Formation of H -Addition Radicals in Adenine Derivatives

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Abstract
The formation of H-addition radicals in monocrystals of 9-methyl adenine, deoxyadenosine.cntdot.H2O and adenosine. HCl by irradiation with X-rays was studied using ESR spectroscopy. In 9-methyl adenine and adenosine.cntdot.HCl, H atoms add exclusively to position C8 of the imidazole part of the purine ring. In deoxyadenosine.cntdot.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 2 equivalent .beta.-protons of 38.0 .+-. 0.5 G and the C2-addition radical by 2 non-equivalent .beta.-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 [Intermediate Neglect of Differential Overlap] calculations. The added H atom comes partly from the H-bonding scheme and partly from the nonexchangeable hydrogens for the C8-addition radical. Together with additional experiments on polycrystalline samples, these results suggest that H 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 Huckel molecular orbital calculations.

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