title: Kinetics and mechanism of the comproportionation reaction between oxoammonium cation and hydroxylamine derived from cyclic nitroxides.
abstract: Cyclic nitroxides demonstrate antioxidative activity in numerous in vitro and in vivo models, which frequently involves the participation of the reduced and oxidized forms of the nitroxide, namely, the hydroxylamine and oxoammonium cation. Generally, cellular reducing equivalents facilitate rapid enzymatic as well as nonenzymatic reduction of nitroxides in the tissue. On the other hand, the reaction of nitroxides with various radicals yields the highly oxidizing oxoammonium cation, which mediates the catalytic effect of nitroxides in selective oxidation of alcohols. Hence, nitroxides might act as both anti- and pro-oxidants. Therefore, the comproportionation reaction between the oxoammonium cation and the hydroxylamine might play a role in lowering the pro-oxidative activity of nitroxides. Although the comproportionation reaction has previously been studied, there is no agreement regarding its kinetic features. We investigated the reaction of the reduced forms of 2,2,6,6-tetramethylpiperidinoxyl (TPO) and 4-OH-2,2,6,6-tetramethylpiperidinoxyl (4-OH-TPO) with the oxoammonium cation derived from TPO at various pHs using rapid-mixing stopped-flow and EPR spectrometry. From the pH dependence of the reaction rate constants we determined the pK(1) of the respective hydroxylamines to be 7.5 and 6.9, respectively. The reduction potentials of the hydroxylamines were determined by cyclic voltammetry, and from their dependence on pH, we obtained the same pK(1) values. The rate constant of the comproportionation reaction does not exceed 20 M(-1) s(-1) in the physiological pH range and, therefore, cannot greatly contribute toward recycling of the nitroxides in the tissue.
authors.affiliation: Department of Pharmaceutics, School of Pharmacy, Hebrew University of Jerusalem, Jerusalem, Israel.
authors.forename: Avner
authors.initials: A
authors.lastname: Israeli
authors.forename: Miriam
authors.initials: M
authors.lastname: Patt
authors.forename: Miriam
authors.initials: M
authors.lastname: Oron
authors.forename: Amram
authors.initials: A
authors.lastname: Samuni
authors.forename: Ron
authors.initials: R
authors.lastname: Kohen
authors.forename: Sara
authors.initials: S
authors.lastname: Goldstein
mesh_terms: D002412:Cations / Q000737:chemistry; D003497:Cyclic N-Oxides / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D006898:Hydroxylamines / Q000737:chemistry*; D007700:Kinetics; D010084:Oxidation-Reduction; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D013997:Time Factors
chemical_list: D002412:Cations; D003497:Cyclic N-Oxides; D005609:Free Radicals; D006898:Hydroxylamines; D000644:Quaternary Ammonium Compounds; C003959:TEMPO