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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.

Primary free radical formations in fructose single crystals X-irradiated at 10 K were investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR induced EPR (EIE) techniques. ENDOR angular variations in the three principal crystallographic planes and a fourth skewed plane allowed the unambiguous determination of five proton hyperfine coupling tensors. From the EIE studies, these hyperfine interactions were assigned to three different radicals, labeled T1, T1* and T2. For the T1 and T1* radicals, the close similarity in hyperfine coupling tensors suggests that they are due to the same type of radical stabilized in two slightly different geometrical conformations. Periodic density functional theory calculations were used to aid the identification of the structure of the radiation-induced radicals. For the T1/T1* radicals a C3 centered hydroxyalkyl radical model formed by a net H abstraction is proposed. The T2 radical is proposed to be a C5 centered hydroxyalkyl radical, formed by a net hydrogen abstraction. For both radicals, a very good agreement between calculated and experimental hyperfine coupling tensors was obtained.

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.

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.

Radiation induced hole transfer and differential recombination of radicals at room temperature and lower temperatures (77 and 12 K) have been studied in crystals of cytosine·HCl doped with 5-methyl...

Radiation-induced free radical formation in the amino acid l-α-alanine has been studied using powder and single-crystal X-, K-, and Q-band electron paramagnetic resonance (EPR) spectroscopy, X-band powder electron−nuclear double resonance (ENDOR), thermal annealing, and EPR spectrum simulations. The spectra obtained after room temperature irradiations are composite, consisting of resonances from mainly three radicals denoted R1, R2, and R3. R1 is the well-known, stable room-temperature species formed by deamination from a protonated alanine anion radical. On the basis of simulations of EPR spectra obtained at X-, K-, and Q-bands, the room-temperature EPR spectrum seems to consist of about 55% of R1. Upon thermal annealing, the R1 resonance disappears faster than those of the other two components. The R2 species is presumably formed in the oxidative chain of radiation-induced events by net H-abstraction from the central alanine carbon atom. Q-band EPR was used to determine the g-tensor of R2. This species ...

X-irradiated Li(N2D5)SO4 single crystals were investigated using EPR and ENDOR spectroscopy. The N-14-ENDOR spectra of the deuterated hydrazine radical cation N2D4.+ were clearly observed at 240 K. N-14 hyperfine (life) and nuclear quadrupole (nqc) tensors of the N2D4.+ cation were determined from angular variation ENDOR measurements in the three orthogonal planes of the crystal. The life tensor obtained for the two equivalent N-14 atoms is very close to that reported in a N2H5HC2O4 single crystal, and the present results support the previous conclusion that the cation has a planar structure at 240 K. The N-14 nqc tensor was estimated using semiempirical methods and also calculated from the field gradients in the LiHzS crystal as evaluated by density functional theory methods. The general agreement with experimental observations further supported the suggested geometrical structure of the N2H4.+ radical. The H-1 and N-14-ENDOR enhancements observed may be due to weakened dipolar interactions between N-14-D-2 and H-1-D-2 leading to increased T-ln relaxations of the N-14 and residual H-1 nuclei of the N2D4.+ cation (and/or other isotopomers) in the deuterated crystals as compared to those in the Li(N2H5)SO4 crystals.

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.

The EPR spectra of single crystals of deoxycytidine 5‘-phosphate monohydrate (5‘dCMP), X-irradiated at 10 K, exhibit signals from several distinct radical species. Analysis of the ENDOR spectra from two of these radicals indicates that these result from oxidation and reduction of the cytosine base. The reduced species exhibits hyperfine coupling to the C6−Hα proton, and an additional small exchangeable hyperfine coupling from the N3−H proton. No additional couplings that may be associated with protonation of the amino group have been observed. Since the native molecule is protonated at N3, it appears that reduction of the cytosine base does not result in any further protonation. The oxidized species exhibits hyperfine couplings to C5−Hα and C1‘−Hβ, and two small exchangeable couplings from the C4−NH2 protons. Since no hyperfine coupling to N3−H was observed, the oxidized species is believed to be the N3−H deprotonated cation. At high X-ray dose there is also evidence for both C5 and C6 H-addition radicals...

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.