title: Prototropic equilibria in DNA containing one-electron oxidized GC: intra-duplex vs. duplex to solvent deprotonation.
abstract: By use of ESR and UV-vis spectral studies, this work identifies the protonation states of one-electron oxidized G:C (viz. G˙+:C, G(N1–H)˙:C(+H+), G(N1–H)˙:C, and G(N2-H)˙:C) in a DNA oligomer d[TGCGCGCA]2. Benchmark ESR and UV-vis spectra from one electron oxidized 1-Me-dGuo are employed to analyze the spectral data obtained in one-electron oxidized d[TGCGCGCA]2 at various pHs. At pH ≥7, the initial site of deprotonation of one-electron oxidized d[TGCGCGCA]2 to the surrounding solvent is found to be at N1 forming G(N1–H)˙:C at 155 K. However, upon annealing to 175 K, the site of deprotonation to the solvent shifts to an equilibrium mixture of G(N1–H)˙:C and G(N2–H)˙:C. For the first time, the presence of G(N2–H)˙:C in a ds DNA-oligomer is shown to be easily distinguished from the other prototropic forms, owing to its readily observable nitrogen hyperfine coupling (Azz(N2) = 16 G). In addition, for the oligomer in H2O, an additional 8 G N2–H proton HFCC is found. This ESR identification is supported by a UV-vis absorption at 630 nm which is characteristic for G(N2–H)˙ in model compounds and oligomers. We find that the extent of photo-conversion to the C1′ sugar radical (C1′˙) in the one-electron oxidized d[TGCGCGCA]2 allows for a clear distinction among the various G:C protonation states which can not be easily distinguished by ESR or UV-vis spectroscopies with this order for the extent of photo-conversion: G˙+:C > G(N1–H)˙:C(+H+) ≫ G(N1–H)˙:C. We propose that it is the G˙+:C form that undergoes deprotonation at the sugar and this requires reprotonation of G within the lifetime of exited state
authors.affiliation: Department of Chemistry, Oakland University, Rochester, MI 48309, USA.
authors.forename: Amitava
authors.initials: A
authors.lastname: Adhikary
authors.forename: Anil
authors.initials: A
authors.lastname: Kumar
authors.forename: Shawn A
authors.initials: SA
authors.lastname: Munafo
authors.forename: Deepti
authors.initials: D
authors.lastname: Khanduri
authors.forename: Michael D
authors.initials: MD
authors.lastname: Sevilla
mesh_terms: D003596:Cytosine / Q000737:chemistry; D004247:DNA / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D006147:Guanine / Q000737:chemistry; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D008962:Models, Theoretical; D010084:Oxidation-Reduction; D011522:Protons; D012997:Solvents / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D011522:Protons; D012997:Solvents; D006147:Guanine; D003596:Cytosine; D004247:DNA