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Defect-engineering of TiO2 can have a major impact on its photocatalytic properties for the degradation of persisting and non-biodegradable pollutants. Herein, a series of intrinsic and extrinsic defects are induced by post annealing of crystalline TiO2 under different reducing atmospheres. A detailed optoelectronic characterization sheds light on the key characteristics of the defect-engineered TiO2 nanopowders that are linked to the photocatalytic performance of the prepared photocatalysts. The photodegradation of a model dye, malachite green, as well as the inactivation of bacterial endospores of the Geobacillus stearothermophilus species were studied in the presence of the developed catalysts under visible light illumination. Our results indicate that a combination of certain defects is necessary for the improvement of the photocatalytic process for water purification and disinfection under visible light.

In the present study, the degradation of methylene blue in non-irradiated TiO2-H2O2 suspensions was investigated. Five commercially available catalysts were characterized (BET surface area, zeta potential, hydrodynamic diameter) and their oxidative power was assessed by means of the degradation of methylene blue. A subsequent EPR study was made to verify and identify potential oxidative radicals. The results showed that all suspensions could degrade methylene blue significantly stronger compared to hydrogen peroxide alone. A broad variation between the different catalysts in their capability to adsorb dye molecules was found which was essential for decomposition of methylene blue in darkness. The highest degradation rate of all samples was found for Degussa P25 at neutral pH. EPR studies of this sample verified the presence of oxygen centred radicals namely hydroxyl ( OH) and superoxide radicals (O2− / OOH). Non-irradiated TiO2-H2O2 systems show great potential not only in dye removal applications but also in the field of disinfection where low concentrations of hydrogen peroxide are required and irradiation may not be feasible.

The structures of the free radicals formed by the irradiation of potassium dithionate (K2S2O6) with 60Co γ-rays and 14N7+ ions were investigated by EPR to further examine a recently proposed LET effect in this material. Two types of SO 3 − radical ions were identified in X-irradiated single crystals by measurements at X- and Q bands. One of these (S1) exhibited 33S hyperfine couplings A⊥ = 12.49, A|| = 15.60 mT, the other (S2) A⊥ = 11.29, A|| = 13.92 mT. The g-factors were nearly isotropic, g⊥ = 2.0010, g|| = 2.0003; and g⊥ = 2.0026, g|| = 2.0008, respectively. The 33S hyperfine coupling tensors and g-tensors were axially symmetric about the trigonal -axis, coinciding with the direction of the S–S bonds of the two non-equivalent S 2 O 6 2 − ions in the crystal. A model for the radiation damage was proposed in which the SO 3 − radical ions retain the orientation of the SO3 groups, aligned along the trigonal axis. The structure of a third main radical species (S3) with g⊥ = 2.0026, g|| = 2.0052 could not be unambiguously assigned, due to undetected 33S features. The relative integrated intensities of S1, S2 and S3 depended on the radiation quality and were approximately estimated as 0.18: 0.65:0.17 for 60Co γ-rays and 0.47: 0.38: 0.15 for 14N7+ ions. Additional weak lines on the low field side of the main signal were tentatively attributed to SO 2 − radical ions. An even weaker strongly anisotropic pair of lines was attributed to SO 3 − radical pairs separated by 0.93–0.95 nm along the trigonal axis.

The photocatalytic behaviour of TiO2 has been thoroughly investigated the past years using UV light to photoactivate TiO2. As this method introduces complications making it difficult to do it economically viable, new pathways to activate TiO2 have been sought. In the present work, reactive oxygen species (ROS) were obtained in a suspension of hydrogen peroxide (H2O2) and titanium dioxide (TiO2) in darkness, offering an alternative method to initiate oxidative behaviour of TiO2. Methylene blue (MB) degradation was chosen as an indicator for measuring the catalytic effect, allowing for a comparison of the reaction kinetics with other methods to create ROS with TiO2. The effects of TiO2 particle effective surface area and concentration of H2O2 were also studied. EPR studies were made to determine the presence of free radicals in the mixture of TiO2/H2O2 in darkness. Under the given lighting conditions, the results demonstrate the relevance of the TiO2 effective surface area and concentration of H2O2 and TiO2 as parameters influencing the chemical catalysis of MB due to the action of hydroxyl (OH ) and/or hydroperoxide radicals (HO2 ).

Purpose: To perform a systematic investigation of the energy dependence of alanine and lilthium formate EPR dosimeters for medium energy x rays. Methods: Lithium formate and alanine EPR dosimeters were exposed to eight different x-ray beam qualities, with nominal potentials ranging from 50 to 200 kV. Following ionometry based on standards of absorbed dose to water, the dosimeters were given two different doses of approximately 3 and 6 Gy for each radiation quality, with three dosimeters for each dose. A reference series was also irradiated to three different dose levels at a {sup 60}Co unit. The dose to water energy response, that is, the dosimeter reading per absorbed dose to water relative to that for {sup 60}Co {gamma}-rays, was estimated for each beam quality. In addition, the energy response was calculated by Monte Carlo simulations and compared to the experimental energy response. Results: The experimental energy response estimates ranged from 0.89 to 0.94 and from 0.68 to 0.90 for lithium formate and alanine, respectively. The uncertainties in the experimental energy response estimates were typically 3%. The relative effectiveness, that is, the ratio of the experimental energy response to that following Monte Carlo simulations was, on average, 0.96 and 0.94 formore » lithium formate and alanine, respectively. Conclusions: This work shows that lithium formate dosimeters are less dependent on x-ray energy than alanine. Furthermore, as the relative effectiveness for both lithium formate and alanine were systematically less than unity, the yield of radiation-induced radicals is decreased following x-irradiation compared to irradiation with {sup 60}Co {gamma}-rays.« less

The primary oxidation product in X-irradiated single crystals of 5-methylcytosine hemihydrate and 5-methylcytosine hydrochloride has been studied at 10 K, using electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) spectroscopies. The radical is characterized by large couplings to the methyl protons and appears to be deprotonated at N1 in both crystal systems. In the hydrochloride crystal the methyl group is completely frozen at 10 K, whereas in the hemihydrate crystal it undergoes tunneling rotation. For the hemihydrate crystal, four ENDOR lines associated with transitions within the A and E rotational states were followed in three planes of rotation. Large ENDOR shifts as measured by saturation of the high- and low-field parts of the EPR spectrum indicate that the rotation is rather slow. Sidebands due to mixing of A and E rotational states are expected for slow rotation and were observed in both the EPR and the EIE spectra. The ENDOR shifts and the sideband frequencies indicate a tunneling splitting between 40 and 60 MHz. Estimates of the barrier to rotation in both crystalline systems were calculated using cluster and single-molecule density functional theory methods, and the results are consistent with those obtained by analysis of the experimental results.

Radical formation and trapping of radicals in X-irradiated crystals of rhamnose at 6 K were investigated using electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques, complemented with periodic density functional theory (DFT) calculations. The two major radical species at 6 K were the O4-centered alkoxy radical and the intermolecularly trapped electron (IMTE), previously also detected by other authors. The current experimental results provided hyperfine coupling constants for these two species in good agreement with the previous data, thus providing a consistency check that improves their credibility. In addition to the O4-centered alkoxy radical and the IMTE, the C3-centered and C5-centered hydroxyalkyl radicals are the most prominent primary species at 6 K. The C3-centered radical appears in two slightly different conformations at 6 K, designated C and D. The C5-centered radical exhibits a coupling to a methyl group with tunneling rotation at 6 K, and analysis of one of the rotational substates (A) of the spin system yielded an understanding of the structure of this radical. Visible light bleaching of the IMTE at 6 K led to the C3-centered radical C, and thermal annealing above 6 K resulted in a conversion of the C to the D conformation. In addition, thermal annealing releases the IMTE, apparently resulting in the formation of the C2-centered radical. It is possible that the thermal decay of the IMTE also contributes to a small part of the C3-centered radical (D) population at 85 K. There are several other products trapped in rhamnose crystals directly after irradiation at 6 K, among which are resonance lines due to the C2 H-abstraction product. However, these other products are minority species and were not fully characterized in the current work.

Single crystals of the 1:1 complex of the nucleic acid base cytosine and the dipeptide N-formylglycine (C· NFG) have been irradiated at 10 and 273 K to doses of about 70 kGy and studied at temperatures between 10 and 293 K using 24 GHz (K-band) and 9.5 GHz (X-band) electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) spectroscopy. In this complex, the cytosine base is hydrogen bonded at positions N3 and N4 to the carboxylic group of the dipeptide, and the N3 position of cytosine has become protonated by the carboxylic group. At 10 K, two major radicals were characterized and identified. One of these (R1) is ascribed to the decarboxylated N-formylglycine one-electron oxidized species. The other (R2) is the N3-protonated cytosine one-electron reduced species. A third minority species (R3) appears to be a different conformation or protonation state of the one-electron reduced cytosine radical. Upon warming, the R2 and R3 radicals decay at about 100 K, and at 295 K, the only cytosine-centered radicals present are the C5 and C6 H-addition radicals (R5, R6). The Rl radical decays at about 150 K, and a glycine backbone radical (R4) grows in slowly. Thus, in the complex, a complete separation of initial oxidation and reduction events occurs, with oxidation localized at the dipeptide moiety, whereas reduction occurs at the nucleic acid base moiety. DFT calculations indicate that this separation is driven by large differences in electron affinities and ionization potentials between the two constituents of the complex. Once the initial oxidation and reduction products are trapped, no further electron transfer between the two constituents of the complex takes place.

Polycrystalline, formates and dithionates are promising materials for EPR dosimetry, as large yields of radiation induced stable radicals are formed with a linear dose response. Rapid spin relaxation rates were detected in many of the substances, indicating that a high microwave power can be applied during EPR acquisition in order to improve sensitivity. Different techniques used to further improve the sensitivity, such as the replacement of Li-7 with Li-6 or exchange of protons with deuterons in the corresponding crystalline matrices and metal ion doping are discussed. It is concluded that formates and dithionates may be up to 10 times as sensitive as L-alpha-alanine. (C) 2004 Elsevier Ltd. All rights reserved.

α-L-Sorbose single crystals were X-irradiated at 295 K (room temperature). A combined electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EI-EPR) study at 120 K revealed a realm of radiation-induced free radicals in this sugar system. In the present work, a pair of closely related radicals is focused on, being dominant immediately after irradiation, but unstable with respect to long time storage or upon warming the samples. A density functional theory (DFT) study was carried out considering the complete hyperfine coupling tensors (principal axes and anisotropic and isotropic couplings) in comparison with the observed electron-proton interactions. This combined approach yielded very plausible models for both radicals, which are formed by a net hydrogen-abstraction from the C3 position of the six-membered sorbose ring. It appears that the difference between the two species is linked to the molecular disorder in the sorbose crystal structure. In addition, DFT calculations of the g tensors were performed for the plausible radical conformations.

Radical formation and hole transfer were investigated in crystals of cytosine.HCl (C.HCl) doped with 0-1.1 mol-% 5-methylcytosine x HCl (5MC x HCl). The doping level was determined by NMR spectroscopy. Crystals and polycrystalline samples were X-irradiated at 295 K, 77 K and 12 K and studied with EPR, ENDOR and FSE spectroscopy at these temperatures. At 295 K the dominant radicals were the so-called 3alphaH radical, formed in 5MC by a net H-abstraction from the methyl group, and the cytosine C6 H-addition (5-yl) radical. At 12 K five radicals were identified. These were the 3alphaH radical, cytosine reduction and oxidation products, and the cytosine C6 and C5 H-addition (5-yl and 6-yl, respectively) radicals. The spectroscopic parameters for the 3alphaH radical are very similar to those of a radical observed previously in the crystalline cytosine derivatives cytidine (CR), 2'deoxycytidine hydrochloride (CdR x HCl), 5'dCMP and 3'CMP as well as in the uracil derivative 2-thiouracil (2-TU). It was shown that amounts of the order of tenths of a percent 5MC x HCl doped into crystals of C.HCl give rise to a considerable yield of 3alphaH radicals after exposure to ionizing radiation both at room temperature and at lower temperatures. This supports a previous suggestion that naturally occurring 5-methylated cytosine impurities may be responsible for the formation of 3alphaH radicals in the crystalline cytosine derivatives CR, CdR.HCl, 5'dCMP and 3'CMP and suggests that the 3alphaH radical in these systems is a 5-methylated base-centered radical. The total radical yield in doped C x HCl crystals increased considerably with the doping level, both at low temperatures and at room temperature, implying that the 3alphaH radical is more stable than the primary cytosine radicals. The relative amounts of the 3alphaH radical were obtained by using simulated benchmark spectra to reconstruct experimental EPR spectra of doped polycrystalline samples. Evidence is presented suggesting that the enhanced yield of the 3alphaH radical in doped samples is due to holes originally formed at cytosine bases and transferred to 5-methylcytosine bases in addition to the 3alphaH radical being less exposed to recombination than other cytosine radicals.

Single crystals of anhydrous thymine (Ta) and cytosine monohydrate (Cm) were irradiated at room temperature using X rays, 20 MeV protons and 35 MeV alpha particles, and the relative distributions of the various radiation-induced stable radicals in the crystals were investigated. These two crystal systems were chosen because of systematic differences in their molecular packing and hydrogen-bonding network. The radicals stabilized in these systems have previously been identified and analyzed by several authors. Experimental EPR spectra could thus be reconstructed based on simulated benchmark spectra using a fitting procedure, yielding the relative amounts of the different radical species. It was found that the relative amounts of a given radical species varied with the type of radiation used, with differences being most prominent between alpha particles and protons or X rays. In Ta, an increased production of hydrogen addition or abstraction radicals was found after exposure to alpha particles. These radicals are believed to be formed predominantly from superexcited states, resulting from the higher density of ionizations along the track of the alpha particle. A corresponding reduction in radicals derived from proton transfer reactions was observed in comparison with exposure to protons or X rays. In Cm, the differences were smaller than for Ta. This is probably due to differences in the arrangement of the crystal lattice between the two systems, with Cm having an extended hydrogen-bonding network promoting fast proton transfers after ionization. Most interesting, however, was the observation that more than 50% of the radical population in Cm not could be accounted for by known radical species. The population with an unknown origin was represented with a broad singlet, having a line width of 1.5 mT and a g value of 2.0045, in the spectral reconstruction procedure.

When single crystals of the 5-membered heterocyclic ring structure 2-oxazolidinone (C3O2NH5) are irradiated at room temperature, the major radical formed (R1) decays during a period of a few hours, leaving a broad, unstructured EPR spectrum not amenable to analysis. Cooling the crystals to about 140 K immediately after irradiation at room temperature allows analysis of the R1 radical by EPR and ENDOR. The radical is formed by a net H-abstraction from one of the two methylene groups of the molecule. A full EPR and ENDOR analysis of four proton interactions (one α-coupling, and three β-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carbon-centered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C1–H bond with respect to the N–C1–C2 plane is estimated to be 13–15° (bending angle θ ≈ 7°). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogen-bonded proton hyperfine interactions.

The structural changes throughout the entire reductive radiation-induced pathway of L-alpha-alanine are solved on an atomistic level with the aid of periodic DFT and nudged elastic band (NEB) simulations. This yields unprecedented information on the conformational changes taking place, including the protonation state of the carboxyl group in the "unstable'' and "stable'' alanine radicals and the internal transformation converting these two radical variants at temperatures above 220 K. The structures of all stable radicals were verified by calculating EPR properties and comparing those with experimental data. The variation of the energy throughout the full radiochemical process provides crucial insight into the reason why these structural changes and rearrangements occur. Starting from electron capture, the excess electron quickly localizes on the carbon of a carboxyl group, which pyramidalizes and receives a proton from the amino group of a neighboring alanine molecule, forming a first stable radical species (up to 150 K). In the temperature interval 150-220 K, this radical deaminates and deprotonates at the carboxyl group, the detached amino group undergoes inversion and its methyl group sustains an internal rotation. This yields the so-called "unstable alanine radical''. Above 220 K, triggered by the attachment of an additional proton on the detached amino group, the radical then undergoes an internal rotation in the reverse direction, giving rise to the "stable alanine radical'', which is the final stage in the reductive radiation-induced decay of alanine.

We review our research of the past decade towards identification of radiation-induced radicals in solid state sugars and sugar phosphates. Detailed models of the radical structures are obtained by combining EPR and ENDOR experiments with DFT calculations of g and proton HF tensors, with agreement in their anisotropy serving as most important criterion. Symmetry-related and Schonland ambiguities, which may hamper such identification, are reviewed. Thermally induced transformations of initial radiation damage into more stable radicals can also be monitored in the EPR (and ENDOR) experiments and in principle provide information on stable radical formation mechanisms. Thermal annealing experi-ments reveal, however, that radical recombination and/or diamagnetic radiation damage is also quite important. Analysis strategies are illustrated with research on sucrose. Results on dipotassium glucose-1-phosphate and trehalose dihydrate, fructose and sorbose are also briefly discussed. Our study demonstrates that radiation damage is strongly regio-selective and that certain general principles govern the stable radical formation.

The process of stabilizing radiation damage in the base pair of 1-methylcytosine (1-MeC):5-fluorouracil (5-FU) has been investigated. The formation of free radicals in purines and pyrimidines is influenced by the matrix in which they are irradiated. Of particular interest are the systems in which two different nucleic acid bases are complexed, providing situations that approximate the close proximity of bases in nucleic acid polymers. Detailed EPR/electron−nuclear double resonance experiments show that only the N3 protonated cytosine anion and the N1 deprotonated uracil cation are observed in single crystals of 1-MeC:5-FU, X-irradiated, and observed at 10 K. Upon warming one observes a radical formed by net hydrogen addition to C6 on uracil, and an allylic radical on the C4−C5−C6 region of the uracil. No cytosine C5 or C6 H-addition radicals are observed. The implications that free radical damage is transferred from the cytosine moiety to the uracil moiety in 1-MeC:5-FU is discussed. Single point calculat...

This study investigates the effects of aliphatic (n-heptane, n-nonane), naphtenic (methylcyclohexane, 1,2,4-trimethylcyclohexane (TMCH)), and aromatic (methylbenzene, 1,2,4-trimethylbenzene (TMB)) hydrocarbons on respiratory burst in human granulocytes. The free radical formation was measured as 2,7-dichlorofluorescein diacetate-amplified (DCF) fluorescence, by electron paramagnetic resonance (EPR) spectroscopy and by hydroxylation of 4-hydroxybenzoate. The chemotactic peptide N-formyl-met-leu-phe (fMLP) and phorbol 12-myristate 13-acetate (PMA), a diacylglycerol analogue, were included as positive controls. DCF fluorescence was elevated in a concentration-dependent manner by C9 hydrocarbons. The C7 hydrocarbons did not stimulate respiratory burst in the concentration range examined. The naphtenic hydrocarbon TMCH showed the strongest effect on respiratory burst and was therefore selected for mechanistic studies of this free radical formation. In the absence of extracellular Ca(2+), fluorescence in response to TMCH and fMLP was reduced by 77 and 90%, respectively. Preincubation of the granulocytes with the protein kinase C inhibitor bisindolylmaleimide reduced the DCF fluorescence stimulated with TMCH, fMLP, and PMA by 82, 56, and 90%, respectively. The phospholipase C inhibitor U73122 lowered the TMCH- and fMLP-activated DCF fluorescence by 87 and 76%. In addition, the TMCH- and fMLP-induced DCF fluorescence, after the preincubation with the phospholipase D modulator n-butanol, was lowered by 83 and 52%, respectively. The importance of protein kinase C, phospholipase C, and phospholipase D for elevation of respiratory burst was also demonstrated by the EPR experiments using the spin trap 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO). Preincubation with the NADPH oxidase inhibitor diphenyleneiodonium and diethyldithiocarbamate, which inhibits superoxide dismutase, led to an almost complete reduction of DCF fluorescence in response to TMCH, fMLP, and PMA. Preincubation with diethyldithiocarbamate led to the elevation of superoxide adducts of DEPMPO. The hydrocarbons stimulated formation of mainly the superoxide (O(*-)(2)) adduct of DEPMPO (DEPMPO-OOH) but also small amounts of the hydroxyl adduct ((*)OH) (DEPMPO-OH). Using 4-hydroxybenzoate as a hydroxyl radical trap confirmed formation of (*)OH after stimulation with the hydrocarbons. In conclusion, our findings indicate that TMCH-activated respiratory burst is dependent on the Ca(2+)-dependent phospholipase C, phospholipase D, and protein kinase C prior to activation of the NADPH oxidase.

The alpha-proton hyperfine coupling observed by electron paramagnetic resonance (EPR) spectroscopy on the radical (CH)-C-.(COOH)(2) in irradiated crystals of malonic acid, CH2(COOH)(2), has served ...

In single crystals of malonic acid grown from heavy water, the methylene protons have been partially exchanged with deuterons. Upon X irradiation at room temperature, the (CD)-C-.(COOD)(2) radical is formed in an amount comparable to the (CH)-C-.(COOD): radical species. In the present work, EPR and ENDOR analyses of the a-deuteron hyperfine coupling (hfc) and nuclear quadrupolar coupling (nqc) tensors at room temperature have been performed. The hyperfine coupling tensor is, when scaled with the differences in the nuclear g-factor, almost identical to the a-proton coupling of the (CH)-C-.(COOH)(2) radical at room temperature. The quadrupolar coupling tensor was found to be virtually coaxial with the hyperfine coupling tensor. The quadrupolar coupling constant is 149.8 +/- 1 kHz, and the asymmetry factor eta = 0.092 +/- 0.020. It is known that, at room temperature, the malonic acid radical exhibits thermal motion between two potential energy minima separated by about +/-12 degrees. Assuming that the observed hfe and nqc tensors are the result of thermal avenging between these two conformations of the radical, a simple two-site jump model was used to estimate the rigid-limit tensors. The most significant result obtained was for the nyc tensor, for which the calculations resulted in a quadrupolar coupling constant of 160 kHz and an asymmetry factor eta = 0.026. These values are fairly close to the nqc parameters for the methylene deuterons in malonic acid at low temperature. The quadrupolar coupling tensor has been theoretically modeled using Slater orbitals and formal electronic populations, as well as electron populations obtained from RHF/CI INDO-type calculations. The simple model to compute the electric field gradient at the alpha-deuteron caused by the charge distribution at the sp(2)-hybridized alpha-carbon was found to be as successful as more advanced methods. Furthermore, density functional theoretical (DFT) calculations for both the malonic acid radical and the native malonic acid molecule have been performed. Field gradients calculated by the DFT method significantly overestimate the quadrupolar tensors for both the alpha-deuteron of the radical and the methylene deuterons of the malonic acid molecule. Calculations using electron populations from the RHF/CI INDO calculations show that contributions to the quadrupolar coupling tensor from electrons and nuclei beyond the nearest-neighbor atom of the Jeuteron are significant.

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.

Single crystals of the amino acid α-glycine N−CH2−COO- were X-ray irradiated at 280 K and studied at 295 K and at 100 K using EPR, ENDOR, and EIE techniques. Five radicals were detected and characterized. Two of these are well known from previous studies, the oxidation product N−ĊH−COO- (radical I) and the reduction product ĊH2−COO- (radical II). It is shown that at 295 K the ENDOR and EIE spectra from radical I are characteristic of efficient W1x cross relaxation induced by rapid rotation of the amino group. This allows for the determination of the absolute signs of the hyperfine coupling constants of radical I. The other three radicals are two geometrical conformations of the product H2N−ĊH−COOH (radicals III and IV) and a species suggested to be the dimer product N−CH2−CO−ĊH−COO- (radical V). Radical III exhibits spectral parameters related to those previously reported by M. Brustolon et al. (J. Phys. Chem. 1997, 101, 4887). However, on the basis of our analysis of ENDOR and EIE spectra from normal as ...

Single crystals of the amino acid l-α-alanine have been X-irradiated at room temperature, and the free radical formation has been studied using X-band and K-band EPR, ENDOR, and EIE (ENDOR-induced EPR) spectroscopy in the temperature interval 220−295 K. Aided by the results from EIE, as well as ENDOR from selected magnetic field positions, nine hyperfine coupling tensors were obtained and assigned to three different radicals. Room-temperature relaxation behaviors characterized by efficient W1x and W1e and by slow W1n relaxation rates allowed for determination of the signs of the various hyperfine couplings from the ENDOR spectra obtained at room temperature. The temperature dependence of the W1x relaxation is qualitatively discussed. The EPR spectra from alanine are dominated by the well-known resonance of the “stable alanine radical”, SAR, formed by a net deamination of the protonated alanine anion. Precise hyperfine coupling tensors due to the α-proton coupling, the methyl group coupling, and a dipolar ...

Single crystals of the amino acid analogue hippuric acid, PhCONHCH2COOH, have been X-irradiated at 295 K and studied using X-band EPR, ENDOR, and ENDOR-induced EPR (EIE) spectroscopy at 295 and 130 K. Two different radical species were observed and characterized. The dominant species is radical R1, PhCONH-•CH2, supposedly formed by net decarboxylation from a pristine oxidation product. The nitrogen hyperfine and quadrupolar interactions yield information on the electronic structure in the nitrogen valence orbitals. The second radical species, radical R2, is formed by a net hydrogen addition to the phenyl entity of hippuric acid. As a reduction product, it may be formed by protonation of the negatively charged anion of the phenyl group, but the alternative mechanism of direct hydrogen addition to the phenyl ring cannot be ruled out. Spectral simulations indicate that radical R1 contributes about 85% of the total EPR spectrum, while the remaining 15% is contributed by radical R2.

Two independent molecules in the crystal structure of the title nucleoside, 1-(2-de­oxy-α-d-ribofuranos­yl)-5-methyl­uracil, C10H14N2O5, form a dimer connected by two inter-base hydrogen bonds. The ring puckering modes are envelope C4′-endo and half-chair C3′-exo-C4′-endo, respectively, which are quite uncommon conformations for 2′-deoxy­riboses.

The crystal structure of the title complex, Na2+·C9H12N3O8P−·6.07H2O·0.27CH4O, in space group P212121 is remarkably similar to the monoclinic P21 structure of sodium cytidine-5′-monophosphate 6.5 hydrate [Borodi et al. (2001). Acta Cryst. E57, m514–m516].

X-ray irradiation of glutarimide single crystal yields radicals by detachment of a hydrogen atom from the carbon atom at meta position with respect to the nitrogen of the glutarimide ring. Two chemically similar but crystallographically different radicals present in the ratio 3:1 were identified using ESR, ENDOR and ENDOR-induced ESR (EIE). The hyperfine couplings (hfc) of the two protons in β position with respect to the radical site of the abundant radical, changed reversibly between 51−38 G and 12–22 G in the temperature range 100–375 K. The α-proton hyperfine couplings of the two radicals were not affected. Chemical exchange, libration motion of the β protons and conformation changes of the glutarimide ring were considered as possible sources of the temperature effects. The latter models are both plausible but cannot be differentiated in the absence of crystallographic data at low temperature.

The crystal structure of the title complex, C4H6N3O+·C9H8NO3−·H2O, has been redetermined as part of an electron paramagnetic resonance (EPR) study of the effect of radiation on such systems. Protonated cytosine and the deprotonated carboxyl­ate group of the amino acid form a typical eight-membered hydrogen-bonded ring motif.

Single crystals of Rochelle salt, [(-)OOC-CHOH-CHOH-COO(-), Na(+) K(+)]center dot 4H(2)O, X-irradiated at 10 K, have been examined using EPR, ENDOR and EIE spectroscopic techniques to characterize the radiation induced radicals stable at that temperature and their reactions upon warming. The one-electron gain product was observed and from the hyperfine interaction with a beta-proton it was unambiguously centered at the C(4) position of the tartrate moiety. An additional nearly isotropic hyperfine structure of about 21 MHz was tentatively assigned to interaction with a sodium ion exhibiting a close contact to O(3) in the crystal. Evidence was obtained that the one-electron reduced radical had become protonated at one of the C(4) bonded carboxyl oxygens, most probably O(4). No evidence for the corresponding C(1)-centered reduction product was found. Two resonance lines (R2, A1) were shown by EIE to belong to a species formed by decarboxylation at C(3), a secondary oxidation product. Two other resonance lines (K1, K2) were assigned to two varieties of another decarboxylation radical, centered at C(2), distinguished by differences in the potassium ion coordination. Furthermore, one other resonance line (A2) was tentatively ascribed to a third decarboxylation radical, centered at the opposite end of the tartrate moiety. The precursor of these products, that is, the one-electron loss product, was not observed after X-irradiation at 10 K. Thermally induced free radical reactions followed by EPR in the temperature range of 12-119K indicate that a water molecule or a hydroxyl ion is eliminated from the one-electron reduction product radical and that a C(3)-centered radical is formed. The reduction and oxidation reaction pathways of hydroxy acid derivatives are discussed.

Single crystals of thymidine and 1-methylthymine were x-irradiated at 10 K and studied by using K-band ESR, ENDOR, and FSE spectroscopy. In thymidine, six primary radicals were identified. The major species were the O4-protonated anion (TI), the radical formed by net H addition to the C6 position of the base (TII), the radical formed by net H abstraction from the methyl group (TIII), and the alkoxy radical (TVI). The two minority radicals were one formed by net H addition to the C5 position of the base (TIV) and one in which the unpaired spin was localized on the sugar moiety, thought to be a C1{prime} H abstraction radical (TV). A small, nonexchangeable coupling in radical TI was shown to be due to a coupling to the methyl group. Evidence is presented that the characteristics of this coupling tensor indicates the protonation state of the thymine anion radical. Upon annealing, the anion decayed at about 40 K with no detectable successor. The alkoxy radical decays in the same temperature range. In 1-methylthymine, three major radical species were identified.

Anhydrous single crystals of thymine have been X-irradiated at 10 K and studied using K-band EPR, ENDOR, and FSE spectroscopy. In addition to several radical pairs, five monoradicals were formed and stabilized at this temperature. It is demonstrated that the dominating radical pair species analyzed previously consists of two radicals, each formed by net hydrogen abstraction from the methyl group. The five monoradicals were identified to be the O4-protonated thymine anion, the thymine-7-yl radical formed by net hydrogen addition to C6, and the 1-yl radical formed by net hydrogen abstraction from N1. Upon thermal annealing, the protonated anion decays before 60 K apparently with no successor, and the 6-yl radical. Possible mechanisms for the formation of these radicals are discussed. These involve processes initiated by primary ionization followed by charge neutralization by protonation/deprotonation or by geminate recombination leading to the formation of hydrogen atoms by homolytic bond scission from molecules in (super) excited states. 23 refs., 5 figs., 3 tabs.

Single crystals of cytosine monohydrate were X-irradiated at 10 K and examined using K-band EPR, ENDOR, and FSE spectroscopy at temperatures between 10 K and room temperature. Three radicals dominated the low-temperature spectra. Radical RI is the cytosine anion protonated at the N3-position. A large coupling to HC6 and smaller couplings to HN3 and to the two amino protons were detected and their characteristics unambiguously established the structure of this radical. Radical RII exhibits major couplings to HC5, N1, and to the nitrogen and the protons of the amino group

Single crystals of 5-methylcytosine hemihydrate and 5-methylcytosine hydrochloride were X-irradiated and studied at 10 K and at higher temperatures using X- and K-band EPR, ENDOR and EIE spectroscopy. In the hemihydrate crystals, four radicals were identified at 10 K, one of them being the recently reported N1-deprotonated one-electron oxidation product (Krivokapić et al., J. Phys. Chem. A 113, 9633-9640, 2009). The other radicals were the 3alphaH radical and the C6 and C5 H-addition radicals (the 5-yl and 6-yl radicals, respectively). After irradiation at 295 K, only the 3alphaH and the 5-yl radicals were observed. In the hydrochloride crystals, at least seven different radicals were present after irradiation at 10 K. These were the N1-deprotonated one-electron oxidation product, the 3alphaH radical, three different one-electron reduction products, and the 5- and 6-yl radicals. DFT calculations were used to assist in assigning the observed couplings. The 3alphaH and 5-yl radicals were dominant after thermal annealing to room temperature. In neither crystal system did the N1-deprotonated oxidation product transform into the 3alphaH radical upon warming. The radical yield was significantly greater after irradiation at 300 K compared to that after irradiation at 10 K followed by warming to 300 K and was also considerably greater in the hydrochloride crystals than in the hemihydrate crystals.

Anhydrous single crystals of thymine have been X-irradiated at 10 K and studied using K-band EPR, ENDOR, and FSE spectroscopy. In addition to several radical pairs, five monoradicals were formed and stabilized at this temperature. It is demonstrated that the dominating radical pair species analyzed previously consists of two radicals, each formed by net hydrogen abstraction from the methyl group. The five monoradicals were identified to be the O4-protonated thymine anion, the thymine-7-yl radical formed by net hydrogen addition to C6, and the 1-yl radical formed by net hydrogen abstraction from N1. Upon thermal annealing, the protonated anion decays before 60 K apparently with no successor, and the 6-yl radical. Possible mechanisms for the formation of these radicals are discussed. These involve processes initiated by primary ionization followed by charge neutralization by protonation/deprotonation or by geminate recombination leading to the formation of hydrogen atoms by homolytic bond scission from molecules in (super) excited states. 23 refs., 5 figs., 3 tabs.

Single crystals of thymidine and 1-methylthymine were x-irradiated at 10 K and studied by using K-band ESR, ENDOR, and FSE spectroscopy. In thymidine, six primary radicals were identified. The major species were the O4-protonated anion (TI), the radical formed by net H addition to the C6 position of the base (TII), the radical formed by net H abstraction from the methyl group (TIII), and the alkoxy radical (TVI). The two minority radicals were one formed by net H addition to the C5 position of the base (TIV) and one in which the unpaired spin was localized on the sugar moiety, thought to be a C1{prime} H abstraction radical (TV). A small, nonexchangeable coupling in radical TI was shown to be due to a coupling to the methyl group. Evidence is presented that the characteristics of this coupling tensor indicates the protonation state of the thymine anion radical. Upon annealing, the anion decayed at about 40 K with no detectable successor. The alkoxy radical decays in the same temperature range. In 1-methylthymine, three major radical species were identified.

One of the major stable radiation-induced radicals in sucrose single crystals (radical T2) has been identified by means of density functional theory (DFT) calculations of electron magnetic resonance parameters. The radical is formed by a net glycosidic bond cleavage, giving rise to a glucose-centered radical with the major part of the spin density residing at the C 1 carbon atom. A concerted formation of a carbonyl group at the C 2 carbon accounts for the relatively small spin density at C 1 and the enhanced g factor anisotropy of the radical, both well-known properties of this radical from several previous experimental investigations. The experimentally determined and DFT calculated proton hyperfine coupling tensors agree very well on all accounts. The influence of the exact geometrical configuration of the radical and its environment on the tensors is explored in an attempt to explain the occurrence and characteristics of radical T3, another major species that is most likely another conformation of T2. No definitive conclusions with regard to the actual structure of T3 could be arrived at from this study. However, the results indicate that, most likely, T3 is identical in chemical structure to T2 and that changes in the orientation of neighboring hydroxy groups or changes in the configuration of the neighboring fructose ring can probably not account for the type and size of the discrepancies between T2 and T3.

Single crystals of L-cysteic acid monohydrate were X-irradiated and studied at 295 K using EPR, ENDOR, and EIE techniques. Three spectroscopically different radicals were observed. These were a deamination radical reduction product (R1), and two oxidation products formed by hydrogen abstraction (radicals R2, R3). R2 and R3 were shown to exhibit the same chemical structure while exhibiting very different geometrical conformations. Cluster DFT calculations at the 6-31G(d,p) level of theory supported the experimental observations for radicals R1 and R2. It was not possible to simulate the R3 radical in any attempted cluster; hence, for this purpose a single molecule approach was used. The precursor radicals for R1, R2, and R3, identified in the low-temperature work on L-cysteic acid monohydrate by Box and Budzinski, were also investigated using DFT calculations. The experimentally determined EPR parameters for the low-temperature decarboxylated cation could only be reproduced correctly within the cluster when the carboxyl group remained in the proximity of the radical. Only one of the two observed low-temperature carboxyl anions (stable at 4 and 48 K) could be successfully simulated by the DFT calculations. Evidence is presented in support of the conclusions that the carboxyl reduction product already is protonated at 4 K and that the irreversible conversion between the two reduction products is brought forward by an umbrella-type inversion of the carboxyl group.

Recently, methylated CpG sites have been established as mutational hotspots in DNA, one reason of which being oxidation of 5-methylcytosine. 1 Up to 4% of the cytosines in DNA are 5-methylcytosine (m 5 C). 2 The ionization potential of m 5 C is comparable to that of guanine and m 5 C may compete with guanine as a hole trap. Holes that are formed by ionizing radiation in the base regions of DNA may thus subsequently transfer to m 5 C. 3 Once oxidized; the m 5 C radical may irreversibly deprotonate at the methyl group and form a highly stable radical known as the 3αH radical 4 with a lifetime sufficiently long to impair normal enzymatic activity. To better understand the electronic properties of m 5 C as a hole trap, and the mechanisms that form the stable end product, the structure of the initial electron-loss product needs to be determined. Single crystals of 5-methylcytosine hemihydrate and 5-methylcytosine hydrochloride have been X-irradiated at 8-10 K and studied at this and elevated temperatures using EPR, ENDOR, and EIE spectroscopy in addition to single molecule- and cluster DFT calculations at the B3LYP level. The primary oxidation radical is stable only at very low temperatures. Both the experimental results and the DFT calculations indicate a structure where a large part of the electron spin is coupled with the methyl protons. At 10 K the methyl group is not able to rotate classically but undergoes a hindered rotation by tunneling through a rotation barrier at a frequency of ≈100 MHz indicating a barrier of 1-2 kcal/mol. The radical appears to deprotonate at different sites in the two distinct crystalline systems, indicating additional influences of the surroundings. The driving force for deprotonation at various sites has been calculated by DFT. Upon thermal annealing, the primary oxidation radical seems to recombine rather than transform into the 3αH radical, suggesting a high barrier for deprotonation at the methyl group. However, the driving force is greater than for deprotonation at other sites and at room temperature the 3αH radical is the only observed oxidation product in m5C. 1) Chen J.X.; Zheng Y.; West M.; Tang M. Cancer Research 1998, 58, 2070-2075. 2) Erlich, M. Oncogene 2002, 21, 5400-5413. 3) Krivokapic, A; Sagstuen, E. Journal of Physical Chemistry A 2003, 107, 9561-9566. 4) Krivokapic, A; Hole, E. O.; Sagstuen, E. Radiation Research 2003, 160, 340-354.

Single crystals of the phosphorylated amino acid L-O-serine phosphate were X-irradiated and studied at 10 K and at 77 K using EPR, ENDOR, and EIE techniques. Two radicals, R1(10 K) and R1(77 K), were detected and characterized as two different geometrical conformations of the protonated reduction productCH-C(OH)(2). R1(10 K) is only observed after irradiation at 10 K, and upon heating to 40 K, R1(10 K) transforms rapidly and irreversibly into R1(77 K). The transition from R1(10 K) to R1(77 K) strongly increases the isotropic hyperfine coupling of the C-CH(beta) coupling (Delta = 32 MHz) and the major C-OH(beta) coupling (Delta = 47 MHz), in sharp contrast to the their much reduced anisotropic hyperfine couplings after the transition. An umbrella-like inversion of the carboxylic acid center, accompanied by minor geometrical adjustments, explains the changes of these observed isotropic and anisotropic couplings. DFT calculations were done on the reduced and protonated L-O-serine phosphate radical at the B3LYP/6-311+G(2df,p)//B3LYP/6-31+G(d) level of theory in order to support the experimental observations. Two different conformations of the anion radical, related by an inversion at the carboxylic center, could be found within the single molecule partial energy-optimization scheme. These two conformations reproduce the experimental hyperfine couplings from radicals R1(10 K) and R1(77 K). A third radical, radical R2, was observed experimentally at both 10 and 77 K and was shown to be due to the decarboxylated L-O-serine phosphate oxidation product, a conclusion fully supported from the DFT calculations. Upon thermal annealing from 77 to 295 K, radicals R1(77 K) and R2 disappeared and all three previously observed room-temperature radicals could be observed. No phosphate-centered radicals could be observed at any temperatures, indicating that the phosphate-ester bond break for one of the room-temperature radicals does not occur by dissociative electron capture at the phosphate group.

As a part of a program to develop an electron paramagnetic resonance (EPR) dosimeter suited for clinical use (doses in the cGy range), polycrystalline samples of lithium formate monohydrate (HCO2Li·H2O), magnesium formate dihydrate (C2H2O4Mg·2H2O), and calcium formate (C2H2O4Ca) have been examined. l -Alanine was included for comparison and reference. Samples were irradiated with 60Co γ-rays and 60–220 kV X-rays. The dosimeter response was assessed using the peak-to-peak amplitude of the first-derivative EPR spectrum. Dose–response curves for the 60Co γ-irradiated samples were constructed, and the dependences of the response on the photon energy, microwave power, and modulation amplitude were studied. Stability of the irradiation products upon storage (signal fading) was also investigated. Lithium formate monohydrate is by far the best candidate of the tested formates, suitable for measuring doses down to approximately 0.1 Gy. Lithium formate monohydrate is more sensitive than alanine by a factor of 5.6–6.8 in the tested photon energy range, it exhibits no zero-dose signal and shows a linear dose response in the dose range from 0.2 to 1000 Gy. Its EPR signal was found unchanged in shape and intensity 1 week after irradiation to 10 Gy. Various less favorable properties rendered the other formates generally unsuitable, although calcium formate exhibits some interesting EPR dosimetric properties.

Single crystals of cytosine monohydrate were used as a model for studying hole transfer in systems related to nucleic acids. Holes (and excess electrons) were induced by ionizing radiation at 15 K. Thiocytosine, imbedded in the cytosine lattice in variable amounts, served as a hole trap. The holes which are stabilized at thiocytosine give rise to an EPR signal well separated from the signals of other radicals present. The ratio of the amount of thiocytosine cations (i.e. trapped holes) to the amount of the remaining radicals is at least 10 times larger than the stochiometric thiocytosine/cytosine ratio, which displays significant hole transfer from cytosine to thiocytosine and subsequent trapping by the latter. The analysis of the hole transfer range in the lattice of cytosine monohydrate, composed of parallel sheets of cytosine bases, is rather complex. Unlike DNA, where each base in a  -stack is flanked by one base on each side, each cytosine base in the present system is surrounded by four other bases in a  -stacked structure. This results in a higher probability of finding a thiocytosine base in a closer proximity to the initially formed hole. Consequently, it seems that the net hole transfer is not larger than about 4 inter-base distances (~13 &Aring ; ; ) in the present system.

Single crystals of deoxycytidine hydrochloride (CdR.HCl) have been X-irradiated at 10 K with doses up to about 150 kGy and studied using 24 GHz (K-band) EPR, ENDOR and FSE spectroscopy. In this system, the cytosine base is protonated at the N3 position. Nine different radicals were characterized and identified. Three of these are ascribed to three versions of the one-electron reduced species, probably differing in their protonation state. Radicals formed by net hydrogen addition to the cytosine C5 and C6 positions were observed at 10 K. The hydrogen-abstraction radical at the deoxyribose C1' position most probably results from initial oxidation of the base. The remaining radical species are all localized to the sugar moiety, representing products formed by net hydrogen abstraction from three of the five available carbons of the deoxyribose sugar. The lack of base-centered oxidation products as well as the structures of the one-electron reduced species is rationalized by considering the specific proton donor-acceptor properties of this crystalline lattice in comparison with similar systems.

Rochelle salt, K+·Na+·C4H4O62−·4H2O, is known for its remarkable ferroelectric state between 255 and 297 K. The current investigation, based on data collected at 105 K, provides very accurate structural information for the low-temperature paraelectric form. Unlike the ferroelectric form, there is only one tartrate molecule in the asymmetric unit, and the structure displays no disorder to large anisotropic atomic displacements.

Using periodic DFT calculations, it is concluded that the stable radiation-induced alanine radical most probably is the result of reductive deamination and protonation of the detached amino group, yielding an NH(4)(+) ammonium ion and a negatively charged radical.

Single crystals of thymidine (Td) and 1-methylthymine (1MT) have been studied using ESR, ENDOR and FSE techniques subsequent to x-irradition at 8 K. Four base-centered radicals were identified in both systems. In addition, an alkoxy radical and a Cl1 H-abstraction radical were observed in Td (Sagstuen et al. 1989).

Since 1985 a continued effort has been made to understand the primary damage caused by direct radiation effects to the constituents of the DNA molecule. The main motivation for initiating these studies was the report from a Swedish group (Graslund et al. 1971). Using ESR spectroscopy it was proposed that in oriented fibres of DNA mainly two radical species are formed and trapped at low temperatures.

Glutarimide single crystals X-irradiated at room temperature were reinvestigated at 150 K with the purpose to obtain information about possible ring opening and other fragmentation processes involving free radicals in this compound, previously only observed to take place in aqueous solutions. In previous work, using EPR,ENDOR and EIE spectroscopy, two H-abstraction radicals present in a 3∶1 relative ratio in the irradiated crystals were identified. In the present work the detection of a third radical species, III, is reported. The g- and hyperfine coupling tensors for all three radicals at 150 K were obtained. Based on simulations of the EPR spectra the relative abundance of the three radicals was estimated to be 60, 25 and 15% for radicals I, II and III, respectively. Radical III is proposed to be of the allyl radical type –CHCH–˙CH– formed formally by a concerted H2 elimination from C2 and C3 of radical I and/or from C3 and C4 of radical II. This proposal and structure is at variance with observations from aqueous solution studies of succinimide, where the CO–CH2 bond was susceptible for rupture. However, the assignment is consistent with density functional theory (DFT) calculations, predicting equivalent hyperfine interactions to the two α-type protons at C2 and C4 in excellent agreement with the experimentally determined hyperfine coupling tensors. EIE results confirmed that both couplings originate from the same radical species. Possible mechanisms for the formation of radical III are discussed.

Single crystals of the co-crystalline complex of 1-methyl-uracil and 9-ethyladenine were X-irradiated and studied using EPR, ENDOR and FSE spectroscopic techniques at 10 K. All together seven radicals were identified, and experimental evidence for at least one more species, as well as for a very low population of radical pairs, is available. Oxidation and reduction products appear to be stabilized at both base constituents of the pair. Of the 1-methyluracil moiety, the product formed by net hydrogen abstraction from the methyl group was observed, together with the 1-methyluracil anion and the 1-methyluracil-5-yl radical. From the 9-ethyladenine moiety, the N3-protonated 9-ethyladenine anion is stabilized. In addition, the 9-ethyladenine cation as well as traces of the amino-deprotonated cation were observed, together with the C8-H hydrogen adduct. The presence of oxidation and reduction products in each of the two bases may indicate that negligible energy transfer takes place between them. This behavior is different from that observed in the similar pair of 1-methylthymine-9-methyladenine. There also seems to be minor proton exchange between the two stacks of molecules: Interbase protonation-deprotonation channeled through the hydrogen-bonding scheme seems to be almost completely suppressed.

Anhydrous single crystals of cytosine hydrochloride (protonated at N3) have been X-irradiated at 10 K and studied using K-band EPR, ENDOR and FSE spectroscopy. At least seven radicals were present at 10 K after X irradiation with a dose of about 150 kGy. Two different protonation states of the one-electron reduced cytosine cation were observed: an amino-protonated species (R1) and the pristine one-electron reduced species (R2) with zero net charge. Apparently three deprotonated versions of the one-electron oxidized cytosine cation were formed: the amino-deprotonated cation (R3), an N3-deprotonated cation (R4) and an N1-deprotonated cation (R5). Finally, two products formed by net hydrogen addition to the cytosine base were observed: a C5 hydrogen-addition radical (R6) and a C6 hydrogen-addition radical (R7). The crystalline lattice of cytosine hydrochloride is characterized in part by a cytosine base initially protonated at the N3-position, thus forming a cytosine base cation, and in part by an extended network of hydrogen bonding involving the chlorine anions. Proton transfer properties of pristine one-electron oxidation and reduction base products in this lattice are discussed and are suggested as explanations of the unusual multitude of positions for deprotonation of the one-electron oxidized species as well as for the two protonation states of the reduction product observed. The magnetic parameters for the amino-protonated species R1 agree well with those extracted from previous studies of cytosine derivatives in frozen solutions and in various glasses.