ENDOR study of a thiocytosine oxidation product in cytosine monohydrate crystals doped with 2-thiocytosine, X-irradiated at 15 K

Abstract

ENDOR spectroscopy was used for the analysis of radiation-induced electron-loss radicals in thiocytosine. The radicals were induced by X-irradiating single crystals of cytosine monohydrate, substitutionally doped with 2-thiocytosine. X-irradiation and measurements were carried out at about 15 K. The electron-loss radicals were found to be associated with the thiocytosine molecules in a proportion much above the thiocytosine/cytosine ratio in the lattice; the concentration enhancement factor was estimated to be more than 45. The significantly elevated spectroscopic splitting factor (g) for the sulfur-centered radicals in the EPR spectra make these radicals easily discriminated from the spectra of other radicals present in the system. The present study is the first detailed ENDOR study of one of the sulfur-centered radicals selectively formed in an appropriately related crystal lattice as a host. Six different intra- and inter-molecular proton couplings were detected. The respective coupling tensors were determined and the couplings were assigned to protons in the thiocytosine N1-deprotonated cation radical (S₁) itself and in its environment. The structure of radical S₁ is analogous to that of the related cytosine radical in irradiated pure crystals of cytosine monohydrate. However, the two radicals differ in their g values and in the spin density distribution. In the thiocytosine-derived radical g_max(S₁) = 2.132, compared with 2.005 for the corresponding radical in cytosine. In radical S₁ most of the spin density (∽65–70%) is located at sulfur and little at the ring atoms: 0.21 at C5 and about 0.14 at N1, compared with spin densities of 0.58 at C5 and 0.30 at N1 in the corresponding radical in cytosine. A concentration enhancement of >45 suggests long-range hole transfer through the lattice. Assuming that this takes place mainly by the transfer through vertically overlapping (stacked) bases, preliminary considerations show that the effectie range for the hole transfer is more than 22 base separations, or more than 7 nm.