Oxidations Of Hydroxylamine And Methylselenocysteine By Metal(?) Complexes:A Kinetic Analysis

The oxidation of hydroxylamine?HONH₂?by Ce?IV?in perchloric acid media took place fast and was followed by stopped-flow spectrometer.A rate law of-d[Ce?IV?]_tot/dt=k'[HONH₂]_tot[Ce?IV?]_tot was proved,where k'stands for the observed second-order rate constant and[Ce?IV?]_tot and[HONH₂]_tot represent the total concentrations of Ce?IV?and HONH₂,respectively.The dependencies of k'on[H⁺]and ionic strength were studied.Rapid scan spectra for the reaction course and reaction stoichiometric measurement unraveled several additional characters.Based on experimental results,a reaction mechanism is proposed which involves Ce⁴⁺,Ce?OH?³⁺,and Ce?OH?₂²⁺attacking on HONH₃⁺in parallel.Moreover,these parallel attacking reactions are the rate-determining steps,generating free radicals·ONH₂ which are rapidly oxidized to nitrate by Ce?IV?.Rate constants of these rate-determining steps have been evaluated;a reactivity trend of Ce⁴⁺>Ce?OH?³⁺>Ce?OH?₂²⁺is obtained but the reactivity differences among them are very small.The rate determining steps are rationalized in terms of an outer-sphere electron transfer,and possible transition states are proposed in the cases of Ce?OH?³⁺and Ce?OH?₂²⁺to account for the small reactivity differences.The kinetic and mechanistic aspects obtained in the present work are compared with the same reaction in sulfuric acid and nitric media studied before.It is concluded that the reaction media can play a dominant role in determining the kinetics,mechanism and the oxidation product?s?of hydroxylamine.Se-methyl L-selenocysteine?MeSeCys?is one of the major organic selenium compounds acquired from the diet by human beings.It has been shown to have anticancer activity and cancerprevention functions.However,its antioxidant activity,largely related to its biological function,has not been well characterized so far.We here report a stopped-flow kinetic study of the reduction of the Pt?IV?anticancer model compounds trans-[PtX₂?CN?₄]^2-?X=Cl;Br?by MeSeCys in a wide pH range.Overall second-order kinetics is established for the redox reactions,and spectrophotometric titrations indicate a1:1 reaction stoichiometry.The MeSeCys is oxidized to its selenoxide form,as identified by high-resolution mass spectra.The proposed reaction mechanism involves parallel attack on one of the trans-coordinated halides of the Pt?IV?complexes by the selenium atom of the various MeSeCys protolytic species.Rate constants for the rate determining steps as well as the pKa values of the various protolytic species of MeSeCys have been determined at 25.0oC and 1.0 M ionic strength.A bridged two-electron transfer mechanism for the rate-determining steps is supported by rapid-scan spectra,activation parameters,and by the much larger reaction rate of[PtBr₂?CN?₄]^2-compared to[PtCl₂?CN?₄]^2-.The experiments indicate that the reduction of[PtX₂?CN?₄]^2-by MeSeCys proceeds via a similar reaction mechanism as L-selenomethionine?SeMet?studied previously.However,there is a large reactivity difference between these two selenium compounds,as a matter of fact the largest one observed so far when compared to other redox systems.It differs between the various protolytic species of MeSeCys and SeMet.The different reactivity of MeSeCys and SeMet in the reduction of various biologically relevant oxidants might account for their disparate efficacies as anticancer agents.