Debije, M. G., Close, D. M. and Bernhard, W. A. Reductive Damage in Directly Ionized DNA: Saturation of the C5 = C6 Bond of Cytosine in d(CGCG)2 Crystals. Radiat. Res. 157, 235 – 242 (2002).
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.
