Two additional carbohydrates are reported whose crystal structures trap electrons intermolecularly in single crystals x irradiated at low temperature, namely sucrose and rhamnose. Five carbohydrate and polyhydroxy compounds are now known which exhibit this phenomenon. The following characteristics of the phenomenon were investigated: (1) the hyperfine couplings of the electron with protons of the polarized hydroxy groups forming the trap; (2) the distances between these protons and the trapped electron; (3) the spin density of the electron at the protons and (4) the relative stabilities of the electron trapped in various crystal structures.
The relationship between γ proton hyperfine couplings and conformation in alkoxy radicals was investigated. It is shown that large γ proton couplings are present in alkoxy radicals in which significant spin polarization of the Cβ–Cγ bonding orbitals is expected. This principle finds application in analyzing the proton hyperfine couplings observed for the alkoxy radical in irradiated deoxycytidine monophosphate. The g tensor and hyperfine coupling tensors which characterize this radical were determined from ESR and ENDOR measurements.
The stable products generated in polycrystalline glycine exposed to ionizing radiation and subsequently dissolved in water were identified by using 13C nuclear magnetic resonance spectroscopy. The study was done on isotopically enriched samples. The results can be correlated with the results of many other studies on irradiated glycine using other methods.Somewhat over two decades ago, Gordy and his collaborators (1), and independently Combrisson and Uebersfeld (2), introduced electron spin resonance (ESR) spectroscopy as a tool for investigating radiation damage in biological compounds. Since then ESR has been employed in innumerable studies of the free radical stages of radiation damage processes in a wide variety of compounds. The power of the ESR technique has been considerably augmented by ENDOR (electron nuclear double resonance) which can provide more exact measurements of the hyperfine couplings of the unpaired electron in a free radical (3). Even before the invention of ESR spectroscopy, mass spectroscopy had found application in radiation research for the analysis of gaseous products. In this communication, we wish to report how 13C nuclear magnetic resonance (NMR) spectroscopy compliments ESR-ENDOR spectroscopy and mass spectroscopy so that a complete description of radiation damage processes in irradiated solids can be obtained.Most of what is known about radiation damage mechanisms has been learned from studies on relatively simple model compounds, such as glycine, which has served as a model compound for numerous studies of radiation damage processes. Thus, mass spectrometer measurements have shown that CO2 and NH3 are produced when polycrystalline glycine is exposed to ionizing radiation. ESR-ENDOR spectroscopy has been essential for identifying the free radical stages of the radiation damage process. Since the initial report of free radical formation in irradiated glycine by the aforementioned investigators (2, 3), numerous additional ESR and ENDOR studies have been reported including many single-crystal studies wherein free radical identifications are more definitive (4-10). The use of isotopically labeled compounds has been an important factor in verifying the identity of certain free radical products by the ESR-ENDOR method. Unstable free radical products can be stabilized by maintaining the sample at sufficiently low temperature, thereby allowing the primary free radical stages of the radiation damage process to be studied. These methods are not applicable to detection of nonvolatile diamagnetic radiation products. It is in this respect that we have found 13C NMR spectroscopy extremely useful.The scheme proposed for the radiation-induced degradation of glycine is shown in Fig. 1 wherein gaseous products, free radical intermediates, and stable diamagnetic products are appropriately distinguished. The degradation scheme has emerged from the work of several research groups and is based on ESR and mass spectroscopic data and on analysis of the stable radiation products by conventional chem...
Alkoxy radicals have been produced by x irradiation in a variety of compounds containing hydroxy groups. The relationship between the g tensor and its correlated hyperfine coupling tensors are explored as an additional means for characterizing alkoxy radicals. A simple model of the spin density distribution in an alkoxy radical is able to account for the main features of the anisotropic coupling. The model places 0.75 spin density in a p orbital of the oxygen atom and a small positive density along the C-H bond axis.
ESR–ENDOR measurements were made on single crystals of 5-nitro-6-methyluracil x-irradiated at 4.2 K. The primary oxidation and reduction products were characterized and identified. The reduced species has a large spin density localized on the nitrogen atom of the nitro group. The hyperfine and quadrupole coupling tensors of the 14N nucleus were determined from ENDOR measurements. The oxidized species is the cation formed by removal of an electron from the uracil ring.
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