ESR and spin trapping of free radicals produced in poly U and poly A using the boron-neutron capture reaction

Powdered samples of poly U and poly A containing sodium tetraborate and controls without tetraborate were irradiated at room temperature with thermal neutrons. Radical formation occurred only in the samples containing tetraborate. Thermal neutrons interact with ¹⁰B (10% naturally occurring in tetraborate) via the boron-neutron capture reaction. This reaction generates α particles and ⁷Li ions with mean kinetic energies of 1.48 and 0.85 MeV, respectively. The identification and location of the free radicals produced by the interaction of these particles with poly U and poly A, was determined by ESR and spin trapping using the spin trap 2-methyl-2-nitrosopropane (MNP). Following dissolution of irradiated poly U in an MNP solution, the spin trapped poly U was hydrolyzed with base (NaOH, pH 12.6). On readjusting the pH to pH 7.5 the spin adduct ESR spectrum showed that in poly U, radical formation occurs at the C(6) carbon of the uracil residues. For poly A the solid state ESR spectrum suggests that radicals are formed at the N(3) and/or N(7) nitrogens and at the C(4) and/or C(5) carbons. However, only the C(5) carbon-centered radicals were spin trapped. In addition, the results for poly A indicate that sugar-phosphate backbone is disrupted by the α particles and ⁷Li ions. The nature of this interactions is still unclear.