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Summary
Polycrystalline samples of several nucleic acid bases co-crystallized with small amounts of thioanalogues as impurites were X-irradiated at 77 K and investigated by electron paramagnetic resonance (EPR) spectroscopy for sulphur (S)-centred radicals. Anhydrous cytosine, cytosine monohydrate and cytosine hydrochloride with 0·01–1% incorporated thiocytosine gave strong EPR signals characteristic of S-centred 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-centred 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.