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 spectrosco py was employed to unravel the components of the spectra in oriented D NA fibers equilibrated in 76% relative humidity of D2O vapor formed up on X irradiation at 77 K and stabilized at that temperature. Using DNA s with different counter-ions (Na+, Li+, Cs+), different base composit ion (calf thymus, Cl. perfringens), substituted thymine bases (deutera ted thymine, 5-halouracil substitution) as well as the copolymer poly( A:U), nine different components were discerned. The spectra of two com ponents, 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 pre vious work. A third component, invoked previously from work with DNA c ontaining deuterated thymine, was also fully characterized and could b e assigned to a thymine anion. For one of the new components, an assig nment to the allyl radical, probably on the thymine base, could be giv en on account of the characteristics of the spectra. Tentative assignm ents are given for three other components which involve an anion on th e adenine and the guanine base, respectively, as well as an oxidation- derived species comprising a glycosidic nitrogen, perhaps on cytosine.