ELECTRON SPIN RESONANCE OF DNA IRRADIATED WITH A HEAVY-ION BEAM (O-16(8- EVIDENCE FOR DAMAGE TO THE DEOXYRIBOSE PHOSPHATE BACKBONE()) )

The free radicals produced from the irradiation of hydrated DNA with a heavy-ion beam have been investigated by ESR spectroscopy. The domina nt free radical species formed after 60 MeV/nucleon O-16(8+) ion-beam irradiations at low temperatures (77 K) are the same as those previous ly identified from studies using low-LET radiation, pyrimidine electro n-gain radicals and purine electron-loss radicals; however, greater re lative amounts of neutral carbon-centered radicals are found with the higher-LET radiation, and a new phosphorus-centered radical is identif ied. The fraction of neutral carbon radicals is also found to increase along the ion-beam track with the highest amounts found in the Bragg peak. The neutral carbon-centered radicals likely arise in part from t he sugar moiety. The G values found for total trapped radicals at 77 K are significantly smaller for the O-16(8+) ion beam than those found for low-LET radiation. The new phosphorus-centered radical species is identified by its large P-31 parallel hyperfine coupling of about 780 G as a phosphoryl radical. This species is produced linearly with dose and is not found in significant amounts in DNA irradiated with low-LE T radiation. The phosphoryl radical must be produced by the fragmentat ion of a P-O bond and suggests the possibility of a direct strand brea k. The yield of phosphoryl species is small (about 0.1% of all radical s); however, it clearly indicates that new mechanisms of damage which are not significant for low-LET radiation must be considered for high- LET radiation. (C) 1996 by Radiation Research Society