cymene)] ? 3H₂O and [NiL₂(H₂O)₂] ? 3H₂O (L: 1‐{4‐[(2‐hydroxy‐3‐methoxybenzylidene)amino]phenyl}ethanone) were prepared and characterized using FT‐IR, ¹H‐ and ¹³C‐NMR, mass spectroscopy, TGA, elemental analysis, X‐ray powder diffraction and magnetic moment techniques. Octahedral geometry for new Ni(II) and Ru(II) complexes was proposed. Thermal decomposition confirmed the existence of lattice and coordinated water molecule in the complexes. To determine the antioxidant properties of Schiff base ligand and its Ni(II), Ru(II) metal complexes, FRAP, CUPRAC, ABTS and DPPH methods of antioxidant assays were used. Moreover, enzyme inhibition of complexes was evaluated against carbonic anhydrase I and II isoenzymes (CA I and CA II) and acetylcholinesterase (AChE). For CA I and CA II, the best inhibition enzymes, was the Ni(II) complex with 62.98±18.41, 86.17±23.62 Ki values, whereas this inhibition effect showed ligand with 24.53±2.66 Ki value for the AChE enzyme.     

Sterically-induced synthesis of 3d–4f one-dimensional compounds: A new route towards 3d–4f single chain magnets

Hexanuclear lanthanide complexes have been used as molecular precursors to built 3d–4f molecular chains. These complexes were originally targeted as building blocks for the synthesis of lanthanides-containing coordination polymers but reacting them with the 3d molecular precursor [Cu(opba)]^2? lead to Ln(III)–Cu(II) hetero-bimetallic chains with general formula [Ln(NO₃)(DMSO)₂Cu(opba)(DMSO)₂]_∞ with Ln = Gd–Er. The reaction mechanism can be explained by a sterically-induced reaction where the attack of the [Cu(opba)]^2? moiety is driven by the hexanuclear lanthanide clusters geometry. Static magnetic properties of the Gd- and Dy-based chains have been investigated as well as the dynamic magnetic properties of the Dy-containing compound. These studies confirmed that this chemical strategy can possibly yield to 3d–4f single chain magnets.     

Light-induced electron-transfer reactions. Part 2. Kinetics of the intra-molecular redox decomposition of tris(oxalato)-cobaltate(III) by irradiation with visible light of aqueous solution containing tris(2,2′-bipyridine)ruthenium(II) ion

The rate of the intra-molecular redox decomposition of the tris(oxalato)cobaltate(III) ion [Co(Ox)₃]^3? is greatly accelerated by irradiation with visible light of aqueous acidic solutions containing the tris(2,2′-bipyridine)ruthenium(II) ion [Ru(bpy)₃]²⁺. The rate of the light-induced reaction in hydrochloric acid with an acidity range 0.05–0.18 mol dm^?3 is of zero-order with respect to the [Co(Ox)₃]^3? ion concentration and is proportional to the light-intensity irradiated and also essentially to the [Ru(bpy)₃]²⁺ ion concentration. Moreover, the rate is independent not only of the oxalate ion concentrations, but also of the acidity over the range 0.05–0.18 mol dm^?3 hydrochloric acid. The ionic-strength dependence, as well as temperature dependence, were extremely small. The [Ru(bpy)₃]²⁺ concentration does not change during the occurrence of the reaction and the tris(2,2′-bipyridine)ruthenium(II) ion acts as a homogeneous catalyzer. However, a dramatic indication that the situation was rather different was found in the stronger acid solutions of 0.5 or 1.0 mol dm^?3 hydrochloric acid, in which the [Ru(bpy)₃]²⁺ concentration decreased greatly immediately after the initiation of reaction and then increased up to the initial concentration. Such a decrease at the initial stage of the reaction disappeared by addition of oxalate before the start of the reaction. A chain mechanism of reaction is proposed to account for these results.     

Superoxidedismutase-mimetic copper(II) complexes containing saccharinate and 4-aminopyridine/4-cyanopyridine

Two copper(II) complexes, [Cu(sac)₂(4-cypy)₂(H₂O)], 1 and [Cu(sac)₂(4-Ampy)₂(H₂O)], 2 (4-cypy: 4-cyanopyridine; 4-Ampy: 4-aminopyridine) were prepared. Physicochemical properties of the complexes were studied by spectroscopic (solution UV–vis, diffuse reflectance and IR) techniques. Structural X-ray diffraction data could be obtained only for [Cu(sac)₂(4-cypy)₂(H₂O)] that it crystallized in the tetragonal space group P4cc with a=b=15.313(1), c=13.240(1) Å, and Z=4 molecules per unit cell. The complex was cited on a crystallographic C₂-axis with the Cu(II) ion in a square–pyramidal environment, coordinated at the pyramid basis to the nitrogen atom of two saccharine anions [d(Cu–N)=2.011(3) Å] and the pyridine N-atom of two 4-cyanopyridine ligands [d(Cu–N)=2.038(4) Å]. The coordination was completed by a water molecule at the pyramid apex [d(Cu–Ow)=2.189(5) Å]. Elemental and spectroscopic analyses revealed an O-saccharinate coordination mode for complex 2 and a square–pyramidal structure. Only complex 2 retained its structure in methanolic solution. However, both complexes were able to catalyze the dismutation of superoxide anion (O₂^?) (pH 7.5) at micromolar concentrations. Therefore, these complexes behaved as useful SOD-mimetic compounds.     

Neutral ruthenium(II) complexes of phenylcyanamido ligands

Neutral Ru(II) complexes with the formula trans-[Ru(trpy*)(L₂)(pcyd)] have been prepared, where trpy* = 4,4′,4″-tri-tert-butyl-terpyridine, L₂ = 2-pyrazinecarboxylato (pca), 2-pyridinecarboxylato (pic), acetylacetonato (acac) and pcyd = 3-chlorophenylcyanamido (3-Clpcyd), 2,3-dichlorophenylcyanamido (2,3-Cl₂pcyd), 2,4,6-trichlorophenylcyanamido (2,4,6- Cl₃pcyd), 2,3,4,5-tetrachlorophenylcyanamido (2,3,4,5-Cl₄pcyd) and 3,4,5-trimethoxyphenylcyanamido (3,4,5-(OMe)₃pcyd). Spectroelectrochemistry was performed on these Ru(II) complexes to obtain the visible absorption spectrum of the Ru(III)–cyanamide ligand-to-metal charge transfer chromophore. The Ru(III)–cyanamide metal–ligand coupling elements of these complexes were compared to other Ru(III)–cyanamide complexes.     

Cu2+ Inhibits Photosystem II Activities but Enhances Photosystem I Quantum Yield of Microcystis aeruginosa

Responses of photosystem I and II activities of Microcystis aeruginosa to various concentrations of Cu²⁺ were simultaneously examined using a Dual-PAM-100 fluorometer. Cell growth and contents of chlorophyll a were significantly inhibited by Cu²⁺. Photosystem II activity [Y(II)] and electron transport [rETR_max(II)] were significantly altered by Cu²⁺. The quantum yield of photosystem II [Y(II)] decreased by 29 % at 100 μg L^?1 Cu²⁺ compared to control. On the contrary, photosystem I was stable under Cu²⁺ stress and showed an obvious increase of quantum yield [Y(I)] and electron transport [rETR_max(I)] due to activation of cyclic electron flow (CEF). Yield of cyclic electron flow [Y(CEF)] was enhanced by 17 % at 100 μg L^?1 Cu²⁺ compared to control. The contribution of linear electron flow to photosystem I [Y(II)/Y(I)] decreased with increasing Cu²⁺ concentration. Yield of cyclic electron flow [Y(CEF)] was negatively correlated with the maximal photosystem II photochemical efficiency (F _v/F _m). In summary, photosystem II was the major target sites of toxicity of Cu²⁺, while photosystem I activity was enhanced under Cu²⁺ stress.     

Heterometallic [AgFe3S4] ferredoxin variants: synthesis, characterization, and the first crystal structure of an engineered heterometallic iron–sulfur protein

Heterometallic [AgFe₃S₄] iron–sulfur clusters assembled in wild-type Pyrococcus furiosus ferredoxin and two variants, D14C and D14H, are characterized. The crystal structure of the [AgFe₃S₄] D14C variant shows that the silver(I) ion is indeed part of the cluster and is coordinated to the thiolate group of residue 14. Cyclic voltammetry shows one redox pair with a reduction potential of +220 mV versus the standard hydrogen electrode which is assigned to the [AgFe₃S₄]^2+/+ couple. The oxidized form of the [AgFe₃S₄] D14C variant is stable in the presence of dioxygen, whereas the oxidized forms of the [AgFe₃S₄] wild type and D14H variants convert to the [Fe₃S₄] ferredoxin form. The monovalent d ¹⁰ silver(I) ion stabilizes the [Fe₃S₄]^+/0 cluster fragment, as opposed to divalent d ¹⁰ metal ions, resulting in more than 0.4 V difference in reduction potentials between the silver(I) and, e.g., zinc(II) heterometallic [MFe₃S₄] ferredoxins. The trend in reduction potentials for the variants containing the [AgFe₃S₄] cluster is wild type ≤ D14C < D14H and shows the same trend as reported for the variants containing the [Fe₃S₄] cluster, but is different from the D14C < D14H < wild type trend reported for the [Fe₄S₄] ferredoxin. The similarity in the reduction potential trend for the variants containing the heterometallic [AgFe₃S₄] cluster and the [Fe₃S₄] cluster can be rationalized in terms of the electrostatic influence of the residue 14 side chains, rather than the dissociation constant of this residue, as is the case for [Fe₄S₄] ferredoxins. The trends in reduction potentials are in line with there being no electronic coupling between the silver(I) ion and the Fe₃S₄ fragment.     

Photochemical NO release from nitrosyl RuII complexes with C-bound imidazoles

The series of nitrosyl complexes trans-[Ru(NH₃)₄L(NO)]Cl₃, L = caffeine, theophylline, imidazole and benzoimidazole in position trans to NO were prepared and their photochemical properties studied. All complexes showed nitric oxide (NO) release under light irradiation at 330–440 nm. Quantum yields for [Ru(NH₃)₄L(H₂O)]³⁺ formation (?_Ru(III)) were sensitive to the natures of L, λ_irr and pH. The major product of the irradiation of trans-[Ru(NH₃)₄L(NO⁺)]³⁺ is the trans-[Ru^III(NH₃)₄L(Cl)]²⁺ and NO as suggested by UV–Vis, electrochemical, and FTIR techniques.     

Synthesis, characterization and kinetic studies of the cis-[RuCl2(cyclen)] complex

We report here the synthesis, characterization and kinetic studies of cis-[RuCl₂(cyclen)]⁺ in aqueous solution, where cyclen is the macrocyclic ligand 1,4,7,10-tetraazacyclododecane. The complex releases one Cl⁻ producing cis-[RuCl(OH)(cyclen)]⁺ in aqueous solution at pH 4.60. The product of this reaction was characterized by Ultraviolet-Visible (UV-Vis) spectrum in comparison to the synthesized cis-[RuCl(OH)(cyclen)](BF₄)·2H₂O. The electrochemical data showed that E_pc of the Ru(III/II) peak increases as the macrocycle ring size decreases and also when the trans conformation is changed to cis. The chloride affinity of Ru(III) depends on the macrocycle ring size since cis-[RuCl₂(cyclam)]⁺ (cyclam=1,4,8,11-tetraazacyclotetradecane) does not release chloride for at least 12 h. The overall effect between cyclam and cyclen reflects the fact that the electron involved in the reduction enters a nonbonding π-d orbital and its energy is affected by the macrocyclic ligand.     

Inner sphere oxidation of L-ascorbic acid by Ru(III) ion and its complexes in aqueous acidic medium

The kinetics of electron transfer from L-ascorbic acid [H₂A] to oxidants, dichlorotetraaquoruthenium(III) [RuCl₂(H₂O)₄]⁺, iminodiacetatoruthenium(III) [Ru(III)IMDA]⁺ and ethylenediaminetetraacetatoruthenate(III) [Ru(III)EDTA]⁻ exhibit a first order dependence both on L-ascorbic acid and oxidants and inverse first order dependence on hydrogen ion concentration. Kinetic, spectroscopic and thermodynamic parameters are reported for the formation of intermediate Ru(III) (1:1) and Ru(III)chelateascorbate (1:1:1) complexes during the oxidation of L-ascorbic acid. The results are interpreted in terms of a mechanism involving a rate-determining inner sphere one electron transfer from L-ascorbic acid to the oxidants used in the present investigation, followed by a subsequent and kinetically rapid transfer of the second electron of ascorbic acid to another molecule of the oxidant. A detailed discussion of the kinetic data, temperature and ionic strength dependence of the oxidation reactions is presented.     

Binding properties of Ru(II) polypyridyl complexes with poly(U)·poly(A)*poly(U) triplex: the ancillary ligand effect on third-strand stabilization

The binding properties of [RuL₂(mip)]²⁺ {where L is 1,10-phenanthroline (phen) or 4,7-dimethyl-1,10-phenanthrollne (4,7-dmp) and mip is 2′-(3″,4″-methylenedioxyphenyl)imidazo[4′,5′-f][1,10]phenanthroline} with regard to the triplex RNA poly(U)·poly(A)*poly(U) were investigated using various biophysical techniques and quantum chemistry calculations. In comparison with [Ru(4,7-dmp)₂(mip)]²⁺, remarkably higher binding affinity of [Ru(phen)₂(mip)]²⁺ for the triplex RNA poly(U)·poly(A)*poly(U) was achieved by changing the ancillary ligands. The stabilization of the Hoogsteen-base-paired third strand was improved by about 10.9 °C by [Ru(phen)₂(mip)]²⁺ against 6.6 °C by [Ru(4,7-dmp)₂(mip)]²⁺. To the best of our knowledge, [Ru(phen)₂(mip)]²⁺ is the first metal complex able to raise the third-strand stabilization of poly(U)·poly(A)*poly(U) from 37.5 to 48.4 °C. The results reveal that the ancillary ligands have an important effect on third-strand stabilization of the triplex RNA poly(U)·poly(A)*poly(U) when metal complexes contain the same intercalative ligands.     

Spectroscopic and magnetochemical studies on the active site copper complex in galactose oxidase

Galactose oxidase is a radical copper oxidase, an enzyme making use of a covalently modified tyrosine residue as a free radical redox cofactor in alcohol oxidation catalysis. We report here a combination of spectroscopic and magnetochemical studies developing insight into the interactions between the active site Cu(II) and two distinct tyrosine ligands in the biological complex. One of the tyrosine ligands (Y495) is coordinated to the Cu(II) metal center as a phenolate in the resting enzyme and serves as a general base to abstract a proton from the coordinated substrate, thus activating it for oxidation. The structure of the resting enzyme is temperature-dependent as a consequence of an internal proton equilibrium associated with this tyrosine that mimics this catalytic proton transfer step. The other tyrosine ligand (Y272) is covalently crosslinked to a cysteine residue forming a tyrosine–cysteine dimer free radical redox site that is required for hydrogen atom abstraction from the activated substrate alkoxide. The presence of the free radical in the oxidized active enzyme results in formation of an EPR-silent Cu(II) complex shown by multifield magnetic saturation experiments to be a diamagnetic singlet arising from antiferromagnetic exchange coupling between the metal and radical spins. A paramagnetic contribution observed at higher temperature may be associated with thermal population of the triplet state, thus permitting an estimate of the magnitude of the isotropic exchange coupling (J>200 cm⁻¹, JS₁·S₂) in this complex. Structural correlations and the possible mechanistic significance of metal–radical coupling in the active enzyme are discussed.