Solid state radiation chemistry of the DNA backbone

Solid-State Radiation Chemistry of DNA: The Bases

The secondary radiation-induced radicals in lithium formate monohydrate were studied using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques complemented with periodic density functional theory (DFT) calculations. Single crystals of lithium formate monohydrate were X irradiated at 77 K and at room temperature. The main radicals present after irradiation at 77 K are the CO(2)(•-) radical (R1), the recently identified protonated electron-gain product, HCOOH(•-) (R2) (Krivokapić et al., Radiat Res 2014: 181:503-11), and a different geometrical conformation of this latter radical, a species that, up until now, has remained unidentified (R3). The successful quantum chemical modeling of R3 confirmed its structure and also provided a possible mechanism for its formation. After irradiation at 295 K, the crystals were investigated both shortly after irradiation and after storage for eight months at room temperature in ambient environments. After long-term storage the CO(2)(•-) radical had significantly decayed and the EPR spectra were dominated by two minority radicals. Both of these radicals are most likely formate-centered π-radicals, and based on the observed EPR parameters (g- and hyperfine coupling tensors) tentative candidates are the CO(•-) radical and the dimer formed by the CO(2)(•-) radical and a neighboring formate molecule yielding the radical (-)O(2)C·O·(•)CH·O(-).

Oxidative stress in clinical situations - fact or fiction?

Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) techniques were used to obtain structural information about the copper(II)-chelidamate complex. Well-resolved nitrogen ENDOR spectra could be recorded from solid solution samples by using selective excitation of spin packets. Evaluation of nuclear quadrupole and dipolar hyperfine interaction of the directly ligated nitrogen allowed for an identification of the bond direction to the copper ion within the eigen frame of the copper g-matrix. Invoking two-dimensional EPR techniques, additional hyperfine interaction with a "distant" nitrogen spin, identified as resulting from the solvent dimethylformamide (DMF), was observed. The experimental data are only consistent with formation of a stable pseudoplanar copper complex with single solvent ligation via its oxygen atom.

Chapter 2 Electron spin resonance

Different pulse Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR) techniques are used to study the electronic and geometric structure of (tetraphenylporphyrinato)cobaltate(II) with pyridine as an axial ligand (cobaltous tetraphenylporphyrin(pyridine), CoTPP(py)). This complex is considered as a model for the heme group in deoxygenated hemoglobin and myoglobin. For the first time, the small hyperfine and nuclear quadrupole interactions of the porphyrin nitrogens of CoTPP(py) are determined using three-pulse electron spin−echo envelope modulation (ESEEM), hyperfine sublevel correlation (HYSCORE), and double nuclear coherence transfer (DONUT)−HYSCORE spectroscopy as well as Davies-ENDOR, and hyperfine correlated ENDOR (HYEND). The assignment of the different cross-peaks in the HYSCORE spectra is shown to be considerably facilitated by the DONUT−HYSCORE experiment. Furthermore, the hyperfine interactions of the surrounding protons are investigated by Davies- and Mims-ENDOR and HYSCORE spectroscopy at X- and S-band microwave frequencies. For the first time, the potential of proton HYSCORE spectroscopy at S-band is demonstrated. Assignment of the observed couplings is facilitated using deuterated pyridine as the axial ligand. The interactions with the ortho and meta protons of pyridine and with the protons of the porphyrin ligand are determined. From the Davies-ENDOR spectra of CoTPP(py) and CoTPP([15N]py), the hyperfine and nuclear quadrupole couplings of the pyridine nitrogen are evaluated. From the hyperfine and nuclear quadrupole parameters, structural (internuclear distances) and electronic information is derived. Comparisons are made with known EPR and ENDOR studies on a number of porphyrin systems with different metal ions. The similarities and differences are discussed in detail.

Electron paramagnetic resonance (EPR) experiments of light activated Ge, Sn and Pb impurities in wurtzite type CdSe crystals are presented. These2S1/2 centres exhibit a large hyperfine structure which shows a variation with temperature. 25 shells of neighbour nuclei are resolved for CdSe∶Sn by means of the Electron nuclear double resonance (ENDOR) technique. All shells belong to one of three distinct shell symmetries of the lattice thus confirming a high symmetry defect. Additional Sn respectively Ge related spectra are described and interpreted as complex centres correlated with lithium.

The distinct spin, optical, and coherence characteristics of solid-state spin defects in semiconductors have positioned them as potential qubits for quantum technologies. Both bulk and two-dimensional materials, with varying structural properties, can serve as crystalline hosts for color centers. In this study, we conduct a comparative analysis of the spin–optical, electron–nuclear, and relaxation properties of nitrogen-bound vacancy defects using electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) techniques. We examine key parameters of the spin Hamiltonian for the nitrogen vacancy (NV−) center in 4H-SiC: D = 1.3 GHz, Azz = 1.1 MHz, and CQ = 2.53 MHz, as well as for the boron vacancy (VB−) in hBN: D = 3.6 GHz, Azz = 85 MHz, and CQ = 2.11 MHz, and their dependence on the material matrix. The spin–spin relaxation times T2 (NV− center: 50 µs and VB−: 15 µs) are influenced by the local nuclear environment and spin diffusion while Rabi oscillation damping times depend on crystal size and the spatial distribution of microwave excitation. The ENDOR absorption width varies significantly among color centers due to differences in crystal structures. These findings underscore the importance of selecting an appropriate material platform for developing quantum registers based on high-spin color centers in quantum information systems.

The crosslinking energy in the radiolysis of polystyrene is 100‐fold that of polyethylene, whereas the free radical yield in the radiation of linear paraffins is only 5–6 times as much as that from alkylaromatic compounds. Therefore, the high radiation stability of polystyrene cannot be explained by the conventional scheme for the action of the benzene rings. In order to explain the high radiation stability of polystyrene the mechanism of the reactions taking place during radiolysis of the polymer must be examined. A hydrogen atom liberated as the result of rupture of a C–H bond under the action of radiation may react in two ways: (1) abstraction of a hydrogen atom from the aliphatic chain, and (2) addition to the double bond of the benzene ring with the formation of a free radical of the cyclohexadienyl type. Owing to the low rate of diffusion in the solid polymers and the high rate of reaction with atomic hydrogen, the radicals formed in this way should be in the immediate vicinity of the primary radical. Interaction of the latter with the radical formed by removal of a hydrogen atom from the aliphatic chain will lead to the formation of crosslinks. The interaction between the primary radical and the radical of the cyclohexadienyl type may take place in the following two ways. The radicals may recombine with each other to form crosslinks or they may engage in a disproportionation reaction. Disproportionation will lead to restoration of the initial state and hence to the dissipation of energy without crosslink formation. This will result in a drop in efficiency of the crosslinking process, the energy for the formation of one crosslink increasing. In order to obtain experimental verification of the above conceptions as to the mechanism of interaction of the primary and cyclohexadienyl radicals, the radiolysis of deuterated toluene C4H5CD3 was studied at the temperature of liquid nitrogen. The addition to the benzene ring of a deuterium atom formed on rupture of a C–D bond under the action of radiation and the subsequent disproportionation of the cyclohexadienyl and primary radicals should, owing to the isotope effect, lead to transition of deuterium atoms from the aliphatic chain to the ring. The results obtained showed that the rate of these processes should exceed by at least 5–6 fold the rate of the processes leading to crosslinkage. It thus follows that the high radiation stability of polystyrene is to a considerable extent associated with disproportionation of the primary and cyclohexadienyl radicals.

Single crystals of cytidine have been x-irradiated and studied by electron nuclear double double resonance, electron nuclear double resonance, and electron spin resonance techniques at room temperature. One radical species was studied in detail and the results show anisotropic couplings of an unpaired electron to three hydrogen nuclei with the following principal values: a1, −66.3 MHz, −43.5 MHz, −20.7 MHz; a2, −68.9 MHz, −43.4 MHz, −20.3 MHz; a3, −37.2 MHz, −27.0 MHz, −13.6 MHz. The g tensor principal values are 2.0031, 2.0025, and 2,0021. These data indicate a radical formed in the ribose sugar group with spin density localized on C4′ and C5′.

Role of the Coordinating Histidine in Altering the Mixed Valency of Cu A: An Electron Nuclear Double Resonance-Electron Paramagnetic Resonance Investigation

ROS: a step closer to elucidating their role in the etiology of light-induced skin disorders.

Radiological protection is based on the assumption that any additional exposure to ionizing radiation produces an increased risk of stochastic adverse health effects. The ICRP has adopted linear no-threshold (LNT) dose-response relationships to derive health risks associated with low doses of radiation and this implies that the risks of cancer and hereditary effects are directly proportional to the dose received, no matter how small the dose. The LNT assumption has been criticised over the years both by those who consider that it underestimates the risk and by those who believe that it is overly conservative. A few years ago, most of the pressure on ICRP came from those who believed that the risks per unit dose adopted at low doses were too small, but in recent years the pressure has been coming from the opposite direction. Strictly speaking, ICRP feels that the most likely form of the dose-response relationship for low LET radiation is linear-quadratic (i.e. sub-linear at low doses), and for cancer they approximate this by two linear relations, that for the low dose region having a slope (risk per unit dose) half of that for the high dose region, represented by a DDREF of 2. Nevertheless, there are those (particularly in the US and France) who argue that there is no direct evidence for stochastic effects being produced by low doses of radiation, and that the appropriate dose-response relationship has a threshold dose below which no risk to health exists, or even that low doses of radiation are beneficial (`radiation hormesis').

Dose rate effects on shape memory epoxy resin during 1 MeV electron irradiation in air

The production of free radicals during the autoxidation of monosaccharides by buffer ions