Specific formation of electron gain and loss centres in X-irradiated oriented fibres of DNA at low temperatures

Electron spin resonance (e.s.r.) spectra have been measured at X- and Q-band frequencies (ca. 9.5 and 34.1 GHz) from six different DNA specimens prepared as oriented A-DNA fibres after irradiation between 4.2 and 77 K. The DNA either varied in its guanine–cytosine base-pair content or contained 5-chloro- or 5-bromo-uracil instead of the natural base thymine. Amended by single-crystal studies at 4.2 K of the 5-halogenouracils and by spin-density calculations and associated spectral simulation, the DNA data have been analysed in terms of two primary species stabilized at low temperatures in thymine-containing DNA which are assigned to the cation of guanine and the anion of thymine. The latter is very probably replaced by the respective 5-halogenouracil anions in the substituted specimen. The average total radical yield is ca. 2.3 per 100 eV. The relative contribution of the guanine cation depends on the base composition.