title: One electron oxidation induced dimerization of 5-hydroxytryptophol: role of 5-hydroxy substitution.
abstract: Reaction of one-electron oxidant (Br(2)(*-)) with tryptophol (TP) and 5-hydroxytryptophol (HTP) have been studied in aqueous solution in the pH range from 3 to 10, employing nanosecond pulse radiolysis technique and the transients detected by kinetic spectrophotometry. One-electron oxidation of TP has produced an indolyl radical that absorbs in the 300-600 nm region with radical pK(a) = 4.9 +/- 0.2, while the reaction with HTP has produced an indoloxyl radical with lambda(max) at 420 nm and radical pK(a) < 3. Hydroxyl radicals ((*)OH) react with these two compounds producing (*)OH radical adducts that undergo water elimination to give one-electron-oxidized indolyl and indoloxyl radical species, respectively. The indoloxyl radicals react with the parent compound to form dimer radicals with an average association constant of (6.7 +/- 0.4) x 10(4) M(-1). No such dimerization is observed with indolyl radical, indicating that the presence of the 5-hydroxy group markedly alters its ability to form a dimer. A possible explanation behind such a difference in reactivity has been supported with ab initio quantum chemical calculations.
authors.affiliation: Radiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India.
authors.forename: G H
authors.initials: GH
authors.lastname: Naik
authors.forename: K Indira
authors.initials: KI
authors.lastname: Priyadarsini
authors.forename: D K
authors.initials: DK
authors.lastname: Maity
authors.forename: Hari
authors.initials: H
authors.lastname: Mohan