Radiation effects on 5-bromo-5,6-dihydrothymine and related compounds: an electron spin resonance study

Abstract

Exposure of 5-bromo-5,6-dihydrothymine, 5-bromo-6-hydroxy-5,6-dihydrothymine, and the two corresponding nucleoside derivatives to ⁶⁰Co γ-rays at 77 K gave paramagnetic centres identified as the 5-yl radicals formed by dissociative electron capture with concurrent formation of bromide ions. In some cases, electron loss centres formed by loss of a C-6 proton from the parent cations were also detected. The 5-yl radicals are of interest since they are related to radicals which may be formed in the radiolysis of DNA. For the hydroxy derivatives, the evidence suggests that they are initially formed in a conformation such that the C-6 proton is constrained to lie close to the radical plane, resulting in unusually small proton hyperfine coupling constants. This result has significant implications for the use of standard e.s.r. parameters in ‘fingerprinting’ the presence of such radicals.