¹³ C nuclear magnetic resonance studies of radiation damage: Radiation-induced degradation of glycine

Abstract

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...

EPR and ENDOR Studies of Single Crystals of α-Glycine X-ray Irradiated at 295 K

“…Two species have been suggested, the σ-radical NH 2 CH 2 COO species 8 (K1) by deprotonation from the amino group, and the electronically rearranged nitrogen centered π-radical species N CH 2 COO - (K2). , The latter seems to relax slowly into a conformational slightly different product K2‘. , Upon 77 K photolysis using various wavelengths of the light, the products HṄCH 2 COO - and N CH 2 were detected . Upon thermal annealing from 77 K, Product K2 or K2‘ has been suggested to convert directly into N CHCOO - (radical I) by intramolecular hydrogen transfer, , but several authors also have suggested an alternative decay route, involving decarboxylation of K2 or K2‘ as an intermediate step. , Thermochemical properties of glycine radicals 40 as well as detectable amounts of CO 2 upon chemical analysis of stable radiation products may support this suggestion. ,, It should be noted that Armstrong et al points out that the radical NH 2 ĊHCOO(H) as detected in aqueous solution 31 are expected products from K2 by spontaneous deprotonation.…”

“…10,39 Thermochemical properties of glycine radicals 40 as well as detectable amounts of CO 2 upon chemical analysis of stable radiation products may support this suggestion. 4,12,41 It should be noted that Armstrong et al 40 points out that the radical NH 2 C ˙HCOO(H) as detected in aqueous solution 31 are expected products from K2 by spontaneous deprotonation.…”

“…The low-temperature irradiation and EPR/ENDOR analysis of glycine crystals have shown that even for this simple amino acid, the radiation chemistry is complex . Iwasaki and co-workers − discussed the radicals formed after 4 K irradiation and measurement, and several other authors ,− have analyzed sequences of reactions taking place and products formed from the initially formed anion and cation radical species. The bulk of experimental data may suggest possible schemes of radical reactions, but even after many years of intensive studies, pieces still are missing in the complex puzzle of mechanistically connecting the various radical products stabilized at different temperatures.…”

Study of radiation induced changes of phosphorus metabolism in mice by 31P NMR spectroscopy

“…They detected a dramatic decline in high-energy phosphates beginning one day after irradiation. Box et al studied effects of radiation on degradation of glycine, a protein building block molecule, by 13 C NMR spectroscopy 21. In addition to NMR spectroscopy methods, magnetic resonance (MR) imaging can be efficient in detecting effects of radiation as well.…”

“…Box et al studied effects of radiation on degradation of glycine, a protein building block molecule, by 13 C NMR spectroscopy. 21 In addition to NMR spectroscopy methods, magnetic resonance (MR) imaging can be efficient in detecting effects of radiation as well. For example 19 F MR imaging was employed to detect accumulation of perfluorooctylbromide in spleen.…”