Kinetic and spectral investigation of the electron and hydrogen adducts of dihydroxy- and dimethyl-substituted pyrimidines: a pulse radiolysis and product analysis study

Abstract

The reactions of hydrated electrons (e_(aq)^−) and hydrogen atoms (H·) with 4,6-dihydroxy-2-methylpyrimidine (DHMP), 2,4-dimethyl-6-hydroxypyrimidine (DMHP), 5,6-dimethyluracil (DMU) and 6-methyluracil (MU) were studied at different pH values using pulse radiolysis. The second-order rate constants obtained for the reaction of e_(aq)^− with these systems are in the range (5–10) × 10^9 dm^3 mol^(−1) s^(−1) at near, neutral pH. At basic pH, the rate constant values were considerably reduced owing to the electrostatic effect between e_(aq)^− and pyrimidine anion. The transient absorption spectra of the electron adducts have distinct absorption maxima at around 300–320 nm. The initial spectrum in the case of DHMP at pH 4.5 was found to undergo a first-order transformation. Based on the spectral characteristics and the yields of methylviologen radical cation (MV^(·+)) resulted from the electron transfer reaction between the electron adducts and MV^(2+), it is proposed that a protonated (at oxygen) electron adduct of DHMP is initially formed which undergoes a proton- and phosphate-catalysed transformation to form a reducing C(5) protonated C(6)-yl radical. Such preferential protonation at C(5) is predominant only with dihydroxypyrimidine systems. At pH 9 and 13, formation of a radical monoanion of DHMP (pK_a ≥13) is proposed. The possible attack of e_(aq)^− is proposed to be at N(1) or N(3) of DMHP. The resulting electron adduct has a pK_a value around 6.0. Similar properties for the electron adducts of DMU and MU [electron attack at O(4)] are proposed. The second-order rate constants for H· with DHMP, DMHP, DMU and MU were in the range (1.7–28) × 10^8 dm^3 mol^(−1) s^(−1). The hydrogen adduct spectra were generally identified as their absorption maxima at 310–380 and 460–510 nm. Formation of C(5)-protonated C(6)-yl radical, the same radical that formed after the H+-and phosphate-catalysed transformation of the electron adduct, is proposed for DHMP. The possibility of the formation of C(5)-yl and C(6)-yl H adducts of DMHP, DMU and MU is discussed. High-performance liquid chromatography coupled with electrospray mass spectrometry (HPLC-ES-MS) has been used to qualitatively analyse the products obtained from the reaction of e_(aq)^− with DHMP and DMHP and the results revealed that the products are mainly derived from the C(5) protonated C(6)-yl radicals via its disproportionation and dimerization reaction. A possible reaction mechanism is proposed for the product formation.