Abstract

Formation and Trapping of Free Radicals in Irradiated Purines: EPR and ENDOR of Hypoxanthine Derivatives Studied as Single Crystals. Sibel Tokdemir

Four different derivatives of hypoxanthine (hypoxanthine·HCl·H₂O, Na⁺·Inosine^–·2.5H₂O, sodium inosine monophosphate, and calcium inosine monophosphate) were irradiated in the form of single crystals with the objective of identifying the radical products. To do so, magnetic resonance methods (EPR, ENDOR experiments and EPR spectrum simulations) were used to study radical products in crystals following x-irradiation at ~10 K without warming, and under conditions of controlled warming. Also, computational chemistry methods were used in combination with the experimental methods to assist in identifying the radical products. Immediately following irradiation at 10 K, at least three different radicals were observed for hypoxanthine·HCl·H₂O. RI was identified at the product of electron addition followed by protonation of the parent at N3. RII was identified as the product of electron loss followed by deprotonation at N7, and RIII was tentatively identified as the product of electron gain followed by protonation at O6. On warming to room temperature, three new radicals were observed: RI and RIII were the products of net H addition to C8 and C2 respectively, while and RII was the product of OH addition to C8. At least five different radical products of Na⁺·Inosine^- were detected immediately after irradiation at 10 K. RI was identified as the electron-loss product of the parent hypoxanthine base, RII was identified as the product of net H-abstraction from C5’of the sugar. RIII and RIV were tentatively identified as the products of net H-addition to O6 (probably via electron addition followed by protonation), and the (doubly-negative) product of electron-gain, respectively. RV, the C8-H addition radical, was the only product detected on warming sodium inosine crystals to room temperature. Because the ENDOR spectra from sodium IMP irradiated at 10K were complex, it was possible to identify only two radicals. RI was identified as the purine base electron-abstraction product, and RII was identified as the O6 hydrogen-addition product. ENDOR spectra could be obtained from calcium IMP only at a few orientations. Thus, all radical identifications in this system are based on EPR spectrum simulations using likely radical structures based on results from other hypoxanthine-based systems.