Structural characterization, electrochemistry, and spectroelectrochemistry of trans-dioxorhenium(V) complex with 4-methoxypyridine, [ReO2(4-MeOpy)4]PF6, and characterization of [ReO2(4-MeOpy)4] generated electrochemically

Crystal structure of [ReO₂(4-MeOpy)₄][PF₆] (4-MeOpy = 4-methoxypyridine) complex has been examined by the single crystal X-ray analytical method. This complex shows a trans-dioxo geometry (average Re-O bond length = 1.766(2) Å) and its equatorial plane is occupied by four 4-MeOpy molecules (average Re-N bond length = 2.156(4) Å). Electrochemical reaction of [ReO₂(4-MeOpy)₄]⁺ in CH₃CN solution containing tetra-n-butylammonium perchlorate as a supporting electrolyte has been studied using cyclic voltammetry at 24 °C. Cyclic voltammograms show one redox couple around 0.65 V (E_pa) and 0.58 V (E_pc) [versus ferrocene/ferrocenium ion redox couple, (Fc/Fc⁺)]. Potential differences between two peaks (ΔE_p) at scan rates in the range from 0.01 to 0.10 V s⁻¹ are 65 mV, which is almost consistent with the theoretical ΔE_p value (59 mV) for the reversible one electron transfer reaction at 24 °C. The ratio of anodic peak currents to cathodic ones is 1.04 ± 0.03 and the (E_pa + E_pc)/2 value is constant, 0.613 ± 0.001 V versus Fc/Fc⁺, regardless of the scan rate. Spectroelectrochemical experiments have also been carried out by applying potentials from 0.40 to 0.77 V versus Fc/Fc⁺ with an optically transparent thin layer electrode. It was found that the UV-visible absorption spectra show clear isosbestic points at 228, 276, and 384 nm, and that the electron stoichiometry is evaluated as 1.03 from the Nernstian plot. These results indicate that the [ReO₂(4-MeOpy)₄]⁺ complex is oxidized reversibly to the [ReO₂(4-MeOpy)₄]²⁺ complex. Furthermore, it was clarified that the [ReO₂(4-MeOpy)₄]²⁺ in CH₃CN has the characteristic absorption bands at 236, 278, 330, 478, and 543 nm and their molar absorption coefficients are 4.3 × 10⁴, 4.5 × 10³, 1.0 × 10⁴, and 6.1 × 10³ M⁻¹ cm⁻¹ (M = mol dm⁻³), respectively.     

Synthesis and characterization of the novel extended TTF-type donors with thiophenic units

Two new TTF-based donors that are dithiolene ligand precursors, 3-{5-[(2-cyanoethyl)thio]-2-(5,6-dihydrothieno[2,3-d][1,3]dithiol-2-ylidene-1,3-dithiol-4-yl)thio}propanenitrile, dtdt (1) and 3-({5-[(2-cyanoethyl)thio]-2-thieno[2,3-d][1,3]dithiol-2-ylidene-1,3-dithiol-4-yl)thio}propanenitrile, α-tdt (2), were synthesized and characterized. The electrochemical properties of these compounds were studied by cyclic voltammetry (CV) in acetonitrile. Compound 1 shows two reversible oxidation process at ¹E_1/2 = 0.639 V and ²E_1/2 = 0.997 V versus Ag/AgCl. This same processes occurs at ¹E_1/2 = 0.612 V and ²E_1/2 = 0.906 V in the case of 2. The crystal structures confirm the ability of these molecules to establish interactions with their neighbours through the peripheral sulfur atoms.     

Measuring the antioxidant capacity of blood plasma using potentiometry

The use of potentiometry to measure plasma antioxidant capacity to contribute to oxidative stress evaluation is presented. In this assay, plasma (n = 60) diluted (0.3 to 1 ml) in phosphate buffer, pH 7.4, NaCl 9%, was submitted to potentiometry. A platinum wire was the working electrode and saturated calomel the reference. The results are presented as the difference between sample and buffer potential (ΔE). ΔE presented a good inverse correlation with added increasing concentrations of ascorbate (2.5−75 μmol/L; R = −0.99), urate (9.0−150 μmol/L; R = −0.99), and bilirubin (0.78−13 μmol/L; R = −0.99). Increase in the antioxidant capacity decreased ΔE. Depletion of the antioxidant capacity by tert-butylhydroperoxide (6.5−50 μmol/L) presented a direct correlation (0.97) with ΔE. Furthermore, ΔE presented an inverse correlation (R = −0.99) with increased antioxidant capacity of plasma (FRAP) induced by the addition of ascorbate (2.5−75 μmol/L). The response of the potentiometric method proved be adequate for measuring the plasma antioxidant depletion induced by acute exhaustive exercise in rats (control, n = 15; exercised, n = 15). This exercise decreased the concentration of urate (p < 0.05), decreased FRAP (p < 0.5), increased TBARS (p < 0.5), and decreased the potentiometer sensor response (p = 6.5 × 10⁻³). These results demonstrate the adequacy of potentiometry for evaluating the antioxidant capacity of blood plasma samples.     

Oxidation of thiocyanate by iron(V) in alkaline medium

The oxidation of thiocyanate by iron(V) (Fe(V)) was studied as a function of pH in alkaline solutions by a premix pulse radiolysis technique. The rates decrease with an increase in pH. The rate law for the oxidation of SCN⁻ by Fe(V) was obtained as −d[Fe(V)]/dt = k₁₀{[H⁺]²/([H⁺]² + K₂[H⁺] + K₂K₃)}[Fe(V)][SCN⁻], where k₁₀ = 5.72 ± 0.19 × 10⁶ M⁻¹ s⁻¹, pK₂ = 7.2, and pK₃ = 10.1. The reaction precedes via a two-electron oxidation, which converts Fe(V) to Fe(III). Thiocyanate reacts approximately 10³× faster with iron(V) than does with iron(VI).     

The synthesis and characterisation of phenolate complexes of Cu(II) and Ni(II) that are capable of supporting a phenoxyl radical ligand

New Cu^II and Ni^II complexes of potentially tridentate N₂O Schiff base ligands 1 and 2 have been synthesised and characterised. [Cu(2)(OH₂)]⁺ possesses a square planar geometry in the solid state whereas [Ni(1)₂] possesses a distorted octahedral geometry in which the amine donors of 1 coordinate weakly to the Ni^II centre. EPR spectroscopy demonstrates that the N₂O₂ coordination sphere of [Cu(2)(OH₂)]⁺ is retained in CH₂Cl₂ solution. [Cu(2)(OH₂)]⁺ exhibits a reversible one electron oxidation at E_1/2 = 0.54 V versus [Fc]⁺/[Fc], the product of which has been characterised by UV-Vis absorption and EPR spectroscopies. The spectroscopic signature of the oxidised product is consistent with the formation of a stable phenoxyl radical ligand bound to a Cu^II centre. [Ni(1)₂] possesses a reversible metal-based oxidation process at E_1/2 = 0.03 V versus [Fc]⁺/[Fc] and a further oxidation, attributed to the generation of a phenoxyl radical centre, at  = 0.44 V versus [Fc]⁺/[Fc]. UV-Vis absorption and EPR spectroscopic studies indicate that the lower potential process is a formal Ni^III/II couple. In contrast, the pro-ligands 1H and 2H exhibit chemically irreversible oxidation processes at  = 0.42 and 0.40 V versus Fc⁺/Fc, respectively, and do not support the formation of stable phenoxyl radical species.     

Aminocarboxylate complexes of vanadium(III): Electronic structure investigation by high-frequency and -field electron paramagnetic resonance spectroscopy

Aminocarboxylate complexes of vanadium(III) are of interest as models for biologically and medicinally relevant forms of this interesting and somewhat neglected ion. The V(III) ion is paramagnetic, but not readily suited to conventional EPR, due to its integer-spin ground state (S = 1) and associated large zero-field splitting (zfs). High-frequency and -field EPR (HFEPR), however, has the ability to study such systems effectively. Three complexes, all previously structurally characterized: Na[V(trdta)] · 3H₂O, Na[V(edta)(H₂O)] · 3H₂O, and [V(nta)(H₂O)₃] · 4H₂O (where trdta stands for trimethylenediamine-N,N,N′,N′-tetraacetate and nta stands for nitrilotriacetate) were studied by HFEPR. All the investigated complexes produced HFEPR responses both in the solid state, and in aqueous solution, but those of [V(nta)(H₂O)₃] · 4H₂O were poorly interpretable. Analysis of multi-frequency HFEPR spectra yielded a set of spin Hamiltonian parameters (including axial and rhombic zfs parameters: D and E, respectively) for these first two complexes as solids: Na[V(trdta)] · 3H₂O: D = 5.60 cm⁻¹, E = 0.85 cm⁻¹, g = 1.95; Na[V(edta)(H₂O)] · 3H₂O: D = 1.4 cm⁻¹, E = 0.14 cm⁻¹, g = 1.97. Spectra in frozen solution yielded similar parameters and showed multiple species in the case of the trdta complex, which are the consequence of the flexibility of this ligand. The EPR spectra obtained in frozen aqueous solution are the first, to our knowledge, of V(III) in solution in general and show the applicability of HFEPR to these systems. In combination with very insightful previous studies of the electronic absorption of these complexes which provided ligand-field parameters, it has been possible to describe the electronic structure of V(III) in [V(trdta)]⁻ and [V(edta)(H₂O)]⁻; the quality of data for [V(nta)(H₂O)₃] does not permit analysis. Qualitatively, six-coordinate V(III) complexes with O,N donor atoms show no electronic absorption band in the NIR region, and exhibit relatively large magnitude zfs (D ? 5 cm⁻¹), while analogous seven-coordinate complexes do have a NIR absorption band and show relatively small magnitude zfs (D < 2 cm⁻¹).     

Formation of N-(1-ferrocenoylpyrrolidine-2-carbonyl)-N,N′-dicyclohexylurea: Dead-end in the preparation of ferrocene-modified peptides

This paper reports the synthesis, characterisation and electrochemical properties of Fc-Pro-DCU 3. The activation of Fc-Pro-OH with DCC in the absence of racemization reducing agent results in the formation of N-(1-ferrocenoylpyrrolidine-2-carbonyl)-N,N′-dicyclohexylurea, as result of O/N-acyl shift in the acylisourea intermediate. Compound 3 exhibits quasi-reversible one-electron oxidation of ferrocenoyl moiety at E_1/2 = 0.62 V.     

trans-[Ru(N-N)2Cl2] species, where N-N is bis(3,5-dimethylpyrazol-1-yl)methane

Using bis(3,5-dimethylpyrazol-1-yl)methane as an N-N donor ligand, a trans-[Ru^III(N-N)₂Cl₂]⁺ core has been isolated from the direct reaction of the ligand with RuCl₃ · xH₂O and characterized structurally for the first time. The core displays a rhombic EPR spectrum and a quasireversible Ru(II/III) couple with an E_1/2 of −0.34 V versus NHE.     

Redox properties of the oxygen-detoxifying flavodiiron protein from the human parasite Giardia intestinalis

Flavodiiron proteins (FDPs) are enzymes identified in prokaryotes and a few pathogenic protozoa, which protect microorganisms by reducing O₂ to H₂O and/or NO to N₂O. Unlike most prokaryotic FDPs, the protozoan enzymes from the human pathogens Giardia intestinalis and Trichomonas vaginalis are selective towards O₂. UV/vis and EPR spectroscopy showed that, differently from the NO-consuming bacterial FDPs, the Giardia FDP contains an FMN with reduction potentials for the formation of the single and the two-electron reduced forms very close to each other (E₁ = −66 ± 15 mV and E₂ = −83 ± 15 mV), a condition favoring destabilization of the semiquinone radical. Giardia FDP contains also a non-heme diiron site with significantly up-shifted reduction potentials (E₁ = +163 ± 20 mV and E₂ = +2 ± 20 mV). These properties are common to the Trichomonas hydrogenosomal FDP, and likely reflect yet undetermined subtle structural differences in the protozoan FDPs, accounting for their marked O₂ specificity.     

Kinetics of the Ca, H, and Mg Interaction with the Ion-Binding Sites of the SR Ca-ATPase

Electrochromic styryl dyes were used to investigate mutually antagonistic effects of Ca²⁺ and H⁺ on binding of the other ion in the E₁ and P-E₂ states of the SR Ca-ATPase. On the cytoplasmic side of the protein in the absence of Mg²⁺ a strictly competitive binding sequence, H₂E₁?HE₁?E₁?CaE₁?Ca₂E₁, was found with two Ca²⁺ ions bound cooperatively. The apparent equilibrium dissociation constants were in the order of K_1/2(2 Ca) = 34 nM, K_1/2(H) = 1 nM and K_1/2(H₂) = 1.32 μM. Up to 2 Mg²⁺ ions were also able to enter the binding sites electrogenically and to compete with the transported substrate ions (K_1/2(Mg) = 165 μM, K_1/2(Mg₂) = 7.4 mM). In the P-E₂ state, with binding sites facing the lumen of the sarcoplasmatic reticulum, the measured concentration dependence of Ca²⁺ and H⁺ binding could be described satisfactorily only with a branched reaction scheme in which a mixed state, P-E₂CaH, exists. From numerical simulations, equilibrium dissociation constants could be determined for Ca²⁺ (0.4 mM and 25 mM) and H⁺ (2 μM and 10 μM). These simulations reproduced all observed antagonistic concentration dependences. The comparison of the dielectric ion binding in the E₁ and P-E₂ conformations indicates that the transition between both conformations is accompanied by a shift of their (dielectric) position.     

Electrochemical behavior of neomycin at DNA-modified gold electrodes

Deoxyribonucleic acid (DNA) modified gold electrodes are prepared by the dry adsorptive method and the electrochemical behavior of neomycin and the influence of Pb(II) are studied by cyclic voltammetry, chronocoulometry, differential pulse voltammetry. It is found that in 0.01 M phosphate-buffered saline (PBS) buffer solutions (pH 7.3) at DNA/Au electrode neomycin exhibits an irreversible cathodic peak (E_p = 0.489 V), which is more positive and less sensitive compared with that at bare gold electrodes (E_p = 0.423 V). In the presence of Pb(II) the peak shifts toward positive with its height increasing. Moreover, the peak height is linear to neomycin concentration over the range of 0.15-57 μM. The interaction of Pb(II)-neomycin complex with calf thymus DNA is also studied by calculating the binding constants (K) of the Pb(II)-neomycin complex to DNA and binding site size (s) from voltammetric data (1.0 × 10⁷ M⁻¹ and 4 bp, respectively).     

Tris-ruthenium(II)/copper(II) multimetallic porphyrin: Synthesis, characterization, DNA binding and supercoiled DNA photocleavage studies

The porphyrin, meso-5-(pentafluorophenyl)-10, 15, 20-tris(4-pyridyl)porphyrin has been used to synthesize two new metalloporphyrin complexes. Insertion of copper(II) into the porphyrin center gives the copper(II) porphyrin. Coordination of three [Ru(bipy)₂Cl]⁺ moieties (where bipy = 2,2′-bipyridine) to the pyridyl nitrogens of the copper(II) porphyrin gives the target complex. Electronic transitions associated with the copper(II) porphyrin and the triruthenium copper(II) porphyrin include an intense Soret band and a less intense Q-band in the visible region of the spectrum. An intense π-π∗ transition in the UV region associated with the bipyridyl groups and a metal to ligand charge transfer (MLCT) band appearing as a shoulder to the Soret band are observed for the ruthenated copper(II) porphyrin. Electrochemical properties associated with the multimetallic complex include a redox couple in the cathodic region with E_1/2 = −0.86 V versus Ag/AgCl attributed to the porphyrin and a redox couple in the anodic region E_1/2 = 0.88 V versus Ag/AgCl due to the Ru^III/II couple. DNA titrations indicate the triruthenium copper(II) porphyrin interacts with DNA potentially through a groove binding mechanism. Irradiation of aqueous solutions of the target complex and supercoiled DNA at a 10:1 base pair to complex ratio with visible light above 400 nm indicates that the complex causes nicking of the DNA helix.     

The response of carotenoids and chlorophylls during virus infection of Emiliania huxleyi (Prymnesiophyceae)

We report the response of carotenoids and chlorophylls during 120 h time series virus infection experiments of the marine coccolithophorid Emiliania huxleyi (Lohm.) Hay et Mohler culture. The response of individual carotenoids to infection varied: Diatoxanthin (Dtx) increased rapidly relative to chlorophyll-a, whereas diadinoxanthin (Ddx) and β-carotene showed a rapid decrease and fucoxanthin and 19′hexanoyloxyfucoxanthin a slight increase. The response of the individual carotenoids reflects their role in epoxy/de-epoxidation cycling, antioxidant protection, biosynthetic conversion and vulnerability to photooxidative destruction. We observed for the first time the operation of the diadinoxanthin cycle occurring in response to viral infection in E. huxleyi with the de-epoxidation ratio (Dtx / (Dtx + Ddx)) increasing exponentially with time (R² = 0.92) and decreasing exponentially with F_V / F_M (R² = 0.97). Our findings contribute to our understanding of the conversion and fate of key biochemical cell constituents in algae and are important in understanding the physiological stress response to virus infection.     

Synthetic, electrochemical and structural aspects of a series of ferrocene-containing dicarbonyl β-diketonato rhodium(I) complexes

Treatment of [Rh(β-diketonato)(cod)] with CO resulted in better yields of [Rh(FcCOCHCOR)(CO)₂] than by treating [Rh(Cl)(CO)₂]₂ with FcCOCH₂COR, R = CF₃ (Hfctfa), CH₃ (Hfca), Ph (Hbfcm, Ph = phenyl) and Fc (Hdfcm, Fc = ferrocenyl). The single crystal structure of the fctfa rhodium(I) complex [C₁₆H₁₀F₃FeO₄Rh], monoclinic, C 2/c(15), a = 13.266(3) Å, b = 19.553(3) Å, c = 13.278(3) Å, β = 100.92(2)°, Z = 8 showed both rotational and translational displacement disorders for the CF₃ group. An electrochemical study revealed that the formal reduction potential, E⁰′, for the electrochemically reversible one electron oxidation of the ferrocenyl group varied between 0.304 (for the fctfa complex) and 0.172 V (for the dfcm complex) versus Fc/Fc⁺ in a manner that could be directly traced to the group electronegativities, χ_R, of the R groups on the β-diketonato ligands, as well as to the values of the free β-diketones. Anodic peak potentials, E_pa,Rh, for the dominant cyclic voltammetry peak associated with rhodium(I) oxidation were between 0.718 (bfcm complex) and 1.022 V (dfcm complex) versus Fc/Fc⁺. Coulometric experiments implicated a second, much less pronounced anodic wave for the apparent two-electron Rh^I oxidation that overlaps with the ferrocenyl anodic wave and that the redox processes associated with these two Rh^I oxidation waves are in slow equilibrium with each other.     

Voltammetry of half-sandwich manganese group complexes of η-PhC3B7H9 and η-C60Bn2PhH2, two ligands that are cyclopentadienyl mimicks

The potentials of a series of one-electron oxidation and reduction reactions have been determined for manganese group half-sandwich complexes of the tricarbadecaboranyl ligand PhC₃B₇H₉ and the penta-organo fullerene ligand C₆₀Bn₂PhH₂ (Bn = benzyl). The anodic processes were studied in CH₂Cl₂ and the cathodic processes were studied in both CH₂Cl₂ and THF, the supporting electrolyte being [NBu₄][B(C₆F₅)₄]. The manganese complex Mn(CO)₂(PMe₃)(PhC₃B₇H₉) (1) is a member of a three-electron transfer series which includes oxidation to 1⁺ (0.51 V versus ferrocene) and successive reductions to 1⁻ (−1.66 V) and 1²⁻ (−1.77 V). Both the oxidation and reduction of the closely-related complex Mn(CO)₂(PPh₃)(PhC₃B₇H₉) (2) are chemically irreversible under slow-scan cyclic voltammetry conditions. The rhenium complex Re(CO)₂(PPh₃)(PhC₃B₇H₉) (3) oxidizes (E_1/2 = 0.82 V versus ferrocene) to a radical cation which, unlike its cyclopentadienyl analogue, shows no evidence of dimerization. Oxidation of the fullerene-based complex Re(CO)₃(C₆₀Bn₂PhH₂) is more facile than that of its cyclopentadienyl analogue, in contrast to previous findings in this class of metal-fullerene derivatives. An electrochemical ligand factor, E_L, of 0.63 is calculated for the PhC₃B₇H₉ ligand in manganese group half-sandwich complexes.     

Hydrolytic cleavage of double-strand DNA by the water-soluble dicobalt(III) complexes of 1,4,7-Triazacyclononane-N-acetate

Three water-soluble dicobalt(III) complexes, [Co₂L₂(µ-OH)₂](ClO₄)₂·5H₂O (1), [Co₂L₂(µ-OH)₂](ClO₄)₂·CH₃OH·H₂O(2); [Co₂L₂(µ-OH)₂](ClO₄)₂·4H₂O(3) (L = 1,4,7-triazacyclononane-N-acetate monoanion), were prepared to serve as nuclease mimics. The complexes were characterized by X-ray, IR and UV-vis spectroscopy as well as ESI-MS. Three complexes exhibit similar structures, just with different solvent molecules. The electrospray mass spectrum of 1 in solution indicates that dinuclear ion [Co₂L₂(µ-OH)₂-H⁺] ⁺ (4) is the active species. In the absence of any reducing agent, the complexes cleave plasmid pBR322 DNA was performed and its hydrolytic mechanism was demonstrated with radical scavengers, anaerobic reaction and T4 ligase. The kinetic aspects of DNA cleavage under pseudo- or true-Michaelis-Menten conditions are also detailed, kinetic parameters (k_cat, K_M) were calculated to be 3.57 h^− 1, 6.92 × 10^− 4 M; 0.28 h^− 1, 1.9 × 10^− 5 M for 4, respectively.     

Structure and magnetic properties of polynuclear chloro- and hydroxo-bridged copper(II) complexes formed by a tetramacrocyclic derivative of 1,4,7-triazacyclononane

Two new copper(II) complexes of the ligand 1,2,4,5-tetrakis(1,4,7-triazacyclononan-1-ylmethyl)benzene (L^dur) have been synthesized and characterized by single crystal X-ray studies. The first, [Cu₄L^dur(μ₂-OH)₄]Cl₂(PF₆)₂ · 8H₂O (1), was isolated from a solution of L^dur and Cu²⁺ at pH 9. Under acidic conditions (pH 3), a polymeric complex, {[Cu₄L^dur(μ₂-Cl)₆](PF₆)₂ · 10H₂O}_n (2), crystallized from solution. In both complexes, each of the four triazacyclononane (tacn) rings of the L^dur ligand facially coordinate to separate metal centres. Pairs of Cu(II) centres are then doubly-bridged by hydroxo groups in 1, leading to tetranuclear complex cation units featuring pairs of isolated copper(II) dimers with Cu₂(μ₂-OH)₂ cores folded at the O?O lines. Two forms of the tetranuclear units, featuring slightly different Cu₂(μ₂-OH)₂ core geometries, are present in equal amounts within the crystal lattice. In complex 2, chloro bridging ligands link pairs of Cu(II) centres from neighbouring tetranuclear units, forming a 1D helical polymeric structure. Variable-temperature magnetic susceptibility measurements suggest that the hydroxo-bridged copper(II) centres within one of the tetranuclear units in 1 are weakly antiferromagnetically coupled (J = −27 cm⁻¹), whilst those in the other interact ferromagnetically (J = +19 cm⁻¹). Similar measurements indicate weak ferromagnetic coupling (J = +16 cm⁻¹) for the chloro-bridged copper(II) centres in 2.