Free Radicals in Irradiated Single Crystals of 5-halo-uracil Derivatives: 5-bromouracil, 5-bromodeoxyuridine and 5-iododeoxyuridine

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Abstract
Electron-spin-resonance spectra of x- or γ-irradiated single crystals of 5-bromouracil (BU), 5-bromodeoxyuridine (BUDR) and 5-iododeoxyuridine (IUDR) were recorded at room temperature as a function of crystal orientation in the magnetic field at 35 GHz (Q-band) and, less extensively, at 10 GHz (X-band). The observed positions of hyperfine lines were fitted to theoretical curves by a computer programme. The results clearly show the existence of two distinct groups of lines, A and B. While the g-factor of group A is close to the free electron value, group B is shifted to lower magnetic fields and is further subject to large variations with changing orientation, both in position and intensity of hyperfine lines. Both groups are correlated to radicals at the pyrimidine group. It is proposed that one of these radicals is formed by hydrogen addition at the oxygen atom bound to C4. Two resonance structures are possible, differing in localization of the unpaired spin density at either C4 or C6. The observed hyperfine splittings of group A fit completely into this scheme for all three substances investigated, if the unpaired spin is assumed to reside mainly at C6 in IUDR and at C4 in BU and BUDR. Two different radicals are proposed to account for group B having in common the main unpaired spin density at C5. The radical structure assigned to BU may be formed by abstraction of hydrogen from N1. It is characterized by two resonance structures with unpaired spin densities at N1 and C5 linked by a conjugated double bond. In BUDR and IUBR, the radical representing group B is produced by addition of a hydrogen atom at C6 resulting in two β-protons at this position.

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