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

Abstract

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 λmax 460 nm at pH 7. A second-order rate constant k(4TU+OH) of 1.7 × 1010 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 × 104 M−1. The reactions of dibromine radical ion (Br2•−) at pH 7, dichlorine radical ion (Cl2•−) at pH 1, and azide radical (N3•) at pH 7 with 4TU have also produced transient with λmax 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 λmax 457 nm by using the time-dependent DFT method for [4TU]2•+ is in agreement with experimental λmax. Theoretical calculations also predicted that compared with [4TU]2•+, 4-thiouridine dimer is more stable, whereas 4-thiothymine dimer is less stable.