Radiation Damage in Solid 5-halouracils: Free Radicals in Single Crystals of 5-fluorouracil

Abstract

X-irradiation at 300 K of single crystals of 5-fluorouracil results in the formation of two different radical species observable by e.s.r. and ENDOR-spectroscopy. One is an alpha-fluoro radical RCF(CH2)R' formed by saturation of the 5,6-double bond of the pyrimidine ring. The principal values of its alpha-fluorine interaction are 170, -9 and -18 G; the isotropic part of the methylene beta-proton couplings are 46.3 and 28 G, respectively; the g-tensor has principal values of 2.0029, 2.0066, and 2.0052. The other radical species is formed by enolization of the C4-carbonyl function. The resulting spin-density distribution gives rise to three observable interactions, an alpha-proton (-5.2, -14.9, -11.2 G) at C6, an OH-proton from the C4--OH group (-1.7, -5.0, -4.1 G) and a residual alpha-fluorine interaction (0,0,11.2G). Irradiation at 77 K yields an e.s.r.-pattern which is tentatively assigned to the molecular anion radical. These findings are related to the radiation chemistry of solid 5-halouracils.