One-Electron Redox Processes in a Cyclic Selenide and a Selenoxide: A Pulse Radiolysis Study

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)-O-center dot radical and specific one-electron oxidant, Br-2(center dot-) radical reacted with DHSred to form similar transients absorbing at 480 nm, which has been identified as a dimer radical cation (DHSred)(2)(center dot+). Secondary electron transfer reactions of the (DHSred)(2)(center dot+) were studied with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS(2-)) and superoxide (O-2(center dot-)) radicals. The bimolecular rate constants for the electron transfer reaction between (DHSred)(2)(center dot+) with ABTS(2-) was determined as 2.4 +/- 0.4 x 10(9) M-1 s(-1). From this reaction, the yield of (DHSred)(2)(center dot+) formed on reaction with (OH)-O-center dot radical was estimated in the presence of varying phosphate concentrations. (DHSred)(2)(center dot+) reacted with O-2(center dot-) 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 (DHSred)(2)(center dot+) was confirmed by the kinetic salt effect. HPLC analysis of the products formed in the reaction of (DHSred)(2)(center dot+) with O-2(center dot-) radicals showed formation of the selenoxide, DHSox. In order to know if a similar mechanism operated during the reduction of DHSox, 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 (DHSox)(center dot-) 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 center dot+. Evidence for this mechanism was obtained by converting DHS center dot+ to (DHSred)(2)(center dot+) with the addition of DHSred to the same system. Quantum chemical calculations provided supporting evidence for some of the redox reactions.