Structure of oxidatively damaged nucleic acid adducts. 3. Tautomerism, ionization and protonation of 8-hydroxyadenosine studied by 15 N NMR spectroscopy

摘要

Natural abundance 15N NMR spectroscopy and ancillary spectroscopic techniques have been employed to study the solution structure of 8-hydroxyadenosine. 8-Hydroxyadenosine is a naturally occurring oxidized nucleic acid adduct that is generally implied to have an 8-hydroxy tautomerlc structure. 15N NMR chemical shifts and coupling constants, however, indicate that the modified base exists as an 8-keto tautomer. The pH dependence of 15N NMR and UV spectra showed the presence of two pKa'S, at 2.9 and 8.7, corresponding to protonation at N1 and ionization at N7, respectively. The latter results in the formation of an 8-enolate structure. Unusual uptield shifts of the 1H and 15H resonances of the NH₂ group, and a reduction in the one-bond coupling constant 1J_N6-H6is indicative of an unfavorable steric or electronic interaction between the NH₂ group and the adjacent N7-H proton. This interaction results in a subtle change in the structure of the NH₂ group. In addition to being a possible mechanism for alteration of hydrogen bonding in oxidized DNA, this type of Interaction gives a better understanding into N7-N9 tautomerism of adenine. Furthermore, the structure of 8-hydroxyadenosine has been related to possible mechanisms for mutations.