The effect of acetylcholine counter-ions on the generation of radicals in the presence of H2O2 in an aqueous medium.

The reactions of acetylcholine halides with H2O2 were studied at room temperature in initially deionized water, i.e., at pH ∼7. Acetylcholine (ACh), which catalyzes the decomposition of hydroperoxides and H2O2 into radicals in an organic medium, virtually does not affect the decomposition of H2O2 in aqueous solutions. In the case of AChI, hydrogen peroxide oxidizes the iodide anion to form diiodine and hydroxyl radical HO•, which was identified using the 5,5-dimethylpyrrolidine N-oxide spin trap. The rate constant of this reaction was estimated from the initial sections of the kinetic curves for the consumption of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL): k1 = (2.0±0.1)•10−3 L mol−1 s−1. A nontrivial combination of radical reactions and redox disproportionation reactions occurs in the AChI—H2O2—TEMPOL system in initially deionized water. In this medium, H2O2, iodine compounds, TEMPOL, and the formed hydroperoxide radicals HO2• have a dual ability to act as oxidants or reducing agents, which results in quasi-stationary concentrations of I2 and TEMPOL being established in the process. The I− oxidation by peroxides in acid medium, known in iodometric titration, generates hydroxide radicals when conducted in neutral solutions (pH ∼7) and can compete with the Fenton reaction in systems in which the presence of metal ions is excluded. [ABSTRACT FROM AUTHOR]