Electron spin resonance and electron nuclear double resonance study of X-irradiated deoxyadenosine: proton transfer behavior of primary ionic radicals.

A study of deoxyadenosine crystals (anhydrous form) after irradiation at 10 K found four base-centered radicals and one sugar-centered radical. Radical R1, thermally stable to about 100 K and photobleachable easily with white light, was the product of deprotonation at the amino group by the primary radical cation. Radical R2, also thermally stable to about 100 K, was the product of protonation at N3 of the primary radical anion. Radical R3, stable to about 170 K, was centered in the deoxyribose moiety and evidently was the result of net hydrogen abstraction from C4'. Radicals R4 and R5 were the C2 and C8 H-addition products with couplings typical of those species. Both R4 and R5 were formed at 10 K and were stable at room temperature. The behavior of R1 in several systems provides additional evidence for significant involvement of the hydrogen-bonding environment in controlling the stabilization (or formation) of radicals resulting directly from ionization, as described previously (Radiat. Res. 131, 272-284, 1992). From comparison of amino-group hydrogen-bonding environments in which radicals with the structure of R1 were stabilized, we conclude that oxygen atoms as proton acceptors are important in permitting the charge and spin separation necessary for radical stabilization. In particular, oxygens of ROH structures seem most efficient by readily permitting a multi-proton shuffle through a mechanism amounting to proton exchange. The collective results show that stabilization of these products is unlikely unless the charge and spin can be separated by at least one intervening molecule.