Prototropic equilibria in DNA containing one- electron oxidized GC: intra-duplex vs. duplex to solvent deprotonation

By use of ESR and UV-vis spectral studies, this work identi. es the protonation states of one-electron oxidized G: C (viz. G(center dot+) : C, G(N1-H)(center dot): C(+H(+)), G(N1-H)(center dot):C, and G(N2-H)(center dot):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 Z7, 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)center dot: C and G(N2-H)center dot: C. For the. rst time, the presence of G(N2-H)center dot: C in a ds DNA-oligomer is shown to be easily distinguished from the other prototropic forms, owing to its readily observable nitrogen hyper. ne coupling (Azz(N2) = 16 G). In addition, for the oligomer in H(2)O, an additional 8 G N2-H proton HFCC is found. This ESR identi. cation is supported by a UV-vis absorption at 630 nm which is characteristic for G(N2-H)(center dot))center dot in model compounds and oligomers. We. nd that the extent of photo-conversion to the C(10) sugar radical (C10) 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 center dot + : C 4 G(N(1)-H)(center dot): C(+H(+)) >> G(N1-H)center dot: C. We propose that it is the G center dot + : C form that undergoes deprotonation at the sugar and this requires reprotonation of G within the lifetime of exited state.