Photogeneration of singlet oxygen (1O2) and free radicals (Sen·-, O·-2) by tetra-brominated hypocrellin B derivative

To improve photodynamic activity of the parent hypocrellin B (HB), a tetra-brominated HB derivative (compound 1) was synthesized in high yield. Compared with HB, compound 1 has enhanced red absorption and high molar extinction coefficients. The photodynamic action of compound 1, especially the generation mechanism and efficiencies of active species (Sen^·-, O^·-₂ and ¹O₂) were studied using electron paramagnetic resonance (EPR) and spectrophotometric methods. In the deoxygenated DMSO solution of compound 1, the semiquinone anion radical of compound 1 is photogenerated via the self-electron transfer between the excited and ground state species. The presence of electron donor significantly promotes the reduction of compound 1. When oxygen is present, superoxide anion radical (O^·-₂) is formed via the electron transfer from Sens^·- to the ground state molecular oxygen. The efficiencies of Sens^·- and O^·-₂ generation by compound 1 are about three and two times as much as that of HB, respectively. Singlet oxygen (¹O₂) can be produced via the energy transfer from triplet compound 1 to ground state oxygen molecules. The quantum yield of singlet oxygen (¹O₂) is 0.54 in CHCl₃ similar to that of HB. Furthermore, it was found that the accumulation of Sens^·- would replace that of O^·-₂ or ¹O₂ with the depletion of oxygen in the sealed system.     

Role of reactive oxygen species in cardiac preconditioning: study with photoactivated Rose Bengal in isolated rat hearts

Oxygen radical scavengers have been shown to prevent the development of ischemic preconditioning, suggesting that reactive oxygen species (ROS) might be involved in this phenomenon. In the present study, we have investigated whether direct exposure to ROS produced by photoactivated Rose Bengal (RB) could mimic the protective effects of ischemic preconditioning.

Methods In vitro generation of ROS from photoactivated RB in a physiological buffer was first characterised by ESR spectroscopy in the presence of 2,2,6,6-tetramethyl-1-piperidone (oxoTEMP) or 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). In a second part of the study, isolated rat hearts were exposed for 2.5 min to photoactivated RB. After 5 min washout, hearts underwent 30 min no-flow normothermic ischemia followed by 30 min of reperfusion.

Results and Conclusions The production of singlet oxygen (¹O₂) by photoactivated RB in the perfusion medium was evidenced by the ESR detection of the nitroxyl radical oxoTEMPO. Histidine completely inhibited oxoTEMPO formation. In addition, the use of DMPO has indicated that (i) superoxide anions (O^·-₂) are produced directly and (ii) hydroxyl radicals (HO^·) are formed indirectly from the successive O^·-₂ dismutation and the Fenton reaction. In the perfusion experiments, myocardial post-ischemic recovery was dramatically impaired in hearts previously exposed to the ROS produced by RB photoactivation (¹O₂, O^·-₂, H₂O₂ and HO^·) as well as when ¹O₂ was removed by histidine (50 mM) addition. However, functional recovery was significantly improved when hearts were exposed to photoactivated RB in presence of superoxide dismutase (10⁵ IU/L) and catalase (10⁶ IU/L).

Further studies are now required to determine whether the cardioprotective effects of Rose Bengal in presence of O^·-₂ and H₂O₂ scavengers are due to singlet oxygen or to other species produced by Rose Bengal degradation.     

Interactions of superoxide anion with enzyme radicals: kinetics of reaction with lysozyme tryptophan radicals and corresponding effects on tyrosine electron transfer

The kinetics of O^·-₂ reaction with semi-oxidized tryptophan radicals in lysozyme, Trp^·(Lyz) have been investigated at various pHs and conformational states by pulse radiolysis. The Trp^·(Lyz) radicals were formed by Br^·-₂ oxidation of the 3-4 exposed Trp residues in the protein. At pH lower than 6.2, the apparent bimolecular rate is about 2 × 10⁸M^-1s^-1; but drops to 8 × 10⁷M^-1s^-1 or less above pH 6.3 and in CTAC micelles. Similarly, the apparent bimolecular rate constant for the intermolecular Trp^·(Lyz) + Trp^·(Lyz) recombination reaction is about (4-7 × 10⁶M^-1s^-1) at/or below pH 6.2 then drops to 1.3-1.6 × 10⁶M^-1s^-1 at higher pH or in micelles. This behavior suggests important conformational and/or microenvironmental rearrangement with pH, leading to less accessible semioxidized Trp^· residues upon Br^·-₂ reaction. The kinetics of Trp^·(Lyz) with ascorbate, a reducing species rather larger than O^·-₂ have been measured for comparison. The well-established long range intramolecular electron transfer from Tyr residues to Trp radicals-leading to the repair of the semi-oxidized Trp^·(Lyz) and formation of the tyrosyl phenoxyl radical is inhibited by the Trp^·(Lyz)+O^·-₂ reaction, as is most of the Trp^·(Lyz)+Trp^·(Lyz) reaction. However, the kinetic behavior of Trp^·(Lyz) suggests that not all oxidized Trp residues are involved in the intermolecular recombination or reaction with O^·-₂. As the kinetics are found to be quite pH sensitive, this study demonstrates the effect of the protein conformation on O^·-₂ reactivity. To our knowledge, this is the first report on the kinetics of a protein-O^·-₂ reaction not involving the detection of change in the redox state of a prosthetic group to probe the reactivity of the superoxide anion.     

Redox reactions of the urate radical/urate couple with the superoxide radical anion, the tryptophan neutral radical and selected flavonoids in neutral aqueous solutions

The kinetics of several processes involving the potential antioxidant role of urate in physiological systems have been investigated by pulse radiolysis. While the monoanionic urate radical, ^·UH^-, can be produced directly by oxidation with ^·Br^-₂ or ^·OH, it can also be generated by oxidation with the neutral tryptophan radical, ^·Trp, with a rate constant of 2 × 10⁷ M^-1s^-1. This radical, ^·UH^-, reacts with ^·O^-₂ with a rate constant of 8 × 10⁸ M^-1s^-1. Also, ^·UH^- is reduced by flavonoids, quercetin and rutin in CTAB micelles at rate constants of 6 × 10⁶ M^-1s^-1 and 1 × 10⁶ M^-1s^-1, respectively. These results can be of value by providing reference data useful in further investigation of the antioxidant character of urate in more complex biological systems.     

Carvedilol inhibition of lipid peroxidation. A new antioxidative mechanism

To define the molecular mechanism(s) of carvedilol inhibition of lipid peroxidation we have utilized model systems that allow us to study the different reactions involved in this complex process.

Carvedilol inhibits the peroxidation of sonicated phosphatidylcholine liposomes triggered by FeCl₂ addition whereas atenolol, pindolol and labetalol are ineffective. The inhibition proved not to be ascribable (a) to an effect on Fe²⁺ autoxidation and thus on the generation of oxygen derived radical initiators; (b) to the scavenging of the inorganic initiators O^·-₂ and ^·OH; (c) to an effect on the reductive cleavage of organic hydroperoxides by FeCl₂; (d) to the scavenging of organic initiators. The observations that (a) carvedilol effectiveness is inversely proportional to the concentration of FeCl₂ and lipid hydroperoxides in the assay; (b) the drug prevents the onset of lipid peroxidation stimulated by FeCl₃ addition and; (c) it can form a complex with Fe³⁺, suggest a molecular mechanism for carvedilol action. It may inhibit lipid peroxidation by binding the Fe³⁺ generated during the oxidation of Fe²⁺ by lipid hydroperoxides in the substrate. The lag time that carvedilol introduces in the peroxidative process would correspond to the time taken for carvedilol to be titrated by Fe³⁺; when the drug is consumed the Fe³⁺ accumulates to reach the critical parameter that stimulates peroxidation. According to this molecular mechanism the antioxidant potency of carvedilol can be ascribed to its ability to bind a species, Fe³⁺, that is a catalyst of the process and to its lipophilic nature that concentrates it in the membranes where Fe³⁺ is generated by a site specific mechanism.     

Nitric oxide and peroxynitrite. The ugly, the uglier and the not so good: a personal view of recent controversies

Nitric oxide, a gaseous free radical, is poorly reactive with most biomolecules but highly reactive with other free radicals. Its ability to scavenge peroxyl and other damaging radicals may make it an important antioxidant in vivo, particular in the cardiovascular system, although this ability has been somewhat eclipsed in the literature by a focus on the toxicity of peroxynitrite, generated by reaction of O^·-₂ with NO^· (or of NO^- with O₂). On balance, experimental and theoretical data support the view that ONOO^- can lead to hydroxyl radical (OH^·) generation at pH 7.4, but it seems unlikely that OH^· contributes much to the cytotoxicity of ONOO^-. The cytotoxicity of ONOO^- may have been over-emphasized: its formation and rapid reaction with antioxidants may provide a mechanism of using NO^· to dispose of excess O^·-₂, or even of using O^·-₂ to dispose of excess NO^·, in order to maintain the correct balance between these radicals in vivo. Injection or instillation of “bolus” ONOO^- into animals has produced tissue injury, however, although more experiments generating ONOO^- at steady rates in vivo are required. The presence of 3-nitrotyrosine in tissues is still frequently taken as evidence of ONOO^- generation in vivo, but abundant evidence now exists to support the view that it is a biomarker of several “reactive nitrogen species”. Another under-addressed problem is the reliability of assays used to detect and measure 3-nitrotyrosine in tissues and body fluids: immunostaining results vary between laboratories and simple HPLC methods are susceptible to artefacts. Exposure of biological material to low pH (e.g. during acidic hydrolysis to liberate nitrotyrosine from proteins) or to H₂O₂ might cause artefactual generation of nitrotyrosine from NO^-₂ in the samples. This may be the origin of some of the very large values for tissue nitrotyrosine levels quoted in the literature. Nitrous acid causes not only tyrosine nitration but also DNA base deamination at low pH: these events are relevant to the human stomach since saliva and many foods are rich in nitrite. Several plant phenolics inhibit nitration and deamination in vitro, an effect that could conceivably contribute to their protective effects against gastric cancer development.     

Reactive oxygen intermediates regulate cellular response to apoptotic stimuli: an hypothesis

Production of reactive oxygen intermediates (ROI) has been thought for a long time to adversely affect the physiology and survival of a cell. There is now a growing body of evidence to suggest that ROI such as superoxide anion (O^·-₂) and hydrogen peroxide (H₂O₂) can influence the growth, as well as death, of animal cells in vitro. The observation that cells release O^·-₂ or its dismutation product H₂O₂, either constitutively in the case of tumor cells or following cytokine stimulation, has led to the speculation that they might possibly serve as intercellular messengers to stimulate proliferation via mechanisms common to natural growth factors. However, as the balance between cell populations in an organism is tightly controlled by the rate of proliferation and death of constituent cells, an increase in cell numbers could reciprocally be viewed as deregulation of cell death. Hence, it is equally important to decipher how ROI influence the response of cells to signals that activate cell death pathway(s). We propose that ROI not only regulate proliferation but also affect cell sensitivity to triggers which activate the cellular suicide program (apoptosis) versus those that cause accidental (necrotic) cell death.     

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.     

Catalysis of plastocyanin electron self-exchange by redox-inert multivalent cations

Electron self-exchange in solutions of the ‘blue’ copper protein plastocyanin is catalysed by the redox-inert multivalent cations Mg²⁺ or Co(NH₃)³⁺₆. Measurements of specific ¹H-NMR line broadening with 50% reduced solutions in the presence of these cations show that electron exchange proceeds through encounters of cation-protein complexes which dissociate at high ionic strength. In the presence of 8mM (5 equivalents/total protein) Co(NH₃)³⁺₆, with 10 mM cacodylate (pH^*6.0) as background electrolyte, the bimolecular rate constant at 25°C is 7 × 10⁴ M⁻¹·s⁻¹. For comparison, the ‘electrostatically screened’ rate constant measured in 0.1 M KCl in the absence of added multivalent cations is ˜ 4 × 10³ M¹·s⁻¹.

Plastocyanin Electron self-exchange NMR Protein-protein interaction Multivalent cation Blue copper protein     

Experimental and theoretical study of a truly functional biomimetic molybdenum oxotransferase analogue system

Density functional theory (DFT) computations at the B3LYP/Lanl2DZ level were used to elucidate the oxygen atom transfer (OAT) and coupled electron proton transfer (CEPT) reaction steps involved in the biomimetic catalytic cycle performed by polymer-supported Mo^VIO₂(NN′)₂ complexes [NN′ = phenyl-(pyrrolato-2-ylmethylene)-amine] with water as oxygen source, trimethyl-phosphane as oxygen acceptor and one-electron oxidising agents. The DFT method employed has been validated against experimental data [X-ray crystal structures of a NN′ ligand and a Mo^VIO₂(NN′)₂ complex as well as kinetic data]. The rate-limiting step in the forward-OAT from [Mo^VIO₂] to PMe₃ is the attack of PMe₃ at an oxo ligand with ΔG^≠ (298 K) = 64.6 kJ mol⁻¹. Dissociation of the product OPMe₃ is facile with ΔG^≠ (298 K) = 26.3 kJ mol⁻¹ giving a mono-oxo [Mo^IVO] complex which fills its coordination sphere with a further PMe₃ substrate with ΔG^≠ (298 K) = 39.2 kJ mol⁻¹. One-electron oxidation to a Mo(V) phosphane complex precedes the coordination of water/hydroxide. Additionally, the comproportionation of [Mo^VIO₂] and [Mo^IVO] to dinuclear oxo-bridged [OMo^V–O–Mo^VO] species has been calculated as the thermodynamic sink in this system and the back-OAT from dmso to mono-oxo [Mo^IVO] to give [Mo^VIO₂] has been shown to involve an equilibrium between stereoisomeric [Mo^VIO₂] complexes with an activation barrier of ΔG^≠ (298 K) = 113.1 kJ mol⁻¹.     

Mechanism of O2 generation in reduction and oxidation cycle of ubiquinones in a model of mitochondrial electron transport systems

O⁻₂ generation in mitochondrial electron transport systems, especially the NADPH-coenzyme Q₁₀ oxidoreductase system, was examined using a model system, NADPH-coenzyme Q₁-NADPH-dependent cytochrome P-450 reductase. One electron reduction of coenzyme Q₁ produces coenzyme Q₁ and O⁻₂ during enzyme-catalyzed reduction and O₂ + coenzyme Q₁ are in equilibrium with O⁻₂ + coenzyme Q₁ in the presence of enough O₂. The coenzyme Q₁ produced can be completely eliminated by superoxide dismutase, identical to bound coenzyme Q₁₀ radical produced in a succinate/fumarate couple-KCN-submitochondrial system in the presence of O₂. Superoxide dismutase promotes electron transfer from reduced enzyme to coenzyme Q₁ by the rapid dismutation of O₂⁻ generated, thereby preventing the reduction of coenzyme Q₁ by O⁻₂. The enzymatic reduction of coenzyme Q₁ to coenzyme Q₁H₂ via coenzyme Q₁ is smoothly achieved under anaerobic conditions. The rate of coenzyme Q₁H₂ autoxidation is extremely slow, i.e., second-order constant for [O₂][coenzyme Q₁H₂] = 1.5 M⁻¹ · s⁻¹ at 258 μM O₂, pH 7.5 and 25°C.     

Ligand induced P---H bond formation and a comparison of the reactivity of two isomers of the carbonyl hydride [Pt2(μ-PBu2)2(H)(CO)(PBu2H)]CF3SO3

The Pt₂ ^(II) isomeric terminal hydrides [(CO)(H)Pt(μ-PBu^t ₂)₂Pt(PBu^t ₂H)]CF₃SO₃ (1a), and [(CO)Pt(μ-PBu^t ₂)₂Pt(PBu^t ₂H)(H)]CF₃SO₃ (1b), react rapidly with 1 atm of carbon monoxide to give the same mixture of two isomers of the Pt₂ ^(I) dicarbonyl [Pt₂(μ-PBu^t ₂)(CO)₂(PBu^t ₂H)₂]CF₃SO₃ (3-Pt); the solid state structure of the isomer bearing the carbonyl ligands pseudo-trans to the bridging phosphide was solved by X-ray diffraction. A remarkable difference was instead found between the reactivity of 1a and 1b towards carbon disulfide or isoprene. In both cases 1b reacts slowly to afford [Pt₂(μ-PBu^t ₂)(μ,η²,η²-CS₂)(PBu^t ₂H)₂]CF₃SO₃ (4-Pt), and [Pt₂(μ-PBu^t ₂)(μ,η²,η²-isoprene) (PBu^t ₂H)₂]CF₃SO₃ (6-Pt), respectively. In the same experimental conditions, 1a is totally inert. A common mechanism, proceeding through the preassociation of the incoming ligand followed by the P---H bond formation between one of the bridging P atoms and the hydride ligand, has been suggested for these reactions.     

Photogeneration of free radicals (*OH and HB*-) and singlet oxygen (1O2) by hypocrellin B in TX-100 micelles microsurroundings

To solve the problems faced in clinical use of hypocrellins, a water-soluble preparation of Hypocrellin B (HB), HB-Triton X-100 (TX-100) micelles, was prepared. To evaluate the photodynamic activity, the free radicals (^•OH and HB^•¯) and singlet oxygen (

1

O

2

) generated via photosensitization of the preparation in aqueous solution were detected by using electron paramagnetic resonance (EPR) and spectrophotometric methods. It was observed that

1

O

2

was formed with a quantum yield of 0.72, similar to that for HB in organic solvents, further, hydroxyl radicals (

•

OH) could also be efficiently produced by the new preparation, which have never before been detected following HB photoactivities. In addition, the semiquinone anion radicals (HB^•-) could also be generated via the self-electron transfer between an excited triplet state and a ground state molecule. The accumulation of HB^•- would replace that of

•

OH or

1

O

2

with the depletion of oxygen in the system. All these findings suggested that the HB-TX-100 micelles could play the photodynamic action through not only the type I mechanism by free radicals (^•OH, O₂^•- and HB^•-) but also the type II mechanism by singlet oxygen (

1

O

2

). It can be concluded further that the new preparation basically maintains the inherent photodynamic activity of HB, or even higher.     

Enhancing effect of melatonin on chemiluminescence accompanying decomposition of hydrogen peroxide in the presence of copper

The oxidation of melatonin (MEL) using the Cu(II) + H₂O₂ + HO⁻ (the Fenton-like reaction) system was investigated by chemiluminescence (CL), fluorescence, spectrophotometric, and EPR spin trapping techniques. The reaction exhibits CL in the 400–730 nm region. The light emission from the Fenton-like reaction was greatly enhanced in the presence of MEL and was strongly dependent on its concentration. The spectrum measured with cut-off filters revealed maxima at around 460, 500, 580–590, 640–650, and 690–700 nm. The band at 460 nm may be due to the excited cleavage product, N¹-acetyl-N²-formyl-5-methoxykynuramine, whereas the bands at 500, 580–590, 640–650, and 700 nm were similar to those observed for singlet molecular oxygen (¹O₂). The effect of reactive oxygen species (ROS) scavengers on the light emission was studied. The CL was strongly inhibited by the ¹O₂ scavengers in a dose-dependent manner; at concentration 1 mM the potency of ¹O₂ scavenging was 5,5-dimethylcyclohexandione-1,3 > methionine > histidine > hydroquinone. The potency of HO^• scavenging by thiourea, tryptophan, cysteine at concentration 5 mM was 79–94%, by 1 mM glutathione and trolox 75 and 94%, respectively, and by 10 mM cimetidine 18%. Specific acceptors of O₂^•− such as p-nitroblue tetrazolium chloride and 4,5-dihydroxy-1,3-benzene disulfonic acid (tiron) at concentration 5 mM decreased the CL by 51 and 95%, respectively, whereas superoxide dismutase (SOD) does not reduce the emission at concentration 2.8 U/ml. At higher concentration SOD substantially enhanced the light emission. Addition of 1360 U/ml catalase and 100 μM desferrioxamine strongly inhibited CL (96 and 90%, respectively). The increased generation of ¹O₂ from the Cu/H₂O₂ system in the presence of MEL was confirmed using the spectrophotometric method based on the bleaching of p-nitrosodimethylaniline and by trapping experiments with 2,2,6,6-tetramethylpiperidine (TEMP) and subsequent electron paramagnetic (EPR) spectroscopy. These findings suggest the increased production of reactive oxygen species (O₂^•−, HO^•, ¹O₂) from the Fenton-like reaction in the presence of MEL. This means that the hormone is not able to act as classical chain-breaking antioxidant even at low concentration, and may show clear prooxidant activity at higher concentrations. In addition, long-lived carbonyl product of the MEL transformation in the triplet state can also be toxic by transferring its energy to organelles and causing a photochemical process.     

One-electron reduction of selenomethionine oxide

Experimental evidence is provided that selenomethionine oxide (MetSeO) is more readily reducible than its sulfur analogue, methionine sulfoxide (MetSO). Pulse radiolysis experiments reveal an efficient reaction of MetSeO with one-electron reductants, such as e^-_aq (k = 1.2 × 10¹⁰M^-1s^-1), CO^·-₂ (k = 5.9 × 10⁸ M^-1s^-1) and (CH₃)₂) C^·OH (k = 3.5 × 10⁷M^-1s^-1), forming an intermediate selenium-nitrogen coupled zwitterionic radical with the positive charge at an intramolecularly formed Se_∴ N 2σ/1σ* three-electron bond, which is characterized by an optical absorption with λ_max at 375 nm, and a half-life of about 70 μs. The same transient is generated upon HO^· radical-induced one-electron oxidation of selenomethionine (MetSe). This radical thus constitutes the redox intermediate between the two oxidation states, MetSeO and MetSe. Time-resolved optical data further indicate sulfur-selenium interactions between the Se_∴ N transient and GSH. The Se_∴ N transient appears to play a key role in the reduction of selenomethionine oxide by glutathione.     

Mechanism of reaction of nitrogen dioxide radical with hydroxycinnamic acid derivatives: A pulse radiolysis study

Nitrogen dioxide radical (NO^·₂) is known as a toxic agent produced in the metabolism of nitrates and nitrites. By the use of the pulse radiolysis technique, the mechanism of the reaction of NO^·₂ radical with hydroxycinnamic acid derivatives (HCA) was studied and the rate constants have been measured. The rate constants were found to be 7.4 × 10⁸, 7.2 × 10⁸, 8.6 × 10⁸ dm³ mol^-1s^-1 for ferulic acid, sinapic acid and caffeic acid, respectively. The reactions produce the corresponding phenoxyl radical.     

Spectroscopic and electrical properties of the [W(C3Se5)3] anion complex (C3Se5 = 1,3-diselenole-2-selone-4,5-diselenolate) and the oxidized species

[NBuⁿ₄]₂[W(C₃Se₅)₃] (C₃Se₅²⁻ = 1,3-diselenole-2-selone-4,5- diselenolate(2−)) was prepared by the reaction of Na₂[C₃Se₅] with WCl₆ in ethanol, followed by addition of [NBuⁿ₄]Br. The cyclic voltammogram in dichloromethane exhibits two oxidation peaks at −0.04 and +0.03 V (versus SCE). The complex reacted with [Fe(C₅Me₅)₂][BF₄], iodine or [TTF]₃[BF₄]₂ (TTF^·+ = the tetrathiafulvalenium radical cation) in acetonitrile to afford the oxidized complexes [Fe(C₅Me₅)₂]_0.5[W(C₃Se₅)₃], [NBuⁿ₄]_0.1[W(C₃Se₅)₃] and [TTF]_0.5[W(C₃Se₅)₃], respectively. Current-controlled electrochemical oxidation of the complex in acetonitrile gave [NBuⁿ₄]_0.6[W(C₃Se₅)₃]. The oxidized complexes exhibit electrical conductivities of 4.7×10 ⁻⁵−1.5×10⁻³ S cm⁻¹ at room temperature measured for compacted pellets. Electronic absorption, IR and ESR spectra of these complexes are discussed.     

Oxygen-copper (II) interplay in the repair of semi-oxidized urate by quercetin bound to human serum albumin

The 1:1 complex of copper (II) and human serum albumin (HSA) slowly reacts with radiolytically generated ^•O₂^- radical-anion at a rate constant of 6.1×10₆ M_-1 s_-1. Absorbance and fluorescence spectroscopies demonstrate that addition of an equimolar portion of quercetin (QH₂) to the solution of the copper (II)-HSA complex induces a relocalization of the copper resulting in a ternary copper (II)-QH₂-HSA complex. This form of quercetin slowly oxidizes in air-saturated solutions. A 10-fold excess urate, a plasma antioxidant, cannot displace copper (II) bound to HSA. In N₂O-saturated solutions the ternary complex form of QH₂ can repair the urate radical with a rate constant of 2.7×10₆ M_-1 s_-1 by an electron transfer reaction similar to that observed in the absence of copper (II). In O₂-saturated solutions and in the absence of copper, HSA-bound QH₂ fails to repair the urate radical because of the fast competitive reaction of ^•O₂^- with urate radicals. However, addition of equimolar copper (II) restores the electron transfer from QH₂ to the urate radical. These contrasting results are tentatively explained either by an enhanced reactivity of copper (II) with ^•O₂^- in the ternary complex or by direct production of quercetin radicals via a copper-catalyzed reduction of the ^•O₂^- radicals by QH₂.     

Chemiluminescent oxidation of ribose catalyzed by horseradish peroxidase in presence of hydrogen peroxide

The reaction of ribose with horseradish peroxidase in the presence of H₂O₂ is accompanied by light emission. The detection of horseradish peroxidase Compound II (FeO²⁺) indicates that the enzyme participates in a normal peroxidatic cycle. Hydrogen peroxide converts horseradish peroxidase into Compound I (FeO³⁺) which in turn is converted into Compound II by abstracting a hydrogen atom from ribose forming a ribosyl radical. In aerated solutions oxygen rapidly adds to the ribosyl radical. Based on the spectral characteristics and the enhancement of the chemiluminescence by chlorophyll-a, xanthene dyes, D₂O and DABCO, it is suggested that the excited species, apparently triplet carbonyls and ¹O₂, are formed from the bimolecular decay of the peroxyl radicals via the Russell mechanism.     

塔克拉玛干沙漠不同区域柽柳沙包土壤盐分分布特征及其影响因素

柽柳(Tamarix chinensis)沙包是塔克拉玛干沙漠特殊的生物地貌景观, 对维持区域生态环境的稳定具有极其重要的作用。该研究采用野外调查与室内分析相结合的方法, 选取且末、阿拉尔、策勒、塔中4个典型区域的柽柳沙包为研究对象, 对柽柳沙包0-500 cm土壤垂直剖面进行采样, 测定土壤pH值、枯落物含量、电导率及HCO₃ ^-、Cl ^-、SO₄ ^2-、Ca ²⁺、Mg ²⁺、K ⁺、Na ⁺含量, 分析柽柳沙包中土壤盐分的空间变化规律及其影响因素。结果表明: 1)从且末、阿拉尔、策勒到塔中, 土壤pH值总体呈升高趋势, 土壤电导率及Na ⁺、Ca ²⁺、Mg ²⁺、SO₄ ^2-含量总体呈降低趋势, K ⁺、Cl ^-、HCO₃ ^-含量没有明显的变化规律。2)盐分在4个样区的垂直分布主要表现为: 且末和策勒样区柽柳沙包的土壤盐分呈表层聚集现象; 阿拉尔和塔中样区柽柳沙包的土壤盐分呈深层聚集现象。随着土层深度的增加, 土壤pH值总体呈升高的趋势, 土壤枯落物含量总体呈降低趋势; 土壤电导率在且末和策勒样区总体呈降低趋势, 阿拉尔样区呈先降低后升高再降低的变化趋势, 而塔中样区呈先升高后降低再升高的变化趋势。3)根据相关性分析和主成分分析, 且末样区土壤枯落物含量、SO₄ ^2-、Na ⁺、K ⁺为影响土壤盐分含量的主要因子, 且土壤盐分以硫酸盐为主; 阿拉尔样区影响土壤盐分组成的主要因子为Cl ^-、Na ⁺; 策勒样区为Cl ^-、K ⁺、Na ⁺; 塔中样区为Cl ^-、Na ⁺、Ca ²⁺、SO₄ ^2-, 且土壤盐分均以氯化物为主。综合分析表明, 不同区域柽柳沙包中土壤盐分存在空间变异性, 柽柳沙包土壤盐分的变化与干旱沙漠地区强烈的蒸发作用、地表风蚀强度、地下水埋深、土壤中枯落物及柽柳的生物积盐效应等因素密切相关, 是影响不同区域土壤盐分分布的关键因子。