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.

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 = 1.2x10(10) M(-1) s(-1)), CO2*- (k = 5.9x10(8) M(-1) s(-1)) and (CH3)2 C*OH (k = 3.5x10(7) M(-1) s(-1)), forming an intermediate selenium-nitrogen coupled zwitterionic radical with the positive charge at an intramolecularly formed Se(three-electron bond)N 2sigma/1sigma* three-electron bond, which is characterized by an optical absorption with lambda(max) at 375 nm, and a half-life of about 70 micros. 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(three-electron bond)N transient and GSH. The Se(three-electron bond)N transient appears to play a key role in the reduction of selenomethionine oxide by glutathione.

One-electron oxidation of ergothioneine and analogues investigated by pulse radiolysis: redox reaction involving ergothioneine and vitamin C.

Redox reactions of endogenous and exogenous sulphur-containing compounds are involved in protection against oxidative damage arising from the incidence and/or treatment of many diseases, including cancer. We have investigated, via pulse radiolysis, the one-electron oxidation of ergothioneine, a molecule with antioxidant properties which is detected at millimolar concentrations in certain tissues and fluids subject to oxidative stress, including erythrocytes and plasma. The spectrum of the transient species, assigned to the product of one-electron oxidation, observed after reaction of ergothioneine with the oxidizing radicals OH., N3. and CCl3O2. has a maximum absorption at 520 nm and is very similar to that obtained by oxidation of analogous molecules such as 2-mercaptoimidazole, 1-methyl-2-mercaptoimidazole, S-methyl- and S,N-dimethyl-ergothioneine. In the presence of vitamin C, the oxidized form of ergothioneine is repaired by a rapid reduction (k = 6.3 x 10(8) M(-1).s(-1)) producing ascorbyl radicals. This co-operative interaction between ergothionine and ascorbate, similar to that previously observed between vitamin E and ascorbate, may contribute to essential biological redox protection.

Free-radical repair by a novel perthiol: reversible hydrogen transfer and perthiyl radical formation.

2-(3-Aminopropyl-amino) ethaneperthiol (RSSH, the perthiol analogue of the thiol radioprotector, WR-1065) reacts with the alpha-hydroxy alkyl radical (CH3)2C.OH by donating a hydrogen atom as indicated by the characterization of perthiyl radicals (RSS.; lambda max approximately 374 nm, epsilon 374 approximately 1680 +/- 20 dm3 mol-1 cm-1) by pulse radiolysis. The perthiyl radical abstracts a hydrogen from the alcohol to establish a reversible hydrogen-transfer equilibrium. This equilibrium lies predominantly on the side of radical repair since the rate constants for the forward and reverse reactions at pH 4 are: kappa(RSSH+(CH3)2C.OH) = (2.4 +/- 0.1) x 10(9) dm3 mol-1 s-1 and kappa(RSS.+(CH3)2CHOH) = (3.8 +/- 0.3) x 10(3) dm3 mol-1 s-1 respectively. The pKa (RSSH<-->RSS(-)+H+) = 6.2 +/- 0.1 was determined from the pH dependence of the rate of perthiol repair. Identical experiments have been performed with WR-1065 allowing a direct comparison of free-radical repair reactivity to be made with the parthiol analogue. At pH approximately 7.4 the reactivities of the thiol and perthiol were similar, both repairing the alcohol radical with a rate constant of approximately (2.4 +/- 0.1) x 10(8) dm3 mol-1 s-1. However, at pH 5 whilst the hydrogen-donation rate of the thiol was 15-20% higher than at pH 7.4, the perthiol reactivity was over an order of magnitude higher. The thermodynamic driving force for the observed enhanced free-radical repair reactivity of RSSH compared to RSH is attributed to the resonance stabilization energy of 8.8 kJ mol-1 within the RSS. radical. These results indicate a possible application of RSSH/RSS- as DNA-targeted antioxidants or chemoprotectors.

Free radical reductive degradation of vic-dibromoalkanes and reaction of bromine atoms with polyunsaturated fatty acids: possible involvement of Br(.) in the 1,2-dibromoethane-induced lipid peroxidation.

Bromine atoms generated upon reductive degradation of 1,2-dibromoethane and various other vic-dibromoalkanes have been shown to react with polyunsaturated fatty acids via both abstraction of bisallylic hydrogen and addition to the double bonds. The abstraction process occurs with 53, 68, and 77% efficiency from linoleic (18:2), linolenic (18:3), and arachidonic acid (20:4), respectively. The corresponding absolute rate constants have been evaluated to be 1.2 x 10(9), 1.8 x 10(9), and 5.1 x 10(9) M-1 s-1 for these three polyunsaturated fatty acids (PUFAs). The rate constants for the bromine atom addition reaction appear to be very similar for all PUFAs and have been determined to (1.2 +/- 0.3) x 10(9) M-1 s-1. Absolute rate constants in the order of 10(5)-10(6) s-1 have also been measured for the bromine atom elimination from various beta-bromoalkyl radicals. The results render bromine atoms a potential initiator for lipid peroxidation, and their reactions may well provide an important chemical basis for the overall toxic action of 1,2-dibromoethane and related substrates.

Oxidation of polyunsaturated fatty acids and lipids through thiyl and sulfonyl radicals: reaction kinetics, and influence of oxygen and structure of thiyl radicals.

Thiyl free radicals have been shown to react with polyunsaturated fatty acids via abstraction of bisallylic hydrogen, forming pentadienyl radicals, and via addition to the double bonds. In the absence of oxygen, the latter pathway leads to regeneration of thiyl radicals through beta-elimination or "repair" of the adduct radicals by thiols. In the presence of oxygen, fixation of thiyl-induced damage occurs through reaction of O2 with the pentadienyl radical (yielding conjugated dienyl peroxyl radicals) and also with the thiyl-to-double bond adduct radical. A quantitative reaction scheme evaluated from these data considers abstraction, addition, rearrangement, and repair reactions, and the evaluation of rate constants for the individual steps. Absolute rate constants have been measured, in particular, for reactions of thiyl free radicals from glutathione, cysteine, homocysteine, N-acetylcysteine, cysteine ethyl ester, penicillamine, captopril, mercaptoethanol, and dithiothreitol with polyunsaturated fatty acids (PUFAs) ranging from 18:2 to 22:6, and the lipids trilinolein and trilinolenin. The rate constants for hydrogen abstraction were found to be typically of the order of 10(7) mol-1 dm3 s-1 and to increase with increasing lipophilicity of the attacking thiyl radical. Thioperoxyl radicals, RSOO., were found to be rather unreactive toward PUFAs, in contrast to the isomer sulfonyl radicals, RSO2., which not only abstract hydrogen from the bisallylic methylene groups of the PUFAs (although only at relatively small yield) but also readily add to the PUFA double bonds (major pathway). Specific information was obtained on the optical properties of the thiyl radical derived from the ACE inhibitor captopril, CpS. (lambda max = 340 nm, epsilon = 460 +/- 50 mol-1 dm3 cm-1), and its conjugate disulfide radical anion (CpS:.SCp) (lambda max = 420 nm).

Reactivity of ebselen and related selenoorganic compounds with 1,2-dichloroethane radical cations and halogenated peroxyl radicals.

The reactivity of ebselen, 2-phenyl-1,2-benzisoselenazol-3(2H)one, and structurally related analogues was studied by pulse radiolysis. The rate constant for the reaction of ebselen with trichloromethylperoxyl radicals was determined to be 2.9 X 10(8) M-1 s-1, while its sulfur analogue, 2-phenyl-1,2-benzisothiazol-3(2H)one, was oxidized at much lower rates, k less than or equal to 10(7) M-1 s-1. Among several derivatives studied, the only other compound that exhibited a high rate constant was 2-(methylseleno)-benzoic acid-N-phenylamide. Oxidation of ebselen by other halogenated peroxyl radicals was also carried out and revealed a direct relationship between rate constant and the degree of halogenation of the oxidant. The transient radicals generated during oxidation of ebselen and the analogues were characterized by optical absorption and conductivity measurements and were attributed to one-electron-oxidized radical cations. The oxidation potentials were determined by cyclic voltammetry. Comparative evaluation of the in vitro behavior during microsomal lipid peroxidation revealed ebselen to be the most potent antioxidant of the compounds investigated, 2-(Methylseleno)-benzoic acid-N-phenylamide, despite its high rate constant for oxidation by halogenated peroxyl radicals, was found to be a poor antioxidant. The rate constant of oxidation of ebselen by trichloromethylperoxyl radicals is comparable to that of alpha-tocopherol under similar conditions, underscoring the potential pharmacological interest of ebselen as an antioxidant.

Reaction of thiyl radicals with alcohols, ethers and polyunsaturated fatty acids: a possible role of thiyl free radicals in thiol mutagenesis?

Possible reactions of thiyl free radicals in biological environment are reviewed. In particular hydrogen transfer processes from model C-H compounds like alcohols and ethers as well as from polyunsaturated fatty acids to thiyl radicals are described to proceed with reasonably high rate constants (10(3)-10(4) and 10(6)-10(7) M-1 s-1, respectively). Thiyl radicals have thus to be considered as potentially hazardous species especially with respect to DNA damage and lipid peroxidation.

Thiyl radical attack on polyunsaturated fatty acids: a possible route to lipid peroxidation.

Absolute rate constants have been measured for the reaction of cysteinyl free radicals, CysS., with linoleic (18:2), linolenic (18:3) and arachidonic acid (20:4) in water/alcohol mixtures using the radiation chemical technique of pulse radiolysis. They are in the order of 10(6)-10(7) M-1 s-1 and increase with the number of biallylic functions, and with the polarity of the solvent. The reaction is shown to be a hydrogen atom abstraction from biallylic C-H bonds and yields pentadienyl radicals. The thiol mediated repair of the latter is considerably slower. Thiyl free radicals must consequently be considered as a potential source of lipid peroxidation.

Reversible H-atom abstraction from alcohols by thiyl radicals: determination of absolute rate constants by pulse radiolysis.

Penicillamine thiyl radicals, PenS., are shown to abstract hydrogen atoms from 2-propanol and to establish an equilibrium PenS. + (CH3)2 CHOH in equilibrium PenSH + (CH3)2 COH. The rate constants for the forward and back reaction have been determined to (1.4 +/- 0.3) x 10(4) and (1.2 +/- 0.3) x 10(8) M-1 s-1, respectively, by means of pulse radiolysis. The data have been obtained from various independent methods which include direct measurements and competitive schemes involving irreversible interception of the alcohol radical by electron acceptors (e.g. CCl4, PNAP) and/or the thiyl radical by antioxidants (e.g. alpha-tocopherol). The results demonstrate that the reaction of carbon-centered radicals with thiols, in radiation biology commonly known as "repair" reaction, may be reversed and thus imply the possibility of thiyl radical induced biological damage.

Rate constants for the reactions of halogenated organic radicals.

Absolute rate constants have been measured by means of pulse radiolysis for the reactions of various halogenated aliphatic compounds (ethane derivatives, including the anaesthetics halothane, enflurane, isoflurane and methoxyflurane) with hydrated electrons and .OH radicals, the reactions of halogenated carbon-centered radicals, derived thereby, with molecular oxygen, and the reactions of halogenated peroxyl radicals with various antioxidants (ascorbate, chlorpromazine, promethazine, propyl gallate, ABTS) in aqueous solutions. All oxygen addition reactions occur essentially diffusion-controlled. This finding is correlated with the stereoelectronic properties of the primary carbon-centred radicals. The oxidative power of the halogenated peroxyl radicals reflects the inductive -I effect of the halogens and accordingly increases with the degree of halogen substitution, with fluorine substituents being particularly effective. The peroxyl radicals derived from freon 113, namely CClF2CClFOO. and CCl2FCF2OO., have been identified as the best oxidants among these species.

On the reaction of molecular oxygen with thiyl radicals: a re-examination.

Absolute rate constants for the addition of oxygen to thiyl radicals, i.e. RS. + O2----RSOO., have been determined by applying a new competition method based on RS. formation via one-electron reduction of the corresponding disulphides, and the competition between RS. reacting with O2 and an electron donor such as ascorbate. Bimolecular rate constants have been obtained for the thiyl radicals derived from cysteine (6.1 X 10(7) mol-1 dm3 s-1), penicillamine (2.5 X 10(7) mol-1 dm3 s-1), homocysteine (8.0 X 10(7) mol-1 dm3 s-1), cysteamine (2.8 X 10(7) mol-1 dm3 s-1), 3-thiopropionic acid (2.2 X 10(8) mol-1 dm3 s-1) and glutathione (3.0 X 10(7) mol-1 dm3 s-1), respectively. The values obtained for the O2 addition to the thiyl radicals from glutathione and cysteine are considerable lower (by about two orders of magnitude) than those previously published. This indicates that the RS. + O2 reaction may be of complex nature and is generally a process which is not solely controlled by the diffusion of the reactants.

Free radical induced degradation of 1,2-dibromoethane. Generation of free Br. atoms.

Free Br. atoms have been found to be generated upon eaq- and .OH radical induced degradation of 1,2-dibromoethane in aqueous solution. The relevant process is beta-bromine cleavage from CH2BrCH2 and CH2BrCHBr radicals, respectively. An absolute rate constant of k = 2.8 x 10(6) s-1 has been determined for the reaction CH2BrCH2----Br. + CH2 = CH2, while an estimate of k approximately 10(6) s-1 can be given for CH2BrCHBr----Br. + CH2 = CHBr. The Br. atoms have been identified through their reaction with Br- to Br2-. (k = 7.7 x 10(9) mol-1dm3s-1) and their reaction with 2,2'-azinobis (3-ethylbenzthiazoline-6-sulphonate) (k = 6.8 x 10(9) mol-1dm3s-1). Ethylene and vinyl bromide have been identified via GC. The results substantiate earlier findings that free radical induced mechanisms can serve as informative probe for metabolic processes.

Radical reactions in aqueous disulphide-thiol systems.

Absolute rate constants have been measured for the reaction of (CH3SSCH3)+. and sulphur centred radical cations of lipoic acid, lip (SS)+., with various thiols including penicillamine, cysteamine and cysteine. Under pulse radiolysis conditions no reactions was observed between the disulphide radical cations and the neutral thiols, RSH, i.e. kappa less than or equal to 10(7) M-1 s-1. Rate constants in the order of 10(9) M-1 s-1, i.e. close to the diffusion controlled limit, were, however, found for the corresponding reactions with the thiolates, RS-. In systems containing lipoate and cysteamine the lip (S therefore S)+. induced oxidation of CyaS- proceeds via CyaS., (CyaS therefore SCya)- and lip (S S)- as intermediates, i.e. results in a cysteamine mediated conversion of an oxidizing lip (S therefore S)+. radical cation to a reducing lip (S S)- radical anion along the reaction route. In other cases the reaction of disulphide radical cations with thiolate anions was found to proceed via an optically absorbing transient (lambda max approximately 380 nm) which is suggested to be an adduct radical. The mechanism of the (RSSR)+. induced oxidation of thiolate appears to depend on the stability of the 3-electron bonded disulphide radical anion.

One electron reduction of CCl4 in oxygenated aqueous solutions: a CCl3O2-free radical mediated formation of Cl- and CO2.

Product yields have been determined after one-electron-induced reduction of CCl4 in aqueous solutions containing t-butanol and various concentrations of oxygen. It was shown that CCl3 radicals add oxygen to form CCl3O2 radicals, which eventually yield three chloride ions and CO2. A constant ratio of G(Cl-)/G(CO2) = 4 is found in solutions containing 1.5 X 10(-4) M or more oxygen. Competing reactions of the CCl3 radical increase this ratio at lower oxygen concentrations. The rate constant for the oxygen addition to CCl3 radicals was determined by pulse radiolysis to 3.3 X 10(9) M-1 s-1. Possible reaction mechanisms leading to the observed end products are discussed.

One-electron reduction of halothane and formation of halide ions in aqueous solutions.

Formation of Br- and, under certain conditions also F- ions has been observed in the radiation chemically induced one-electron reduction of the anesthetic halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) in aqueous solutions. The initial step is the release of Br- and formation of the 2-chloro-1,1,1-trifluoroethyl radical. The latter can react via competing pathways including H-atom abstraction, addition of molecular oxygen and further reduction by an antioxidant. All of these three competitive routes lead to different product patterns. High yields of F- ions are observed under anaerobic conditions in the presence of antioxidants such as ascorbate, propyl-gallate, etc. The fluoride elimination is strongly pH-dependent and seems to occur in various steps after initiation through reduction of the (CF3CHCl) radical. The implication for biochemical studies on the metabolism of halothane under different oxygen concentrations is discussed.

Oxidation of methionine by X2.- in aqueous solution and characterization of some S therefore X three-electron bonded intermediates. A pulse radiolysis study.

The primary steps of the oxidation of methionine, Met, by X2.- (X = Cl; Br, I, SCN) have been investigated using pulse radiolysis techniques. In principle, the mechanism follows the same pattern which has been established for the OH radical induced oxidation. It is characterized by a primary attack at the sulphur atom and the formation of sulphur-centred radical cations S+. and S therefore (+) S as key intermediates. At pH greater than pKa of the carboxyl group these can then oxidize the amino function intramolecularly, which subsequently leads to irreversible decarboxylation. An additional important intermediate is a S therefore X radical with a three-electron bond between sulphur and halide. This radical is linked to the OH . radical induced mechanism through the equilibrium S therefore X + Met in equilibrium with S therefore (+) S + X-, and in addition exists in the equilibria X2.- + Met in equilibrium with S therefore X + X-, S therefore X in equilibrium with S+. + X- and S therefore X in equilibrium with Met + X.. The S therefore X- species absorb at 410, 400, and 390 nm for X = I, Br, and Cl, respectively. Absolute rate constants have been measured for the reactions S therefore (+) S + I- (k = 1.0 x 10(10) mol-1 ls-1, pH 1.4), Br2.- + Met (k = 2.5 x 10(9), 1.7 x 10(9), and 2.0 x 10(9) mol-1 ls-1 at pH 1, 5, and 11, respectively) and Cl2.- + Met (k = 3.9 x 10(9) mol-1 ls-1, pH 1). Methionine is also oxidized by (SCN)2.- whereas any significant oxidation by I2.- is not indicated. N-acetylmethionine, a model compound for a sulphur-containing peptide, and some other methionine derivatives are oxidized by X2.- in the same way, that is, through electrophilic addition at the sulphur function. The results require reinterpretation of some data published in the literature and are discussed in view of a 'selective free radical attack'.