title: One-electron redox processes in a cyclic selenide and a selenoxide: a pulse radiolysis study.
abstract: One-electron redox reactions of cyclic selenium compounds, DL-trans-3,4-dihydroxy-1-selenolane (DHS(red)), and DL-trans-3,4-dihydroxy-1-selenolane oxide (DHS(ox)) were carried out in aqueous solutions using nanosecond pulse radiolysis, and the resultant transients were detected by absorption spectroscopy. Both *OH radical and specific one-electron oxidant, Br(2)(*-) radical reacted with DHS(red) to form similar transients absorbing at 480 nm, which has been identified as a dimer radical cation (DHS(red))(2)(*+). Secondary electron transfer reactions of the (DHS(red))(2)(*+) were studied with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS(2-)) and superoxide (O(2)(*-)) radicals. The bimolecular rate constants for the electron transfer reaction between (DHS(red))(2)(*+) with ABTS(2-) was determined as 2.4 +/- 0.4 x 10(9) M(-1) s(-1). From this reaction, the yield of (DHS(red))(2)(*+) formed on reaction with *OH radical was estimated in the presence of varying phosphate concentrations. (DHS(red))(2)(*+) reacted with O(2)(*-) radical with a bimolecular rate constant of 2.7 +/- 0.1 x 10(9) M(-1) s(-1) at pH 7. From the same reaction, the positive charge on (DHS(red))(2)(*+) was confirmed by the kinetic salt effect. HPLC analysis of the products formed in the reaction of (DHS(red))(2)(*+) with O(2)(*-) radicals showed formation of the selenoxide, DHS(ox). In order to know if a similar mechanism operated during the reduction of DHS(ox), its reactions with e(aq)(-) were studied at pH 7. The rate constant for this reaction was determined as 5.6 +/- 0.9 x 10(9) M(-1) s(-1), and no transient absorption could be observed in the wavelength region from 280 to 700 nm. It is proposed that the radical anion (DHS(ox))(*-) formed by a one-electron reduction would get protonated to form a hydroxyl radical adduct, which in presence of proton donors, would undergo dehydration to form DHS(*+). Evidence for this mechanism was obtained by converting DHS(*+) to (DHS(red))(2)(*+) with the addition of DHS(red) to the same system. Quantum chemical calculations provided supporting evidence for some of the redox reactions.
authors.affiliation: Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India. beenam@barc.gov.in
authors.forename: Beena G
authors.initials: BG
authors.lastname: Singh
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Thomas
authors.forename: Fumio
authors.initials: F
authors.lastname: Kumakura
authors.forename: Kenichi
authors.initials: K
authors.lastname: Dedachi
authors.forename: Michio
authors.initials: M
authors.lastname: Iwaoka
authors.forename: K Indira
authors.initials: KI
authors.lastname: Priyadarsini
mesh_terms: D002851:Chromatography, High Pressure Liquid; D004583:Electrons; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry*; D017665:Hydroxyl Radical / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D016566:Organoselenium Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D011675:Pulse Radiolysis / Q000379:methods*; D011789:Quantum Theory
chemical_list: C553726:3,4-dihydroxy-1-selenolane; C531309:3,4-dihydroxy-1-selenolane oxide; D006573:Heterocyclic Compounds, 1-Ring; D016566:Organoselenium Compounds; D017665:Hydroxyl Radical