EPR and ENDOR Study of Radiation-Induced Radical Formation in Purines: Sodium Inosine Crystals X-irradiated at 10 K

Abstract

X-irradiated single crystals of sodium inosine (Na(+)*Inosine(-)*2.5H(2)O), in which the hypoxanthine base is present as the N1-deprotonated anion, were investigated using K-band (24 GHz) electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and ENDOR induced EPR (EIE) techniques at 10 K. At least five different radicals were present immediately after irradiation at 10 K. R1, which decayed upon warming the crystals to 50 K, was identified as the electron-loss product of the parent N1-deprotonated hypoxanthine base. Hyperfine couplings to HC8 and HC2 were fully characterized with ENDOR spectroscopy, and the identification was supported by DFT calculations. R2, which also decayed on warming to 50 K, exhibited nearly equal couplings to HC2 and HC8. Taken in combination with an extensive set of DFT calculations, the experimental results indicate that R2 is the (doubly negative) product of electron-gain by the initially anionic N1-deprotonated hypoxanthine parent. R3, which exhibited hyperfine coupling only to HC8 could not be identified. R4, which persisted on annealing to 260 K, exhibited one large alpha-proton hyperfine coupling which was fully characterized by ENDOR. Based on DFT calculations and the experimental data, R4 was identified as the product of net H-abstraction from C5'. The remaining HC5' was the source of the measured alpha-proton coupling. R5, present at low temperature and the only observable radical after warming the crystals to room temperature, was identified as the C8-H addition radical. The alpha-coupling to HC2 and beta-couplings to the pair of C8 methlyene protons were fully characterized by ENDOR.