Free-radical probes of condensed-phase aspects of radiation damage mechanisms in DNA constituents: 5-halouracils

Reactions of water radiolysis intermediates with pyrimidine and purine bases in aqueous BeF2 glasses: an EPR study

Free radical formation by reaction of water radiolysis intermediates produced by X-irradiation in aqueous glasses containing 7 M BeF2 at 77 K with pyrimidine and purine constituents of DNA (10 mM) was studied by electron paramagnetic resonance (EPR) spectroscopy. Reactions of electrons and .OH radicals were observed; H. form a minority and their contribution was difficult to establish. The electrons form substrate radicals at 77 K, while .OH radicals, stabilized in the matrix at 77 K, become mobile at about 140K and in part react with solute molecules. The radicals formed by both reactions were characterized after isolation of the corresponding components by thermal annealing up to about 190 K and spectra simulation using literature parameters whenever possible. The spectra from cytosine gave strong evidence for heteroatom protonation following electron addition while those from adenine were somewhat less clear. The spectra of uracil and all methylated pyrimidines gave no evidence for the protonation state of the electron adduct. For the .OH radical, the reaction with uracil and cytosine was found to be addition to the 5,6-double bond, in line with studies using aqueous solutions. For all methylated pyrimidines, however, H-abstraction from the methyl group was dominant. .OH addition to adenine was found to take place at C2.

Long-range hole migration in irradiated crystals of nucleic acid bases. An EPR study

Polycrystalline samples of several nucleic acid bases co-crystallized with small amounts of thioanalogues as impurities were X-irradiated at 77 K and investigated by electron paramagnetic resonance (EPR) spectroscopy for sulphur (S)-centered radicals. Anhydrous cytosine, cytosine monohydrate and cytosine hydrochloride with 0.01-1% incorporated thiocytosine gave strong EPR signals characteristic of S-centered radicals, with spectroscopic parameters different from those for irradiated thiocytosine alone. Likewise, in guanine hydrochloride doped with thioguanine (0.1-1%), strong signals of the radicals associated with thioguanine were observed. For the thymine-thiothymine systems (anhydrous and monohydrate) no characteristic S-centered radicals were observed after the same treatment. This was also the case for samples from non-matching bases and thiols (e.g. cytosine-thioguanine or guanine-thiocytosine), for combinations in the form of frozen glasses or frozen aqueous solutions and for almost all freeze-dried samples composed of the bases containing a small amount of thioanalogues. These observations are discussed in terms of the long-range migration of holes within the stacked base systems.