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. Copyright © 2013 John Wiley & Sons, Ltd.