E.S.R. and ENDOR of an α-chloro radical in X-irradiated 5-chlorodeoxyuridine single crystals

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Abstract
The most prominent radical formed from X-irradiation of the nucleic acid constituent analogue 5-chlorodeoxyuridine at room temperature is shown to be an α-chloro radical formed by hydrogen addition to C6. The E.S.R. analysis of the 35Cl hyperfine interaction combined with theoretical simulation of the spin hamiltonian yields tensor components axx =46·98 MHz, ayy =-10·98 MHz and azz =-17·01 MHz with a quadrupole coupling constant of eqQ=72 MHz. The principal g-tensor values are gxx =2·0012, gyy =2·00862 and gzz =2·00687. Three additional hyperfine interactions in the radical are observed combining E.S.R. and ENDOR spectroscopy. Besides the two nearly equivalent β-protons on C6 with principal values of 103·39 MHz and 110·12 MHz, there is hyperfine interaction with the 14N nucleus of nitrogen N3 (axx =9·81 MHz, ayy =azz ≃ 0 MHz) and with the proton of the hydrogen bonded to N3. The latter interaction has tensor components of 2·65 MHz, -10·80 MHz and -8·09 MHz as obtained from ENDOR data. The chlorine hyperfine coupling parameters are related to those observed in other α-chloro radicals. The mechanism of the formation of the radical in 5-chlorodeoxyuridine is discussed briefly.

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