title: One-electron oxidation of gemcitabine and analogs: mechanism of formation of C3' and C2' sugar radicals.
abstract: 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 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.
authors.affiliation: Department of Chemistry, Oakland University , Rochester, Michigan 48309, United States.
authors.forename: Amitava
authors.initials: A
authors.lastname: Adhikary
authors.forename: Anil
authors.initials: A
authors.lastname: Kumar
authors.forename: Ramanjaneyulu
authors.initials: R
authors.lastname: Rayala
authors.forename: Ragda M
authors.initials: RM
authors.lastname: Hindi
authors.forename: Ananya
authors.initials: A
authors.lastname: Adhikary
authors.forename: Stanislaw F
authors.initials: SF
authors.lastname: Wnuk
authors.forename: Michael D
authors.initials: MD
authors.lastname: Sevilla
mesh_terms: D002241:Carbohydrates / Q000737:chemistry*; D003841:Deoxycytidine / Q000031:analogs & derivatives* / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry; D010084:Oxidation-Reduction; D000093542:Gemcitabine
chemical_list: D002241:Carbohydrates; D005609:Free Radicals; D003841:Deoxycytidine; D000093542:Gemcitabine