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

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]2, where C* = 5-Me-C. Results: We report S-Me-l'-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(-1- 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 cat- ion radical is kinetically controlled and sugar C-H bond energies are not the only controlling factors in these deprotonations. [ABSTRACT FROM AUTHOR]