lntramolecularly formed S : . N three-electron-bonded radical cations have been identified as transient intermediates in the 'OH radical-induced oxidation of 3-(methylthio)propylamine, methionine, and methionine ethyl ester. A similarly bonded radical is also indicated in the reduction of dehydromethionine by hydrated electrons. Intramolecular S : . 0-bonded intermediates are suggested to be formed in the'OHinduced oxidation of 4-(methylthio) butyric acid and S-methylcysteine. All these three-electron-bonded species exhibit optical absorptions with maxima in the 380-400 nm range. The observable S:.Obonded transient from S-methylcysteine is presumed to be protonated at oxygen. Favourable steric structures, particularly five-membered rings, significantly increase the probability of formation and the overall stability of a particular three-electron bond. Under comparable conditions, i.e. structure, substituents, consecutive chemistry, etc., the S : .
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.
Time-resolved pulse radiolysis studies have been carried out on the RzS+. + R2S (R2S:.SR2)+ equilibrium and associated decay kinetics of sulfur centered radical cations. -Equilibrium constants of K = 2.0. 10' and 5.4.10' mol-' dm3 at 20°C have been determined for the species with R = CH3 and (CH,)'CH, respectively. Absolute rate constants have been measured for the back reaction of the above equilibrium: k p l = 1.5. lo4 s-' and 5.6. lo6 s-', respectively, at 20°C. Further rate constants refer to the deprotonation of RzS+. by various proton acceptors (phosphate, acetate and water). -AH * = 57 kJ mol-' and 17 kJ mol-' were determined for the dissociation of the sulfur-sulfur 3-electron bond in (MezS.'.SMe2)+ and (i-PrzS.'.S i-Prz)+, respectively. On this basis the S:.S bond energies were estimated to 5 8 0 kJ mol-' and 5 40 kJ mol-' for these two sDecies. The kinetic and thermodynamic results substantiate Drevious conclusions on the electronic structure and relative stability of S.
Absolute rate constants for the reaction of the thiyl radicals derived from cysteamine, cysteine, glutathione, penicillamine, and 2-mercaptopropionylglycine (Thiola) with the phenothiazines, promethazine and chlorpromazine and with ascorbate ion, have been measured by pulse radiolysis and have been found to lie in the range 0.03-1.3 x 109 I mol-I s-I. The reactions are considered to occur by an electron transfer mechanism. The possibility that, in biochemical systems, glutathione can act as a catalyst linking hydrogen atom and electron transfer reactions is suggested.Phenothiazines are widely used in psychiatric therapy. However, in spite of their considerable success, they may present several undesirable side effects, such as skin photosensitisation and visual disturbance.'e2 In vitro studies have shown that the radical-cation formed by one-electron oxidation of chlorpromazine (CZ") is a potent inhibitor of the enzymes ATPase and uridine diphosphate glucose NAD + oxido-red~ctase.~'~
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