An EPR Study of the Transfer and Trapping of Holes Produced by Radiation in Guanine (Thioguanine) Hydrochloride Single Crystals

Abstract

Single crystals of guanine hydrochloride monohydrate, guanine hydrochloride dihydrate and anhydrous guanine dihydrochloride, doped with thioguanine, were irradiated with X and gamma rays. In all three systems the dominant radicals were associated with thioguanine. In the former two systems the stabilized species is the thiyl radical, formed by initial loss of an electron at some of the guanines in the crystal lattice, followed by hole migration to thioguanine and subsequent deprotonation of the radical formed. In the anhydrous guanine(thioguanine) dihydrochloride, that process is followed by acquisition of a chlorine ion. In the guanine hydrochloride monohydrate and guanine hydrochloride dihydrate lattices, systems of interacting closely spaced stacked bases and strings of chloride ions might support the migration of electrons and/or holes. In anhydrous guanine dihydrochloride, neither the bases nor the Cl- ions alone are capable of providing the means for the long-range electron, energy and spin transfer. It is the interchangeable sequence of the charged bases and the Cl- ions that makes the supporting strings or networks. The ultimate chlorination of the thioguanine-centered electron-loss radicals depends mainly on the availability of the Cl- ions and the space for their accommodation in the vicinity of the sulfur atom.

Exploring the mechanisms for the radical induced damage of 6‐thioguanine

“…An earlier ENDOR study on guanine crystal indicates that the guanine cation deprotonates at the exocyclic nitrogen (N10 position) at 10 K . Further, EPR experiments on irradiated single crystals of guanine hydrochloride and thioguanine hydrochloride monohydrates revealed that the radicals formed from H‐transfer from these systems are of π type . Hence, at this point, it is important to understand the reactivity between σ and π state of radical ( 17 ).…”

EPR study of the free radicals in the single crystals of 2-thiothymine γ-irradiated at 300K

“…In the single crystals of nucleic acids and their constituents, as well as in the single crystals of 2-thiocytosine [4], 6-thioguanine [7] and 2-thiouracil [11], c-irradiated at low temperature (77 K), only radicals of the p type were observed. Also, radiation-induced sulfur-centered free radicals, stabilized on the base thioanalogs molecules, are always of the cationic origin.…”

“…One of the free radicals associated with sulfur atom of 2-thiothymine is the cation radical of r-electron configuration, formed by a loss of an electron from 2-thiothymine and subsequent deprotonation at N(3) [12]. Sulfur-centered r radical was the first observed radical of r-electron configuration in all nucleic-acid bases or their thioanalogs in ordered structure that is not associated with three-electron bond formed by the interaction with electron-donating group, like Cl À in some hydrochlorinated matrices [5,7]. The other sulfur-centered radical is of p-electron configuration and anionic origin, primarily formed by the capture of an electron at S(2) and subsequently neutralized by the formation of the N 0 (3)-S(2) hydrogen bonding between two adjacent 2-thiothymine molecules, with the H 0 (3) participating in protonation of an anion [13].…”

“…Thioanalogs of the nucleic-acid bases are also known to be good traps of the radiation energy. In a number of studies, it has been shown that these substances, imbedded in small quantities in ordered structures of the nucleic-acid bases, represent preferred sites for localization of the migrating electron/holes [4][5][6][7][8][9][10].…”

5-Methyl-2-thiouracil

“…The thio-analogues of nucleic acid bases absorb light at long wavelengths and can be selectively photoactivated into the electronic triplet state, which leads to their high af®nity for crosslinking to other nucleic acid bases and amino acid residues with which they are in contact. It has also been shown that the thio-analogues of nucleic acid bases behave as good traps for the excess energy emitted by ionizing radiation (Sankovic  et al, 1991;Herak & Hu È ttermann, 1991;Herak et al, 1999Herak et al, , 2000.…”