E.s.r.-spectroscopy of radiation-produced radicals in 9-ethyl adenine

SummaryRadical formation in monocrystals of 9-ethyl adenine by irradiation with X-rays at room temperature has been studied using e.s.r.-spectroscopy at 9·5 GHz and 35 GHz. The two types of spectra found were attributed to hydrogen abstraction and addition radicals. It was possible to separate the correlated spectra and to assign definite structures to the two species by analysing the orientational variations of the spectra. The spectra of the abstraction radical were obtained under conditions of power saturation of the addition radical at reduced temperature (77 K). The site of hydrogen dissociation and of maximum unpaired spin density was localized at the C-10-carbon atom bound to N-9 of the purine. The methyl β-protons give isotropic splittings of 24·0 G each. The abstraction radical could be bleached by illumination with U.V.-light, leaving the addition radical unchanged. Hydrogen addition was concluded on the basis of isotropic hyperfine interactions from two β-protons of 39·5 G each and anisotropic splittings from two nitrogen nuclei with axial symmetry. Theoretical splittings calculated with the INDO method indicate addition at the C-8-atom. Principal values of the nitrogen splittings perpendicular and parallel to the purine ring are 27·0 G and 0·0 G for N-7, 6·6 G and 0·0 G for N-9.