Radiation chemistry of solid-state carbohydrates using EMR

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

Dynamic nuclear polarization and spin-diffusion in non-conducting solids

There has been much renewed interest in dynamic nuclear polarization (DNP), particularly in the context of solid state biomolecular NMR and more recently dissolution DNP techniques for liquids. This paper reviews the role of spin diffusion in polarizing nuclear spins and discusses the role of the spin diffusion barrier, before going on to discuss some recent results.Comment: submitted to Applied Magnetic Resonance. The article should appear in a special issue that is being published in connection with the DNP Symposium help in Nottingham in August 200

Radical Formation in X-Irradiated Single Crystals of Guanine Hydrochloride Monohydrate. II. ESR and ENDOR in the Range 10-77 K

In a study of guanine·HCl·H2O (Gm) single crystals X-irradiated at temperatures between 10 and 77 K, three radical species were found and characterized by ESR and ENDOR spectroscopy. All three are primary products in that they were present immediately following irradiation at T \u3c 10 K. Radical I, which apparently can exist in two slightly different conformations, was identified as the product of electron gain by the parent molecule and subsequent protonation at O6. Radical I decayed only after warming the crystals beyond 250 K. Radical II was the guanine cation previously reported (D. M. Close, E. Sagstuen, and W. H. Nelson, J. Chem. Phys. 82, 4386 (1985)); however, ENDOR data are reported here which confirm the previous results. The guanine cation in Gm resulted from electron loss from the parent and subsequent deprotonation at N7. It is proposed that Radical III results from OH attack at C8 of the parent molecule, followed by rupture of the C8-N9 bond and ring opening. The OH radicals thought to produce Radical III result from electron loss by the cocrystallized water molecules. The reaction leading to Radical III, unusual in solid-state radiation chemistry, is thought to be mediated by the specific hydrogen bonding network in this crystal

Radical Formation in X-Irradiated Single Crystals of Guanine Hydrochloride Monohydrate. III. Secondary Radicals and Reaction Mechanisms

This work involves an ESR and ENDOR study of the reactions of the three primary radicals observed in X-irradiated single crystals of guanine hydrochloride monohydrate. Radical I, the O6-protonated anion, decays at 250 K yielding a stable room temperature radical (Radical V). The experimental evidence indicates that Radical V results from H-abstraction at N9 of a neighboring molecule. Radical II, the N7-deprotonated cation, decays at 60 K with no detectable successor. Radical IV, a C8 H-addition radical, is formed when an imidazole ring-opened radical (Radical III) decays at 150 K. The added H-atom was found to be from an easily exchangeable source. It is proposed that Radical III decays by the formation of a diamagnetic formamida molecule and an H-atom. It is important to note that Radical IV, the purine H-addition radical, is the result of basic oxidation events. Previous assumptions have been that purine H-addition radicals result either from reduction, i.e., protonation of a pristine anion, or from the \u27excitation pathway\u27, by addition of H-atoms dissociated from superexcited purine bases

Cervicogenic headache and spinal manipulative therapy : a retrospective case series

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Radiation Damage to DNA Base Pairs. I. Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Study of Single Crystals of the Complex 1-Methylthymine·9-Methyladenine X-Irradiated at 10 K

Single crystals of the complex 1-methylthymine·9-methyladenine were X- irradiated at 10 and at 65 K and studied in the temperature range 10 to 290 K using K-band EPR, ENDOR and field-swept ENDOR (FSE) techniques. The EPR and ENDOR spectra are dominated by two major and four minor resonances. The two major resonances are: MTMA1, the well-known radical formed by net hydrogen abstraction from the C5 methyl group of the thymine moiety, and MTMA2, the radical formed by net hydrogen abstraction from the NI methyl group of the thymine moiety. The latter product has not been observed previously in any 1- methylthymine derivative. The four minor resonances are: MTMA3, the anion of 1-methylthymine, possibly protonated at the 04 position; MTMA4, the well- known species formed by net hydrogen addition to C6 of the thymine moiety; MTMA5, the species formed by net hydrogen addition to C2 of the adenine moiety; and MTMA6, the species formed by net hydrogen addition to C8 of the adenine moiety. Radical MTMA3, the O4-protonated thymine anion, was clearly detected at 10 K, but upon thermal annealing at 40 K the lines began to disappear. In crystals irradiated at 65 K MTMA3 was only weakly present. Radical MTMA2 decayed at about 250 K with no detectable successor, and radical MTMA5 disappeared at about 180 K. It was not possible to learn from the data if MTMA5 transformed into MTMA6. The radical distribution in the 1- methylthymine·9-methyladenine crystal system is different from that in crystals of the individual components. Reasons for this behavior are discussed in light of the hydrogen bonding schemes and molecular stacking interactions in each of the crystals. An important feature is the concept of excited-state transfer from the adenine to the thymine moiety, followed by dehydrogenation at the thymine N1-methyl group, the mechanism resulting in radical MTMA2