FREE-RADICALS FROM ORIENTED DNA FIBERS AFTER IRRADIATION AT 77 K - CONTINUOUS-WAVE AND PULSED ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY FORM D2O-EQUILIBRATED SPECIMENS

Combined continuous-wave and field-sweep electron spin echo spectroscopy was employed to unravel the components of the spectra in oriented DNA fibers equilibrated in 76% relative humidity of D2O vapor formed upon X irradiation at 77 K and stabilized at that temperature. Using DNAs with different counter-ions (Na+, Li+, Cs+), different base composition (calf thymus, Cl. perfringens), substituted thymine bases (deuterated thymine, 5-halouracil substitution) as well as the copolymer poly(A:U), nine different components were discerned. The spectra of two components, known before partially, were fully characterized. One of them should probably be associated with a cytosine anion, the other with a guanine cation, the latter connection having already been made in previous work. A third component, invoked previously from work with DNA containing deuterated thymine, was also fully characterized and could be assigned to a thymine anion. For one of the new components, an assignment to the allyl radical, probably on the thymine base, could be given on account of the characteristics of the spectra. Tentative assignments are given for three other components which involve an anion on the adenine and the guanine base, respectively, as well as an oxidation-derived species comprising a glycosidic nitrogen, perhaps on cytosine.