Pulse Radiolysis Studies on Reactions of Hydroxyl Radicals with Selenocystine Derivatives

Abstract

Reactions of hydroxyl radicals (^•OH) with selenocystine (SeCys) and two of its analogues, diselenodipropionic\nacid (SeP) and selenocystamine (SeA), have been studied in aqueous solutions at pHs of 1, 7, and 10 using\nthe pulse radiolysis technique coupled with absorption detection. All of these diselenides react with ^•OH\nradicals with rate constants of ∼10¹⁰ M^-1 s^-1, producing diselenide radical cations (∼1−5 μs after the pulse),\nwith an absorption maximum at 560 nm, by elimination of H₂O or OH^- from hydroxyl radical adducts.\nAssignment of the 560 nm band to the diselenide radical cation was made by comparing the transient spectra\nwith those produced upon reaction of diselenides with specific one-electron oxidants, Cl₂^•- (pH 1) and Br₂^•-\nradicals (pHs of 7 and 10). SeP having a carboxylic acid functionality showed quantitative conversion of\nhydroxyl radical adducts to radical cations. The compounds SeCys and SeA, having an amino functional\ngroup, in addition to the radical cations, produced a new transient with λ_max at 460 nm, at later time scales\n(∼20−40 μs after the pulse). The rate and yield of formation of the 460 nm band increased with increasing\nconcentrations of either SeCys or SeA. In analogy with similar studies reported for analogous disulfides, the\n460 nm transient absorption band has been assigned to a triselenide radical adduct. The one-electron reduction\npotentials of the compounds were estimated to be 0.96, 1.3, and 1.6 V versus NHE, respectively, for SeP,\nSeCys, and SeA at pH 7. From these studies, it has been concluded that the electron-donating carboxylic acid\ngroup decreases the reduction potential and facilitates quantitative conversion of hydroxyl radical adducts to\nradical cations, while the electron-withdrawing NH₃⁺ group not only increases the reduction potential but\nalso leads to fragmentation of the hydroxyl radical adduct to selenyl radicals, which are converted to triselenide\nradical adducts.