Dimer radical cation of 4‐thiouracil: a pulse radiolysis and theoretical study

Pulse radiolysis with optical absorption detection has been used to study the reactions of hydroxyl radical (OH • ) with 4‐thiouracil (4TU) in aqueous medium. The transient absorption spectrum for the reaction of OH • with 4TU is characterized by λ max 460 nm at pH 7. A second‐order rate constant k (4 TU + OH) of 1.7 × 10 10  M −1  s −1 is determined via competition kinetics method. The transient is envisaged as a dimer radical cation [4TU] 2 •+ , formed via the reaction of an initially formed radical cation [4TU] •+ with another 4TU. The formation constant of [4TU] 2 •+ is 1.8 × 10 4  M −1 . The reactions of dibromine radical ion (Br 2 •− ) at pH 7, dichlorine radical ion (Cl 2 •− ) at pH 1, and azide radical (N 3 • ) at pH 7 with 4TU have also produced transient with λ max 460 nm. Density functional theory (DFT) studies at BHandHLYP/6–311 + G(d,p) level in aqueous phase showed that [4TU] 2 •+ is characterized by a two‐centerthree electron (2c‐3e) [−S∴S−] bond. The interaction energy of [−S∴S−] bond in [4TU] 2 •+ is −13.01 kcal mol −1 . The predicted λ max 457 nm by using the time‐dependent DFT method for [4TU] 2 •+ is in agreement with experimental λ max . Theoretical calculations also predicted that compared with [4TU] 2 •+ , 4‐thiouridine dimer is more stable, whereas 4‐thiothymine dimer is less stable. Copyright © 2013 John Wiley & Sons, Ltd.