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Gemcitabine is a modified cytidine analog having two fluorine atoms at the 2?-position of the ribose ring. It has been proposed that gemcitabine inhibits RNR activity by producing a C3?? intermediate via direct H3?-atom abstraction followed by loss of HF to yield a C2?? with 3?-keto moiety. Direct detection of C3?? and C2?? during RNR inactivation by gemcitabine still remains elusive. To test the influence of 2?- substitution on radical site formation, electron spin resonance (ESR) studies are carried out on one-electron oxidized gemcitabine and other 2?-modified analogs, i.e., 2?-deoxy-2?-fluoro-2?-C-methylcytidine (MeFdC) and 2?-fluoro-2?-deoxycytidine (2?-FdC). ESR line components from two anisotropic ?-2?-F-atom hyperfine couplings identify the C3?? formation in one-electron oxidized gemcitabine, but no further reaction to C2?? is found. One-electron oxidized 2?-FdC is unreactive toward C3?? or C2?? formation. In one-electron oxidized MeFdC, ESR studies show C2?? production presumably from a very unstable C3?? precursor. The experimentally observed hyperfine couplings for C2?? and C3?? match well with the theoretically predicted ones. C3?? to C2?? conversion in one-electron oxidized gemcitabine and MeFdC has theoretically been modeled by first considering the C3?? and H3O+ formation via H3?-proton deprotonation and the subsequent C2?? formation via HF loss induced by this proximate H3O+. Theoretical calculations show that in gemcitabine, C3?? to C2?? conversion in the presence of a proximate H3O+ has a barrier in agreement with the experimentally observed lack of C3?? to C2?? conversion. In contrast, in MeFdC, the loss of HF from C3?? in the presence of a proximate H3O+ is barrierless resulting in C2?? formation which agrees with the experimentally observed rapid C2?? formation.

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Journal of the American Chemical Society