Krešimir Sanković

An EPR study of the transfer and trapping of holes produced by radiation in guanine(thioguanine) hydrochloride single crystals.

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.

Postirradiation Long-Range Energy Transfer in a Single Crystal of Cytosine Monohydrate: An EPR Study

Thiocytosine molecules incorporated in the cytosine monohydrate crystal lattice act as traps for both electrons and holes. The radiation-induced cytosine ion radicals, C(+) and C(-), release their charge upon heating. The excess electrons and holes migrate long distances in the crystal lattice. The migration of holes has been demonstrated by the postirradiation, thermally activated accumulation of thiocytosine cation radicals, T(+), and the migration of electrons by formation of the S-centered radicals of an anionic nature. It is estimated that the migration length of the holes is at least 30 interbase distances, and the migration length of the electrons is more than 100 interbase distances. The selective formation of the cationic and anionic trap radicals, depending on the trap concentration, is discussed in terms of differences between the migration of electrons and holes.

Radiation energy transfer and trapping in single crystals of hemihydrate and hydrochloride of 5-methylcytosine doped with 5-methylthiocytosine : An EPR study

Abstract Two different crystals of 5-methylcytosine with 5-methylthiocytosine as impuritiy, irradiated with ionising radiation, exhibit quite different types of paramagnetic centres associated with thio impurities. In 5-methylcytosine hemihydrate there are two types of radicals with the unpaired spin located mainly on sulphur. Both are of thiyl structure, presumably derived by a loss of an electron from 5-methylthiocytosine. In doped 5-methylcytosine hydrochloride two distinct paramagnetic species with high spin density on chlorine have been detected. One of them has been spectroscopically characterised and assigned to an electron-deficient species with the unpaired electron shared by a sulphur and a chlorine atom, with some delocalization over the pyrimidine ring. Both crystal lattices of 5-methylcytosine support migration of electrons/holes. In the hemihydrate lattice, the base stacking interaction, although weak in comparison to that in DNA or some other molecular crystals of the nucleic-acid bases, might be responsible for that. In the hydrochloride lattice, the chlorine-ion strings or layers might provide the means for the hole transport. Both neutral and positively charged 5-methylcytosine is a good radiation energy trap (hole trap).

Sigma radicals in gamma-irradiated single crystals of 2-thiothymine

Thioanalogs of the nucleic-acid bases are known photosensitive probes and good traps of radiation energy. Radiation-induced sulfur-centered free radicals stabilized on the base thioanalogs molecules are of the cationic origin. Their electronic configuration is of the π type, unless an electron-donating group, like Cl−, is added to the sulfur atom, forming the σσ* “ three-electron” S∴Cl bond. The σ radical observed in irradiated 2-thiothymine is formed simply by deprotonation of the pristine cation radical. The 2σ character of the radical is determined from the nature of the g tensor and the A(14N) coupling tensor and their relation to the radical skeleton. The deprotonation of the cation radical takes place at N(3) rather than at N(1), generally observed in the irradiated pyrimidine natural bases and their other thioanalogs.

ESR spectroscopy of the sulphur-centered radicals derived from thiocytosine

Abstract Cytosine: thiocytosine crystals are found to be suitable for the observation of radiation-energy transfer to the sulphur-containing species. At low temperatures the thiocytosine cations are formed by a transfer of excitations to thiocytosine in the host matrix. At elevated temperatures the thiocytosine protonated anions are observed. The latter radicals are formed by an electron transfer from the cytosine anion in a thermally activated process, with a subsequent protonation on S-2. The principal elements of the g tensors ( g xx =2.132, g yy =2.004 and g zz =2.002 for the cation and g xx =2.066, g yy =2.008 and g zz =2.000 for the protonated anion) are in agreement with the g values for other sulphur-centered radicals of similar structures.

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

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 gmax(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.

5-Methyl-2-thiouracil.

The molecular structure of the title compound, also known as 2-thiothymine [systematic name: 2,3-dihydro-5-methyl-2-thioxopyrimidin-4(1H)-one], C(5)H(6)N(2)OS, is similar to that of thymine, with only small changes in the ring structure, apart from a significant difference at the substitution site [S=C = 1.674 (1) A]. The molecules are connected by hydrogen bonds, with N-H.O = 2.755 (2) A and N-H.S = 3.352 (1) A. The hydrogen-bond network is different from that in thymine, since it involves all the donor and acceptor atoms.

ENDOR study of the chlorinated thiocytosine radical in a crystal matrix

Interaction of protons with the unpaired electron in the σ2σ*1 three-electron S∴Cl bond has been investigated by electron nuclear double resonance (ENDOR) spectroscopy. The S∴Cl bond in the chlorinated thiocytosine radical, imbedded in the crystal lattice of cytosine hydrochloride, interacts significantly with at least seven protons. Five of these interactions have been analyzed. From the nature of the coupling tensors, it is concluded that three couplings are due to protons being constituents of the chlorine–thiocytosine radical, and the remaining two are due to protons belonging to neighboring cytosine molecules. The nature of the dipolar component of the coupling tensors is not consistent with the location of the entire electron spin density in the S∴Cl bond. From isotropic as well as anisotropic hyperfine coupling tensor components, it is concluded that the unpaired electron is somewhat delocalized over the radical, with a major portion being concentrated in the C(2)–S–Cl region but with a significant fraction occupying a π* molecular orbital extending over the cytosine ring atoms. Density functional theory molecular orbital calculations support such a kind of spin distribution. The data also suggest that the radical remains protonated at both the N(1) and N(3) positions.