Reaction of oxide radical ion (O^•-) with substituted pyrimidines

Abstract

Pulse radiolysis technique has been used to investigate the reaction of oxide radical ion (O^•-) with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 5,6-dimethyl uracil (DMU) and 6-methyl uracil (MU) in strongly alkaline medium. The second-order rate constants for the reaction of O^•- with these compounds are in the range 2-5 × 10⁸ dm³ mol^-1 s^-1. The transient absorption spectra obtained with DHMP have two maxima at 290 and 370 nm and with DMHP have maxima at 310 and 470 nm. The transient spectrum from DMU is characterized by its absorption maxima at 310 and 520 nm and that of MU by its single maximum at 425 nm. The intermediate species were found to react with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) with high G(TMPD^•+) values ranged between 3.9 × 10^-7 molJ^-1 and 4.8 × 10^-7 molJ^-1. These radicals undergo decay by second-order kinetics (2k/ε = 1.0-1.7 × 10⁶ s^-1). The reaction of O^•- with the selected pyrimidines is proposed to proceed through a hydrogen abstraction from the methyl group forming allyl type radicals. These are mainly oxidizing radicals and hence readily undergo electron transfer reactions with TMPD.