Radiation Damage in Solid 5-Halouracils: Deoxyribose Ring Opening in Single Crystals of 5-Chloro- and 5-Bromodeoxyuridine

• Published in: Radiation research 1979-12, Vol.80 (3), p.447-463
• Main Authors: Oloff, H., Haindl, E., Hüttermann, J.
• Format: Article
• Language: English
• Subjects: Crystals; Electron paramagnetic resonance; Free radicals; Hydrogen; Irradiation; Molecules; Protons; Pyrimidines; Single crystals; Tensors
• ISSN: 0033-7587; 1938-5404
• DOI: 10.2307/3574987

X irradiation of single crystals of 5-chlorodeoxyuridine (ClUdR) and 5-bromodeoxyuridine (BUdR) at 300 K produces three different free-radical species which are isostructural in the two-crystal systems. One is the α-halogen type ${\rm R}\dot{{\rm C}}{\rm Hal}({\rm CH}_{2}){\rm R}^{\prime}$ described previously (1, 2). Of the remaining free radicals, one has been analyzed in detail by ESR-ENDOR techniques. It exhibits a g-tensor with elements 2.0007, 2.0042, and 2.0065 and three hyperfine interactions, A (4.6 G, 6.6 G, 4.3 G), B (-4.8 G, -0.1 G, -5.1 G), and C (2.0 G, 0.7 G, 5.6 G). These spectral parameters are assigned to a free-radical structure 0 = CH(Ċ-OH)CHRR′ at the ${\rm O}_{5^{\prime}}{\rm C}_{5^{\prime}}{\rm C}_{4^{\prime}}{\rm C}_{3^{\prime}}$ site of the deoxyribose group resulting from ring opening subsequent to fragmentation of a precursor alkoxy radical at the ${\rm O}_{5^{\prime}}$ locus. The third radical species is inaccessible to ESR studies but was found to give rise to a distinct α-proton ENDOR-resonance line from which, as a tentative assignment, enolization of the ${\rm C}_{4}\text{-pyrimidine}$ base-located carbonyl function can be derived. These structures are discussed in relation to the radiosensitizing properties of 5-halouracils when replacing thymine residues in DNA.