Reactions of 5-methylcytosine cation radicals in DNA and model systems: Thermal deprotonation from the 5-methyl group vs. excited state deprotonation from sugar

摘要

Purpose: To study the formation and subsequent reactions of the 5-methyl-2′-deoxycytidine cation radical (5-Me-2′- dC?+) in nucleosides and DNA-oligomers and compare to one-electron oxidized thymidine. Materials and methods: Employing electron spin resonance (ESR), cation radical formation and its reactions were investigated in 5-Me-2′-dC, thymidine (Thd) and their derivatives, in fully double-stranded (ds) d[GC*GC*GC*GC*]2 and in the 5-Me-C/A mismatched, d[GGAC*AAGC:CCTAATCG], where C* = 5-Me-C. Results: We report 5-Me-2′-dC?+ production by one-electron oxidation of 5-Me-2′-dC by Cl2?- via annealing in the dark at 155 K. Progressive annealing of 5-Me-2′-dC?+ at 155 K produces the allylic radical (C-CH2?). However, photoexcitation of 5-Me-2′-dC?+ by 405 nm laser or by photoflood lamp leads to only C3′? formation. Photoexcitation of N3-deprotonated thyminyl radical in Thd and its 5′-nucleotides leads to C3′? formation but not in 3′-TMP which resulted in the allylic radical (U-CH2?) and C5′? production. For excited 5-Me-2′,3′- ddC?+, absence of the 3′-OH group does not prevent C3′? formation. For d[GC*GC*GC*GC*]2 and d[GGAC*AAGC:CCTAATCG], intra-base paired proton transferred form of G cation radical (G(N1-H)?: C(+ H+)) is found with no observable 5-Me-2′-dC?+ formation. Photoexcitation of (G(N1-H)?:C(+ H+)) in d[GC*GC*GC*GC*]2 produced only C1′? and not the expected photoproducts from 5-Me-2′-dC?+. However, photoexcitation of (G(N1-H)?:C(+ H+)) in d[GGAC*AAGC:CCTAATCG] led to C5′? and C1′? formation. Conclusions: C-CH2? formation from 5-Me-2′-dC?+ occurs via ground state deprotonation from C5-methyl group on the base. In the excited 5-Me-2′-dC?+ and 5-Me-2′,3′- ddC?+, spin and charge localization at C3′ followed by deprotonation leads to C3′? formation. Thus, deprotonation from C3′ in the excited cation radical is kinetically controlled and sugar C-H bond energies are not the only controlling factors in these deprotonations.