Coulometric Oxidation Research Articles

H2 quantification based on selective pre-concentration and oxidative stripping at Pd modified microelectrodes

A concept for amperometric detection and quantification of H2 employing a palladium microsensor is demonstrated. Resistive sensors exploit the outstanding affinity of Pd toward H2 absorption leading to a detectable modulation in resistivity, while amperometric sensors are generally non-Pd-based and detect the electrochemical oxidation of H2 at the electrode surface. The latter method requires the use of a porous membrane in order to ensure that the electrode reaction is limited by the diffusion of H2 from the gas phase to the sensing electrode. We introduce direct quantification of dissolved H2 in aqueous electrolytes that relies on a pre-concentration mechanism at Pd modified microelectrodes and the subsequent amperometric or coulometric oxidation of H2 from bulk Pd. Due to the straightforward data analysis, this method allows for precise determination of H2 concentration in solution, with the maximum sensitivity obtained by adjusting the pre-concentration time.

Single-Step Rapid Diagnosis of Dopamine and Serotonin Metabolism Disorders.

Early diagnosis of dopamine and serotonin metabolic defects is of importance notably because of the availability of therapeutic strategies able to prevent the associated progressive brain dysfunction. The diagnosis of these diseases relies on the determination of monoamine metabolites and pterins in cerebrospinal fluid (CSF). Current methods involve at least two high-performance liquid chromatography runs of CSF analysis. The first one is devoted to the quantification of dopamine and serotonin metabolites and the second one to the quantification of pterins. Here, we describe a single-step method to measure monoamine neurotransmitter metabolites and pterins of interest in less than 10 min by ultrahigh-performance liquid chromatography coupled to sequential coulometric oxidation and fluorescence detections. All target compounds were quantified in CSF with a small volume (50 μL) and a single filtration step for sample preparation and analysis. After validation, the proposed method was applied to the determination of age-related reference ranges in the CSF of target compounds from a series of 1372 samples collected in France from 2008 to 2014. In the same period, the results obtained for 19 CSF samples from patients with known neurotransmitter disorders and 115 CSF samples with known immune system activation confirmed the expected pattern of changes in monoamine metabolites and pterins.

Pterin determination in cerebrospinal fluid: state of the art

Abstract The analysis of pterins in the cerebrospinal fluid (CSF) is mandatory for the etiologic diagnosis of inborn errors of dopamine and serotonin metabolism. The success of the available therapeutic strategies for preventing the ongoing brain dysfunction is tightly dependent of the early diagnosis of these neurotransmitter disorders. Previous methods of pterins determination in the CSF have in common at least one reversed phase HPLC step coupled to electrochemical or fluorescence detection (FD). They differ in the oxidation procedure of the reduced forms of pterins into their oxidized fluorescent counterparts. Most of the methods using the FD include at least one offline chemical oxidation procedure and cannot allow the direct quantification of tetrahydrobiopterin (BH4). A recent method proposed a single step simultaneous quantification of all forms of pterins including BH4 by HPLC coupled to FD after post-column coulometric oxidation. Nowadays, recent advances in mass spectrometry (MS), notably in term of sensitivity, allow the direct unambiguous determination of all forms of pterins in the CSF by LC-MS/MS.

On the mechanism of electrochemical vesicle cytometry: chromaffin cell vesicles and liposomes.

The mechanism of mammalian vesicle rupture onto the surface of a polarized carbon fiber microelectrode during electrochemical vesicle cytometry is investigated. It appears that following adsorption to the surface of the polarized electrode, electroporation leads to the formation of a pore at the interface between a vesicle and the electrode and this is shown to be potential dependent. The chemical cargo is then released through this pore to be oxidized at the electrode surface. This makes it possible to quantify the contents as it restricts diffusion away from the electrode and coulometric oxidation takes place. Using a bottom up approach, lipid-only transmitter-loaded liposomes were used to mimic native vesicles and the rupture events occurred much faster in comparison with native vesicles. Liposomes with added peptide in the membrane result in rupture events with a lower duration than that of liposomes and faster in comparison to native vesicles. Diffusional models have been developed and suggest that the trend in pore size is dependent on soft nanoparticle size and diffusion of the content in the nanometer vesicle. In addition, it appears that proteins form a barrier for the membrane to reach the electrode and need to move out of the way to allow close contact and electroporation. The protein dense core in vesicles matrixes is also important in the dynamics of the events in that it significantly slows diffusion through the vesicle.

Simultaneous Determination of All Forms of Biopterin and Neopterin in Cerebrospinal Fluid

In humans, genetic defects of the synthesis or regeneration of tetrahydrobiopterin (BH4), an essential cofactor in hydroxylation reactions, are associated with severe neurological disorders. The diagnosis of these conditions relies on the determination of BH4, dihydrobiopterin (BH2), and dihydroneopterin (NH2) in cerebrospinal fluid (CSF). As MS/MS is less sensitive than fluorescence detection (FD) for this purpose, the most widely used method since 1980 involves two HPLC runs including two differential off-line chemical oxidation procedures aiming to transform the reduced pterins into their fully oxidized fluorescent counterparts, biopterin (B) and neopterin (N). However, this tedious and time-consuming two-step indirect method underestimates BH4, BH2, and NH2 concentrations. Direct quantification of BH4 is essential for studying its metabolism and for monitoring the efficacy of BH4 supplementation in patients with genetic defects. Here we describe a single step method to simultaneously measure BH4, BH2, B, NH2, and N in CSF by HPLC coupled to FD after postcolumn coulometric oxidation. All target pterins were quantified in CSF with a small volume (100 μL), and a single filtration step for sample preparation and analysis. As compared to the most widely used method in more than 100 CSF samples, this new assay is the easiest route for accurately determining in a single run BH4, BH2, and NH2 in CSF in deficit situations as well as for monitoring the efficacy of the treatment.

Structure, fluorescence, redox properties and theoretical interpretation of heteroleptic copper(I) and silver(I) complexes of N-[(2-pyridyl)methyliden]-6-coumarin and triphenylphosphine

[MX(L)(PPh3)] (L=N-[(2-pyridyl)methyliden]-6-coumarin; M=Cu(I), Ag(I); X=Cl, Br, I, N3, SCN) are characterized by spectroscopic data. The X-ray structure of [CuCl(L)(PPh3)] shows CuN2PCl distorted tetrahedral coordination (L acts N,N chelator). The complexes show emission and the emission efficiency is dependent on nature of M and X; Cu(I) complexes (d10) are more emissive than Ag(I) complexes which may be due to heavy atom effect of silver. [Cu(SCN)(L)(PPh3)] shows highest emission intensity in the series. The cyclic voltammogram of [CuX(L)(PPh3)] shows Cu(II)/Cu(I) quasireversible redox couple while [AgX(L)(PPh3)] exhibits deposition of Ag(0) on the electrode surface together with irreversible ligand reduction. Coulometric oxidation of [CuX(L)(PPh3)] has synthesised copper(II) derivatives. Theoretical calculations are applied in modelling of complexes studied and UV–Vis spectra simulation.

Differential coulometric oxidation following post column-switching high pressure liquid chromatography for fluorescence measurement of unmetabolized folic acid in human plasma

Although many countries have fortified their grain supplies with folic acid (FA) to decrease the incidence of neural tube defects, others have not due to concerns that this synthetic folate might have some adverse effects. Persistent unmetabolized FA has been found even in plasma from fasted subjects. To facilitate measurement of low levels of folic acid in human plasma, post-column coulometric oxidative cleavage was used to convert poorly fluorescent FA into a highly fluorescent compound determined to be 6-formyl-pterin. To minimize sample work-up and maximize recovery, column-switching HPLC transferred a window of eluate containing the FA from the first column (C8) onto a second column (phenyl-hexyl). The pH of two mobile phases were adjusted to be above and then below a pK of the FA α-carboxyl group, thus promoting separation from compounds coeluting from the C8-column. This permitted sample preparation using only a simple high recovery protein precipitation. Definitive identification of FA in human plasma was accomplished by duplicate injections of sample with the electrochemical voltage set above and below its half-potential. The LOD (S/N=3) was 0.10nM. The intra- and inter-assay CV's were 2.3% and 5%, respectively. Comparison of these results with those obtained by HPLC/MS/MS with stable isotope internal standard showed a slope of 1.00±0.019. This simple, sensitive, and repeatable assay facilitates a more thorough investigation of the response of various human populations to folic acid intake. Post-column differential coulometric electrochemistry can expand the variety of compounds amenable to fluorescence detection.

Ultrasensitive DNA detection based on coulometric measurement of enzymatic silver deposition on gold nanoparticle-modified screen-printed carbon electrode

An ultrasensitive DNA detection method based on coulometric measurement of enzymatic silver deposition on gold nanoparticle (AuNP)-modified screen-printed carbon electrode (SPCE) was developed. The DNA sensor was prepared by self-assembly of a hairpin probe DNA, dually labeled with thiol at its 5′ end and biotin at its 3′ end, on AuNPs on SPCEs. The immobilized probe can shielded the biotin from being approached by a bulky alkaline phosphatase-linked streptavidin (Sv-ALP) conjugate. In the presence of the target DNA, hybridization caused conformational change in the probe DNA and forced biotin away from the surface of the AuNPs. Sv-ALP was bound to the biotin and catalyzed the hydrolysis of ascorbic acid 2-phosphate to form ascorbic acid. The latter reduced the silver ions for the deposition of metallic silver on the electrode surface. The deposited silver was then measured by coulometric analysis in the H2SO4 solution. A considerable increase in signal was obtained because of the accumulation and near-exhaustive coulometric oxidation of metallic silver. Under optimal conditions, the dynamic detection range of the target DNA was from 3.0×10−17 to 1.0×10−14molL−1, and the detection limit was 1.5×10−17molL−1. Further, the DNA sensor exhibited selectivity against single-base mismatched DNA.

Structures, redox behavior, antibacterial activity and correlation with electronic structure of the complexes of nickel triad with 3-(2-(alkylthio)phenylazo)-2,4-pentanedione

3-(2-(Alkylthio)phenylazo)-2,4-pentanedione (HL), an O, N, S donor ligand, is used for the synthesis of Ni(II), Pd(II) and Pt(II) complexes. The spectroscopic (IR, UV–Vis, and NMR) data determine the structure. The single crystal X-ray diffraction measurement of [Ni(L) 2] and [Pt(L)Cl] has confirmed the structures. Coulometric oxidation of [Ni(L) 2] and EPR spectra thereof show formation of Ni(III) state. DFT computation has calculated the electronic configuration and has explained the spectral and redox properties of the complexes. The compounds are screened for their in vitro anti-bacterial activity using Gram-positive and Gram-negative bacteria ( Bacillus subtilis UC564, Escherichia coli TG1, Staphylococcus aureus Bang25, Pseudomonas aeruginosa C/1/7, Salmonella typhi NCTC62, Salmonella paratyphi NCTC A2, Shigella dysenteriae 8NCTC599/52, Streptococcus faecalis S2, Vibrio cholerae DN7 and Mricococcus luteus AGD1). The minimum inhibitory concentration is determined for the compounds. The effect of the structure of the investigated compounds on the antibacterial activity is discussed.

Quantitation of Parent Drug and Its Unstable Metabolites by in Situ Coulometric Oxidation and Liquid Chromatography−Tandem Mass Spectrometry

Recent FDA and ICH guidances on safety testing of drug metabolites have challenged the way we traditionally think about quantitative bioanalytical methods. Such assays, in general, require a reference standard for each analyte to construct calibration curves and prepare quality control samples. However, early in the drug development process, metabolite standards may not be readily available, and if they are inherently unstable, they are difficult to synthesize or purify. In this paper, we describe a novel in-line method for producing and then quantifying a very unstable metabolite which is based upon the in situ postcolumn coulometric oxidation of the parent drug. Lacking any metabolite standards, the feasibility of simultaneously quantifying a development drug (compound A) and its unstable hydroxylated metabolites (metabolite B) was investigated. Reference standards for these ostensibly major human metabolites could not be reliably obtained due to rapid degradation upon purification and/or subsequent storage. Following high-performance liquid chromatography (HPLC) separation, parent drug and its [(13)C(3)-(15)N] isotopically labeled internal standard were quantitatively converted to equal amounts of a diastereomeric pair of hydroxylated metabolites using a postcolumn coulometric electrochemical cell before reaching the mass spectrometer. The concentration of the injected parent (which is equal to the total concentration of the in-line generated metabolites since the conversion to metabolite is quantitative) and the tandem mass spectrometry (MS/MS) signals of the electrochemically generated metabolites were used to construct a calibration curve for quantifying both the parent drug and its hydroxylated metabolites. Plasma extracts from humans dosed with compound A contained chromatographically distinct liquid chromatography-mass spectrometry (LC-MS) signals (m/z 538) for in vivo formed hydroxylated metabolites and the electrochemically oxidized parent drug which had been converted in-line into its chemically identical twin. Both peaks in this study sample could be quantified using a single calibration curve obtained under the same coulometric conditions using known amounts of the parent drug. Although no attempt was made to fully validate a bioanalytical method, the practicality of this in situ quantification approach was further confirmed by the preliminary bioanalytical analysis of a selection of plasma samples collected following oral administration (50 mg) of compound A in a clinical study.

Abstract 4829: Plasma coenzyme Q10 levels and postmenopausal breast cancer risk: The Multiethnic Cohort Study

Abstract Background: Coenzyme Q10 (CoQ10) is a component of the mitochondrial electron transport chain and is considered an important cellular antioxidant. Lower circulating CoQ10 levels are reported in patients with breast cancer, but epidemiologic evidence from large prospective studies is lacking. We examined the association of plasma CoQ10 levels with postmenopausal breast cancer risk in prospectively collected blood samples. Methods: Plasma levels of total CoQ10 were measured by HPLC with UV detection after precolumn coulometric oxidation among 160 incident postmenopausal breast cancer cases and 289 controls in the Multiethnic Cohort Study. Cases and controls were individually-matched on age, sex, ethnicity, study location (Hawaii or California), hormone replacement therapy use, date/time of collection and hours of fasting. Conditional logistic regression was used to compute odds ratios (ORs) and 95% confidence intervals (CIs). Results: Plasma CoQ10 levels were positively associated with breast cancer risk among postmenopausal women in the current study. The OR for the highest versus the lowest tertile was 1.6 (95% CI: 1.0 - 2.6; P = 0.05). The strongest association was observed for postmenopausal women under age 60 (OR for the highest versus the lowest tertile: 3.9; 95% CI: 1.1 - 14.0). Results were similar after adjusting for breast cancer risk factors. Conclusions: Higher CoQ10 levels in postmenopausal women are associated with increased breast cancer risk in this multiethnic population. Future studies are needed to define the origin of circulating CoQ10, its physiological meaning, and potential role in cancer etiology and progression. This work was supported by NIH Grants CA132149, CA33619. One of the authors (WC) was supported by NIH Training Grant CA90956. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4829.

Abstract 874: Low plasma coenzyme Q10 levels and breast cancer risk in Chinese women: Influence of menopausal status

Abstract Coenzyme Q10 (CoQ10) is an essential component of mitochondrial ATP synthesis and an important cellular antioxidant. Decreased circulating levels of CoQ10 have been associated with increased cancer incidence and poor prognosis for a number of cancer types. Limited clinical data suggest that supplemental CoQ10 may benefit breast cancer patients, as evidenced by regression of cancer and increased survival. We prospectively examined the association of plasma CoQ10 with breast cancer risk in a nested case-control study within the Shanghai Women's Health Study. Pre-diagnostic plasma samples were obtained from 373 cases and 740 age-matched controls and analyzed for total CoQ10 by HPLC with UV detection after precolumn coulometric oxidation. A risk reduction of borderline significance was associated with increasing plasma CoQ10 level using a conditional logistic regression model adjusted for age. Analysis by menopausal status revealed approximately a 50% reduction in odds ratio for pre-menopausal women in the highest quartiles of plasma CoQ10 compared to those with the lowest (p for trend = 0.01). In contrast, no significant reduction in risk was observed for post-menopausal women with increasing plasma CoQ10. Adjustment of the model for age, education, age at menarche, age at 1st live birth, months of breast feeding, history of fibroadenoma, first degree family cancer history, smoking history, and use of hormone replacement therapy yielded similar results: a significant inverse trend for breast cancer risk with increasing plasma CoQ10 in pre-menopausal women (p = 0.03) and no association for post-menopausal women. The data suggest that future epidemiologic studies of CoQ10 and breast cancer for pre-menopausal women are warranted and highlight the need for greater understanding of the origins and physiologic meaning of circulating CoQ10 in humans. This work was supported by NIH Grants CA 132149 (RVC), CA106591 (QD), and CA70867 (WZ). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 874.

Diphenoxido‐Bridged CoII and ZnII Complexes of Tripodal N2O2 Ligands: Stabilisation of MII‐Coordinated Phenoxyl Radical Species

AbstractThree new tripodal ligands with an N2O2 donor set, namely2‐tert‐butyl‐6‐({(2‐hydroxybenzyl)[2‐(2‐pyridyl)ethyl]amino}methyl)‐4‐methylphenol (H2L1), 2‐tert‐butyl‐6‐({(2‐hydroxybenzyl)[2‐(2‐pyridyl)ethyl]amino}methyl)‐4‐methoxyphenol (H2L2) and 2‐tert‐butyl‐6‐({[2‐(dimethylamino)ethyl](2‐hydroxybenzyl)amino}methyl)‐4‐methoxyphenol (H2L3) have been synthesised. Treatment of the ligands with Co(CH3CO2)2·4H2O or [Zn(H2O)6][ClO4]2 in the presence of Et3N provides the corresponding CoII and ZnII complexes of composition [MII2(L1)2] [M = Co (1) (single‐crystals are a solvate with the composition [CoII2(L1)2]·2CHCl3, i.e. 1·2CHCl3); M = Zn (2)], [MII2(L2)2] [M = Co (3); Zn (4)] and [CoII2(L3)2] (5). Crystallographic analyses reveal that the complexes have closely similar diphenoxido‐bridged structures. Each metal centre assumes MIIN2O3 coordination. The geometry around each metal ion in 1·2CHCl3 (τ = 0.76), 2 (τ = 0.77), 3 (τ = 0.74), 4 (τ = 0.76) and 5 [one CoII (τ = 0.49) and the other CoII (τ = 0.63)] is intermediate between ideal square‐pyramidal (τ = 0) and trigonal‐bipyramidal (τ = 1). Temperature‐dependent magnetic studies reveal weak intramolecular antiferromagnetic exchange couplings for all the three CoII complexes(–J = 1.84, 1.32 and 5.70 cm–1 for 1, 3 and 5, respectively). Spectroscopic properties of the complexes have also been investigated. Cyclic voltammetric (CV) measurements of 1 and 2 show an irreversible oxidative response at Epa (anodic peak potential) in the range 0.60–0.75 V relative to the SCE (saturated calomel electrode), whereas two successive quasi‐reversible oxidative responses can be observed for 3–5 at E1/2 values in the range 0.40–0.53 V relative to the SCE. Oxidative responses are due to the formation of MII‐coordinated phenoxyl radical species. The metal‐coordinated phenoxyl radical species, generated by two‐electron coulometric oxidation of 3–5, were characterised by CV and by adsorption and EPR spectroscopy. The stability of such species was determined by measuring the decay constant (absorption spectroscopy), which reveals that the phenoxyl radical species of 5 is more stable than that of 3 and 4. EPR spectroscopic studies (120 K) of coulometrically generated two‐electron oxidised species of 4 in CH2Cl2 (containing 0.1 M TBAP) at 298 K reveal a combination of an isotropic S = ${1 \over 2}$ signal at g = 2 [(phenolato)(phenoxyl radical)‐coordinated dizinc(II) complex] and a spin‐triplet resonance (S = 1) that gives rise to the symmetric split‐line pattern [bis(phenoxyl radical)‐cordinated dizinc(II) complex]. To pinpoint the site of oxidation (metal‐ or ligand‐centred) in each case, DFT calculations were performed at the B3LYP level of theory.

N, N′-Bis(aryl)pyridine-2,6-dicarboxamide complexes of ruthenium: Synthesis, structure and redox properties

Reaction of five N, N′-bis(aryl)pyridine-2,6-dicarboxamides (H 2L-R, where H 2 denotes the two acidic protons and R (R = OCH 3, CH 3, H, Cl and NO 2) the para substituent in the aryl fragment) with [Ru(trpy)Cl 3](trpy = 2,2′,2″-terpyridine) in refluxing ethanol in the presence of a base (NEt 3) affords a group of complexes of the type [Ru II(trpy)(L-R)], each of which contains an amide ligand coordinated to the metal center as a dianionic tridentate N,N,N-donor along with a terpyridine ligand. Structure of the [Ru II(trpy)(L-Cl)] complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru II(trpy)(L-R)] complexes shows a Ru(II)–Ru(III) oxidation within 0.16–0.33 V versus SCE. An oxidation of the coordinated amide ligand is also observed within 0.94–1.33 V versus SCE and a reduction of coordinated terpyridine ligand within −1.10 to −1.15 V versus SCE. Constant potential coulometric oxidation of the [Ru II(trpy)(L-R)] complexes produces the corresponding [Ru III(trpy)(L-R)] + complexes, which have been isolated as the perchlorate salts. Structure of the [Ru III(trpy)(L-CH 3)]ClO 4 complex has been determined by X-ray crystallography. All the Ru(III) complexes are one-electron paramagnetic, and show anisotropic ESR spectra at 77 K and intense LMCT transitions in the visible region. A weak ligand-field band has also been shown by all the [Ru III(trpy)(L-R)]ClO 4 complexes near 1600 nm.

First Ruthenium Complex of Glyoxalbis(N-phenyl)osazone (LNHPhH2): Synthesis, X-ray Structure, Spectra, and Density Functional Theory Calculations of (LNHPhH2)Ru(PPh3)2Cl2

The first ruthenium complex containing the parent osazone ligand, glyoxalbis(N-phenyl)osazone (L(NHPh)H(2)), is reported. The complex (L(NHPh)H(2))Ru(PPh(3))(2)Cl(2) (1) was characterized with mass, IR, (1)H NMR, and UV-vis spectroscopy as well as with theoretical calculations. Density functional theory calculations on the model compound (L(NHPh)H(2))Ru(PMe(3))(2)Cl(2) (2) reproduce the geometrical features observed for 1 and verify that it formally contains a ruthenium(II) metal center coordinated by a neutral osazone. Subsequent bonding analyses identify pi-interactions between the occupied orbitals of the metal fragment and the LUMO of the osazone, which results in transfer of approximately 0.3 electrons from the metal to the ligand. The complex 1 absorbs strongly at 405 nm, which is assigned to a ruthenium-to-ligand charge-transfer band on the basis of results of theoretical calculations. Complex 1 is also electroactive and displays a single one-electron oxidation wave at 0.39 V; coulometric oxidation gives the oxidized species [1]+ as a [PF(6)](-) salt. Simulation of the EPR spectra of [1][PF(6)], a one-electron paramagnetic species, affords g-tensor parameters g(x) = 2.2649, g(y) = 2.0560, and g(z) = 1.9064 consistent with a ruthenium(III) description for [1](+), thereby confirming a metal-centered redox reaction.

Catecholamine analysis with microcolumn LC-peroxyoxalate chemiluminescence reaction detection

Background Plasma catecholamines (CAs) are widely used as an index of sympathetic nervous system activity. In addition, CAs are known to be metabolized by catechol- O-methyltransferase (COMT) to produce their 3- O-methyl metabolites. We previously established a sensitive determination method of CAs and their 3- O-methyl metabolites using HPLC-peroxyoxalate chemiluminescence (POCL) reaction detection system. In this study, a microcolumn (100 × 1.0 mm I.D.) was used for separation to obtain higher sensitivity and shorter analysis time. Methods The system included automated precolumn ion-exchange extraction of amines, followed by separation on an ODS column, coulometric oxidation, fluorescence derivatization with ethylenediamine, and finally POCL reaction detection. Results The detection limits for CAs and their 3- O-methyl metabolites were 0.3–2.0 fmol. The analysis time was about 35 min, about half that of previously reported results. The method developed was used in monitoring changes in CAs and 3- O-methyl metabolite concentrations in human plasma during exercise. Conclusion The simultaneous determination method for concentrations of CAs and their 3- O-methyl metabolites in human plasma was developed using micro LC-peroxyoxalate chemiluminescence detection. We were successful in quantitating the changes in plasma CAs and their 3- O-methyl metabolites during exercise.

Ruthenium-mediated Selective Aromatic Thiolation in the Complex [RuII{o-S—C6H4-(p-R-)—N=N—C3H2NN(1)—R′}2]. Synthesis, Spectroscopic, e.p.r., Electro-chemical Characterization and Spectro-electrochemical Correlation

The reaction of ctc-[Ru(RaaiR′)2Cl2] (3a–3i) [RaaiR′=1-alkyl-2-(arylazo)imidazole, p-R—C6H4—N=N— C3H2NN(1)—R′, R=H, OMe, NO2, R′=Me, Et, Bz] with KS2COR′′ (R′′=Me, Et, Pr, Bu or CH2Ph) in boiling dimethylformamide afforded [RuII{o-S—C6H4(p-R-)—N=N—C3H2NN(1)—R′}2] (4a–4i), where the ortho-carbon atom of the pendant phenyl ring of both ligands has been selectively and directedly thiolated. The newly formed tridentate thiolate ligands are bound in a meridional fashion. The solution electronic spectra exhibit a strong MLCT band near 700 nm and near 550 nm, respectively in DCM. The molecular geometry of the complexes in solution has been determined by H n.m.r. spectroscopy. Cyclic voltammograms show a Ru(II)/Ru(III) couple near 0.4 V and an irreversible oxidation response near 1.0 V due to oxidation of the coordinated thiol group, along with two successive reversible ligand reductions in the range −0.80–0.87 V (one electron), −1.38–1.42 V (one electron). Coulometric oxidation of the complexes at 0.6 V versus SCE in CH2Cl2 produced an unstable Ru(III) congener. When R=Me the presence of trivalent ruthenium was proved by a rhombic e.p.r. spectrum having g1=2.349, g2=2.310.

Variable Coordination Mode of Chloranilic Acid. Synthesis, Structure, and Electrochemical Properties of Some Osmium Complexes

Reaction of chloranilic acid (H2ca) with [Os(bpy)2 Br2] (bpy = 2,2'-bipyridine) affords a dinuclear complex of type [{Os(bpy)2}2 (ca)]2+, isolated as the perchlorate salt. A similar reaction of H2ca with [Os(PPh3)2 (pap)Br2] (pap = 2-(phenylazo)pyridine) affords a dinuclear complex of type [{Os(PPh3)2 (pap)}2 (ca)]2+ (isolated as the perchlorate salt) and a mononuclear complex of type [Os(PPh3)2 (pap)(ca)]. Reaction of H2ca with [Os(PPh3)2(CO)2(HCOO)2] gives a dinuclear complex of type [{Os(PPh3)2(CO)2}2 (r-ca)], where r-ca is the two electron reduced form of the chloranilate ligand. The structures of the [{Os(PPh3)2 (pap)}2 (ca)](ClO4)2, [Os(PPh3)2 (pap)(ca)], and [{Os(PPh3)2(CO)2}2 (r-ca)] complexes have been determined by X-ray crystallography. In the [{Os(bpy)2}2 (ca)]2+ and [{Os(PPh3)2 (pap)}2 (ca)]2+ complexes, the chloranilate dianion is serving as a tetradentate bridging ligand. In the [Os(PPh3)2 (pap)(ca)] complex, the chloranilate dianion is serving as a bidentate chelating ligand. In the [{Os(PPh3)2(CO)2}2 (r-ca)] complex, the reduced form of the chloranilate ligand (r-ca(4-)) is serving as a tetradentate bridging ligand. All the four complexes are diamagnetic and show intense metal-to-ligand charge-transfer transitions in the visible region. The [Os(PPh3)2 (pap)(ca)] complex shows an Os(II)-Os(III) oxidation, followed by an Os(III)-Os(IV) oxidation on the positive side of a standard calomel electrode. The three dinuclear complexes show two successive oxidations on the positive side of SCE. The mixed-valent Os(II)-Os(III) species have been generated in the case of the two chloranilate-bridged complexes by coulometric oxidation of the homovalent Os(II)-Os(II) species. The mixed-valent Os(II)-Os(III) species show intense intervalence charge-transfer transitions in the near-IR region.

Structure and properties of bivalent nickel and copper complexes with pyrazine-amide-thioether coordination: stabilization of trivalent nickel

Acyclic pyrazine-2-carboxamide and thioether containing hexadentate ligand 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-dithiobutane (H(2)bpzctb), in its deprotonated form, has afforded light brown [Ni(II)(bpzctb)](1)(S=1) and green [Cu(II)(bpzctb)](2)(S=1/2) complexes. The crystal structures of 1.CH(3)OH and 2.CH(2)Cl(2) revealed that in these complexes the ligand coordinates in a hexadentate mode, affording examples of distorted octahedral M(II)N(2)(pyrazine)N'(2)(amide)S(2)(thioether) coordination. Each complex exhibits in CH(2)Cl(2) a reversible to quasireversible cyclic voltammetric response, corresponding to the Ni(III)/Ni(II)(1) and Cu(II)/Cu(I)(2) redox process. The E(1/2) values reveal that the complexes of bpzctb(2-) are uniformly more anodic by approximately 0.2 V than those of the corresponding complexes with the analogous pyridine ligand, 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane (H(2)bpctb), attesting that compared to pyridine, pyrazine is a better stabilizer of the Ni(ii) or Cu(i) state. Coulometric oxidation of the previously reported complex [Ni(II)(bpctb)] and 1 generates [Ni(III)(bpctb)](+) and [Ni(III)(bpzctb)](+) species, which exhibit a LMCT transition in the 470--480 nm region and axial EPR spectra corresponding to a tetragonally elongated octahedral geometry. Complex 2 exhibits EPR spectra characteristic of the d(z(2)) ground state.

Chemically induced cyclometalation of 2-(arylazo)phenols. Synthesis, characterization, and redox properties of a family of organoosmium complexes.

Reaction of 2-(arylazo)phenols (H(2)ap-R; R = OCH(3), CH(3), H, Cl, and NO(2)) with [Os(PPh(3))(2)(CO)(2)(HCOO)(2)] affords a family of organometallic complexes of osmium(II) of type [Os(PPh(3))(2)(CO)(ap-R)] where the 2-(arylazo)phenolate ligand is coordinated to the metal center as a tridentate C,N,O-donor. Structure of the [Os(PPh(3))(2)(CO)(ap-H)] complex has been determined by X-ray crystallography. All the [Os(PPh(3))(2)(CO)(ap-R)] complexes are diamagnetic and show characteristic (1)H NMR signals and intense MLCT transitions in the visible region. They also show emission in the visible region at ambient temperature. Cyclic voltammetry on the [Os(PPh(3))(2)(CO)(ap-R)] complexes shows a reversible Os(II)-Os(III) oxidation within 0.39-0.73 V vs SCE, followed by a reversible Os(III)-Os(IV) oxidation within 1.06-1.61 V vs SCE. Coulometric oxidation of the [Os(PPh(3))(2)(CO)(ap-R)] complexes generates the [Os(III)(PPh(3))(2)(CO)(ap-R)](+) complexes, which have been isolated as the hexafluorophosphate salts. The [Os(III)(PPh(3))(2)(CO)(ap-R)]PF(6) complexes are one-electron paramagnetic and show axial ESR spectra. In solution they behave as 1:1 electrolytes and show intense LMCT transitions in the visible region. The [Os(III)(PPh(3))(2)(CO)(ap-R)]PF(6) complexes have been observed to serve as mild one-electron oxidants in a nonaqueous medium.