Specific One-electron Oxidants Research Articles

Pulse Radiolysis Studies on Reactions of Hydroxyl Radicals with Selenocystine Derivatives

Reactions of hydroxyl radicals (*OH) with selenocystine (SeCys) and two of its analogues, diselenodipropionic acid (SeP) and selenocystamine (SeA), have been studied in aqueous solutions at pHs of 1, 7, and 10 using the pulse radiolysis technique coupled with absorption detection. All of these diselenides react with *OH radicals with rate constants of approximately 10(10) M(-1) s(-1), producing diselenide radical cations ( approximately 1-5 micros after the pulse), with an absorption maximum at 560 nm, by elimination of H(2)O or OH(-) from hydroxyl radical adducts. Assignment of the 560 nm band to the diselenide radical cation was made by comparing the transient spectra with those produced upon reaction of diselenides with specific one-electron oxidants, Cl(2)(*-) (pH 1) and Br(2)(*-) radicals (pHs of 7 and 10). SeP having a carboxylic acid functionality showed quantitative conversion of hydroxyl radical adducts to radical cations. The compounds SeCys and SeA, having an amino functional group, in addition to the radical cations, produced a new transient with lambda(max) at 460 nm, at later time scales ( approximately 20-40 micros after the pulse). The rate and yield of formation of the 460 nm band increased with increasing concentrations of either SeCys or SeA. In analogy with similar studies reported for analogous disulfides, the 460 nm transient absorption band has been assigned to a triselenide radical adduct. The one-electron reduction potentials of the compounds were estimated to be 0.96, 1.3, and 1.6 V versus NHE, respectively, for SeP, SeCys, and SeA at pH 7. From these studies, it has been concluded that the electron-donating carboxylic acid group decreases the reduction potential and facilitates quantitative conversion of hydroxyl radical adducts to radical cations, while the electron-withdrawing NH(3)(+) group not only increases the reduction potential but also leads to fragmentation of the hydroxyl radical adduct to selenyl radicals, which are converted to triselenide radical adducts.

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.

Effect of pH on One-Electron Oxidation Chemistry of Organoselenium Compounds in Aqueous Solutions

Pulse radiolysis coupled with absorption detection has been employed to study one-electron oxidation of selenomethionine (SeM), selenocystine (SeCys), methyl selenocysteine (MeSeCys), and selenourea (SeU) in aqueous solutions. Hydroxyl radicals (*OH) in the pH range from 1 to 7 and specific one-electron oxidants Cl2*- (pH 1) and Br2*- (pH 7) have been used to carry out the oxidation reactions. The bimolecular rate constants for these reactions were reported to be in the range of 2 x 10(9) to 10 x 10(9) M(-1) s(-1). Reactions of oxidizing radicals with all these compounds produced selenium-centered radical cations. The structure and stability of the radical cation were found to depend mainly on the substituent and pH. SeM, at pH 7, produced a monomer radical cation (lambdamax approximately 380 nm), while at pH 1, a dimer radical cation was formed by the interaction between oxidized and parent SeM (lambdamax approximately 480 nm). Similarly, SeCys, at pH 7, on one-electron oxidation, produced a monomer radical cation (lambdamax approximately 460 nm), while at pH 1, the reaction produced a transient species with (lambdamax approximately 560 nm), which is also a monomer radical cation. MeSeCys on one-electron oxidation in the pH range from 1 to 7 produced monomer radical cations (lambdamax approximately 350 nm), while at pH < 0, the reaction produced dimer radical cations (lambdamax approximately 460 nm). SeU at all the pH ranges produced dimer radical cations (lambdamax approximately 410 nm). The association constants of the dimer radical cations of SeM, MeSeCys, and SeU were determined by following absorption changes at lambdamax as a function of concentration. From these studies it is concluded that formation of monomer and dimer radical cations mainly depends on the substitution, pH, and the heteroatoms like N and O. The availability of a lone pair on an N or O atom at the beta or gamma position results in monomer radical cations having intramolecular stabilization. When such a lone pair is not available, the monomer radical cation is converted into a dimer radical cation which acquires intermolecular stabilization by the other selenium atom. The pH dependency confirms the role of protonation on stabilization. The oxidation chemistry of these selenium compounds is compared with that of their sulfur analogues.

Pulse radiolysis studies of mangiferin: A C- glycosyl xanthone isolated from Mangifera indica

Pulse radiolysis technique has been employed to study the reaction of different oxidizing and reducing radicals with mangiferin. The reaction of OH radical showed the formation of transient species absorbing in 380–390 and 470–480 nm region. The reaction with specific one-electron oxidants (N 3 , CCl 3O 2 ) also showed the formation of similar transient absorption bands and is assigned to phenoxyl radicals. The p K a values of the transient species have been determined to be 6.3 and 11.9. One-electron oxidation potential of mangiferin at pH 9 has been found to be 0.62 V vs. NHE. The reaction of e aq − showed the formation of transient species with λ max at 340 nm, which is assigned to the ketyl anion radical formed on addition of e aq − at carbonyl site. Reactions of one-electron oxidised mangiferin radicals with ascorbic acid have also been studied.

One-Electron Oxidation of Selenourea in Aqueous Solution

One-electron oxidation of selenourea in aqueous solution has been studied using pulse radiolysis, cyclic voltammetry, and ab initio quantum chemical techniques. Specific one-electron oxidants (N3•,...

Different channels of ·OH radical reaction with tryptophanol in aqueous solutions: A pulse radiolysis study

The pulse radiolysis technique has been employed to study the reaction of ·OH radical with tryptophanol (TPN). Reactions of specific one-electron oxidants like Br2· - and N3· and ·H atom were carried out to understand the contribution of different channels of · OH radical reaction with TPN. The studies were carried out in the pH range 3 to 10. One-electron oxidation of TPN (pH 3) produced radical cation absorbing at 570 nm. However, at higher pH, deprotonation of TPN cation radical takes place from N(1) position and indolyl radical absorbing at 520 nm with a pKa value of 3.6 is formed. Redox titration with TMPD, ABTS2- and MV2+ was performed to determine the total yield of oxidizing and reducing radicals produced during ·OH reaction.

Investigations on the nature of the transient species formed on pulse radiolysis of a aqueous solution of 4-(methylthio)benzoic acid

The transient absorption spectrum (λmax = 320, 400 and 550 nm) obtained on reaction of OH radicals with 4-(methylthio)benzoic acid is assigned to a solute radical cation with a positive charge on the benzene ring. The reaction with specific one-electron oxidants also produced similar spectrum and the oxidation potential for the formation of solute radical cation is estimated to be between 1.4 and 1.6 V vs NHE. The reaction of eaq- with the solute showed the formation of a transient absorption band at 320 nm and is assigned to solute radical anion with reduction potential more negative than-1.5 V.

Reactions of oxidizing radicals with 2- and 3-aminopyridines: a pulse radiolysis study

Reactions of OH radicals and some one-electron oxidants with 2-aminopyridine (2-AmPy) and 3-aminopyridine (3-AmPy) were studied in aqueous solutions using pulse radiolysis technique. The OH adduct of 2-AmPy at pH 9 has an absorption maximum at 360 nm along with a weak absorption band in the visible region and was found to be reactive with oxygen. The rate constant for its reaction with O 2 was determined to be 1.0×10 8 dm 3 mol −1 s −1. At pH 4 also, the OH adduct of 2-AmPy has an absorption band at 360 nm. However, there are differences in the absorption at other wavelengths. From the plot of ΔOD vs. pH at 340 nm, the p K a of the OH adduct was determined to be 6.5. Among the specific oxidants, only SO 4 − radicals were able to oxidize 2-AmPy. In the case of 3-aminopyridine (3-AmPy), the transient species formed by OH radical reaction at pH 9 has an absorption maximum at 410 nm with shoulder bands on both the sides. Its absorption spectrum at pH 4 was different indicating the existence of a p K value for the OH adduct. p K a of 3-AmPy-OH radical adduct species was evaluated to be 5.7. This adduct species was also found to be reactive with oxygen ( k=7.6×10 6 dm 3 mol −1 s −1). Specific one-electron oxidants like N 3 , Br 2 − C 2 − and SO 4 − were able to oxidize 3-AmPy indicating that it is easier to oxidize 3-AmPy as compared to 2-AmPy.

Spectral, Kinetics, and Redox Studies on the Transients Formed on Pulse Radiolysis of Aqueous Solution of (4-Methylthiophenyl)methanol

Abstract Pulse radiolysis technique has been employed to investigate the nature and the redox properties of the transient species generated on radiolysis of aqueous solution of (4-methylthiophenyl)methanol (MTPM). Pulse radiolysis in 1,2-dichloroethane, reaction with specific one-electron oxidants (Cl2•−, SO4•−, Tl2+, Br•) and reaction of •OH radicals in acidic solution showed absorption bands at 320 and 545 nm; these are assigned to solute radical cation with positive charge on the benzene ring. OH-adduct was observed in neutral solution. The oxidation potential for MTPM/MTPM•+ couple is determined to be 1.55 ± 0.04 V vs NHE. eaq− reacts with a bimolecular rate constant of 1.5 × 109 dm3 mol−1 s−1 and the transient absorption bands (λmax = 320, 470 nm) are assigned to the solute radical anion. E0 value for MTPM/MTPM•− couple is determined to be −1.84 ± 0.04 V vs NHE. The redox properties of the transient species formed on reaction with •OH, •H, and O•− have been evaluated.

Evidence for the Role of Electron-Withdrawing Power of Functional Groups and [H+] for Electron-Transfer Reaction in Substituted Alkyl Sulfides

The pulse radiolysis technique has been employed to demonstrate the effects of the electron-withdrawing power of functional groups and the H+ concentration on the nature of •OH radical reaction with substituted alkyl sulfides. The intermediate OH adduct and α-thioalkyl radical could be detected in substituted sulfides having a functional group of high electron-withdrawing power. The concentration of H+ required for the formation of solute radical cations appears to correlate with the electron-withdrawing power of the functional group. The reactivity of eaq- toward dialkyl sulfides increases upon the introduction of strongly electron-withdrawing groups which effectively reduce the electron density at sulfur. The transient absorption band (λmax = 310 nm) observed from the reaction of •OH radicals with methyl thioacetic acid (MTA) is assigned to the α-thioalkyl radical formed via an intermediate OH adduct. In highly acidic solutions ([HClO4] ≥ 3 mol dm-3), •OH radicals are able to react with MTA to form dimer radical cations (λmax = 490 nm). The specific one-electron oxidants (Cl2•-, Br2•-, and SO4•-) undergo electron-transfer reaction with the solute; however, the transient absorption band of the dimer radical cation at 490 nm could not be observed, which may be due to unstable nature of the transient species in neutral and slightly acidic solutions. The oxidation potential is determined to be 1.56 V. The decay kinetics of the solute dimer radical cation is discussed in detail, and deprotonation of the solute radical cation is found to be the rate-determining step. The stability constant for the dimer radical cation has been determined to be 10 dm3 mol-1 at 25 °C. The transient species (λmax = 390 nm, k = 3.3 × 109 dm3 mol-1 s-1), formed from the reaction of Br• atom with the solute, is assigned to a three-electron-bonded Br• adduct.

Formation, excited state and electron transfer properties of the phenoxyl radical of 4,4′-thiodiphenol

The transient absorption bands ( λ max = 335, 500, 650–800 nm) formed on pulse radiolysis of N 2O-saturated basic aqueous solution of 4,4′-thiodiphenol (TDPH) are assigned to phenoxyl radical (TDP ⋅) formed on one-electron oxidation of phenoxide ion (TDP −) of TDPH. Specific one-electron oxidants (N ⋅ 3, I ⋅− 2, Br ⋅, CCl 3OO ⋅) also produced phenoxyl radicals with similar transient absorption spectrum. Phenoxyl radicals are able to undergo electron transfer reaction with N, N, N′, N′-tetramethyl-p-phenylenediamine (TMPD) with a bimolecular rate constant of 4.5 × 10 9 dm 3 mol −1 s −1. Pulse radiolysis of N 2-saturated TDPH in benzene showed absorption bands at 330, 500, 650–800 nm. The second-order decay of these bands was not affected by oxygen and anthracene, well-known triplet quenchers and the transient spectrum is assigned to phenoxyl radicals formed on fast decay of solute triplets with a G value of 0.29. Benzophenone triplet is able to undergo energy transfer with TDPH with a bimolecular rate constant of 6 × 10 9 dm 3 mol −1 s −1 and formed benzophenone ketyl radical ( λ = 545 nm) and phenoxyl radical (TDP ⋅, λ = 500 nm) on H ⋅ atom transfer from TDPH to benzophenone in an intermediate complex. The G value of phenoxyl radicals was determined to be 1.44. Picosecond laser flash photolysis of TDPH in acetonitrile failed to form any transient absorption in 450–900 nm region, supporting that only a small fraction of TDPH triplet decay to phenoxyls. Pulse radiolysis and picosecond laser flash photolysis studies indicate that the triplets of TDPH are highly unstable. The oxidation potential of TDP −/ TDP ⋅, H + couple is estimated to be between 0.265 and 1.03 V.

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

Investigations on the nature and redox properties of the transients formed on pulse radiolysis of aqueous solutions of 2-(phenylthio)ethanol

The radical cation of 2-(phenylthio)ethanol (PTE)+, generated on reaction of specific one-electron oxidants Cl2-, Tl2+, SO4-, CCl3OO, Br2-, by pulse radiolysis in aqueous solutions of PTE exhibits absorption bands at 315 and 530 nm. Pulse radiolysis of PTE in 1,2-dichloroethane also produced a similar transient absorption spectrum. The hydroxyl radicals are observed to react with a bimolecular rate constant of 7.5×109 d mol-1 s-1 and form absorption bands at 300, 365 and 530 nm. While the 530 nm band decayed by first order kinetics with k=2.1×104 s-1, other bands showed mixed kinetics. O- reacts exclusively by H atom abstraction forming a transient absorption band in 290–330 nm region, H reacts both by H abstraction and H-adduct formation. Based on these studies, OH radicals are inferred to react by electron transfer, H abstraction and OH-adduct formation. The radical cation reacts with electron donors, I-, N3-, with a high rate constant value. In neutral solutions, eaq- reacts with a bimolecular rate constant of 7.1×108 d mol-1 s-1 and the transient absorption band at 360 nm (ε=1.4×103 d mol-1 cm-1) is assigned to H-adduct formed on protonation of radical anion (pKa=7.9). In basic solutions, the radical anion has very small absorption at 360 nm (ε=0.6×103 d mol-1 cm-1). The radical anion formed on reaction of eaq- with PTE at pH 12 is able to transfer an electron to MV2+ whereas at neutral pH, electron transfer is not possible. The reduction potential for the PTE/PTE- couple is determined to be -1.23 V.

Physico-Chemical Properties and Antioxidant Activities of Methoxy Phenols

A few structurally related phenols, dehydrozingerone (DZ), bromopentenone (BP), eugenol (EG) and isoeugenol (IEG), derived from plant products show antioxidant properties by inhibiting lipid peroxidation in membrane models. The phenoxyl radicals produced by the scavenging of free radicals during the inhibition of lipid peroxidation, were also generated by specific one-electron oxidants using pulse radiolysis. The radical lifetimes (second order rate constants for radical-radical reactions), reactivities with hydroxyl and model peroxyl radicals and the one-electron reduction potentials with respect to the standard couples were quantified. These results along with their lipophilicity data were correlated with their antioxidant activity (IC 50 values).

Spectral, Kinetic, and Redox Studies on the Transients Formed on Reaction of Hydroxyl Radicals with Thioanisole

The hydroxyl radicals are observed to react with thioanisole (TA) with a bimolecular rate constant of 3.5 × 109 dm3 mol-1 s-1 and form transient optical absorption bands with λmax = 310, 365, 530, and 740 nm. The transient absorption bands at 310, 530, and 740 nm are also formed on reaction of specific one-electron oxidants such as CCl3OO•, Br2•-, and Cl2•- with thioanisole and are assigned to solute radical cations with positive charge on the benzene ring. Based on the studies with specific one-electron oxidants, ≈73% of •OH radicals are inferred to react with thioanisole by electron transfer and 27% by OH-adduct formation. The oxidation potential for the TA/TA•+ couple, based on the equilibrium established with the Br2•- / 2Br- couple, is determined to be 1.53 V vs NHE. The OH-adduct (λmax = 365 nm) is observed to undergo acid-catalyzed dehydration and form a solute radical cation. A small fraction of it is converted to sulfur-centered dimer radical cations and the major fraction into the monomer radical cations with the positive charge on the benzene ring. The monomer radical cations, formed in neutral aqueous solutions, are a strong one-electron oxidant and are able to oxidize I- with a bimolecular rate constant value of 6.4 × 109 dm3 mol-1 s-1.

Reaction of H, OH,O-and specific one-electron oxidants with 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(1,8) (2H,5H)-acridinedione Ground- and excited-state and transient acid[ndash ]base properties

Acridinedione dye is able to exist in neutral, protonated and deprotonated forms in acid–base conditions. The neutral form shows an absorption at 400 nm and emits at 463 nm. The absorption under acidic conditions is blue shifted and the emission intensity at 463 nm decreases with the formation of a new emission peak. In basic solution, the dye is deprotonated and absorbs at 470 nm and emits at 525 nm. The ground- and excited-state pKa have been calculated. The pulse radiolysis experiments were carried out with specific one-electron oxidants, such as Br2˙−, N3˙, Cl2˙−, I2˙−. They show transient absorption bands at 305 and 630 nm. Pulse radiolysis of acidic solutions of the dye show transient bands with absorption maxima at 315 and 545 nm and are assigned to a solute radical cation, which deprotonates in neutral solutions. The reaction of ˙H, ˙OH and O˙− with the dye has been studied and the transient absorption bands in the neutral solution are assigned to a neutral carbon centred radical, which deprotonates in basic solution. The pKa values of the transient species are determined. The redox potential for the one-electron oxidation of the dye is determined as 1.01 ± 0.02 V vs. NHE which compares well with the value obtained by an electrochemical method.