Marija Bonifačić

Formation of positive ions and other primary species in the oxidation of sulphides by hydroxyl radicals

The oxidation of simple aliphatic sulphides [MeSMe, EtSEt, (CH2)4S] by hydroxyl radicals occurs via a complex reaction mechanism. The first step is addition of the OH· to sulphur to form R2ṠOH radicals. At low sulphide concentrations ( 1 ms) positive ions. The stable oxidation product, sulphoxide, has been identified.

DETERMINATION OF ABSOLUTE RATE CONSTANTS FOR THE REVERSIBLE HYDROGEN-ATOM TRANSFER BETWEEN THIYL RADICALS AND ALCOHOLS OR ETHERS

Absolute rate constants have been determined for the reversible hydrogen-transfer process R˙+ RSH ⇄ RH + RS˙ by pulse radiolysis, mainly through direct observation of the RS˙ radical formation kinetics in water–RH (1 : 1, v/v) mixtures. The thiols investigated were penicillamine and glutathione; the RH hydrogen donors were methanol, ethanol, propan-1-ol, propan-2-ol, ethylene glycol, tetrahydrofuran and 1,4-dioxane with the abstracted hydrogen being located α to the hydroxy or alkoxy function. Rate constants for the forward reaction of the above equilibrium (in radiation biology referred to as ‘repair’ reaction) were typically of the order of 107– 108 dm3 mol–1 s–1 while hydrogen abstraction from RH by thiyl radicals (reverse process) occurred with rate constants of the order of 103– 104 dm3 mol–1 s–1. This yields equilibrium constants of the order of 104. Based on these data, standard reduction potentials could be evaluated for the R′R″C˙OH/H+//R′R″CHOH, R′R″CO/H+//R′R″C˙(OH) and R′R″CO//R′R′C˙O– couples from methanol, ethanol and propan-2-ol. Effective hydrogen-atom abstraction by RS˙ required activation by neighbouring groups of the C—H bond to be cleaved in RH. No such process was observed for the RS˙ reaction with —CH3 groups, e.g. in 2-methylpropan-2-ol. Several halogenated hydrocarbons, including some anaesthetics (e.g. halothane) and Fe(CN)63– have been tested with respect to their ability to disturb the (CH3)2C˙OH + RSH ⇄(CH3)2CHOH + RS˙ equilibrium through an irreversible electron-transfer reaction with the reducing α-hydroxyl radical, thereby drawing the equilibrium to the lefthand side. The respective efficiencies are found to be related to the electronegativities of the electron acceptors. The results are briefly discussed in terms of their biological relevance.

Stabilization of oxidized sulphur centres by halide ions. Formation and properties of R2S∴X radicals in aqueous solutions

In the presence of halide ions, stabilization of oxidized sulphur centres in organic sulphides can occur via formation of three electron S∴X bonds to yield R2S∴X radicals. An equilibrium is found to exist between these species, X2–˙ radical anions, and (R2S)2+˙ radical cations. Equilibrium constants for the processes X2–˙+ R2S ⇌ R2S∴X + X–, R2S∴X + R2S ⇌(R2S)2+˙+ X–, R2S∴X ⇌ R2S+˙+ X–, and R2S∴X ⇌ R2S + X˙ have been determined, together with the optical properties of R2S∴X, where R = Me and Et, and X = Cl, Br, and I. The absolute stability of the S∴X bond increases as the difference in electronegativity between S and X decreases and is thus highest for R2S∴I. In addition, rate constants for formation and decay processes of R2S∴X are reported.

One-electron reduction of selenomethionine oxide

Experimental evidence is provided that selenomethionine oxide (MetSeO) is more readily reducible than its sulfur analogue, methionine sulfoxide (MetSO). Pulse radiolysis experiments reveal an efficient reaction of MetSeO with one-electron reductants, such as e-aq (k = 1.2 × 1010M-1s-1), CO·-2 (k = 5.9 × 108 M-1s-1) and (CH3)2) C·OH (k = 3.5 × 107M-1s-1), forming an intermediate selenium-nitrogen coupled zwitterionic radical with the positive charge at an intramolecularly formed Se∴ N 2σ/1σ* three-electron bond, which is characterized by an optical absorption with λmax at 375 nm, and a half-life of about 70 μs. The same transient is generated upon HO· radical-induced one-electron oxidation of selenomethionine (MetSe). This radical thus constitutes the redox intermediate between the two oxidation states, MetSeO and MetSe. Time-resolved optical data further indicate sulfur-selenium interactions between the Se∴ N transient and GSH. The Se∴ N transient appears to play a key role in the reduction of selenomethionine oxide by glutathione.

One-electron redox potentials of RSSR+˙–RSSR couples from dimethyl disulphide and lipoic acid

One-electron redox potentials have been measured for three (RSSR)+˙/RSSR couples by reference to (SCN)2–˙/2 SCN– and/or I2–˙/2 I– in pulse radiolysis experiments: E°(CH3SSCH3+˙/CH3SSCH3)+(1.391 ± 0.003) V; E°[lip(SS)COOH+˙/lip(SS)COOH]+(1.13 ± 0.01) V; and E°[lip(SS)COO–+˙/lip(SS)COO–]+(1.10 ± 0.01) V [lip(SS)= lipoic acid]. The paper also includes equilibrium constants for the underlying RSSR + X2–˙⇄ RSSR+˙+ 2 X– equilibria (X = SCN or I), and rate constants for the respective back and/or forward reactions. The results are discussed in the light of structural considerations and in relation to other redox couples involving sulphur-centred radical species.

Reaction of a stable N ∴ N bonded radical cation with free radicals generated by pulse radiolysis: exceedingly rapid hydrogen abstraction from C–H bonds

Absolute rate constants have been measured for the reaction of the N ∴ N three-electron bonded 1,6-diazabicyclo[4.4.4]dodecane radical cation ([4.4.4]+˙) with various free radicals produced by means of pulse radiolysis. Reduction and oxidation reactions occur with rate constants generally somewhat below the diffusion limit. This is considered to reflect the inwardly oriented structure of the [4.4.4]+˙. High rate constants (ca. 109 mol–1 dm3 s–1) have been measured for hydrogen-atom abstraction from [4.4.4]+˙ by almost all radicals except eeq–. The most remarkable of these reactions appears to be H-atom abstraction by a thiyl radical [(CH3)3CS˙], which occurs with k 3.2 × 109 mol–1 dm3 s–1. This indicates highly labile C–H bonds in [4.4.4]+˙, which are considered to be those located on CH2 groups α to the nitrogen atoms. The fast radical–radical H-atom transfer is considered to be energetically assisted by favourable stereoelectronics and least heavy atom motion.

On the hydrolysis of AgII, TlII, SnIII and CuIII

Using pulse radiolysis with optical, conductometric and polarographic detection it is shown that the oxidation of Tl+, Ag+, Cu2+ and Sn2+ by hydroxyl radicals proceeds via on OH adduct: Men++·OH → Me(OH)n+. Depending on pH, the OH adduct may react with a proton followed by water elimination, may add OH– or react with water followed by elimination of a proton. The pK values for the equilibria Me(n+1)+⇌ Me(OH)n+⇌ Me(OH)(n–1)+⇌… decrease with increasing state of oxidation of Me. The oxidative properties decrease with increasing state of hydrolysis.

Quantification of iodide oxidation by trichloromethyl peroxyl radicals and I− + I2 ⇌ I3− equilibrium in alcohol/water mixtures

The molar absorption coefficient (e) of I3− at λmax, and the equilibrium constant (Keq) for the equilibrium I2 + I− ⇌ I3− were determined for various alcohol/water mixtures by using standard analytical procedures, i.e. measurement of optical densities via UV spectrophotometry. The stability of I3− is found to increase significantly with increasing amount of alcohol with Keq ranging from 7.1 × 102 dm3 mol−1 in H2O to, e.g., 2.5 × 104 dm3 mol−1 in 70% (v/v) of 2-propanol/H2O mixture. Also, by increasing the amount of alcohol, the position of λmax for I3− is shifted to longer wavelengths (from 350 to 357 nm). This red-shift is accompanied by a drop in e from 25 400 dm3 mol−1 cm−1 in pure H2O to, e.g., 14 000 dm3 mol−1 cm−1 in a 70% (v/v) of a 2-propanol/H2O mixture. The obtained data were applied for quantitative determination of I3− by means of the time-resolved technique of pulse radiolysis, where this product was formed via oxidation of a total of three iodide ions by each CCl3OO˙. The overall process afforded formation of one equivalent each of I2 (complexed by I− to I3−) and I˙ (complexed to I2˙− and suffering as such disproportionation). An excellent agreement was obtained with the data from the standard analytical measurements. Furthermore, the radiation chemical yields of I3− generated in these pulse radiolysis experiments (e.g., G(I3−) = 0.96 μmol J−1 in 2-propanol/H2O mixtures) are in complete correspondence with a previously suggested multi-electron oxidation mechanism. Complementary steady-state product analysis of γ-irradiated solutions confirmed the formation of I3− in high yields, with limiting values of G ≅ 0.9 μmol J−1 in 2-propanol/water mixtures over wide ranges of pH, iodide, CCl4, CH2Cl2 and 2-propanol concentrations. All the results also strongly corroborate corresponding oxidation mechanisms of organic sulfides and selenides by halogenated peroxyl radicals.

Halogenated peroxyl radicals as multi-electron oxidants: pulse radiolysis study on the reaction of trichloromethyl peroxyl radicals with iodide

CCl3OO˙ radicals are shown to oxidize three equivalents of iodide ions to yield one I2 molecule (showing up as I3– in aqueous solution) and one I˙ atom (showing up as I2˙–) per peroxyl unit; the mechanism is considered to proceed via a protonated CCl3OO˙I– adduct radical as the primary intermediate and subsequent nucleophilic (SN2) attack by a second iodide accompanied by electron localization in the hydroperoxide moiety leads to I2 and CCl3O˙ with the latter being responsible for the oxidation of the third iodide.

On the equilibrium between thallium(II) hydroxide and hydroxothallium(II) ion: a pulse-radiolysis study

The oxidation of Tl+ by OH˙ radicals leads to an addition product [Tl(OH)]+ which exists in the equilibria [Tl(OH)]+⇌ Tl2++[OH]– and Tl[OH]2⇌[Tl(OH)]++[OH]–. The previously unrecorded pK of the latter equilibrium has been determined as 7.7 ± 0.2. The optical-absorption spectrum of Tl[OH]2 shows a maximum at 370 nm (Iµ 3.8 × 103 dm3 mol–1 cm–1) and a band rising towards the u.v. below 300 nm. Thallium(II) hydroxide, in contrast to Tl2+ and [Tl(OH)]+, does not exhibit good oxidizing properties. It may even transfer an electron to suitable acceptors such as tetrantromethane.