Reductive damage in directly ionized DNA : saturation of the C5-C6 bond of cytosine in d(CGCG)2 crystals

Electron paramagnetic resonance (EPR) was used to study an oligodeoxynucleotide duplex of d(CGCG)2 that is known to crystallize in Z-form. After X irradiation at 4 K, EPR data were collected on single crystals and polycrystalline samples as a function of annealing temperature and dose. A radical produced by the net gain of a hydrogen atom at C6 and a proton at N3, Cyt(C6+H, N3+H+)+¿·, is identified. This radical had not been positively identified in polymeric DNA previously. The Cyt(C6+H, N3+H+)+¿· makes up about 4¿% of the total radical population at 4 K, increasing to about 10¿–¿15¿% after the DNA is annealed to 240 K. There appears to be neither an increase nor a decrease in the absolute concentration of Cyt(C6+H, N3+H+)+¿· upon annealing from 4 K to 240 K. Additionally, the presence of another radical, one due to the net gain of hydrogen at C5 of cytosine, the Cyt(C5+H)·, is implicated. Together, these two radicals appear to account for 60¿–¿80¿% of the reduced species in DNA that has been irradiated at 4 K and annealed to 240 K.