One-ElectronOxidation of Gemcitabine and Analogs:Mechanism of Formation of C3′ and C2′ Sugar Radicals

摘要

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 β-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 presumablyfrom a very unstable C3′• precursor. The experimentallyobserved hyperfine couplings for C2′• and C3′•match well with the theoretically predicted ones. C3′•to C2′• conversion in one-electron oxidized gemcitabineand MeFdC has theoretically been modeled by first considering theC3′• and H3O⁺ formation via H3′-protondeprotonation and the subsequent C2′• formation viaHF loss induced by this proximate H3O⁺. Theoreticalcalculations show that in gemcitabine, C3′• to C2′•conversion in the presence of a proximate H3O⁺ has a barrier in agreement with the experimentally observedlack of C3′• to C2′• conversion. In contrast,in MeFdC, the loss of HF from C3′• in the presence ofa proximate H3O⁺ is barrierless resulting inC2′• formation which agrees with the experimentallyobserved rapid C2′• formation.