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 up to 300 K as well as after storage at 300 K. Individual radical patterns were isolated from the spectra by comuter 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 C_5.The latter species is also present in cytosine which, in addition displays the pattern due to H-addition at the carboxyl oxygen C_2. Adenosine exhibits two H-addition radicals, one at C_2, the other at C_8. 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 contain already the secondary radicals but exhibit a large linewidth. This feature is attributed to dipolar coupling caused by radicals in close proximity. (orig.)SIGLEAvailable from TIB Hannover: RO 801(93-09) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman