Reactions of captopril and epicaptopril with transition metal ions and hydroxyl radicals: An EPR spectroscopy study

In the present study, using the technique of EPR spin trapping with DMPO a spin trap, we demonstrated formation of thiyl radicals from thiol-containing angiotensin converting enzyme (ACE) inhibitor captopril (CAP) and from its stereoisomer epicaptopril (EPICAP), a non-ACE inhibitor, in the process of ^.OH radical scavenging. Splitting constants of DMPO/thiyl radical adducts were identical for both thiols and were a_N = 15.3 G, and a_H = 16.2 G. Bimolecular rate constants for the reaction of CAP and EPICAP with ^.OH radicals were close to a diffusion-controlled rate (≈ 2 × 10¹⁰ M⁻¹s⁻¹). Our data also show that both CAP and EPICAP reduce Fe(III) ions and that their respective thiyl radicals are formed in this reaction. In the presence of Fe(III), H₂O₂, and CAP, or EPICAP, ^.OH radicals were produced by a thiol-driven Fenton mechanism. Copper(II) ions were also reduced by these thiols, but no thiyl radicals could be detected in these reactions, and no ^.OH or other Fenton oxidants were observed in the presence of H₂O₂. Our data show direct evidence that thiol groups of CAP and EPICAP are involved in scavenging of ^.OH radicals. The direct ^.OH radical scavenging, together with the reductive “repair” of other sites of ^.OH radical attack, may contribute to the known protective effect of CAP against ischemia/reperfusion-induced arrhythmias. The formation of reactive thiyl radicals in the reactions of the studied compounds with ^.OH radicals and with Fe(III) ions may play a role in some of the known adverse effects of CAP.     

Radical-scavenging activity of butylated hydroxytoluene (BHT) and its metabolites

To clarify the radical-scavenging activity of butylated hydroxytoluene (BHT), a food additive, stoichiometric factors (n) and inhibition rate constants (k_inh) were determined for 2,6-di-tert-butyl-4-methylphenol (BHT) and its metabolites 2,6-di-tert-butyl-p-benzoquinone (BHT-Q), 3,5-di-tert-butyl-4-hydroxybenzaldehyde (BHA-CHO) and 3,5-di-tert-butyl-4-hydroperoxy-4-methyl-2,5-cyclohexadiene-1-one (BHT-OOH). Values of n and k_inh were determined from differential scanning calorimetry (DSC) monitoring of the polymerization of methyl methacrylate (MMA) initiated by 2,2′-azobis(isobutyronitrile) (AIBN) or benzoyl peroxide (BPO) at 70 °C in the presence or absence of antioxidants (BHT-related compounds). The n values declined in the order BHT (1–2) > BHT-CHO, BHT-OOH (0.1–0.3) > BHT-Q (0). The n value for BHT with AIBN was approximately 1.0, suggesting dimerization of BHT. The k_inh values declined in the order BHT-Q ((3.5–4.6)×10⁴ M⁻¹ s⁻¹) > BHT-OOH (0.7–1.9×10⁴ M⁻¹ s⁻¹) > BHT-CHO ((0.4–1.7)×10⁴ M⁻¹ s⁻¹) > BHT ((0.1–0.2)×10⁴ M⁻¹ s⁻¹). The k_inh for metabolites was greater than that for the parent BHT. Growing MMA radicals initiated by BPO were suppressed much more efficiently by BHT or BHT-Q compared with those initiated by AIBN. BHT was effective as a chain-breaking antioxidant.     

Interaction of melanin with carbon- and oxygen-centered radicals from methanol and ethanol

The interaction of dopa-melanin (DM) and cysteinyldopa-melanin (CDM) with carbon- and oxygen-centered radicals generated by benzophenone-photosensitized hydrogen abstraction from ethanol, or by pulse radiolysis of aqueous solutions of methanol and ethanol, is reported. Photosensitized formation of carbon-centered radicals and their interaction with melanin was monitored by electron paramagnetic resonance (EPR) spin trapping using DMPO, and via the melanin free radical signal itself. In the pulse radiolysis experiments, the interaction of DM or CDM with hydroxymethyl, hydroxyethyl, and the corresponding methanol peroxyl radical was monitored by recording time-dependent changes of the melanin absorbance at selected wavelengths. The data indicate that both melanins are good scavengers of carbon-centered radicals, with corresponding rate constants in the range of 10⁷ to 10⁸ M⁻¹ s⁻¹. Significantly, compared to DM, CDM is also an exceptionally efficient scavenger of oxygen-centered radicals derived from methanol with corresponding rate constants of 2.7 × 10⁴ and 2 × 10⁶, M⁻¹ s⁻¹ for DM and CDM, respectively. The results are discussed with reference to the potential role of melanin in protecting the integrity of melanosomes by inhibiting peroxidation of lipid components of the organelle membrane.     

Simulation of the molecular and electronic structure of heterofullerenes C55Y5 (Y=Si, Ge, Sn, B, Al, N, P, SiH, GeH, SnH) and their η-π-complexes with Li by the MNDO method

Qualitative estimates of the relative stability of hypothetical heterofullerenes C₅₅Y₅ (Y=Si, Ge, Sn, B, Al, N, P, SiH, GeH, SnH) and some η⁵-π-complexes LiC₅₅Y₅ were carried out by the MNDO method. Atoms Y (or groups XH) are assumed to substitute those C atoms in fullerene C₆₀ which are located at the -positions of a separated pentagonal face (pent^*) of this polyhedral molecule. It is shown that the spin densities in radicals C₅₅Y₅ (Y=SiH, GeH, SnH, B, Al, N, P) are localized on the separated pentagon atoms and the Li-pentagonal face (Li-pent^*) bonds in η⁵-π-complexes of these radicals with the Li atom are considerably stronger than Li-pent^* bonds in complexes [η⁵-π-LiC₆₀]⁺ and [η⁵-π-LiC₆₀] of unsubstituted C₆₀. In addition, it is established that the Li-pent^* bond energies in η⁵-π-complexes LiC₅₅B₅ and LiC₅₅Al₅ exceed the energy of the Li-pent^* bond in the η⁵-π-complex LiC₆₀H₅ studied earlier. In contrast, the energies of similar bonds for Y=N, P are close to the energy of the Li-pent^* bond in the η⁵-π-complex LiC₆₀H₅.     

Free radical scavenging behavior of folic acid: evidence for possible antioxidant activity

The free radical scavenging properties and possible antioxidant activity of folic acid are reported. Pulse radiolysis technique is employed to study the one-electron oxidation of folic acid in homogeneous aqueous solution. The radicals used for this study are CCl₃O₂^•, N₃^•, SO₄^•−, Br₂^•−, √OH, and O^•−. All these radicals react with folic acid under ambient condition at an almost diffusion-controlled rate producing two types of transients. The first transient absorption maximum is around 430 nm, which decays, and a simultaneous growth at around 390 nm is observed. Considering the chemical structure of folic acid, the absorption maximum at 430 nm has been assigned to a phenoxyl radical. The latter one is proposed to be a delocalized molecular radical. A permanent product has been observed in the oxidation of folic acid with CCl₃O₂^• and N₃^• radicals, with a broad absorption band around 370–400 nm. The bimolecular rate constants for all the radical-induced oxidation reactions of folic acid have been measured. Folic acid is seen to scavenge these radicals very efficiently. In the reaction of thiyl radicals with folic acid, it has been observed that folic acid can not only scavenge thiyl radicals but can also repair these thiols at physiological pH. While carrying out the lipid peroxidation study, in spite of the fact that folic acid is considerably soluble in water, we observed a significant inhibition property in microsomal lipid peroxidation. A suitable mechanism for oxidation of folic acid and repair of thiyl radicals by folic acid has been proposed.     

Kinetics of the reactions of nitrogen dioxide with glutathione,cysteine, and uric acid at physiological pH

Nitrogen dioxide (NO₂^•) is a key biological oxidant. It can be derived from peroxynitrite via the interaction of nitric oxide with superoxide, from nitrite with peroxidases, or from autoxidation of nitric oxide. In this study, submicromolar concentrations of NO₂^• were generated in < 1 μs using pulse radiolysis, and the kinetics of scavenging NO₂^• by glutathione, cysteine, or uric acid were monitored by spectrophotometry. The formation of the urate radical was observed directly, while the production of the oxidizing radical obtained on reaction of NO₂^• with the thiols (the thiyl radical) was monitored via oxidation of 2,2′-azino-bis-(3-ethylthiazoline-6-sulfonic acid). At pH 7.4, rate constants for reaction of NO₂^• with glutathione, cysteine, and urate were estimated as 2 × 10⁷, 5 × 10⁷, and 2 × 10⁷ M⁻¹ s⁻¹, respectively. The variation of these rate constants with pH indicated that thiolate reacted much faster than undissociated thiol. The dissociation of urate also accelerated reaction with NO₂^• at pH > 8. The thiyl radical from GSH reacted with urate with a rate constant of 3 × 10⁷ M⁻¹ s⁻¹. The implications of these values are: (i) the lifetime of NO₂^• in cytosol is < 10 μs; (ii) thiols are the dominant ‘sink’ for NO₂^• in cells/tissue, whereas urate is also a major scavenger in plasma; (iii) the diffusion distance of NO₂^• is 0.2 μm in the cytoplasm and < 0.8 μm in plasma; (iv) urate protects GSH against depletion on oxidative challenge from NO₂^•; and (v) reactions between NO₂^• and thiols/urate severely limit the likelihood of reaction of NO₂^• with NO• to form N₂O₃ in the cytoplasm.     

Hydroxyl Radical-Induced Reactions in Polyadenylic Acid as Studied by Pulse Radiolysis: Part I. Transformation Reactions of Two Isomeric OH-Adducts

The absorption spectra of polyadenylic acid (polyA) radicals in N₂0 saturated aqueous solution have been measured as a function of time (up to 15 s) following an 0.4μS electron pulse. The spectra and their changes were analysed by comparison with those from monomeric adenine derivatives (nucleosides and nucleotides) which had been studied by Steenken.¹

The reaction of OH^· radicals with the adenine moiety in poly A results in the formation of two hvdroxvl adducts at the positions C-4 [polyA40H^·] and C-8 [polyA80H^·]. Each OH-adduct undergoes a unimol-ecular transformation reaction before any bimolecular or other unimolecular decay occurs. These reactions are characterized by different rate constants and _pH dependencies. The polyA40H^· adduct undergoes a dehydration reaction to yield a neutral N⁶ centered radical (rate constant K_deh= 1.4 × 10⁴s^-1 at ^pH7.3). This reaction is strongly inhibited by H⁺. In comparison with the analogous reactions in adenosine phosphates, the kinetic _pK value for its inhibition is two ^pH units higher. This shift is the result of the counter ion condensation or double-strand formation. The polyA80H^· adduct undergoes an imidazole ring opening reaction to yield an enol type of formamidopyrimidine radical with the resulting base damage (k^r.o. = 3.5 × 10⁴ s ^-1 at pH7.3). This reaction in contrast is strongly catalysed by H⁺and OH^-, similar as for adenosine but different compared to the nucleotides.     

The potential role of microzooplankton in a northwestern Australian pelagic food web

The role of microzooplankton in waters adjacent to Australia's North West Cape (21°49'S 114°14'E) was studied during the austral summers 1997/1998 and 1998/1999. We estimated microzooplankton abundance and biomass at a shallow (∼20 m) shelf station and at a shelf break station (∼80 m). Microzooplankton were placed into six categories: four ciliate groups (strombidiids, strobilidiids, tintinnids, “other ciliates”), dinoflagellates, and sarcodines. Total microzooplankton abundances ranged between 0.14×10³ l^-1 and 3.4×10³ l^-1. The most abundant groups were the dinoflagellates (mean 459±73 standard error l^-1) and strombidiids (mean 334±42 standard error l^-1). Total microzooplankton biomass ranged between 0.03 and 1.70 µg C l^-1 (mean 0.33±0.05 standard error l^-1). Redundancy analysis indicated differences in microzooplankton community composition between stations and sampling years but no differences with sampling depth. The microzooplankton community showed considerable variability between adjacent sampling dates, reinforcing the conclusion of earlier studies that this area is a dynamic environment. Ciliate production on the shelf was estimated to be 1.05 µg C l^-1day^-1 (∼20 mg C m^-2day^-1) and 0.79 µg C l^-1day^-1(∼70 mg C m^-2day^-1) at the shelf break. Ciliate production near North West Cape was two- to six-fold higher than the rate of secondary production by juvenile copepods. Despite this, ciliate grazing appears to account for only ∼5% of primary production and ciliates do not appear to be a major conduit between primary producers and higher trophic levels in these waters.     

Reduction of daunorubicin in the presence of sulfur-containing peptides

Daunorubicin, an anthracycline antitumor antibiotic, was reduced in the presence of reduced (GSH) or oxidized (GSSG) glutathione to evaluate the possibilities of detoxification or of potentiation of the drug by these compounds. The reductants were ^.COO⁻ free radicals produced by γ radiolysis. In both cases, the final product is 7-deoxydaunomycinone, i.e., the same as without glutathione. The reduction yield is also the same as without GSH or GSSG (0.23 μmol·J⁻¹). No glutathione depletion was observed. Limits for the rate constants of some possible nonenzymatic detoxification reactions are given. To evaluate the possible interactions of daunorubicin with sulfur-containing proteins, the reduction of this drug by ^.COO⁻ free radicals was also studied in the presence of a polypeptide containing two disulfide bridge are, respectively, 0.23 μmol·J⁻¹ 7-deoxydaunomycinone. The yields of reduction of the drug and of a protein disulfide bridge are, respectively, 0.23 μmol·J⁻¹ and ≤ 6 nmol·J⁻¹. These values indicate thet disulfide radical anions of the protein can reduce the drug, giving back the disulfide bridge, but that the drug transients niether oxidize nor reduce the protein.     

On the Specificity of Allopurinol and Oxypurinol as Inhibitors of Xanthine Oxidase. A Pulse Radiolysis Determination of Rate Constants for Reaction of Allopurinol and Oxypurinol with Hydroxyl Radicals

Allopurinol has been employed as a “specific” inhihitor of xanthine oxidase in studies of hypoxic/ reoxygenation injury. Pulse radiolysis was used to establish rate constants for the reactions of allopurinol and its major metabolite oxypurinol with hydroxyl radicals: values were (1.45 ± 0.241 × 10⁹ M^-1 s^-1 for allopurinol and (4.95 ± 0.84) × 10⁹ M^-1 s^-1 for oxypurinol. These rate constants show that, in view of the amounts of allopurinol that have been used in animal studies. hydroxyl radical scavenging by this molecule could contribute to its biological actions. especially if animals are pre-treated with allopurinol. so allowing oxypurinol to form. The ability of allopurinol to protect tissues not containing xanthine oxidase against reoxygenation injury may be related to radical scavenging by allopurinol and oxypurinol.     

Kinetics of Parallel Electron Transfer from β-Carotene to Phenoxyl Radical and Adduct Formation Between Phenoxyl Radical and β-Carotene

Phenoxyl radicals generated by laser flash photolysis were found to react with β-carotene with concomitant β-carotene bleaching in two parallel reactions with similar rates: (i) formation of a β-carotene adduct with a (pseudo) first order rate constant of 1-1.5 ± 10⁴ s^-1 with absorption maximum around 800 nm, and (ii) formation of a β-carotene radical cation with a (pseudo) first order rate constant of 2-3 ± 10⁴ s^-1 with absorption maximum around 920 nm. Both β-carotene radicals decay on a similar time scale and have virtually disappeared after 100 ms, the β-carotene adduct by a second order process. Oxygen had no effect on β-carotene bleaching or radical formation and decay. The reduction of phenoxyl radicals by β-carotene may prove important for an understanding of how β-carotene acts as an antioxidant.     

Free radical scavenging actions of metallothionein isoforms I and II

By employing electron spin resonance spectroscopy, we examined the free radicals scavenging effects of hepatic metallothionein (MT) isoforms I and II (MTs-I and II) on four types of free radicals. Solutions of 0.15mM of MT-I and 0.3mM of MT-II were found to scavenge the 1,1-diphenyl-2-picrylhydrazyl radicals (1.30 × 10¹⁵ spins/ml) completely. In addition, both isoforms exhibited total scavenging action against the hydroxyl radicals (1.75 × 10¹⁵ spins/ml) generated in a Fenton reaction. Similarly, 0.3mM of MT-I scavenged almost 90% of the superoxide (2.22 × 10¹⁵ spins/ml) generated by the hypoxanthine and xanthine oxidase system, while a 0.3mM MT-II solution could only scavenge 40% of it. By using 2,2,6,6-tetramethyl-4-piperidone as a “spin-trap” for the reactive oxygen species (containing singlet oxygen, superoxide and hydroxyl radicals) generated by photosensitized oxidation of riboflavin and measuring the relative signal intensities of the resulting stable nitroxide adduct, 2,2,6,6-tetramethyl-4-piperidine-1-oxyl, we observed that MT-II (0.3 mM) could scavenge 92%, while MT-I at 0.15 mM μl/ml concentrations could completely scavenge all the reactive species (2.15 × 10¹⁵ spins/ml) generated.

The results of these studies suggest that although both isoforms of MT are able to scavenge free radicals, the MT-I appears to be a superior scavenger of superoxide and 1,1 diphenyl-2-picrylhydrazyl radicals.     

Rapid scavenging of peroxynitrous acid by monohydroascorbate

The reaction of peroxynitrous acid with monohydroascorbate, over the concentration range of 250 μM to 50 mM of monohydroascorbate at pH 5.8 and at 25°C, was reinvestigated and the rate constant of the reaction found to be much higher than reported earlier (Bartlett, D.; Church, D. F.; Bounds, P. L.; Koppenol, W. H. The kinetics of oxidation of L-ascorbic acid by peroxynitrite. Free Radic. Biol. Med. 18:85–92; 1995; Squadrito, G. L.; Jin, X.; Pryor, W. A. Stopped-flow kinetics of the reaction of ascorbic acid with peroxynitrite. Arch. Biochem. Biophys. 322:53–59; 1995). The new rate constants at pH 5.8 are k₁ = 1 × 10⁶ M⁻¹ s⁻¹ and k₋₁ = 500 s⁻¹ for 25°C and k₁ = 1.5 × 10⁶ M⁻¹ s⁻¹ and k₋₁ = 1 × 10³ s⁻¹ for 37°C. These values indicate that even at low monohydroascorbate concentrations most of peroxynitrous acid forms an adduct with this antioxidant. The mechanism of the reaction involves formation of an intermediate, which decays to a second intermediate with an absorption maximum at 345 nm. At low monohydroascorbate concentrations, the second intermediate decays to nitrate and monohydroascorbate, while at monohydroascorbate concentrations greater than 4 mM, this second intermediate reacts with a second monohydroascorbate to form nitrite, dehydroascorbate, and monohydroascorbate. EPR experiments indicate that the yield of the ascorbyl radical is 0.24% relative to the initial peroxynitrous acid concentration, and that this small amount of ascorbyl radicals is formed concomitantly with the decrease of the absorption at 345 nm. Thus, the ascorbyl radical is not a primary reaction product. Under the conditions of these experiments, no homolysis of peroxynitrous acid to nitrogen dioxide and hydroxyl radical was observed. Aside from monohydroascorbate's ability to “repair” oxidatively modified biomolecules, it may play a role as scavenger of peroxynitrous acid.     

Electron Transfer Reactions in RB90745, A Bioreductive Drug Having Both Aromatic N-Oxide and Nitroarene Moieties

The bifunctional hypoxia-specific cytotoxin RB90745, has a nitroimidazole moiety attached to an imidazo[1,2,-a]quinoxaline mono-N-oxide with a spacer/linking group. The reduction chemistry of the drug was studied by pulse radiolysis using the one electron reductant CO₂^˙-. As N-oxides and nitro compounds react with CO₂^˙- at diffusion controlled rates, initial reaction produced a mixture of the nitro radical (λ_max 410 nm) and the N-oxide radical (λ_max 550 nm) in a few microseconds. Subsequently an intramolecular electron transfer (IET) was observed (k = 1.0 ± 0.25 × 10³ s^-1 at pH 5-9), from the N-oxide to the more electron-affinic nitro group. This was confirmed by the first order decay rate of the radical at 550 nm and formation at 410 nm, which was independent of both the concentration of the parent compound and the radicals. The rates of electron transfer and the decay kinetics of the nitro anion radicals were pH dependent and three different pK_aS could be estimated for the one electron reduced species: 5.6 (nitroimidazole group) and 4.3, and 7.6 (N-oxide function). The radicals react with oxygen with rate constants of 3.1 × 10⁷ and 2.8 × 10⁶ dm³ mol^-1 s^-1 observed at 575 nm and 410 nm respectively. Steady state radiolysis studies indicated four electron stoichiometry for the reduction of the compound.     

Nitrosylmyoglobin as antioxidant--kinetics and proposed mechanism for reduction of hydroperoxides

Nitrosylmyoglobin (MbFe^IINO), which is believed to have a protective role during ischemia and reperfusion injury, was oxidized by tert-butyl hydroperoxide (t-BuOOH), and by hydrogen peroxide (H₂O₂) to the nitrite anion and metmyoglobin (MbFe^III). Further characterization of the reaction of MbFe^IINO with excess of t-BuOOH was investigated with respect to reaction stoichiometry, temperature and pH dependence. It was found that the reaction between MbFe^IINO with excess of t-BuOOH followed a simple stoichiometry and had moderate pH and temperature dependence with the activation parameters ΔH^‡ = 57.4 ± 1.4 kJ mol^- 1 and ΔS^‡ = - 112.0 ± 5.1 J mol^- 1 K^- 1, which is consistent with an associative reaction mechanism. Moreover, t-BuOOH-induced oxidation of MbFe^IINO did not result in any detectable formation of the hypervalent myoglobin (Mb) species, i.e. perferrylmyoglobin, (^√MbFe^IV = O) or ferrylmyoglobin (MbFe^IV = O), and hereby differed from H₂O₂-induced oxidation of MbFe^IINO, which results in the formation of MbFe^IV = O. Based on the obtained results and on published data, different mechanisms for the reaction of the MbFe^IINO with t-BuOOH and H₂O₂ are proposed.     

Free Radical Transients in Photobleaching of Xanthophylls and Carotenes

Carotenoicls in chloroform and carbon tetrachloriclc photobleach upon nanosecond laser flash photolysis in two steps: instantaneously and in a second-order reaction. The rate constant for second-order reaction (first-order in a solvent derived radical and first-order in (excess) ccirotenoid) is largest for carotenes (9.8·10⁸ M^-1 s^-1 for β-carotene), intermediate for hydroxylated carotenoids, and smallest for carbonyl containing carotenoids (1.0·10⁸ M^-1 s^-1 for astaxanthin) in chloroform at 20°C. Near infrared, ibsorbing transients are formed concomitant with pliotohleaching in chloroform (not detected in cxbon tetrachloride). A species formed instantaneously is tentatively identified as either a carotenoid/solvent adduct or an ion-pair. A second species is formed by decay of the instantaneously formed species and is identified as the carotenoid radical cation. This species is formed in a first-order reaction with a rate constant of approx. 5·10⁴ s^-1 and absorbing at longer wavelength than the precursor. The lifetime (second-order decay) of the interniediates appears to be longest for the carotenoids with the longest conjugated system. The results indicate that carotenes are better antioxidants than xantliophylls as the carotenes, at least in the present lipophilic solvents, react faster with free radicals.     

Rate of electron transfer between plastocyanin, cytochrome ƒ, related proteins and artificial redox reagents in solution

The rate of electron transfer between reduced cytochrome ƒ and plastocyanin (both purified from parsley) has been measured as k = 3.6 · 10⁷ M⁻¹ · s⁻¹, at 298 °K and pH 7.0, with activation parameters ΔH^‡ = 44 kJ · mole⁻¹ and ΔS^‡ = +46 J · mole⁻¹ · °K⁻¹. Replacement of cytochrome ƒ with red algal cytochrome c-553, Pseudomonas cytochrome c-551 and mammalian cytochrome c gave rates at least 30 times slower: k = 5 · 10⁵, 7.5 · 10⁵ and 1.0 · 10⁶ M⁻¹ · s⁻¹, respectively.

Similar measurements made with azurin instead of plastocyanin gave k = 6 · 10⁶ and approx. 2 · 10⁷ M⁻¹ · s⁻¹ for reaction of reduced azurin with cytochrome ƒ and algal cytochrome respectively.

Rate constants of 115 and 80 M⁻¹ · s⁻¹ were found for reduction of plastocyanin by ascorbate and hydroquinone at 298 °K and pH 7.0. The rate constants for the oxidation of plastocyanin, cytochrome ƒ, Pseudomonas cytochrome c-551 and red algal cytochrome c-553 by ferricyanide were found to be between 3 · 10⁴ and 8 · 10⁴ M⁻¹ · s⁻¹.

The results are discussed in relation to photosynthetic electron transport.     

Antioxidant action of sodium diethyldithiocarbamate: Reaction with hydrogen peroxide and superoxide radical

The oxidation of sodium diethyldithiocarbamate (DDC) by hydrogen peroxide or superoxide radicals has been investigated. Hydrogen peroxide oxidizes DDC, leading to the formation of a hydrated form of disulfiram, a dimer of DDC having a disulfide group. In equimolar conditions, the overall process appears as a first-order reaction (k = 0.025±0.005 s⁻¹), the first step being a second-order reaction (k = 5.0±0.1mol⁻¹.1. s⁻¹). No radical intermediate was observed in this process. In the presence of an excess of any of the reagents, the hydrated form of disulfiram transforms into different products corresponding to the fixation of oxygen by sulfur atoms or replacement of C = S group by ketone function, in the presence of an excess of hydrogen peroxide. Superoxide anions (produced by steady-state ⁶⁰Co γ-radiolysis) oxidize DDC, yielding similar products to those obtained with hydrogen peroxide with a maximum oxidation G-value of 0.3 μmol.J⁻¹. The rate constant k(O₂^·− + DDC) is equal to 900 mol⁻¹. 1. s⁻¹.     

The diffusion-controlled reaction of semioxidized tryptophan with the superoxide radical anion

From pulse radiolysis measurements in oxygenated aqueous solution, the semioxidized tryptophan radical (Trp·— formed by the one-electron oxidation of Trp by Br₂^- radical—has been shown to oxidize the superoxide radical anion with a rate constant of k = 2 × 10⁹ M⁻¹ s⁻¹. Proof of this reaction is found in addition of superoxide dismutase (SOD) to the system, which totally eliminates the contribution of the Trp^· + O₂^- mechanism to Trp^· decay. Little, if any, reaction of molecular oxygen with Trp^· may be observed on the time scale of the pulse radiolysis experiment.     

Monovalent copper as a potential catalyst for formation of acetaldehyde via the migration of methyl radicals to the coordinated carbonyl in the complex (CO)Cu-CH3

Cu_aq⁺ forms stable complexes with carbon monoxide in aqueous solutions. Furthermore it reacts very fast with aliphatic radicals. The reaction of Cu(CO)_maq⁺ with methyl radicals, ^•CH₃ was studied using the pulse-radiolysis technique. The results point out that methyl radicals react with Cu(CO)_aq⁺ to form an unstable intermediate with a Cu^II-C σ bond identified as (CO)Cu^II-CH₃⁺, k = (1.1±0.2) × 10⁹ M⁻¹ s⁻¹. This intermediate has a strong LMCT charge transfer band (λ_max = 385 nm, _max = 2500 M⁻¹ cm⁻¹) which is similar to the absorption bands of other transient complexes with Cu^II-alkyl σ bonds. The coordinated carbon monoxide in (CO)Cu^II-CH₃⁺ inserts into the copper—carbon bond (or rather the coordinated methyl migrates to the coordinated carbon monoxide ligand) at a rate of (3.0±0.8) × 10² s⁻¹ to form the copperacetyl complex (CO)_mCu^II-C(CH₃)=O⁺ (λ_max = 480 nm, _max = 2100 M⁻¹ cm⁻¹). The rate of formation of (CO)Cu^II-CH₃⁺ and of the insertion reaction are pH independent. The complex (CO)_mCu^II-C(CH₃)=O⁺ is also unstable and decomposes heterolytically to yield acetaldehyde and Cu_aq²⁺ as the final stable products. This reaction is slightly pH dependent. The same reactivity pattern has been observed for the Cu(CO_naq⁺ complexes (n = 2 or 3). The results clearly point out that CO remains coordinated to transient complexes of the type Cu^II-alkyl.