An experimental and theoretical study of the kinetics and mechanism of hydroxyl radical reaction with 2-aminopyrimidine

Abstract

Oxidation of 2-aminopyrimidine (2Apy) by hydroxyl radicals (˙OH) in aqueous medium has been studied using pulse radiolysis coupled to optical absorption and M05-2X/6-311++G(d,p) level DFT calculations aimed at elucidation of the reaction mechanism. The rate constant (k) 2.76 ± 0.06 × 10⁹ M⁻¹ s⁻¹ determined by the pulse radiolysis method for the title reaction at neutral pH reflects a diffusion-controlled process. The maximum absorption wavelength (λ_max 330 and 550 nm) of transient(s) remains intact in the pH range 10.5–7. Radical adduct formation (RAF) reactions with N1 (N3 is an identical site), C2, C4 (C6 is an identical site) and C5 as the target atoms, hydrogen transfer (HT) reaction with amino group and single electron transfer (SET) of 2Apy with ˙OH were modelled theoretically. Lowest Gibbs free energy of activation (ΔG^≠) in solution, 1.72 kcal mol⁻¹, was calculated for RAF at C5; the computed k value for this process is 2.0 × 10⁹ M⁻¹ s⁻¹ using the transition state theory (TST). The relative yield (87%) of this major product predicted from the computed k value shows excellent agreement with the results of the pulse radiolysis redox titrations. Subsequently, the possibilities for other RAF reactions, HT and SET were ruled out. As an extension to the above work, we have carried out a theoretical survey of the reactivity of ˙OH with 4-aminopyrimidine (4Apy); wherein the RAF at C5 is proposed as the most probable mechanism with a ΔG^≠ of 2.15 kcal mol⁻¹.