Free radicals from polycrystalline pyrimidines and purines upon heavy ion bombardment at low temperatures: an electron spin resonance study

The formation of free radicals in polycrystalline samples of the DNA constituents thymine, cytosine, and adenosine after bombardment with heavy ions at about 100 K was investigated by electron spin resonance (ESR) spectroscopy. Spectra were observed at 77 K after irradiation at 100 K, upon annealing to 300 K and after storage at 300 K. Individual radical patterns were isolated from the spectra by computer manipulation and assigned to structures by powder-simulations based on literature data. The spectra of thymine contain an allyl radical, the octet pattern of the 5-thymyl radical and contributions of the 6-yl radical formed by net hydrogen gain at carbon C5. The latter species is also present in cytosine which in addition displays the pattern due to H-addition at the carboxyl oxygen C2. Adenosine exhibits two H-addition radicals, one at C2, the other at C8. Additionally, the spectra of all DNA subunits studied contain as a radical component a Gaussian singlet of about 0.9 mT line width. The spectra obtained at low temperature already contain the secondary radicals but exhibit a large linewidth. This feature is attributed to dipolar coupling caused by radicals in close proximity.