OH Spin Research Articles

A Comparative DFT Study on the Antioxidant Activity of Some Novel 3-Hydroxypyridine-4-One Derivatives.

The current study on the antioxidant activity of kojic acid and 3-hydroxypyridine-4-one derivatives was performed by implementation of density functional theory calculations with the B3LYP hybrid functional and the 6-311++G** basis set in Polarizable Continuum Model of solvation. Compounds under evaluation were previously synthesized by our research group. The DPPH scavenging effect and the IC50 values in mM concentrations were evaluated. Subsequently, various electronic and energetic descriptors such as HOMO and LUMO energy gaps, bonding dissociation enthalpy of an OH bond, ionization potential, electron affinity, hardness, softness, NBOs and spin density of radical and neutral species were used to study antioxidant properties of investigated compounds. The computations detected two compounds, HP3 and HP4, with significant antioxidant activity. Energetic descriptors indicated that the SPLET mechanism is preferred over the other antioxidation mechanism and computational results were in accordance with the experimental results.

Synthesis, antioxidant activity, and density functional theory study of some novel 4-[(benzo[d]thiazol-2-ylimino)methyl]phenol derivatives: a comparative approach for the explanation of their radical scavenging activities.

Background and purpose:Radicals produced by Fenton and Haber-Weiss reactions play detrimental roles in our body. Some oxidized proteins as toxic configurations are identified in amyloid-β deposits. These deposits mostly occur in conditions, such as Alzheimer’s disease. Here, we report the synthesis, evaluation of the antioxidant activity, and implementation of density functional theory (DFT) calculations of some4- [(benzo[d]thiazol-2-ylimino) methyl]phenol derivatives. The aim of this study was to provide a comparative theoretical-experimental approach to explain the antioxidant activities of the compounds.Experimental approach:Compounds were synthesized by the reaction between para hydroxybenzaldehyde and aminobenzothiazole derivatives. The scavenging activity of the compounds was evaluated. Various electronic and energetic descriptors such as high occupied molecular orbital and low unoccupied molecular orbital energy gaps, bonding dissociation enthalpy of OH bond, ionization potential, electron affinity, hardness, softness, and spin density of the radical and neutral species were calculated. DFT calculations with B3LYP hybrid functional and 6-311++ G** basis set in the polarizable continuum model were utilized to obtain these descriptors.Findings/Results:Ascorbic acid showed the best DPPH scavenging activity. However, 4d and 4c showed promising antioxidant activity. The values of EHOMO for 4c and 4d were closer to zero, thus, they showed the best scavenging activities. The computational results were in accordance with the experimental ones. The energetic descriptors indicated that the sequential proton loss-electron transfer mechanism is preferred over other mechanisms.Conclusion and implication:Antioxidant activity of 4-[(Benzo[d]thiazol-2-ylimino) methyl]phenol derivatives confirmed by experimental and theoretical documents proves them as novel antioxidants against amyloid-β based disease.

ATP Mimics pH-Dependent Dual Peroxidase-Catalase Activities Driving H 2 O 2 Decomposition

Adenosine triphosphate (ATP) is produced mainly in the mitochondrion, and its primary task is to function as a ubiquitous energy currency to meet the cellular metabolic demands in biological system...

Nuclear spin/parity dependent spectroscopy and predissociation dynamics in vOH = 2 ← 0 overtone excited Ne-H2O clusters: Theory and experiment.

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H2O complexes (D0(ortho) = 34.66 cm-1 and D0(para) = 31.67 cm-1) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02-〉 and |02+〉) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level potential energy surface [CCSD(T)/VnZf12 (n = 3, 4)], corrected for basis set superposition error and extrapolated to the complete basis set (CBS) limit, are employed to successfully predict and assign Π-Σ, Σ-Σ, and Σ-Π infrared bands in the spectra, where Σ or Π represent approximate projections of the body-fixed H2O angular momentum along the Ne-H2O internuclear axis. IR-UV pump-probe experimental capabilities permit real-time measurements of the vibrational predissociation dynamics, which indicate facile intramolecular vibrational energy transfer from the H2O vOH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τprediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H2O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H2O bands are surprisingly absent despite ab initio predictions of Q-branch intensities with S/N > 20-40. Such behavior signals the presence of highly selective nuclear spin ortho-para predissociation dynamics in the upper state, for which we offer a simple mechanism based on Ne-atom mediated intramolecular vibrational relaxation in the H2O subunit (i.e., |02±〉 → {|01±〉; v2 = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π1/2,3/2), and lambda doublet product distributions.

ESR spin trapping determination of the hydroperoxide concentration in polyethylene oxide (PEO) in aqueous solution

Initial hydroperoxide (ROOH), which exists even in freshly produced polymers, acts as an initiator for the oxidative degradation of the polymer. Electron spin resonance (ESR) with the spin trapping agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to investigate the presence of ‘initial ROOH’ in a polyethylene oxide (PEO) sample. The degradation of PEO was investigated by heating or irradiating mixtures of PEO/DMPO/H2O with or without dissolved oxygen by ultraviolet-visible (UV-vis, λ > 250 nm) and visible (λ > 350 nm) light. The concentration of ‘initial ROOH’ and the photolysis and thermolysis rate constants were estimated from the concentrations of the DMPO–H, DMPO–C–PEO, and DMPO–OH spin adducts. Knowledge of the oxidative degradation mechanism of polymers allows the ‘initial ROOH’ concentration to be used as a quality control indicator for polymers.

Inhibition of Fe2+- and Fe3+- induced hydroxyl radical production by the iron-chelating drug deferiprone

Deferiprone (L1) is an effective iron-chelating drug that is widely used for the treatment of iron-overload diseases. It is known that in aqueous solutions Fe2+ and Fe3+ ions can produce hydroxyl radicals via Fenton and photo-Fenton reactions. Although previous studies with Fe2+ have reported ferroxidase activity by L1 followed by the formation of Fe3+ chelate complexes and potential inhibition of Fenton reaction, no detailed data are available on the molecular antioxidant mechanisms involved. Similarly, in vitro studies have also shown that L1–Fe3+ complexes exhibit intense absorption bands up to 800nm and might be potential sources of phototoxicity. In this study we have applied an EPR spin trapping technique to answer two questions: (1) does L1 inhibit the Fenton reaction catalyzed by Fe2+ and Fe3+ ions and (2) does UV–Vis irradiation of the L1–Fe3+ complex result in the formation of reactive oxygen species. PBN and TMIO spin traps were used for detection of oxygen free radicals, and TEMP was used to trap singlet oxygen if it was formed via energy transfer from L1 in the triplet excited state. It was demonstrated that irradiation of Fe3+ aqua complexes by UV and visible light in the presence of spin traps results in the appearance of an EPR signal of the OH spin adduct (TMIO–OH, a(N)=14.15G, a(H)=16.25G; PBN–OH, a(N)=16.0G, a(H)=2.7G). The presence of L1 completely inhibited the OH radical production. The mechanism of OH spin adduct formation was confirmed by the detection of methyl radicals in the presence of dimethyl sulfoxide. No formation of singlet oxygen was detected under irradiation of L1 or its iron complexes. Furthermore, the interaction of L1 with Fe2+ ions completely inhibited hydroxyl radical production in the presence of hydrogen peroxide. These findings confirm an antioxidant targeting potential of L1 in diseases related to oxidative damage.

Reaction of the carbonate radical with the spin-trap 5,5-dimethyl-1-pyrroline- N-oxide in chemical and cellular systems: Pulse radiolysis, electron paramagnetic resonance, and kinetic-competition studies

Carbonate radicals (CO 3 − ) can be formed biologically by the reaction of OH with bicarbonate, the decomposition of the peroxynitrite–carbon dioxide adduct (ONOOCO 2 −), and enzymatic activities, i.e., peroxidase activity of CuZnSOD and xanthine oxidase turnover in the presence of bicarbonate. It has been reported that the spin-trap DMPO reacts with CO 3 − to yield transient species to yield finally the DMPO–OH spin adduct. In this study, the kinetics of reaction of CO 3 − with DMPO were studied by pulse radiolysis, yielding a second-order rate constant of 2.5 × 10 6 M − 1 s − 1 . A Fenton system, composed of Fe II–DTPA plus H 2O 2, generated OH that was trapped by DMPO; the presence of 50–500 mM bicarbonate, expected to convert OH to CO 3 − , markedly inhibited DMPO–OH formation. This was demonstrated to be due mainly to a fast reaction of CO 3 − with Fe II–DTPA ( k = 6.1 × 10 8 M − 1 s − 1 ), supported by kinetic analysis. Generation of CO 3 − by the Fenton system was further proved by analysis of tyrosine oxidation products: the presence of bicarbonate caused a dose-dependent inhibition of 3,4-dihydroxiphenylalanine with a concomitant increase of 3,3′-dityrosine yields, and the presence of DMPO inhibited tyrosine oxidation, in agreement with the rate constants with OH or CO 3 − . Similarly, the formation of CO 3 − by CuZnSOD/H 2O 2/bicarbonate and peroxynitrite–carbon dioxide was supported by DMPO hydroxylation and kinetic competition data. Finally, the reaction of CO 3 − with DMPO to yield DMPO–OH was shown in peroxynitrite-forming macrophages. In conclusion, CO 3 − reacts quite rapidly with DMPO and may contribute to DMPO–OH yields in chemical and cellular systems; in turn, the extent of oxidation of other target molecules (such as tyrosine) by CO 3 − will be sensitive to the presence of DMPO.

Degradation of high-molecular-weight hyaluronan by hydrogen peroxide in the presence of cupric ions

Dynamic viscosity ( η) of the high-molecular-weight hyaluronan (HA) solution was measured by a Brookfield rotational viscometer equipped with a Teflon cup and spindle of coaxial cylindrical geometry. The decrease of η of the HA solution, indicating degradation of the biopolymer, was induced by a system containing H 2O 2 alone or H 2O 2 plus CuCl 2. The reaction system H 2O 2 plus CuCl 2 as investigated by EPR spin-trapping technique revealed the formation of a four-line EPR signal characteristic of a DMPO–OH spin adduct. Thus, hydroxyl radicals are implicated in degradation of high-molecular-weight HA by the system containing H 2O 2 and CuCl 2.

Effects of Hydrodynamic Conditions on OH Radical Production at Ti/Pt Anodes During Electrochemical Treatment

The effects of hydrodynamic conditions on the production of hydroxyl radicals (•OH) during electrochemical treatment using a titanium coated by platinum anode and a stainless steel cathode are discussed in this paper. The sample used was ultra-pure water containing 200 mM of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The amount of •OH for treatment was measured using electron spin resonance spectrometry coupled with DMPO spin trapping. Under constant hydrodynamic conditions, the production rate of the •OH spin adduct (DMPO-OH) increased with the terminal potential. This result was caused by the change in the electric current density at the anode, which was proportional to the rate of production of DMPO-OH. Increasing the linear velocity of water in an electrolytic cell promoted the production of DMPO-OH by two effects, namely, by the enhancement of ion transportation and the promotion of DMPO transfer from the bulk to the anode. The former effect emerged when the DMPO concentration near the anode was not insignificant in comparison with the DMPO concentration in the bulk, that is, when the overall rate-determining step was the electron transfer at the anode. The latter effect emerged when the DMPO concentration near the anode was much lower than the DMPO concentration in the bulk, that is, when the overall rate-determining step was the diffusion of DMPO from the bulk to the anode. In addition, the latter effect was found to be proportional to the square root of the linear velocity of water in the electrolytic cell.

Theoretical studies of intersystem crossing effects in the O(3P, 1D)+H2 reaction

We have studied the influence of intersystem crossing on the reaction dynamics of the O+H2 reaction by performing trajectory surface hopping (TSH) calculations with accurate potential-energy surfaces and global spin–orbit coupling surfaces that we have generated using a four state model proposed by Hoffmann and Schatz. In the TSH calculations, we develop a new mixed representation that treats the reactant and product asymptotes in the adiabatic representation, and the singlet–triplet crossing region in the diabatic representation. This representation thus correctly describes O and OH fine structure-resolved cross sections, and it also treats intersystem crossing effects arising from the singlet–triplet crossing. Our calculations are based on the 1 3A′ and 1 3A″ states of Walch and Kuppermann, and the 1 1A′ state of Dobbyn and Knowles. The globally determined spin–orbit coupling matrix is derived from complete active space self-consistent field calculations using the two-electron Breit–Pauli Hamiltonian. Our dynamics calculations show that the triplet O+H2 cross section is modestly increased (up to 20% at collision energies >10 kcal/mol above the reactive threshold) by intersystem crossing, and product rotational excitation is also increased. In addition, we find that the OH spin–orbit distributions favor the Π3/22 state by a 2:1 ratio over Π1/22. This result is consistent with observations for O atom reactions with alkanes.

The role of the DMPO-hydrated electron spin adduct in DMPO- [rad]OH spin trapping

Time-resolved in situ radiolysis ESR (electron spin resonance, equivalently EPR, electron paramagnetic resonance) studies have shown that the scavenging of radiolytically produced hydroxyl radical in nitrous oxide-saturated aqueous solutions containing 2 mM DMPO is essentially quantitative (94% of the theoretical yield) at 100 μs after the electron pulse [1]. This result appeared to conflict with earlier results using continuous cobalt-60 γ radiolysis and hydrogen peroxide photolysis, where factors of 35 and 33% were obtained, respectively [2,3]. To investigate this discrepancy, nitrogen-saturated aqueous solutions containing 15 mM DMPO were cobalt-60 γ irradiated (dose rate = 223 Gy/min) for periods of 0.25–6 min, and ESR absorption spectra were observed ∼ 30 s after irradiation. A rapid, pseudo-first-order termination reaction of the protonated DMPO-hydrated electron adduct (DMPO-H) with DMPO-OH was observed for the first time. The rate constant for the reaction of DMPO-H with DMPO-OH is 2.44 × 10 2 (± 2.2 × 10 1) M −1 s −1. In low-dose radiolysis experiments, this reaction lowers the observed yield of DMPO-OH to 44% of the radiation-chemical OH radical yield (G = 2.8), in good agreement with the earlier results [2,3]. In the absence of the DMPO-H radical, the DMPO-OH exhibits second-order radical termination kinetics, 2k T = 22 (± 2) M −1 s −1 at initial DMPO-OH concentrations ≥ 13 μM, with first-order termination kinetics observed at lower concentrations, in agreement with earlier literature reports [4].

Biological effects of menadione photochemistry: effects of menadione on biological systems may not involve classical oxidant production

Because cell-mediated reduction of menadione leads to the generation of reactive oxygen species (ROS), this quinone is widely used to investigate the effects of ROS on cellular functions. We report that A549 human lung epithelial cells exposed to menadione demonstrate a dose-dependent increase in both intracellular calcium ([Ca(2+)](i)) and ROS formation. The concentrations of menadione required to initiate these two events are markedly different, with ROS detection requiring higher levels of menadione. Modulators of antioxidant defences (e.g. buthionine sulphoximine, 3-amino-1,2,4-triazole) have little effect on the [Ca(2+)](i) response to menadione, suggesting that ROS formation does not account for menadione-dependent alterations in [Ca(2+)](i). Additional evidence suggests that menadione photochemistry may be responsible for the observed [Ca(2+)](i) effects. Specifically: (a) EPR studies with the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) show that light exposure (maximum effect at 340 nm) stimulates menadione-dependent formation of the DMPO/(.)OH spin adduct that was not sensitive to antioxidant interventions; (b) DMPO inhibits menadione and light-dependent increases in [Ca(2+)](i); and (c) light (maximum effect at 340 nm) augments the deleterious effects of menadione on cell viability as determined by (51)Cr release. These photo effects do not appear to involve formation of singlet oxygen by menadione, but rather are the result of the oxidizing chemistry initiated by menadione in the triplet state. This work demonstrates that menadione species generated by photo-irradiation can exert biological effects on cellular functions and points to the potential importance of photochemistry in studies of menadione-mediated cell damage.

Biological effects of menadione photochemistry: effects of menadione on biological systems may not involve classical oxidant production

Because cell-mediated reduction of menadione leads to the generation of reactive oxygen species (ROS), this quinone is widely used to investigate the effects of ROS on cellular functions. We report that A549 human lung epithelial cells exposed to menadione demonstrate a dose-dependent increase in both intracellular calcium ([Ca2+]i) and ROS formation. The concentrations of menadione required to initiate these two events are markedly different, with ROS detection requiring higher levels of menadione. Modulators of antioxidant defences (e.g. buthionine sulphoximine, 3-amino-1,2,4-triazole) have little effect on the [Ca2+]i response to menadione, suggesting that ROS formation does not account for menadione-dependent alterations in [Ca2+]i. Additional evidence suggests that menadione photochemistry may be responsible for the observed [Ca2+]i effects. Specifically: (a) EPR studies with the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) show that light exposure (maximum effect at 340nm) stimulates menadione-dependent formation of the DMPO/•OH spin adduct that was not sensitive to antioxidant interventions; (b) DMPO inhibits menadione and light-dependent increases in [Ca2+]i; and (c) light (maximum effect at 340nm) augments the deleterious effects of menadione on cell viability as determined by 51Cr release. These photo effects do not appear to involve formation of singlet oxygen by menadione, but rather are the result of the oxidizing chemistry initiated by menadione in the triplet state. This work demonstrates that menadione species generated by photo-irradiation can exert biological effects on cellular functions and points to the potential importance of photochemistry in studies of menadione-mediated cell damage.

Bond-breaking in quantum state selected clusters: Inelastic and nonadiabatic intracluster collision dynamics in Ar–H2O→Ar+H(2S)+OH(2Π1/2,3/2±;N)

High-resolution vibrationally mediated IR+UV photodissociation methods are used to investigate the dynamics of H–OH bond breaking in quantum state selected H2O and Ar–H2O van der Waal complexes prepared in a slit supersonic jet expansion. This capability is based on the following strategy: (i) Specific rovibrational quantum states of the Ar–H2O dimer and H2O monomer are optically selected in the second overtone (vOH=3←0) region with an injection seeded, Fourier transform limited (Δν≈160 MHz) optical parametric oscillator. (ii) Selective H–OH bond cleavage of the vibrationally excited H2O subunit in the cluster is achieved by 248 nm or 222 nm UV photolysis. (iii) Multibody collision dynamics between the H, OH, and Ar photofragments are probed via laser induced fluorescence (LIF) on the asymptotic OH rotational, lambda-doublet and spin–orbit distributions. Comparison between cluster (Ar–H2O) and monomer (H2O) data explicitly samples the influence of the Ar “solvent” on the UV photolysis dynamics and in particular highlights the dominant role of intracluster collisions as the fragments recoil. Most importantly, the OH fine structure distributions are found to be dramatically different for Ar–H2O vs H2O photolysis, indicating the major contribution of nonadiabatic events in the photofragmentation dynamics.

The Existence of a Lysosomal Redox Chain and the Role of Ubiquinone

Several studies concerning the distribution of ubiquinone (UQ) in the cell report a preferential accumulation of this biogenic quinone in mitochondria, plasma membranes, Golgi vesicles, and lysosomes. Except for mitochondria, no recent comprehensive experimental evidence exists on the particular function of UQ in these subcellular organelles. The aim of a recent study was to elucidate whether UQ is an active part of an electron-transfer system in lysosomes. In the present work, a lysosomal fraction was prepared from a light mitochondrial fraction of rat liver by isopycnic centrifugation. The purity of our preparation was verified by estimation of the respective marker enzymes. Analysis of lysosomes for putative redox carriers and redox processes in lysosomes was carried out by optical spectroscopy, HPLC, oxymetry, and ESR techniques. UQ was detected in an amount of 2.2 nmol/mg of protein in lysosomes. Furthermore, a b-type cytochrome and a flavin–adenine dinucleotide (FAD) were identified as other potential electron carriers. Since NADH was reported to serve as a substrate of UQ redox chains in plasma membranes, we also tested this reductant in lysosomes. Our experiments demonstrate a NADH-dependent reduction of UQ by two subsequent one-electron-transfer steps giving rise to the presence of ubisemiquinone and an increase of the ubiquinol pool in lysosomes. Lysosomal NADH oxidation was accompanied by an approximately equimolar oxygen consumption, suggesting that O2 acts as a terminal acceptor of this redox chain. DMPO/•OH spin adducts were detected by ESR in NADH-supplemented lysosomes, suggesting a univalent reduction of oxygen. The kinetic analysis of redox changes in lysosomes revealed that electron carriers operate in the sequence NADH > FAD > cytochrome b > ubiquinone > oxygen. By using the basic spin label TEMPAMINE, we showed that the NADH-related redox chain in lysosomes supports proton accumulation in lysosomes. In contrast to the hypothesis that UQ in lysosomes is simply a waste product of autophagy in the cell, we demonstrated that this lipophilic electron carrier is a native constituent of a lysosomal electron transport chain, which promotes proton translocation across the lysosomal membrane.

Oxygen-17-induced proton relaxation rates for alcohols and alcohol solutions

The use of 17O enriched samples of alcohols to measure the correlation time of the OH internuclear vector works well when the hydroxyl proton exchanges rapidly. For alcohols such as methanol and ethanol the hydroxyl exchange rate for neat samples is relatively slow even at room temperature and significant systematic errors result if slow exchange effects are not considered. For slow exchange the hydroxyl proton, 17OH, signal is a relatively complex function of the chemical exchange rate of the hydroxyl proton, the OH spin coupling (about 80 Hz for alcohols and water) and the relaxation time for the oxygen. The OH linewidth can become so large due to scalar relaxation with the rapidly relaxing oxygen nucleus that the signal becomes very difficult to detect. For neat 17O enriched ethanol at room temperature the oxygen relaxation time is about 3.0 ms and the hydroxyl proton linewidth is over 1000 Hz.

Endogenous superoxide dismutase levels regulate iron-dependent hydroxyl radical formation in Escherichia coli exposed to hydrogen peroxide.

Aerobic organisms contain antioxidant enzymes, such as superoxide dismutase (SOD) and catalase, to protect them from both direct and indirect effects of reactive oxygen species, such as O2.- and H2O2. Previous work by others has shown that Escherichia coli mutants lacking SOD not only are more susceptible to DNA damage and killing by H2O2 but also contain larger pools of intracellular free iron. The present study investigated if SOD-deficient E. coli cells are exposed to increased levels of hydroxyl radical (.OH) as a consequence of the reaction of H2O2 with this increased iron pool. When the parental E. coli strain AB1157 was exposed to H2O2 in the presence of an alpha-(4-pyridyl-1-oxide)-N-tert-butyl-nitrone (4-POBN)-ethanol spin-trapping system, the 4-POBN-.CH(CH3)OH spin adduct was detectable by electron paramagnetic resonance (EPR) spectroscopy, indicating .OH production. When the isogenic E. coli mutant JI132, lacking both Fe- and Mn-containing SODs, was exposed to H2O2 in a similar manner, the magnitude of .OH spin trapped was significantly greater than with the control strain. Preincubation of the bacteria with the iron chelator deferoxamine markedly inhibited the magnitude of .OH spin trapped. Exogenous SOD failed to inhibit .OH formation, indicating the need for intracellular SOD. Redox-active iron, defined as EPR-detectable ascorbyl radical, was greater in the SOD-deficient strain than in the control strain. These studies (i) extend recent data from others demonstrating increased levels of iron in E. coli SOD mutants and (ii) support the hypothesis that a resulting increase in .OH formation generated by Fenton chemistry is responsible for the observed enhancement of DNA damage and the increased susceptibility to H2O2-mediated killing seen in these mutants lacking SOD.

Uranium reactions with hydrogen peroxide studied by EPR–spin trapping with DMPO †

Uranyl nitrate [UO2(NO3)2] (1.0 × 10–3M) is reacted with hydrogen peroxide (H2O2, 5 × 10–3M) in the presence of the spin trap 5,5-dimethyl-4,5-dihydro-3H-pyrrole N-oxide (DMPO, 5 × 10–2M) in acidic solutions. The reaction generates a 1∶2∶2∶1 quartet with hyperfine coupling constants, aN = aHβ = 1.50 mT. These values are consistent with reported values for the DMPO–OH spin adduct. It is possible that the uranous ion (UO2+), which corresponds to UIV, generates hydroxyl radicals (˙OH) to give the observed DMPO–OH. This is suggested by two observations: (1) the intensity of the EPR spectrum is dependent on pH, reaching a maximum at pH = 0.6, which is consistent with the formation of UO2+ from UO22+ at lower pH values; and (2) the uranyl ion (UO22+) corresponds to UVI and cannot be further oxidized. To determine whether ˙OH radicals generate DMPO–OH, the reaction is carried out in the presence of varying concentrations of ethanol (EtOH). The results indicate that at a low ratio of [EtOH]/[DMPO] the EPR signal corresponds to DMPO–OH, while at a [EtOH]/[DMPO] ratio ca. 1 the signal is mixed and equally intense for DMPO–OH and the hydroxyethyl adduct to DMPO (DMPO–EtOH). When the [EtOH]/[DMPO] ratio is ca. 10 the EPR signal is mainly that of the DMPO–EtOH adduct. This confirms that uranium reacts with H2O2 generating hydroxyl radicals.

Photosensitization by Harmine: An ESR Spin Trapping Study on the Generation of the Superoxide Anion Radical

Harmine was in situ UV-irradiated in the presence of 5,5-dimethyl-1-pyrroline- N-oxide (DMPO), α-phenyl- N-tert-butyl nitrone (PBN), or α-(4-pyridyl-1-oxide) N-tert-butyl nitrone (4-POBN) as spin traps and observed by electron spin resonance (ESR) spectroscopy. The superoxide radical (O2·−) was detected as the corresponding DMPO, PBN, or 4-POBN spin adduct in dimethylsulfoxide (DMSO) or acetonitrile solution. The detection of these spin adducts was prevented by the addition of superoxide dismutase (SOD) to the solution. Also, the O2·− adduct formation was inhibited by the addition of antioxidants as cysteine methyl ester or ascorbic acid, in a dose-dependent manner. The studies carried out in aqueous buffered solution did not allow superoxide radical adduct detection. In the presence of DMPO as a spin trap, the DMPO–OH spin adduct was detected. On the other hand, classical scavengers of the hydroxyl radical such as mannitol or glycerol abolished DMPO–OH spin adduct detection. Other scavengers such as ethanol, DMSO, or sodium azide inhibited the trapping of hydroxyl radicals and resulted in the formation of new radical adducts. No PBN or 4-POBN spin adducts were detected in aqueous solution. The origin of the hydroxyl radical is discussed.

In Situ Radiolysis Time-Resolved ESR Studies of Spin Trapping by DMPO: Reevaluation of Hydroxyl Radical and Hydrated Electron Trapping Rates and Spin Adduct Yields

The second-order rate constants for the reaction of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) with radiolytically produced hydroxyl radicals and hydrated electrons have been measured in aqueous solution by direct observation of spin adduct initial yield using time-resolved electron spin resonance. The rate constants are 2.8 × 109 mol-1 dm3 s-1 for the DMPO−hydroxyl radical addition reaction and 3.2 × 109 mol-1 dm3 s-1 for the reaction of DMPO and hydrated electron, using sodium formate and chloroacetic acid as competitive scavengers of the hydroxyl radical and hydrated electron, respectively. The hydrated electron−DMPO competition study determined the fraction of DMPO−H produced directly from radiolytically produced hydrogen atoms as 0.082 of the total DMPO−H yield, indicating that approximately half of the hydrogen atoms react with DMPO to produce non-aminoxyl products. The fraction of the total hydroxyl radical yield leading to DMPO−OH spin adduct was determined to be 0.94, using the bleach of 2,2,6,6-tet...