| Author(s) |
Singh, B. G.; Thomas, E.; Kumakura, F.; Dedachi, K.; Iwaoka, M.; Priyadarsini, K. I. |
|
One-electron redox reactions of cyclic selenium compounds,
DL-trans-3,4-dihydroxy-1-selenolane (DHSred), and
DL-trans-3,4-dihydroxy-1-selenolane oxide (DHSox) 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, Br2·- radical reacted with DHSred to form similar transients absorbing at 480 nm, which has been identified as a dimer radical cation (DHSred)2·+. Secondary electron transfer reactions of the (DHSred)2·+ were studied with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid)
(ABTS2-) and superoxide (O2·-) radicals. The bimolecular rate constants for the electron transfer reaction between
(DHSred)2·+ with ABTS2- was determined as 2.4
± 0.4 x 109 M-1 s-1. From this reaction, the yield of
(DHSred)2·+ formed on reaction with <·OH radical was estimated in the presence of varying phosphate concentrations.
(DHSred)2·+ reacted with O2·-
radical with a bimolecular rate constant of 2.7 ± 0.1 x 109 M-1 s-1 at pH 7. From the same reaction, the positive charge on (DHSred)2·+ was confirmed by the kinetic salt effect. HPLC analysis of the products formed in the reaction of
(DHSred)2·+ with O2·-
radicals showed formation of the selenoxide, DHSox. In order to know if a similar mechanism operated during the reduction of DHSox, its reactions with eaq- were studied at pH 7. The rate constant for this reaction was determined as 5.6
± 0.9 x 109 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 (DHSox)·-
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 (DHSred)2·+ with the addition of DHSred to the same system. Quantum chemical calculations provided supporting evidence for some of the redox reactions. |