The formation of H-aduition radicals in monocrystals of 9-methyl adenine, deoxyadenosine monohydrate, and adenosine hydrochloride by irradiation with X-rays has been studied using ESR spectroscopy. In 9-methyl adenine and adenosine-HCl, hydrogen atoms add exclusively to position C8 of the imidazole part of the purine ring. On the other hand, in deoxyadenosine · H2O crystals, H-addition radicals at position C2 of the pyrimidine part of the purine ring occur together with H-addition radicals at position C8. Both radicals could be isolated by using their differential stability under warming or illumination with light. The C8-addition radical is characterized by two equivalent β-protons of 38.0 ± 0.5 G and the C2-addition radical by two non-equivalent β-protons of 34.0 ±1.0 and 50.0 ± 1.0 G. The nitrogen splittings perpendicular to the purine ring are 27.0 and 6.6 G for the C2-addition radical and 20.5 and 9.2 G for the C2-addition radical. The coupling constants of both radicals are in agreement with INDO calculations. It is further shown that the added hydrogen atom comes partly from the hydrogen bonding scheme and partly from the nonexchangeable hydrogens for the C8-addition radical. Together with additional experiments on polycrystalline samples, these results lead to the conclusion that hydrogen atoms add non-selectively to C2 and C8 of the neutral molecules, whereas protons add predominantly to C8 of anion radicals. This is supported by Hückel molecular orbital calculations.
The Equally Important Role of Adenine Derivatives to That of Pyrimidine Derivatives in Near-0 eV Electron-Induced DNA Lesions