ESR and ENDOR of sulfur substituted nucleosides, 6-methylmercaptopurine riboside at 300 K

Single crystals of the sulfur containing DNA nucleoside analog 6-methylmercaptopurine riboside have been prepared and irradiated by x rays. Electron spin resonance and electron–nuclear double resonance techniques have been used to study the radiation induced radicals at room temperatures. Three radical species (I, II, III) were observed to be stable at room temperature. Radical species I and II showed little or no hyperfine structure, but had large g-value variations about average values of I, 2.0275; and II, 2.0112. These two radical species are attributed to π-electron monosulfide radicals formed by rupture of the S–CH3 bond. Radical species III is formed by H addition at C2 of the purine ring. This radical has g value and hyperfine principal values: 2.0052, 2.0048, 2.0028; H8, −8.9, −6.3, −3.3 G; N3, 20.5, 1.1, 1.1 G; N1, 7.0, 0.5, 0.5 G; CH2, 40.0 G.