N2O-saturated Aqueous Solutions Research Articles

Anti- and pro-oxidant properties of sodium 2-mercaptoethane sulphonate (Mesna)

The •OH and the NO2• radicals generated pulse radiolytically in N2O-saturated aqueous solution at pH 8–8.5 oxidize Mesna to form the corresponding thiyl radicals which on reaction with thiolate ions form an RSSR•− type of transient with λmax = 420 nm. The rate constants for the formation of these transients were determined. In the absence of O2 at pH=6, the RS• radicals formed show an absorption maximum at 360 nm and an e=200±50 dm3 mol−1 cm−1. The rate constant k (•OH+RSH) was 6×109 dm3 mol−1 s−1 as determined from competition kinetics. In the presence of O2 the Mesna thiyl radical was seen to rapidly add oxygen to form an RSOO• type of species with λmax = 535 nm, e=700±50 dm3 mol−1 cm−1 and k (RS•+O2)=1.3×108 dm3 mol−1 s−1. Both the RS• and the RSOO• radicals formed by the oxidation of Mesna were able to abstract H-atoms from ascorbate ions and k(RS• +AH−)=~k(RSOO•+AH−)=~6−7×108 dm3 mol−1 s−1-. Moderately strong oxidants like CCl3OO• and the (CH3)3CO• radicals, having a reduction potential of +1.4−1.6 V vs NHE were unable to oxidize Mesna. The results thus reflect on the pro- and anti-oxidant properties of Mesna.

Pulse radiolytic investigation of the reaction of hydroxyl radicals with gentamycin

Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous solutions of the aminoglycoside antibiotic, gentamycin. Using the pulse radiolysis technique, the rate constant of OH radicals with gentamycin determined was 1.2·109 dm3·mol−1·s−1. Upon.OH attack a transient species with an absorption maximum at 270 nm is observed which decays by second-order kinetics within the solute concentration range of 3.2·10−5 to 1·10−3mol·dm−3. Transient species undergoes transformation to a permanent product absorbing between 260 and 340 nm with maximum absorption at 300 nm. Rate constant of the reaction of bimolecular decay of gentamycin radicals, k (Gen.+Gen.) was found to be ≈ 1.4·107 dm3·mol−1·s−1.

Hydroxyl radical-induced cross-linking of thymine and lysine: identification of the primary structure and mechanism.

Hydroxyl radical-induced formation of a cross-link of thymine (Thy) and lysine (Lys) in the gamma-radiolysis of N2O-saturated aqueous solution was studied. A Thy-Lys cross-link (I) of the formal structure that OH radical and 4-carbon-centered Lys radical added respectively to C(5) and C(6) positions of Thy was isolated by a preparative HPLC and identified by a FAB-HRMS. The primary cross-link I was dehydrated by treatment with HCl at 120 degrees C to yield the secondary structure (II) possessing a C(5)-C(6) double bond in the Thy moiety: the latter structure II was reported previously (Dizdaroglu, M.; Gajewski, E. Cancer Res. 1989, 49, 3463-3467). A pulse radiolysis study with a redox titration method indicated that 4-carbon centered Lys radical intermediate was of neutral redox reactivity in contrast to reducing reactivity of 5-hydroxy-5,6-dihydrothymin-6-yl radical intermediate. The cross-link I could be formed by a conventional radical recombination mechanism, but not by an ionic recombination mechanism involving a redox reaction between the radical intermediates.

Reaction of OH radicals with benzoquinone in aqueous solutions. A pulse radiolysis study

Hydroxyl radicals have been generated by pulse radiolysis in N2O-saturated aqueous solutions. Their addition to 1,4-benzoquinone BQ (k3 = 6.6 × 109 dm3 mol–1 s–1 by competition with thiocyanate) in neutral solution leads to a build-up of optical absorption that shows different rates at wavelengths at around 330 and at >400 nm. At 330 nm the rate of build-up is proportional to the benzoquinone concentration, and its rate constant agrees with the value (k3) obtained by competition. At the longer wavelengths, it becomes independent of benzoquinone concentration beyond 4 × 10–4 mol dm–3 (k6 = 6.9 × 105 s–1). Kinetic analysis in the ns time-range show that the primarily-generated benzoquinone–OH-adduct radical 1 undergoes rapid (k4 = 2.5 × 106 s–1) keto–enol tautomerization yielding the 2,4-dihydroxyphenoxyl radical 2. To gain support for this proposed reaction, radical 2 [pKa(2) ≈ 4.9 ± 0.2] has been independently generated by one-electron oxidation of 1,2,4-trihydroxybenzene using ˙OH in acidic or N3˙ in neutral and basic solution. Its absorption characteristics compare favourably with those observed in the benzoquinone system. On the basis of spectrophotometric and conductometric data it is proposed that in neutral solution the radical anion 2a is rapidly oxidized by benzoquinone itself (k15 2 × 109 dm3 mol–1) into the end product 2-hydroxy-1,4-benzoquinone anion 4a [pKa(4) = 4.1 ± 0.1; λmax(4) = 380 nm; λmax(4a) = 482 nm] and the semibenzoquinone radical anion 3a. The latter decays bimolecularly into benzoquinone and hydroquinone (2k16 = 3.1 × 108 dm3 mol–1 s–1). In acidic solution the rate of oxidation of 2 by benzoquinone is considerably slower (k13 ⩽ 2.4 × 107 dm3 mol–1 s–1). The assignment of the final product to 2-hydroxy-1,4-benzoquinone 4 has been confirmed by the absorption characteristics and pKa value of the authentic material obtained by the two-electron electrochemical oxidation of 1,2,4-trihydroxybenzene.

184 Pentafuside (T-20), a novel inhibitor of HIV-1 fusion: Pharmacokinetics in rodents, monkeys and man: T. Venetta, B. DiMassimo, M.R. Johnson, D.M. Lambert, M.A. Recny, S. Hopkins and M. Saag* Trimeris, Inc., P. O. Box 13963, Research Triangle Park, NC 27709 *AIDS Outpatient Clinic, University of Alabama at Birmingham, Birmingham, Alabama 35294

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.

Significance of the intramolecular transformation of glutathione thiyl radicals to α-aminoalkyl radicals. Thermochemical and biological implications

R. Zhao, J. Lind, G. Merényi and T. E. Eriksen, J. Chem. Soc., Perkin Trans. 2, 1997, 569 DOI: 10.1039/A605727F

Reactions between hydroxyl-radical-induced 7,8-dihydro-8-oxo-2'-deoxyguanosine precursor and the spin trap alpha-phenyl-N-tert-butylnitrone.

An N2O-saturated aqueous solution containing 2'-dG and the spin trap agent PBN was examined by ESR and HPLC-ECD methods after X irradiation. ESR examination showed that the ESR spectrum obtained consisted of signals due to the PBN-OH and PBN-H adducts. The signal intensity of PBN-H adducts was larger in the presence of 2'-dG than in the absence of 2'dG, while that of PBN-OH adducts was smaller in the presence of 2'-dG than in the absence of 2'-dG. When the OH-radical-induced 8-oxodG was measured by HPLC-ECD, the yield of 8-oxodG was found to be enhanced about twofold in the presence of PBN. By contrast, usual OH-radical scavengers (DMSO, sodium formate and mannitol) inhibited the formation of 8-oxodG beyond expectation. The enhancement of the yield of PBN-H adducts by 2'-dG and the enhancement of the 8-oxodG formation by PBN were explained by the electron transfer and subsequent proton transfer reactions from OH-radical-induced guanine-N7 radicals to PBN to form 8-oxodG and PBN-H. The present study led us to conclude that PBN reacted with the precursor radical of 8-oxodG to accelerate the formation of 8-oxodG, while OH-radical scavengers reacted with it to diminish the formation of 8-oxodG.

Single-Strand Breaks in Double-Stranded DNA Irradiated in Anoxic Solution: Contribution of tert-Butanol Radicals

Yields of single-strand breaks induced by 60Co gamma or pulse irradiation in double-stranded calf thymus DNA have been measured in N2O-saturated aqueous solution as a function of the concentration of tert-butanol. The yields were found to be dependent on dose rate. The experimental data were analyzed using a theoretical model based on non-homogeneous scavenging kinetics. It is concluded from this analysis that after 60Co gamma irradiation in the absence of oxygen, aside from breaks caused by hydroxyl radicals, additional breaks occur which are initiated by hydrogen atoms and secondary radicals of tert-butanol. The efficiency of hydrogen atoms in causing single-strand breaks in double-stranded calf thymus DNA was determined to be 2.3%, while the rate constant for the reaction of tert-butanol radicals with DNA and their efficiency in causing single-strand breaks was determined to be 4.1 x 10(3) dm3 mol-1 s-1 and 2%, respectively.

Disproportionation of Cr V generated by the radiation-induced reduction of Cr VI in aqueous solution containing formate: a pulse radiolysis study

The kinetics of the radiation-induced reduction of hexavalent CrVI to trivalent CrIII, through the unstable states CrV and CrIV, in N2O-saturated aqueous solution in the presence of formate, has been studied by pulse radiolysis with optical measurements in the pH range ca. 2–5 and conductometric measurements at pH 3. The absorption spectrum of CrV has been obtained in the pH range ca. 2–9 by comparing the observed and expected bleaching yields resulting from the one-electron reduction of CrVI by CO2˙– or eaq–. The observed second-order rate constant for the disproportionation of CrV, determined from the rate of partial regeneration of CrVI, shows a bell-shaped dependence on pH with a maximum value at pH 3.3. This pH dependence is consistent with a rearrangement of the coordination shell of CrV from tetrahedral, CrVtet, to octahedral, CrVoct, in two sequential proton assisted steps followed by disproportionation between CrVtet and CrVoct with k= 3.4 × 108 dm3 mol–1 s–1. The intermediate form of CrV generated in the first of these steps does not contribute to the disproportionation reaction. Based on the bell-shaped dependence on pH of the observed rate constant for disproportionation of CrV, the forms of CrV in acidic solution are assigned as H3CrO4(pKa= 2.7), H2CrO4–(pKa= 3.8), and HCrO42–. At pH 6.8 the spectral data indicate the presence of CrO43– and pKa= 7.0 for HCrO42–.

Oxidation of benzyl radicals by Fe(CN)6 3?

Hydroxyl radicals and their anions [O˙– radicals, pKa(˙OH)= 11.9] have been generated radiolytically in N2O-saturated aqueous solution and reacted with 4-chlorotoluene. While the ˙OH radical mainly produces hydroxycyclohexadienyl-type radicals, the O˙– radical practically only abstracts H atoms from the methyl group yielding 4-chlorobenzyl radicals (k= 1.9 × 109 dm3 mol–1 s–1; determined by pulse radiolysis). The radicals formed by ˙OH radical attack at pH 7 (mainly hydroxycyclohexadienyl-type radicals) are oxidized by Fe(CN)63–(k= 1.8 × 107 dm3 mol–1 s–1) giving rise to the following products (G values in units of 10–7 mol J–1 are given in parentheses): 4-chloro-2-hydroxytoluene (2.9), 4-chloro-3-hydroxytoluene (2.4), 4-chlorobenzyl alcohol (0.05), 4-chlorobenzaldehyde (0.1) and 4-chloro-2- hydroxybenzaldehyde (0.15). The 4-chlorobenzyl radical is the main species formed at pH 13.7 and is oxidized by Fe(CN)63– with a similar rate constant (k= 4.2 × 107 dm3 mol–1 s–1), the major products being 4-chlorobenzaldehyde (3.7), 4-chloro-2-hydroxybenzaldehyde (0.65) and 4-chlorobenzyl alcohol (0.5). From ˙OH radical attack (ca. 10% at this pH), 4-chloro-2-hydroxytoluene (0.4) and 4-chloro-3- hydroxytoluene (0.3) are also formed. It is suggested that the oxidation of the 4-chlorobenzyl radical by Fe(CN)63– yields in the first step a carbocation which cyclizes by deprotonation. The resulting cyclohexadienyl-type radical undergoes β-fragmentation yielding the 4-chlorobenzyloxyl radical. A 1,2-H shift and subsequent oxidation leads to 4-chlorobenzaldehyde. The unsubstituted benzyl radical is also oxidized by Fe(CN)63– yielding benzaldehyde in high yields.

Termination of•CH2OH/CH2O•-Radicals in Aqueous Solution

Hydroxymethyl radical •CH2OH and its radical anion •CH2O- (pKa 10.7) were generated pulse-radiolytically in N2O-saturated aqueous solutions of methanol. The overall decay is by second-order kinetics. At pH ≤ 8 (•CH2OH, 2k = 1.7 × 109 dm3 mol-1 s-1), one observes ethylene glycol and formaldehyde plus methanol in a disproportionation/recombination ratio of 0.17. At pH ≥ 12 (•CH2O-) the rate constant is 2k = 0.5 × 109 dm3 mol-1 s-1. From the pKa and the dependence of the rate of bimolecular decay on pH, a mixed-termination rate constant k(•CH2OH + •CH2O-) = 1.2 × 109 dm3 mol-1 s-1 is calculated. With increasing pH, the yield of ethylene glycol decreases while that of formaldehyde (plus methanol) shows a corresponding increase. Ethylene glycol is no longer formed at pH ≥ 11.3. Quantum-chemical calculations indicate that •CH2O- possesses considerable spin density also at oxygen (mesomeric structure: -CH2O•). Since in their bimolecular termination reactions the •CH2O- radicals can no longer disproportionate by...

Kinetics and Products of OH Radical Addition to Baicalin

The reaction of OH radicals with baicalin was studied by pulse and gamma radiolysis in N2O-saturated aqueous solutions. The addition of an OH radical to baicalin produced an intermediate species that shows a maximum UV absorbance band at 420 nm and decays with a unimolecular rate constant of 1.25 x 10(4) s-1 and a bimolecular rate constant of 6.1 x 10(8) mol-1 dm3 s-1. Four major final products were obtained by gamma irradiation, and their structures were identified. Two of them (P1 and P2) are suggested to originate from the unimolecular reaction and the other two (P3 and P4) from the bimolecular reaction of the radicals.

Formation of 5-hydroxy-5,6-dihydrouracil and release of undamaged uracil as a result of poly(U) irradiation in N2O-saturated aqueous solution.

The radiation chemical yields of 5-hydroxy-5,6-dihydrouracil and uracil were measured on radiolysis of N2O-saturated solutions containing I mmol dm-3 poly(U) and were found to be 0.04 and 0.31 mumol J-1 respectively. From the 5-hydroxy-5,6-dihydrouracil yield in the presence of cysteamine and 2-mercaptoethanol, it is shown that only 20% of the OH-radicals, which attack poly(U) results in the formation of 5-hydroxy uracil adducts. It has been proposed that as yet unknown reducing radicals are formed in addition to known reducing radicals (the 5-OH-adducts of uracil, the H-adducts of uracil, and sugar radicals) as a result of the reaction of OH-radicals with poly(U).

Reaction of the OH radical with tetrachloroethene and trichloroacetaldehyde (hydrate) in oxygen-free aqueous solution

Hydroxyl radicals are generated radiolytically in N2O-saturated aqueous solutions and are reacted with either tetrachloroethene or trichloroacetaldehyde hydrate. The primary products after irradiation are tetrachlorosuccinic acid (G= 2.3 × 10–7 mol J–1 and 2.7 × 10–7 mol J–1, respectively) and chloride ions (G= 11.6 × 10–7 mol J–1 and 7.3 × 10–7 mol J–1, respectively). The former decomposes into trichloroacrylic acid and carbon dioxide (Ea= 115 kJ mol–1). From tetrachloroethene some dichloroacetic acid is also formed (G= 0.2 × 10–7 mol J –1). Trichloroacetic acid is not among the products.Kinetic studies were carried out by pulse radiolysis. In the case of trichloroacetaldehyde hydrate there is a jump in conductivity increase of G(H+)≈ 2 ×G(˙OH) within ⩽ 5 µs and no further increase in conductivity at later times. It is concluded that the OH-radicals abstract the carbon-bound H-atom and the resulting CCl3C(OH)˙2 radicals rapidly eliminate HCl forming ˙CCl2CO2H radicals which dissociate and dimerize yielding tetrachlorosuccinic acid.In the case of the reaction of the OH-radical with tetrachloroethene the conductivity build-up is biphasic. In the first step one equivalent H+ is formed when the primary ˙CCl2CCl2OH radical eliminates HCl yielding the ˙CCl2C(O)Cl radical (k > 7 × 105 s–1). Upon dimerization (2 k= 6.9 × 108 dm3 mol–1 s–1) Cl(O)CCCl2CCl2C(O)Cl is formed which hydrolyses (k= 5 s–1). Only one conductivity equivalent is liberated, and it is concluded that the ensuing monoanion rapidly eliminates a chloride ion (no conductivity change) yielding tetrachlorosuccinic anhydride. It subsequently hydrolyses at a later stage, not resolved by pulse radiolysis, to tetrachlorosuccinic acid.

1 H NMR Studies of g-Irradiated Polynucleotides and DNA in N 2 O-Saturated Aqueous Solutions: Release of Undamaged and Modified Bases

In the 400-MHz 1H NMR spectra of gamma-irradiated, N2O-saturated aqueous solutions of polyuridylic acid (polyU), polycytidylic acid (polyC), polyadenylic acid (polyA), and single-stranded DNA from calf thymus, well-resolved signals of the undamaged free bases were identified. Upon incubation of the solutions at 95 degrees C for 2 h after irradiation, the amount of release of undamaged bases increased and in the case of polyA and DNA additional signals in the region 7.7-8.3 ppm due to 4,6-diamino-5-(formylamino)-pyrimidine and 8-hydroxyadenine (8-OH-adenine) were identified. Probably because of their low solubility guanine and modified guanine bases were not detected, whereas the lack of signals of low molecular weight pyrimidine derivatives is explained by the stability of the corresponding glycosidic bonds toward hydrolysis under our conditions. From the spectral intensities G values for release of unaltered and modified nucleic bases were estimated for radiation doses corresponding to approximately 2.5% conversion for polyU and polyC, to approximately 6% conversion for polyA and to approximately 30-60% conversion for DNA. The NMR data were in agreement with G values reported for release of undamaged bases from irradiated polyU and for double-strand DNA obtained on the basis of chromatographic procedures in combination with UV absorption (D.J. Deeble, D. Schulz, and C. von Sonntag, Int. J. Radiat. Biol. 49, 915-926, 1986; J.F. Ward and I. Kuo, Radiat. Res. 66, 485-498, 1976). Broadening of the NMR peaks of the polymers at high radiation doses (> 10 kGy) was dependent on the magnetic field strength. Therefore, it is ascribed to chemical shift heterogeneity caused by a variety of irradiation products with strongly overlapping signals rather than to a decrease in internal mobility caused by crosslinking of the polymer chains.

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 N2O saturated aqueous solution have been measured as a function of time (up to 15 s) following an 0.4 microsecond 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. The reaction of OH. radicals with the adenine moiety in polyA results in the formation of two hydroxyl adducts at the positions C-4 [polyA4OH.] and C-8 [polyA8OH.]. Each OH-adduct undergoes a unimolecular transformation reaction before any bimolecular or other unimolecular decay occurs. These reactions are characterized by different rate constants and pH dependencies. The polyA4OH. adduct undergoes a dehydration reaction to yield a neutral N6 centered radical (rate constant kdeh = 1.4 x 10(4)s-1 at pH 7.3). 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 polyA8OH. adduct undergoes an imidazole ring opening reaction to yield an enol type of formamidopyrimidine radical with the resulting base damage (kr.o. = 3.5 x 10(4)s-1 at pH 7.3). This reaction in contrast is strongly catalysed by H+ and OH-, similar as for adenosine but different compared to the nucleotides.

Structure–reactivity studies on the nature of transients formed on reaction of ˙OH radicals with chloroiodoalkanes in aqueous solution

The transient optical absorption bands (λmax= 315 and 345 nm) formed on reaction of ˙OH radicals with 1-chloro-3-iodopropane in N2O-saturated neutral aqueous solutions have been assigned to an OH-adduct formed between iodine atom and ˙OH radicals (t½= 1.5 µs, ε350= 2.46 × 103 dm3 mol–1 cm–1). In acidic solutions, iodine-centred radical cations (λmax= 345 nm, t½= 4.0 µs, ε345= 2.41 × 103 dm3 mol–1 cm–1) are formed at low solute concentrations and dimer radical cations (λmax= 430 nm, t½= 16.0µs, ε430= 6.7 × 103 dm3 mol–1 cm–1) at high solute concentrations. In chloroiodomethane, the ˙OH radicals do not seem to form OH-adducts in neutral aqueous solutions. In acidic solutions, intramolecular radical cation (λmax= 430 nm, t½= 8.0 µs, ε430= 2.63 × 103 dm3 mol–1 cm–1) with oxidized iodine and chlorine is observed. The intermolecular dimer radical cations are formed at high solute concentration (λmax= 500 nm).

Pulse radiolysis investigations on electron-transfer reactions in aqueous solutions of substituted alkyl sulfides

The transient optical absorption band (λmax= 345 nm, t½= 4.0 µs, Iµ= 2.6 × 103 dm3 mol–1 cm–1) formed on pulse radiolysis of N2O-saturated dilute neutral aqueous solutions of dimethyl 3,3′-thiodipropionate (DTDP) is assigned to an ˙OH-adduct. The rate constant for the reaction of ˙OH radicals has been determined to be 1.4 × 1010 dm3 mol–1 s–1. At higher solute concentrations the ˙OH-adduct is converted to sulfur-centred dimer radical cations (λmax= 510 nm, Iµ= 4.9 × 103 dm3 mol–1 cm–1, t½= 14.0 µs). In acidic solutions, only dimer radical cations are formed. In neutral aqueous solutions of 3,3′-thiodipropanol (TDP)˙OH radicals are able to bring about one-electron oxidation, forming intramolecular radical cations with p-orbital overlap between the oxidized sulfur and oxygen (λmax= 420 nm, t½= 17 µs, Iµ= 2.6 × 103 dm3 mol–1 cm–1). In acidic solutions, sulfur-centred dimer radical cations are formed (λmax= 510 nm, t½= 12 µs, Iµ= 6.3 × 103 dm3 mol–1 cm–1). In the presence of halide ions, the oxidized sulfur in DTDP and TDP stabilizes by formation of an S∴X type species.

Pulse radiolysis investigations on the reactions of primary radiolytic species of water with atropine

On pulse radiolysis of N2O saturated aqueous solutions of atropine, an optical absorption band (λmax at 320 nm,e=2.81·103 dm3·mol−1·cm−1) was observed, which is assigned to the product of reaction of OH radicals with the solute. This absorption decayed following second order kinetics with a rate constant of 4.5·108 dm3·mol−1·s−1. The rate constant for the reaction of OH radicals with atropine as estimated by following the build-up kinetics is 2.7·109 dm3·mol−1·s−1. The H atoms also reacted with this compound to produce a transient absorption band behaving similarly to the one observed in the case of reaction with OH radicals. The transient species formed in both cases is assigned to a radical derived by H atom abstraction by H/OH radicals from the parent compound. This radical was unreactive towards 2-mercaptoethanol. e aq − was found to react with atropine forming a transient band with λmax at 310 nm (e=3.55·103 dm3·mol−1). Its decay was also second order with a rate constant of 1.64·109 dm3·mol−1·s−1. The bimolecular rate constant for the reaction of e aq − with atropine as estimated from the decay of e aq − absorption at 720 nm is 3.9·109 dm3·mol−1·s−1. Specific one-electron oxidizing and reducing agents (such as Cl 2 − , Tl2+, SO 4 − and (CH3)2COH, CO 2 − , respectively) failed to oxidize or reduce this compound in aqoues solutions. The radical anion of atropine formed by its reaction with e aq − was found to reduce thionine and methyl viologen with bimolecular rate constant of 3.8·109 and 3.2·109 dm3·mol−1·s−1, respectively.

Hydroxyl radical induced cross-linking of cytosine and tyrosine in nucleohistone

Hydroxyl radical induced formation of a DNA-protein cross-link involving cytosine and tyrosine in nucleohistone in buffered aqueous solution is reported. The technique of gas chromatography-mass spectrometry was used for this investigation. A gamma-irradiated aqueous mixture of cytosine and tyrosine was first investigated in order to obtain gas chromatographic-mass spectrometric properties of possible cytosine-tyrosine cross-links. One cross-link was observed, and its structure was identified as the product from the formation of a covalent bond between carbon 6 of cytosine and carbon 3 of tyrosine. With the use of gas chromatography-mass spectrometry with selected-ion monitoring, this cytosine-tyrosine cross-link was identified in acidic hydrolysates of calf thymus nucleohistone gamma-irradiated in N2O-saturated aqueous solution. The yield of this DNA-protein cross-link in nucleohistone was found to be a linear function of the radiation dose in the range of 100-500 Gy (J.kg-1). This yield amounted to 0.05 nmol.J-1. Mechanisms underlying the formation of the cytosine-tyrosine cross-link in nucleohistone were proposed to involve radical-radical and/or radical addition reactions of hydroxyl adduct radicals of cytosine and tyrosine moieties, forming a covalent bond between carbon 6 of cytosine and carbon 3 of tyrosine. When oxygen was present in irradiated solutions, no cytosine-tyrosine cross-links were observed.