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

One-electron redox reactions of cyclic selenium compounds, DL-<i>trans</i>-3,4-dihydroxy-1-selenolane (DHS<sub>red</sub>), and DL-<i>trans</i>-3,4-dihydroxy-1-selenolane oxide (DHS<sub>ox</sub>) were carried out in aqueous solutions using nanosecond pulse radiolysis, and the resultant transients were detected by absorption spectroscopy. Both <sup>•</sup>OH radical and specific one-electron oxidant, Br<sub>2</sub><sup>•−</sup> radical reacted with DHS<sub>red</sub> to form similar transients absorbing at 480 nm, which has been identified as a dimer radical cation (DHS<sub>red</sub>)<sub>2</sub><sup>•+</sup>. Secondary electron transfer reactions of the (DHS<sub>red</sub>)<sub>2</sub><sup>•+</sup> were studied with 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS<sup>2−</sup>) and superoxide (O<sub>2</sub><sup>•−</sup>) radicals. The bimolecular rate constants for the electron transfer reaction between (DHS<sub>red</sub>)<sub>2</sub><sup>•+</sup> with ABTS<sup>2-</sup> was determined as 2.4 ± 0.4 × 10<sup>9</sup> M<sup>−1</sup> s<sup>−1</sup>. From this reaction, the yield of (DHS<sub>red</sub>)<sub>2</sub><sup>•+</sup> formed on reaction with <sup>•</sup>OH radical was estimated in the presence of varying phosphate concentrations. (DHS<sub>red</sub>)<sub>2</sub><sup>•+</sup> reacted with O<sub>2</sub><sup>•−</sup> radical with a bimolecular rate constant of 2.7 ± 0.1 × 10<sup>9</sup> M<sup>−1</sup> s<sup>−1</sup> at pH 7. From the same reaction, the positive charge on (DHS<sub>red</sub>)<sub>2</sub><sup>•+</sup> was confirmed by the kinetic salt effect. HPLC analysis of the products formed in the reaction of (DHS<sub>red</sub>)<sub>2</sub><sup>•+</sup> with O<sub>2</sub><sup>•−</sup> radicals showed formation of the selenoxide, DHS<sub>ox</sub>. In order to know if a similar mechanism operated during the reduction of DHS<sub>ox</sub>, its reactions with e<sub>aq</sub><sup>−</sup> were studied at pH 7. The rate constant for this reaction was determined as 5.6 ± 0.9 × 10<sup>9</sup> M<sup>−1</sup> s<sup>−1</sup>, and no transient absorption could be observed in the wavelength region from 280 to 700 nm. It is proposed that the radical anion (DHS<sub>ox</sub>)<sup>•−</sup> 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<sup>•+</sup>. Evidence for this mechanism was obtained by converting DHS<sup>•+</sup> to (DHS<sub>red</sub>)<sub>2</sub><sup>•+</sup> with the addition of DHS<sub>red</sub> to the same system. Quantum chemical calculations provided supporting evidence for some of the redox reactions.