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.