Free Radicals from Polycrystalline Pyrimidines and Purines upon Heavy Ion Bombardment at Low Temperatures: An Electron Spin Resonance Study

Abstract

The formation of free radicals in polycrystalline samples of the DNA constituents thymine, cytosine, and adenosine after bombardment with heavy ions at about 100K was investigated by electron spin resonance (ESR) spectroscopy. Spectra were observed at 77K after irradiation at 100K, upon annealing to 300K and after storage at 300K. 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.