N2O-saturated Aqueous Solutions Research Articles

Radiolytic degradation of 4-hydroxybenzoate in aerated and deoxygenated aqueous solutions

ABSTRACT The radiolytic degradation of 4-hydroxybenzoate (4-HBA–) in aerated, oxygen-free and N2O-saturated aqueous solutions at concentrations of 0.10 and 0.25 mmol/dm3 were gamma irradiated at different doses in a source of Co-60. The results show that ·OH adds predominantly to the third position of the aromatic ring, and elimination of the acid group leads to the degradation of 4-HBA–. With an N2O-saturated 0.10 mmol/dm3 4-HBA– solution, total degradation occurred at 1.6 kGy, and with a 0.25 mmol/dm3 solution, total degradation occurred at 3.5 kGy. In the aerated and oxygen-free 0.25 mmol/dm3 4-HBA– solutions, the behavior was similar, degradation occurring at a dose of 13.1 kGy. At the concentration of 0.10 mmol/dm3, total degradation occurred at 7.0 kGy, with small amounts of radiolytic products and byproducts. We propose a mechanism for the degradation of 4-HBA– caused by water radicals produced in the three environments, leading to formation of the identified stable products. Oxidation was followed by chemical oxygen demand (COD), which decreased as the 4-HBA− concentration increased. The kinetics showed a pseudo-first-order behavior. The rate constant of degradation was similar for the solutions with and without oxygen.

Mechanistic study of the acidic aqueous solution of chrome alum chemical dosimeter: Steady state, pulse radiolysis and conductivity studies

It is crucial to have an efficient and reliable approach to ensure an optimum traceability in high dose range and high dose rate dosimetry for radiation processing (food irradiation, surgery devices sterilization …). Chemical solution dosimeters are well suited for this purpose. By using γ-irradiation and pulse radiolysis, we evaluated the performances of the chrome alum KCr(SO4)2·12H2O based on the radiation induced oxidation of Cr3+ to Cr6+ in N2O-saturated aqueous solutions buffered at pH 4 by the hydroxyl radical (•OH). With combination of spectrophotometric and conductometric measurements, a mechanism has been put forward to elucidate the oxidation of Cr3+ leading to Cr6+. The suggested mechanism involves an initial adduct formation, [Cr3+ – •OH], that enables inner-sphere electron transfer. As far as dosimetry is concerned, this study showed that the absorbance at 350 nm of the radiation induced Cr6+ increases linearly with the dose with a yield of G (Cr6+) = 0.128 μmol J−1. No dose rate effect was observed up to 80 kGy min−1. This aqueous solution could be used for chemical dosimetry, mainly in the high dose range (up to 5 kGy) and high dose rate (up to 80 kGy min−1). The main advantage of using this aqueous solution is its equivalence to human tissue in terms of radiation absorption properties.

Dynamics of radiolytic formation of CdSe quantum dots in aqueous solution containing different alcohols: A pulse radiolysis study

The dynamics of radiolytic formation of CdSe quantum dots in aqueous solution containing equimolar ammoniated cadmium sulphate, Cd(NH3)4SO4 and sodium selenosulphate, Na2SeSO3 as precursors for Cd and Se respectively, have been investigated in the presence of different aliphatic alcohols, namely methanol, ethanol, 1-propanol, 2-propanol or 1-butanol by electron pulse radiolysis studies. The mechanism of CdSe quantum dots formation was studied in N2O saturated aqueous solutions to investigate the formation through reactions with different aliphatic alcohols. The formation of the CdSe quantum dots was found to proceed via the formation of short lived transient intermediate species having and absorption at around 500 nm, which are formed through the reaction of different aliphatic alcohol radicals with the cadmium and selenium precursors. The rate constants for the formation and decay of these intermediate species have been determined. Optical properties of these quantum dots were found to be different in the presence of different alcohols.

A pulse radiolysis study of the dynamics of ascorbic acid free radicals within a liposomal environment.

The dynamics of free-radical species in a model cellular system are examined by measuring the formation and decay of ascorbate radicals within a liposome with pulse radiolysis techniques. Upon pulse radiolysis of an N2O-saturated aqueous solution containing ascorbate-loaded liposome vesicles, ascorbate radicals are formed by the reaction of OH(·) radicals with ascorbate in unilamellar vesicles exclusively, irrespective of the presence of vesicle lipids. The radicals are found to decay rapidly compared with the decay kinetics in an aqueous solution. The distinct radical reaction kinetics in the vesicles and in bulk solution are characterized, and the kinetic data are analyzed.

Radiation‐Induced Synthesis of Nanostructured Conjugated Polymers in Aqueous Solution: Fundamental Effect of Oxidizing Species

Synthesis of conjugated poly(3,4-ethylenedioxythiophene) (PEDOT) polymers is achieved through the radiolysis of N2O-saturated aqueous solutions of 3,4-ethylenedioxythiophene by using two different oxidizing species: HO(·) (hydroxyl) and N3(·) (azide) radicals. Both oxidative species lead to self-assembled polymers that are evidenced in solution by cryotransmission electron microscopy and UV/Vis absorption spectroscopy and, after centrifugation and deposition, by scanning electron microscopy and attenuated total reflectance FTIR techniques. Whereas HO(·) radicals lead to PEDOT-OH globular nanostructures with hydrophilic properties, N3(·) radicals enable the formation of amphiphilic PEDOT-N3 fibrillar nanostructures. These results, which highlight the differences in the intermolecular interaction behaviors of the two kinds of PEDOT polymers, are discussed in terms of polymerization mechanisms.

Radiolytic formation of non-toxic Cr(III) from toxic Cr(VI) in formate containing aqueous solutions: A system for water treatment

Toxic hexavalent chromium Cr(VI) in the form of potassium dichromate was radiolytically reduced to non-toxic trivalent chromium Cr(III) in N2O-saturated aqueous solutions containing formate. This reduction by the electron donor (CO2H/CO2−) produced by continuous radiolysis of water, was a linear function of the absorbed dose. This reaction was pH and dose rate dependent. pH was an important parameter in the reduction, as it affects both chemical speciation of Cr(VI) and formate. Possible mechanisms related to dose rate dependence of removal of Cr(VI) are presented. At pH 3 a decrease in the radiation induced reduction of Cr(VI) was observed with increasing hydrogen peroxide concentration. A mechanism to account for this variation is proposed. These findings suggest that irradiation of Cr(VI) solutions in presence of formate can be effective, economical and simple means for treatment of waste water contaminated with hexavalent Cr(VI).

Investigation of reactions postulated to occur during inhibition of ribonucleotide reductases by 2′-azido-2′-deoxynucleotides

Model 3′-azido-3′-deoxynucleosides with thiol or vicinal dithiol substituents at C2′ or C5′ were synthesized to study reactions postulated to occur during inhibition of ribonucleotide reductases by 2′-azido-2′-deoxynucleotides. Esterification of 5′-(tert-butyldiphenylsilyl)-3′-azido-3′-deoxyadenosine and 3′-azido-3′-deoxythymidine (AZT) with 2,3-S-isopropylidene-2,3-dimercaptopropanoic acid or N-Boc-S-trityl-L-cysteine and deprotection gave 3′-azido-3′-deoxy-2′-O-(2,3-dimercaptopropanoyl or cysteinyl)adenosine and the 3′-azido-3′-deoxy-5′-O-(2,3-dimercaptopropanoyl or cysteinyl)thymidine analogs. Density functional calculations predicted that intramolecular reactions between generated thiyl radicals and an azido group on such model compounds would be exothermic by 33.6–41.2 kcal/mol and have low energy barriers of 10.4–13.5 kcal/mol. Reduction of the azido group occurred to give 3′-amino-3′-deoxythymidine, which was postulated to occur with thiyl radicals generated by treatment of 3′-azido-3′-deoxy-5′-O-(2,3-dimercaptopropanoyl)thymidine with 2,2′-azobis-(2-methyl-2-propionamidine) dihydrochloride. Gamma radiolysis of N2O-saturated aqueous solutions of AZT and cysteine produced 3′-amino-3′-deoxythymidine and thymine most likely by both radical and ionic processes.

A surprisingly complex aqueous chemistry of the simplest amino acid. A pulse radiolysis and theoretical study on H/D kinetic isotope effects in the reaction of glycine anions with hydroxyl radicals

A pulse radiolysis study was carried out of the reaction rate constants and kinetic isotope effects of hydroxyl-radical-induced H/D abstraction from the most-simple alpha-amino acid glycine in its anionic form in water. The rate constants and yields of three predominantly formed radical products, glycyl (NH2-*CH-CO2-), aminomethyl (NH2-*CH2), and aminyl (*NH-CH2-CO2-) radicals, as well as of their partially or fully deuterated analogs, were found to be of comparable magnitude. The primary, secondary, and primary/secondary H/D kinetic isotope effects on the rate constants were determined with respect to each of the three radicals. The unusual variety of products for such an elementary reaction between two small and simple species indicates a complex mechanism with several reactions taking place simultaneously. Thus, a theoretical modeling of the reaction mechanism and kinetics in the gas- and aqueous phase was performed by using the unrestricted density functional theory with the BB1K functional (employing the polarizable continuum model for the aqueous phase), unrestricted coupled cluster UCCSD(T) method, and improved canonical variational theory. Several hydrogen-bonded prereaction complexes and transition states were detected. In particular, the calculations pointed to a significant mechanistic role of the three-electron two-orbital (sigma/sigma* N therefore O) hemibonded prereaction complexes in the aqueous phase. A good agreement with the experimental rate constants and kinetic isotope effects was achieved by downshifting the calculated reaction barriers by 3 kcal mol(-1) and damping the NH(D) stretching frequency by a factor of 0.86.

Radical scavenging reactions of chlorogenic acid: A pulse radiolysis study

At near neutral pH (approx. 5.5), the OH-adduct of chlorogenic acid (CGA), formed on pulse radiolysis of N2O-saturated aqueous CGA solutions (λ max = 400 and 450 nm) with k = 9 × 109 dm3 mol−1 s−1, rapidly eliminates water (k = 1 × 103 s−1) to give a resonance-stabilized phenoxyl type of radical. Oxygen rapidly adds to the OH-adduct of CGA (pH 5.5) to form a peroxyl type of radical (k = 6 × 107 dm3 mol−1 s−1). At pH 10.5, where both the hydroxyl groups of CGA are deprotonated, the rate of reaction of · OH radicals with CGA was essentially the same as at pH 5.5, although there was a marked shift in the absorption maximum to approx. 500 nm. The CGA phenoxyl radical formed with more specific one-electron oxidants, viz., Br 2 ·− and N 3 · radicals show an absorption maximum at 385 and 500 nm, k ranging from 1–5.5 × 109 dm3 mol−1 s−1. Reactions of other one-electron oxidants, viz., NO 2 · , NO· and CCl3OO· radicals, are also discussed. Repair rates of thymidine, cytidine and guanosine radicals generated pulse radiolytically at pH 9.5 by CGA are in the range of (0.7–3) × 109 dm3 mol−1 s−1.

Synthesis of hydrogels by radiation-induced cross-linking of Pluronic® F127 in N2O-saturated aqueous solution

Pluronic F127, a tri-block copolymer of poly(ethylene oxide) and poly(propylene oxide) of a nominal formula EO100-PO65-EO100, when subjected to the action of ionizing radiation in oxygen-free, N2O-saturated aqueous solution, undergoes predominantly cross-linking by intermolecular recombination of polymer-derived radicals. This manifests itself as an increase in average molecular weight and, at sufficiently high doses, as a formation of permanent hydrogel. Cross-linking occurs with high yields. Side reactions include chain scission and traces of oxidation processes. Polymer concentration and temperature influence the yields of radiation-induced reactions. This indicates that radiolysis mechanism depends on the spatial arrangement of Pluronic molecules, which can be organized into single chains, micelles, and physical gel. Macroscopic hydrogels of Pluronic are thermo-sensitive (phase transition temperature behavior with the transition temperature, at the tested conditions, of ca. 27 °C). Pluronic gels are prone to oxidative degradation, both in an aqueous environment and in the dry state, especially at elevated temperatures. Copyright © 2006 John Wiley & Sons, Ltd.

Water-Assisted Intramolecular Electron Transfer from the Ring to the Side Chain in N,N,N′,N′-Tetraalkyl-para-phenylenediamine Radicals-The Reverse of Side Chain Deprotonation of Radical Cations

not on the side chain but on the ring. We have now found evidence for the protonation of a side chain carbon atom (a reaction which involves a neutral radical), electron transfer from the ring to the side chain providing the (high) electron density necessary for the protonation on carbon to occur. N2O-saturated aqueous solutions at pH 7 ‐ 12 containing N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD; used as the dihydrochloride) were irradiated with the 100 ns pulses from a 3 MeV van de Graaff electron accelerator, and the time-resolved changes in the transmission or of the conductance of the solution monitored. Irradiation of water leads to the radicals . OH (45 %), H . (10 %), and e ˇ (45 %). The latter reacts with N2O within 20 ns to yield another . OH radical. In the presence of 0.2 ‐ 0.5 mm TMPD, its radical cation TMPD .a is produced, whose lmax values are at 330, 565, and 610 nm

Reactions of phenol-OH-adduct radicals. Phenoxyl radical formation by water elimination vs. oxidation by dioxygen

The reactions of OH radicals generated radiolytically in N2O-saturated aqueous solutions with phenol have been examined, focusing special attention on the addition of dioxygen and the competing dehydration reactions of the OH-adduct radicals. Using the pulse radiolysis technique, the overall rate constant of dioxygen addition to the dihydroxycyclohexadienyl radicals was determined to be k = 1.2 × 109 dm3 mol−1 s−1. This dioxygen addition reaction was found to be practically irreversible, in contrast to other hydroxycyclohexadienyl radicals. The so-formed dihydroxycyclohexadienylperoxyl radicals subsequently eliminate HO2˙/O2˙−. By using tetranitromethane (TNM) as a probe for O2˙− (formation of the nitroform anion), the overall rate constant of HO2˙-elimination (mainly of the para- and ortho-isomers, formation of hydroquinone and catechol, respectively) was determined to be 1.3 × 105 s−1. The rate constant of the dehydration of the p-OH-adduct in neutral to acidic solution was determined by monitoring the absorbance build-up of the phenoxyl radical to be k7 = 1.8 × 103 + 1.7 × 109 × [H+] s−1, and that of the o-OH-adduct to be k6 = 1.1 × 108 × [H+] s−1 (the uncatalysed reaction is too slow to be measurable by this technique). The HPO42−-catalysed dehydration rate constant of the p-OH-adduct was similarly determined to be 5.8 × 107 dm3 mol−1 s−1. Based on the above rate constant of dioxygen addition, the rate constant of the proton-catalysed dehydration of the p-OH-adduct was determined by the competition of these two reactions on the yield of hydroquinone to be 1.0 × 109 dm3 mol−1 s−1, and similarly that of the ortho-OH-adduct (on the yield of catechol) to be 2.1 × 108 dm3 mol−1 s−1.

Formation of 5-Hydroxy-5,6-Dihydrothymidine and cis-5,6-Dihydroxy-5,6-Dihydrothymidine in the Radiolysis of N2O-Saturated Aqueous Solutions of Thymidine, Thymidine 5"-Monophosphate, and DNA

The yields of formation of 5-hydroxy-5,6-dihydrothymidine and cis-5,6-dihydroxy-5,6-dihydrothymidine in the radiolysis of thymidine, thymidine 5"-monophosphate, and DNA in N2O-saturated aqueous solutions were measured in order to study the mechanism of nucleic acid radiolysis. The above compounds were found to be main radiolysis products upon irradiation of thymidine and thymidine 5"-monophosphate. However, these compounds were formed in very low yields upon irradiation of DNA, and they amounted to less than 2% of the degradation yield of DNA thymine. The yield of the 5-hydroxythymidin-6-yl radical was evaluated by determining the amount of 5-hydroxy-5,6-dihydrothymidine formed in the radiolysis of the above compounds in the presence of cysteamine.

Free-radical-induced chain breakage and depolymerization of poly(methacrylic acid): equilibrium polymerization in aqueous solution at room temperature.

Hydroxyl radicals, generated by ionizing radiation in N2O saturated aqueous solutions, abstract H atoms from poly(methacrylic acid) at the methyl and methylene groups, and radicals 1 and 2 are formed, respectively. The reactions of the poly(methacrylic acid) radicals were investigated by pulse radiolysis (using optical and conductometric detection), EPR, product analysis, and kinetic simulations. The conductometric detection allowed us to measure the rate of chain scission and monomer release. Under conditions in which the polymer is largely deprotonated, the primary radical 1 abstracts a hydrogen (k= 3.5 x 10(2)s(-1)) from the methylene group, and this yields the more stable secondary radical 2. This radical undergoes chain scission by beta-fragmentation (k= 1.8 s(-1)), and the terminal (end-of-chain) radical 3 is formed. The polymer radicals terminate only slowly (2k= 80 dm3mol(-1)s(-1)). This allows effective depolymerization (depropagation) to take place (k=0.1 s(-1)). The yield of monomer release is higher than the original radical yield by up to two orders of magnitude. Once monomer is formed, it reacts with 3 (propagation, k= 15 dm3mol(-1)s(-1)), and a situation close to an equilibrium radical polymerization is approached. From these data, the equilibrium monomer concentration is calculated at 6.7 x 10(-3) mol dm(-3) at room temperature. The standard entropy of propagation is estimated at -185 to -150 J mol(-1)K(-1). Because the monomer reaches concentrations in the millimolar range, the *OH radicals increasingly react with monomers (results in oligomerization) rather than with the polymer. This effect is reflected by, for example, a lowering of chain-scission yields upon prolonged irradiation. In acid solutions, the decay of the polymer radicals becomes much faster (estimated at about 10(7)dm3mol(-1)s(-1) at pH3.5), and monomer release is no longer observed.

Stability Constants and Decay of Aqua‐Copper(III) – A Study by Pulse Radiolysis with Conductometric and Optical Detection

Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous solutions and reacted with Cu ions. Pulse radiolysis with conductometric detection showed that the aqua-CuIII species thus formed (k = 3.2 × 108 dm3 mol–1 s–1) is uncharged at pH 5, i.e. [Cu(OH)3]aq, and from the changes in conductance as a function of pH it has been calculated that Cu(OH) must have a pK value of 4.8 and Cu(OH) a value of 3.7. At pH 2.5 there is no strong evidence for the presence of Cu. Aqua-Cu(OH)3 decays bimolecularly to give Cu and H2O2. Cu(OH) [and possibly also Cu(OH)] are in equilibrium with Cu and OH radicals [K(Cu(OH)) ≈ 3.7 × 104 dm3 mol–1]. Rapid electron transfer occurs from Cu to [Cu(OH)3]aq (k = 3.5 × 109 dm3 mol–1 s–1).

Hydroxyl-radical-induced reactions of poly(vinyl methyl ether): a pulse radiolysis, EPR and product study in deoxygenated and oxygenated aqueous solutions

Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous solutions in the presence of poly(vinyl methyl ether) (PVME, 6 × 104 Da, 10−3–10−2 mol dm−3 in monomer units). As measured by pulse radiolysis, they react (k = 2.2 × 108 dm3 mol−1 s−1) with PVME by giving rise to mainly α-alkoxyalkyl radicals (≈72%) that reduce (k ≈ 2 × 109 dm3 mol−1 s−1) Fe(CN)63−, IrCl62− or tetranitromethane. Based on the formaldehyde yield in the presence of the latter two oxidants (≈40% of ˙OH), it is concluded that OH radicals undergo H-abstraction at ROCH2–H, R3C–H and R2HC–H with probabilities of ≈40, ≈32 and ≈28%, respectively. The momentary rate constant of the decay of the PVME radicals depends on the number of radicals per polymer chain and drops as they decay. The yield of intermolecular crosslinks, as measured by an increase in the molecular weight, strongly increases with decreasing dose rate, and it is concluded that the majority of crosslinks occur intramolecularly, even at the lowest dose rate used [0.0015 Gy s−1, G(intermolecular crosslinks) = 0.62 × 10−7 mol J−1]. In the presence of dioxygen, the primary PVME radicals are converted into their corresponding peroxyl radicals. They undergo efficient autoxidation via intramolecular H-abstraction [e.g.G(dioxygen uptake) ≈ 110 × 10−7 mol J−1 at 0.0015 Gy s−1]. Most of the hydroperoxides are unstable, i.e. the high dioxygen-uptake yield is not matched by the organic hydroperoxide yield [G(organic hydroperoxide) ≈ 34 × 10−7 mol J−1 at 0.0015 Gy s−1]. As a consequence of the instability of some of the organic hydroperoxides, chain scission also strongly depends on dose rate [e.g.G(chain breaks) ≈ 15 × 10−7 mol J−1 at 0.0015 Gy s−1], and prompt chain breakage due to processes occurring in the bimolecular decay of the peroxyl radicals is minor compared to the former pathway.

The reaction of methyl radicals with hydrogen peroxide

Methyl radicals (plus methanesulfinic acid) were generated radiolytically in N2O-saturated aqueous solutions by reacting the OH radicals thus formed with dimethyl sulfoxide. Upon addition of hydrogen peroxide the methyl radicals abstract an H-atom from hydrogen peroxide. With increasing hydrogen peroxide concentrations the methane yield increases while that of ethane drops. The yield of methanesulfinic acid remains unaffected. At constant hydrogen peroxide concentration, the ethane yield increases with increasing dose rate. The system has been successfully modelled using k(˙CH3 + H2O2) = 2.7 × 104 dm3 mol–1 s–1. As the corresponding rate constant in the gas phase is reported to be 3.3 × 107 dm3 mol–1 s–1, the water solvent appears to have a dramatic effect. Some aspects of the mechanism of H-abstraction from H2O2 are briefly discussed.

Spectral, kinetics, and redox properties of the transients formed on one-electron oxidation of 4,4?-thiodiphenol: A pulse radiolysis study

The transient absorption bands (λmax = 330, 525 nm, kf = 5 × 109 dm3 mol−1 s−1) obtained on pulse radiolysis of N2O-saturated neutral aqueous solution of 4,4′-thiodiphenol (TDPH) are due to the reaction of TDPH with ·OH radicals and are assigned to phenoxyl radical formed on fast deprotonation of the solute radical cation. The reaction of specific one-electron oxidants (Cl2·−, Br2·−, N3·, TI2+, CCl3OO·) with TDPH also produced similar transient absorption bands. The phenoxyl radicals are also produced on pulse radiolysis of N2-saturated solution of TDPH in 1,2-dichloroethane. The nature of transient absorption spectrum obtained on reaction of ·OH radicals with TDPH is not affected in acidic solutions, showing that OH-adduct is not formed in neutral solutions. The oxidation potential for the formation of phenoxyl radical is determined to be 0.98 V. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 603–610, 1999

Radical Cations in the OH-Radical-Induced Oxidation of Thiourea and Tetramethylthiourea in Aqueous Solution

Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous solutions of thiourea and tetramethylthiourea. The rate constant of the reaction of OH radicals with thiourea (tetramethylthiourea) has been determined using 2-propanol as well as NaN3 as competitors to be 1.2 × 1010 dm3 mol-1 s-1 (8.0 × 109 dm3 mol-1 s-1). A transient appears after a short induction period and shows a well-defined absorption spectrum with λmax = 400 nm (ε = 7400 dm3 mol-1 cm-1); that of tetramethylthiourea has λmax = 450 nm (ε = 6560 dm3 mol-1 cm-1). Using conductometric detection, it has been shown that, in both cases, OH- and a positively charged species are produced. These results indicate that a radical cation is formed. These intermediates with λmax = 400 nm (450 nm) are not the primary radical cations, since the intensity of the absorbance depends on the substrate concentration. The absorbance build-up follows a complex kinetics best described by the reversible formation of a dimeric radical cation by addition of a primary radical cation to a molecule of thiourea. The equilibrium constant for this addition has been determined by competition kinetics to be 5.5 × 105 dm3 mol-1 for thiourea (7.6 × 104 dm3 mol-1 for tetramethylthiourea). In the bimolecular decay of the dimeric radical cation (thiourea, 2k = 9.0 × 108 dm3 mol-1 s-1; tetramethylthiourea, 1.3 × 109 dm3 mol-1 s-1), formamidine (tetramethylformamidine) disulfide is formed. In basic solutions of thiourea, the absorbance at 400 nm of the dimeric radical cation decays rapidly, giving rise (5.9 × 107 dm3 mol-1 s-1) to a new intermediate with a broad maximum at 510 nm (ε = 750 dm3 mol-1 cm-1). This reaction is not observed in tetramethylthiourea. The absorption at 510 nm is attributed to the formation of a dimeric radical anion, via neutralization of the dimeric radical cation and subsequent deprotonation of the neutral dimeric radical. The primary radical cation of thiourea is deprotonated by OH- (2.8 × 109 dm3 mol-1 s-1) to give a neutral thiyl radical. The latter reacts rapidly with thiourea, yielding a dimeric radical, which is identical to the species from the reaction of OH- with the dimeric radical cation. The dimeric radical cations of thiourea and tetramethylthiourea are strong oxidants and readily oxidize the superoxide radical (4.5 × 109 dm3 mol-1 s-1 for thiourea and 3.8 × 109 dm3 mol-1 s-1 for tetramethylthiourea), phenolate ion (3 × 108 dm3 mol-1 s-1 for tetramethylthiourea), and even azide ion (4 × 106 dm3 mol-1 s-1 for thiourea and ∼106 dm3 mol-1 s-1 for tetramethylthiourea). With O2, the dimeric radical cation of thiourea reacts relatively slowly (1.2 × 107 dm3 mol-1 s-1) and reversibly (2 × 103 s-1).

Substrate specificities of the ntg1 and ntg2 proteins of Saccharomyces cerevisiae for oxidized DNA bases are not identical.

Two genes of Saccharomyces cerevisiae, NTG1 and NTG2, encode proteins with a significant sequence homology to the endonuclease III of Escherichia coli. The Ntg1 and Ntg2 proteins were overexpressed in E.coli and purified to apparent homogeneity. The substrate specificity of Ntg1 and Ntg2 proteins for modified bases in oxidatively damaged DNA was investigated using gas chromatography/isotope-dilution mass spectrometry. The substrate used was calf-thymus DNA exposed to gamma-radiation in N2O-saturated aqueous solution. The results reveal excision by Ntg1 and Ntg2 proteins of six pyrimidine-derived lesions, 5-hydroxy-6-hydrothymine, 5-hydroxy-6-hydrouracil, 5-hydroxy-5-methylhydantoin, 5-hydroxyuracil, 5-hydroxycytosine and thymine glycol, and two purine-derived lesions, 2,6-diamino-4-hydroxy-5-formamidopyrimidine and 4,6-diamino-5-formamidopyrimidine from gamma-irradiated DNA. In contrast, Ntg1 and Ntg2 proteins do not release 8-hydroxyguanine or 8-hydroxyadenine from gamma-irradiated DNA. The Ntg1 and Ntg2 proteins also release 2, 6-diamino-4-hydroxy-5-N-methylformamido-pyrimidine from damaged poly(dG-dC).poly(dG-dC). Excision was measured as a function of enzyme concentration and time. Furthermore, kinetic parameters were determined for each lesion. The results show that kinetic constants varied among the different lesions for the same enzyme. We also investigated the capacity of the Ntg1 and Ntg2 proteins to cleave 34mer DNA duplexes containing a single 8-OH-Gua residue mispaired with each of the four DNA bases. The results show that the Ntg1 protein preferentially cleaves a DNA duplex containing 8-OH-Gua mispaired with a guanine. Moreover, the Ntg1 protein releases free 8-OH-Gua from 8-OH-Gua/Gua duplex but not from duplexes containing 8-OH-Gua mispaired with adenine, thymine or cytosine. In contrast, the Ntg2 protein does not incise duplexes containing 8-OH-Gua mispaired with any of the four DNA bases. These results demonstrate that substrate specificities of the Ntg1 and Ntg2 proteins are similar but not identical and clearly different from that of the endonuclease III of E.coli and its homologues in Schizosaccharomyces pombe or human cells.