Nature of the transient species formed in pulse radiolysis of n-allylthiourea in aqueous solutions

Reactions of e⁻_aq, OH radicals and H atoms were studied with n-allylthiourea (NATU) using pulse radiolysis. Hydrated electrons reacted with NATU (k = 2.8×10⁹ dm³ mol⁻¹ s⁻¹) giving a transient species which did not have any significant absorption above 300 nm. It was found to transfer electrons to methyl viologen. At pH 6.8, the reduction potential of NATU has been determined to be −0.527 V versus NHE. At pH 6.8, OH radicals were found to react with NATU, giving a transient species having absorption maxima at 400–410 nm and continuously increasing absorption below 290 nm. Absorption at 400–410 nm was found to increase with parent concentration, from which the equilibrium constant for dimer radical cation formation has been estimated to be 4.9×10³ dm³ mol⁻¹. H atoms were found to react with NATU with a rate constant of 5 × 10⁹ dm³ mol⁻¹ s⁻¹, giving a transient species having an absorption maximum at 310 nm, which has been assigned to H-atom addition to the double bond in the allyl group. Acetoneketyl radicals reacted with NATU at acidic pH values and the species formed underwent reaction with parent NATU molecule. Reaction of Cl^.−₂ radicals (k = 4.6 × 10⁹ dm³ mol⁻¹ s⁻¹) at pH 1 was found to give a transient species with λ_max at 400 nm. At the same pH, reaction of OH radicals also gave transient species, having a similar spectrum, but the yield was lower. This showed that OH radicals react with NATU by two mechanisms, viz., one-electron oxidation, as well as addition to the allylic double bond. From the absorbance values at 410 nm, it has been estimated that around 38% of the OH radicals abstract H atoms and the remaining 62% of the OH radicals add to the allylic double bond.     

Reactions of melatonin with radicals in deoxygenated aqueous solution

Reactions of melatonin (N-acetyl-5-methoxytryptamine) with radiolytically generated radicals were studied. Reaction of melatonin with OH radicals is diffusion-controlled (k=1.2·10¹⁰ dm³ mol⁻¹·s⁻¹), the main (but not the only one) intermediate being the indolyl-type radical, while the rate constant for the reaction with hydrated electrons isk=4.3·10⁸ dm³·mol⁻¹·s⁻¹. Melatonin is capable of scavenging tert-butanol radicals, while its reactivity towards polymer radicals of poly(acrylic acid) and poly(vinyl pyrrolidone) is very low.     

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.     

Scavenging of reactive oxygen radicals by resveratrol: antioxidant effect

Pulse radiolysis of resveratrol was carried out in aqueous solutions at pH ranging from 6.5 to 10.5. The one-electron oxidized species formed by the N^•₃ radicals at pH 6.5 and 10.5 were essentially the same with λ_max at 420 nm and rate constant varying marginally (k = (5−6.5) × 10⁹ dm³ mol⁻¹ s⁻¹). The nature of the transients formed by NO^•₂, NO^• radical reaction at pH 10.5 was the same as that with N^•₃, due to the similarity in decay rates and the absorption maximum. Reaction of ^•OH radical with resveratrol at pH 7 gives an absorption maximum at 380 nm, attributed to the formation of carbon centered radical. The repair rates for the thymidine and guanosine radicals by resveratrol were approx. 1 × 10⁹ dm³ mol⁻¹ s⁻¹, while the repair rate for tryptophan was lower by nearly an order of magnitude (k = 2 × 10⁸ dm³ mol⁻¹ s⁻¹). The superoxide radical anion was scavenged by resveratrol, as well as by the Cu–resveratrol complex with k = 2 × 10⁷ and 1.5 × 10⁹ dm³ mol⁻¹ s⁻¹, respectively. Its reduction potential was also measured by cyclic voltammetry.     

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 λ_max = 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 ε=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 λ_max = 535 nm, ε=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.     

Reaction mechanism studies on isoquinoline with hydroxyl radical in aqueous solutions

The reaction mechanism between isoquinoline and ·OH radical in aqueous dilute solutions under different conditions was studied by pulse radiolysis. The main characteristic peaks in these transient absorption spectra were attributed and the growth-decay trends of several transient species were investigated. Under neutral or alkaline conditions, the reaction of ·OH radical and isoquinoline produces OH-adducts with respective rate constants of 3.4 × 10⁹ and 6.6 × 10⁹ mol⁻¹ · dm³ · s⁻¹ while under acidic conditions, the isoquinoline was firstly protonated and then ·OH added to the benzene ring and produced protonated isoquinoline OH-adducts with a rate constant of 3.9 × 10⁹ mol⁻¹ · dm³ · s⁻¹. With a better understanding on radiolysis of isoquinoline, this study is of help for its degradation and for environmental protection. __________ Translated from Journal of Fudan University (Natural Science), 2006, 45(6): 774–778 [8BD1;81EA: 590D;65E6;5B66;62A5;(自然科学版)]     

Reactive oxygen and nitrogen species (RONS) scavenging reactions of o-vanillin: Pulse radiolysis and stopped flow studies

Reactions of peroxyl radicals and peroxynitrite with o-vanillin (2-hydroxy 3-methoxy benzaldehyde), a positional isomer of the well-known dietary compound vanillin, were studied to understand the mechanisms of its free radical scavenging action. Trichloromethylperoxyl radicals (CCl₃O ₂ ^· ) were used as model peroxyl radicals and their reactions with o-vanillin were studied using nanosecond pulse radiolysis technique with absorption detection. The reaction produced a transient with a bimolecular rate constant of approx. 10⁵ M⁻¹s⁻¹, having absorption in the 400–500 nm region with a maximum at 450 nm. This spectrum looked significantly different from that of phenoxyl radicals of o-vanillin produced by the one-electron oxidation by azide radicals. The spectra and decay kinetics suggest that peroxyl radical reacts with o-vanillin mainly by forming a radical adduct. Peroxynitrite reactions with o-vanillin at pH 6.8 were studied using a stopped-flow spectrophotometer. o-Vanillin reacts with peroxynitrite with a bimolecular rate constant of 3 × 10³ M⁻¹s⁻¹. The reaction produced an intermediate having absorption in the wavelength region of 300–500 nm with a absorption maximum at 420 nm, that subsequently decayed in 20 s with a first-order decay constant of 0.09 s⁻¹. The studies indicate that o-vanillin is a very efficient scavenger of peroxynitrite, but not a very good scavenger of peroxyl radical. The reactions take place through the aldehyde and the phenolic OH group and are significantly different from other phenolic compounds.     

Kinetic, spectral and redox properties of the transients formed on one-electron reduction of 3, 3, 6, 6-tetramethyl-3, 4, 6, 7, 9-pentahydro-10-phenyl (1, 8) (2H, 5H) acridinedione in aqueous-organic mixed solvent system: A pulse radiolysis study

The phenyl substituted acridine-1,8-dione (AD) dye reacts with (CH₃)₂*COH radicals with a bimolecular rate constant of 0.6 × 10⁸ dm³ mol⁻¹ s⁻¹ in acidic aqueous-organic mixed solvent system. The transient optical absorption band (λ_max = 465 nm, ɛ = 6.8 × 10² dm³ mol⁻¹ cm⁻¹) is assigned to ADH* formed on protonation of the radical anion. In basic solutions, (CH₃)₂*COH radicals react with a bimolecular rate constant of 4.6 × 10⁸ dm³ mol⁻¹ s⁻¹ and the transient optical absorption band (λ_max = 490 nm, ɛ = 10.4 × 10³ dm³ mol⁻¹ cm⁻¹) is assigned to radical anion, AD*⁻, which has a pK_a value of 8.0. The reduction potential value of the AD/AD*⁻ couple is estimated to be between −0.99 and −1.15 V vs NHE by pulse radiolysis studies. The cyclic voltammetric studies showed the peak potential close to −1.2 V vs Ag/AgCl.     

Reactions of H atoms and OH radicals with ascorbic acid: A pulse radiolysis Fourier transform ESR study

Using pulse radiolysis, free radicals of ascorbic acid were generated by reactions of the primary radicals H and OH in acidic and basic aqueous solutions. The formation and the decay of several radicals of ascorbic acid were detected by time resolved Fourier transform electron spin resonance within a time interval of 100 ns to 1 ms. The rate constant of addition of H atoms to ascorbic acid (1.3·10⁸ dm³· mol⁻¹·s⁻¹) was directly determined by the change of line width of the low field line of the H atom in the presence of ascorbic acid. The addition of OH radicals to ascorbic acid results in different radical structures, detected by highly resolved Fourier transform ESR spectra.     

Radiation chemical investigations on aqueous solutions of C60(OH)18

The ultraviolet-visible absorption spectrum of C₆₀(OH)₁₈ in water showed an absorption band with λ_max = 215 nm and other characteristic absorption bands of C₆₀ are not observed. The singlet-singlet and triplet-triplet absorption bands are not observed in the 400–900 nm region. It has low reactivity with e_aq⁻ and formed an absorption band with λ_max = 580 nm. The hydroxyl radicals react with a bimolecular rate constant of 2.4×10⁹ dm³ mol⁻¹ s⁻¹ and showed an absorption band at 540 nm.     

Redox chemistry of o- and m -hydroxycinnamic acids: A pulse radiolysis study

Radiation chemical reactions of^•OH, O^•−, N₃ ^•and e _aq ^t- witho- and m-hydroxycinnamic acids were studied. The second-orderrateconstantsforthereaction of^•OH with ortho and meta isomers in buffer solution at pH7 are 3.9±0.2 × 10⁹ and 4.4 ± 0.3 × 10⁹ dm³ mol^-1 s^-1 respectively. At pH 3 the rate with the ortho isomer was halved (1.6 ± 0.4 × 10⁹ dm³ mol^-1 s^-1) but it was unaffected in the case of meta isomer (k = 4.2±0.6 × 10⁹dm³mol^-1 s^-1). The rate constant in the reaction of N₃ ^• with the ortho isomer is lower by an order of magnitude (k = 4.9 ± 0.4 × 10⁸ dm³ mol^-1s^-1). The rates of the reaction of e _aq ^t- with ortho and meta isomers were found to be diffusion controlled. The transient absorption spectrum measured in the^•OH witho-hydroxycinnamic acid exhibited an absorption maximum at 360 nm and in meta isomer the spectrum was blue-shifted (330 nm) with a shoulder at 390 nm. A peak at 420 nm was observed in the reaction of O^bb−with theo-isomer whereas the meta isomer has a maximum at 390 and a broad shoulder at 450 nm. In the reaction of the absorption peaks were centred at 370–380 nm in both the isomers. The underlying reaction mechanism is discussed.     

Reactions of hydroxyl radical (·OH) and oxide radical anion (O·−) with 2-aminopurine in aqueous medium

Pulse radiolysis has been used to investigate the reaction of hydroxyl radical (^·OH) and oxide radical anion (O^·−) with 2-aminopurine (2AP), a fluorescent analogue of adenine, in aqueous medium. The second-order rate constant for the reaction of ^·OH with 2AP was determined to be 3 × 10⁹ dm³ mol⁻¹s⁻¹ and for the reaction of O^·− it was 7.1 × 10⁸ dm³ mol⁻¹s⁻¹. The transient absorption spectrum obtained in the reaction of ^·OH at pH 7 has absorption maxima at 370 and 470 nm. The spectrum undergoes a time-dependent transformation at higher time-scale. The intermediate species was found to react with N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD). The yield of TMPD^·+ was calculated in terms of G(TMPD^·+) to be 3.3 × 10⁻⁷ mol J⁻¹ at pH 7. The ^·OH reactions were also carried out at pH 10 and the transient absorption spectra have λ _max at 400 and 480 nm. The transient spectra obtained in the reaction of O^·− at pH ≈14 have maxima at 400 and 480 nm. The transient intermediate species at pH 7 are assigned to the formation of 2AP-4-OH^· (54%), 2AP-5-OH^· (7%) and 2AP-8-OH^· (39%) based on the spectral evidence and TMPD^·+ build-up. Both 2AP-4-OH^· and 2AP-5-OH^· undergo OH⁻ elimination to form a radical cation. At higher pH (pH 10), the dehydration reaction of these OH-adducts leads to a N-centered radical (2AP-N(9)^·, 71%). Formation of 2AP-8-OH^· (29%) is also proposed at this pH. In the reaction of O^·− with 2AP, it is proposed that a similar nitrogen centered 2AP-N(9)^· radical is formed by an electron-transfer reaction at N(9).     

Studies of the reaction of the hydroxyl radical with the oxalate ion in an acidic aqueous solution by pulse radiolysis

The reaction of the · OH radical with the oxalate ion in an acidic aqueous solution was studied by pulse radiolysis. The rate constant for the reaction of formation of the radical HOOC-COO·(λ_max = 250 nm, ɛ = 1800 L mol⁻¹ cm⁻¹) is (5.0±0.5)·107 L mol⁻¹ s⁻¹. In the reaction with the hydrogen ion (k = 1.1·10⁷ L mol⁻¹ s⁻¹), the radical HOOC-COO· is transformed into a nonidentified radical designated arbitrarily as H⁺(HOOC-COO)· (λ_max = 260 nm, ɛ = 4000 L mol⁻¹ cm⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1165–1167, June, 2008.     

Pulse radiolysis of butyl acrylate in aqueous solution

The pulse radiolysis of n-butyl acrylate (nBA) in aqueous solution was studied. The rate constant of the reaction of nBA with hydroxyl radicals was calculated as 1.5×10¹⁰ dm³ mol⁻¹ s⁻¹. The absorption spectrum of the OH^·–nBA adduct appeared to have a broad maximum at 300 nm. This spectrum was attributed to the α-carbon centred radicals. It decayed with the first-order rate constant k=1.5×10⁴ s⁻¹ (pH 10.8). The rate constant of the nBA reaction with hydrated electrons was determined as k=1.6×10¹⁰ dm³ mol⁻¹ s⁻¹. The spectrum of H^·–nBA adduct was similar to that recorded for OH^· adduct. It decayed with first-order kinetics at k=1.0×10⁴ s⁻¹. Spectra of the electron adduct were characterised by the band with a maximum at 285 nm (pH 10.0) or at 280 nm (pH 4.0) with ϵ=10 500 dm³ mol⁻¹ cm⁻¹. In acidic solution, radical anion formed upon addition of hydrated electrons to the nBA molecule, undergoes fast, reversible protonation. The decay of the reversibly protonated electron adduct was a second-order process at k=2.5×10⁹ dm³ mol⁻¹ s⁻¹. This reaction took place at the carbonyl oxygen. Slow, irreversible protonation of the electron adduct at high pH takes place at the β-carbon atom at k=2.9×10⁴ s⁻¹.     

Photochromic transformations in solutions of 8,8′-diquinolyl disulfide and di(mercaptoquinolinato)nickel(<Emphasis Type="SmallCaps">II</Emphasis>)

The nature of intermediate species and their reactions were studied by laser pulse photolysis for a photochromic system consisting of 8,8′-diquinolyl disulfide (RSSR) and a planar Ni^II complex di(mercaptoquinolinato)nickel(II) (Ni(SR)₂) in toluene and benzene solutions. Under exposure to laser radiation, disulfide RSSR dissociates to two RS^· radicals, whose spectrum has an intense absorption band with a maximum at λ = 400 nm (ε = 8400 L mol⁻¹ cm⁻¹). The radicals disappear by recombination (2k _rec = 4.6 · 10⁹ L mol⁻¹ s⁻¹). In the presence of the Ni(SR)₂ complex, coordination of the radical (k _coord = 4.4 · 10⁹ L mol⁻¹ s⁻¹) competes with recombination to form a radical complex RS^· Ni(SR)₂ having an intense absorption band with a maximum at 460 nm (ε = 16 600 L mol⁻¹ cm⁻¹). This species decays in the second-order reaction (2k = 4.6 · 10⁴ L mol⁻¹ s⁻¹). Since the photochromic system returns to the initial state, the reaction of two radical complexes is assumed to produce radical recombination and reduction of the disulfide and Ni(SR)₂ complex. Analysis of the kinetic data showed that some RS^· radicals decay in the microsecond time interval due to the reaction with the RS^· Ni(SR)₂ radical complex (k = 3.1 · 10⁹ L mol⁻¹ s⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2291–2300, October, 2005.     

Effect of electron-withdrawing power of the substituted group on?OH radical reaction with substituted aryl sulphides: A pulse radiolysis study

In neutral aqueous solution of (phenylthio)acetic acid, hydroxyl radical is observed to react with a bimolecular rate constant of 7.2 × 10^-1 dm³ mol⁻s⁻ and the transient absorption bands are assigned to^•OH radical addition to benzene and sulphur with a rough estimated values of 50 and 40% respectively. The reaction of the^•OH radical with diphenyl sulphide (k = 4.3 × 10⁸ dm³ mol⁻¹ s⁻¹) is observed to take place with formation of solute radical cation, OH-adduct at sulphur and benzene with estimated values of about 12, 28 and 60% respectively. The transient absorption bands observed on reaction of^•OH radical, in neutral aqueous solution of 4-(methylthio)phenyl acetic acid, are assigned to solute radical cation (λ_max = 550 and 730 nm), OH-adduct at sulphur (λ_max = 360 nm) and addition at benzene ring (λ_max = 320 nm). The fraction of^•OH radical reacting to form solute radical cation is observed to depend on the electron-withdrawing power of substituted group. In acidic solutions, depending on the concentration of acid and electron-withdrawing power, solute radical cation is the only transient species formed on reaction of^•OH radical with the sulphides studied.     

Decay kinetics of benzophenone oxide in the liquid phase

The kinetics of self-termination of benzophenone oxide (BPO) in the liquid phase was studied by flash photolysis. The extinction coefficient of BPO (ε) was found to be virtually independent of the solvent nature, ε=(1.9±0.1)·10³ L mol⁻¹ cm⁻¹. The rate constant of the BPO self-temination increases from 1.8·10⁷ (MeCN) and 7.4·10⁷ (C₆H₆) to 1.5·10⁹ (n-decane) and 2.0·10⁹ L mol⁻¹ s⁻¹ (n-pentane) at 293±2 K. Solvation of BPO promotes a polar state of the molecule in MeCN and C₆H₆. In nonpolar hydrocarbons, a great contribution is made by the biradical structure resulting in an increase in the rate constant and a shift of the absorption maximum to the long-wave region (from 410 nm in MeCN to 425 nm inn-pentane). In solutions of benzene and acetonitrile, benzophenone oxide reacts with the parent diazo compound with a rate constant of (2–4)·10⁵ L mol⁻¹ s⁻¹ (293±2 K) along with the self-termination. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1329–1332, July, 1998.     

Pulse radiolysis study of redox reactions of safranine T in sodium dodecylsulphate (SDS) micellar medium

Reaction of hydrated electrons with safranine T (SF⁺), a phenazine dye useful as sensitizer in photogalvanic cell and the transient semireduced species formed by this reaction have been studied in SDS micellar medium using the technique of pulse radiolysis. Thee _aq ⁻ reaction with SF⁺ in the micellar environment was only marginally slower (5.1 × 10⁹ dm³ mol⁻¹ s⁻¹) as compared to that in homogeneous aqueous medium (2.2 × 10¹⁰ dm³ mol⁻¹ s⁻¹) explicable on the basis of our finding that although a large fraction of the dye gets localized near the micelle Stern layer where the molecule experiences a dielectric.constant of ≈40, a small but significant concentration of the dye exists in the aqueous bulk as charge pair complex with the anionic surfactant monomer (association constant for the formation of the complex being 2.8 × 10⁴ dm³ mol⁻¹). The transient semireduced absorption band observed in the micellar medium showed a red shift of ≈ 50 nm and also the decay of the transient, which was very fast with 2k = 1 × 10⁹ dm³ mol⁻¹ s⁻¹ in aqueous medium, was stable in the SDS micellar medium over a few tens of milliseconds suggesting that the radical is incorporated deeper than the parent molecule in the SDS micelle. The effect of this stability on the photogalvanic conversion needs to be examined.     

Photochemical properties of a new kind of anti-cancer drug: <Emphasis Type="Italic">N</Emphasis>-glycoside compound

Due to the nontoxicity and efficient anti-cancer activity, more and more attention has been paid to N-glycoside compounds. Laser photolysis of N-(α-D-glucopyranoside) salicyloyl hydrazine (NGSH) has been performed for the first time. The research results show that NGSH has high photosensitivity and is vulnerable to be photo-ionized via a monophotonic process with a quantum yield of 0.02, generating NGSH^+· and hydrated electrons. Under the aerobic condition of cells, the hydrated electrons are very easy to combine with oxygen to generate ¹O₂ and O₂ ⁻, both of which are powerful oxidants that can kill the cancer cells. In addition, NGSH^+· can be changed into neutral radicals by deprotonation with a pK _a value of 4.02 and its decay constant was determined to be 2.55×10⁹dm³·mol⁻¹·s⁻¹. NGSH also can be oxidized by SO₄ ^−· with a rate constant of 1.76×10⁹ dm³·mol⁻¹·s⁻¹, which further confirms the results of photoionization. All of these results suggest that this new N-glycoside compound might be useful for cancer treatment. The same contribution to the work Supported by the National Natural Science Foundation of China (Grant Nos. 30570376 and 50673078) and Shanghai Project (Grant Nos. 06JC14068 and 08ZZ21)     

Laser flash photolysis of carboxymethylcellulose in an aqueous solution

Laser flash photolysis with excitation at 248 nm was used to study photochemically derived changes of carboxymethylcellulose (CMC) in aqueous solutions. Transient absorption spectra of solutions after photolysis revealed a broad band with a maximum of approximately 720 nm, which could be ascribed to the signal of the hydrated electron. The interaction of the hydrated electron with CMC was slow (<10⁷ dm³ mol^?1 s^?1), but the OH radical, formed by the decomposition of H₂O₂, reacted with CMC at a high rate constant (9.5–11.0 × 10⁸ dm³ mol^?1 s^?1). The rate constant of the reactions of CMC with hydroxyl radicals depended on the conformation of the macromolecules, which was determined by the pH of the solution. Transient absorption was recorded at a wavelength shorter than 370 nm for CMC solutions photolyzed in the presence of H₂O₂. As a result of OH attack, long‐lived radicals were formed on CMC. The recombination of macroradicals led to the formation of crosslinking bonds between side‐chain groups, and as a result of it an insoluble gel arose in low‐pH solutions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 505–518, 2005