Free radicals from lyophilized 'dry' DNA bombarded with heavy-ions as studied by electron spin resonance spectroscopy

To investigate the number and species of free radicals from dry DNA after bombardment with heavy-ions at low temperature in comparison with X-irradiation at 77 K by electron spin resonance (ESR) spectroscopy.

Solid DNA samples were either X-irradiated at 77 K as cylinder samples or heavy-ion-bombarded (LET range 1500-12400keV/microm) at the Gesellschaft für Schwerionenforschung (GSI Darmstadt) as very thin tablets. Data acquisition was on a Bruker ESP 380 ESR-spectrometer (X-band, 9.5 GHz). Data analysis involved computer treatment of spectra.

Spectra from heavy-ion-bombarded samples were found to reveal the same free radical species with about the same relative contributions for most components as those from X-irradiated samples at comparable doses. Only two components, a thymine allyl radical and a Cl' deoxyribose located species, were enhanced after heavy-ion bombardment. Dose effects in both cases involved a higher relative amount of deoxyribose-derived free radicals. The analysis of G (total free radicals, taken from dose-response curves) were typically smaller than those for X-rays but showed no clear dependence on LET.

The differing biological response to high-LET particle bombardment compared with low-LET irradiation is not strongly reflected by the chemical structure and total number of initial free radicals. It might rather derive from an inhomogeneous distribution of energy deposition (resulting in 'clustered damages') or from effects at higher chemical and biological levels (e.g. product formation and repair) which is not strongly apparent in the free radical characteristics obtained by ESR-spectroscopy.