Temperature-dependent free radical reaction in water

Abstract

Temperature-dependent free radical reactions were investigated using nitroxyl radicals as redox probes. Reactions of two types of nitroxyl radicals, TEMPOL (4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl) and carbamoyl-PROXYL (3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl), were tested in this paper. Heating a solution containing a nitroxyl radical and a reduced form of glutathione (GSH) caused temperature-dependent decay of electron paramagnetic resonance (EPR) signal of the nitroxyl radical. Heating a solution of the corresponding hydroxylamine form of the nitroxyl radical showed EPR signal recovery. The GSH-dependent reduction of nitroxyl radicals at 70°C was suppressed by antioxidants, spin trapping agents, and/or bubbling N2 gas, although heating carbamoyl-PROXYL with GSH showed temporarily enhanced signal decay by bubbling N2 gas. Since SOD could restrict the GSH-dependent EPR signal decay of TEMPOL, O2•− is related with this reaction. O2•− was probably generated from dissolved oxygen in the reaction mixture. Oxidation of the hydroxylamines at 70°C was also suppressed by bubbling N2 gas. Heating a solution of spin trapping agent, DMPO (5,5-dimethyl-1-pyrroline-N-oxide) showed a temperature-dependent increase of the EPR signal of the hydroxyl radical adduct of DMPO. Synthesis of hydroxyl radical adduct of DMPO at 70°C was suppressed by antioxidants and/or bubbling N2 gas. The results suggested that heating an aqueous solution containing oxygen can generate O2•−.

Amplification of glutathione-mediated oxidative stress by catalase in an aqueous solution at hyperthermal temperatures

“…The oxoamonium cation form of TEMPOL then could react with GS • to make a stable non-paramagnetic complex and as a result TEMPOL was two-electron-reduced. (13) On the other hand, NBT can be directly reduced by O 2 •− to give blue formazan dye. Strong reductants, such as ascorbic acid, can also reduce NBT and give the blue formazan (data not shown).…”

“…(13,14) Incubating a reaction mixture containing 4-hydroxyl-2,2,6,6-tetramethylpiperidine- N -oxyl (TEMPOL) and GSH at 44°C, the TEMPOL shows a steep reduction following a short time delay. This characteristic TEMPOL reduction was halted by adding superoxide dismutase (SOD) to the reaction mixture or bubbling N 2 gas through the reaction mixture.…”

Method for assessing X-ray-induced hydroxyl radical-scavenging activity of biological compounds/materials

“…Since DMPO-OH is a nitroxyl radical, the method may be disrupted by hydrogen donors, such as NAD(P)H, and/or some thiol compounds, such as GSH. (19,20) Applying other detection methods, such as luminescence probes, may make the method available for thiol compounds and other inapplicable cases.…”

Evaluation of Antioxidative Effects of Sesamin on the In vivo Hepatic Reducing Abilities by a Radiofrequency ESR Method

“…TEMPOL, which is in the nitroxyl group (>N-O·), appears to be quickly converted into hydroxylamine (>N-OH) and a non-paramagnetic compound through one-electron reductions and other reactions that occur within biological systems. 26,27 In our study, we found there was a rapid decrease of the signal intensity of TEMPOL after the reduction of TEMPOL to hydroxylamine. We confirmed that the decay rate of TEMPOL follows pseudo-first-order kinetics, meaning that the initial phase of the semi-logarithmic plots of the signal intensity against time will give a straight line (R > 0.995).…”