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Electron nuclear double resonance studies of free radicals trapped in x-irradiated single crystals of phenazine, hippuric acid and n-acetylglycine Singh, Baishnab Charan


The technique of electron nuclear double resonance (ENDOR) spectroscopy has been used to determine the identity and structure of radicals trapped in room temperature X-irradiated single crystals of some biologically important compounds, namely, phenazine, hippuric acid and N-acetylglycine. The ESR spectra of phenazine which had not previously been reported, were too complex to be analyzed. However, the high resolution of ENDOR has made it possible to study all the proton hyperfine interactions in detail. The radicals are formed as a result of the addition of atomic hydrogen to the nitrogen atom of phenazine molecule. The appearance of twice the number of certain ENDOR lines than expected from the crystal symmetry was interpreted by postulating that the radical was stabilized in two different conformations. In hippuric acid (N-Benzoylglycine), the stable N-Benzoylaminomethyl radical formed by room temperature X-irradiation of single crystals has been identified and its structure established by detailed ENDOR studies at 77 K. Besides proton signals, ENDOR transitions arising from the hyperfine and quadrupole interactions of the ¹⁴N nucleus were detected and a detailed analysis is presented. The assignment of exchangeable protons has been accomplished by use of partially deuterated crystals. The radical is most likely formed from oxidised hippuric acid by deprotonation and subsequent decarboxylation. Electronic and molecular structure of the stable radical produced by room temperature X-irradiation of single crystals of N-acetylglycine has been reinvestigated by detailed ESR and ENDOR studies. The results indicate that the radical structure and the nature of the wavefunction of the unpaired electron are significantly different from those reported in a previous 'negative' ENDOR study. In agreement with earlier ESR and ENDOR studies, it is found that the stable radical at room temperature is CH₃CONHCHCOOH. ENDOR lines due to the α-proton, three equivalent methyl protons and six exchangeable protons have been identified and the hyperfine tensors determined. According to the findings, the radical exists in four slightly different conformations. Isotopic substitution with ¹⁵N and deuteration of the exchangeable protons were helpful in the analysis of the complex ESR and ENDOR spectra. It has been conclusively shown that the multiplet structure in the ESR spectrum of irradiated N- acetylglycine arises from the interaction of the unpaired electron with protons of the methyl group and the ¹⁴N nucleus. It has been also demonstrated that the radical concentration in the crystal has to be very carefully controlled for the success of an ENDOR investigation. In order to confirm the radical identification INDO-MO calculations have been performed on all the radicals reported here. Excellent agreement has been obtained between the observed spin densities and those computed by the INDO-MO calculations.

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