ONE-ELECTRON-REDUCTION POTENTIALS OF PYRIMIDINE-BASES, NUCLEOSIDES, AND NUCLEOTIDES IN AQUEOUS-SOLUTION - CONSEQUENCES FOR DNA REDOX CHEMISTRY

The reduction potentials in aqueous solution of the pyrimidine bases, nucleosides, and nucleotides of uracil (U) and thymine (T) were determined using the technique of pulse radiolysis with time-resolved spectrophotometric detection. The electron adducts of U and T were found to undergo reversible electron exchange with a series of ring-substituted N-methylpyridinium cations with known reduction potential. From the concentrations of the pyrimidine electron adducts and the reduced N-methylpyridinium compounds at electron-transfer equilibrium, the thermodynamical equilibrium constants were obtained and from these the reduction potentials. The results show U and T and their nucleosides and nucleotides to have very similar reduction potentials, approximately -1.1 V/NHE at pH 8, i.e., the effect of methylation at C5, C6, or of substitution at N1 is small, less-than-or-equal-to 0.1 V. In the case of cytosine (C) the electron adduct is protonated (probably at N3), even up to pH 13. The protonated adduct (C(H).) undergoes a reversible electron transfer with the N-methylpyridinium cations. This is accompanied in one direction by transfer of a proton but by that of a water molecule in the other direction. As a result of the protonation of the electron adduct, the effective ease of reduction of C in aqueous solution is similar to that of U and T. It is suggested that in DNA the tendency for C.- to be protonated (by its complementary base G) is larger by greater-than-or-equal-to 10 orders of magnitude than that for protonation of T.- by its complementary base A. This results in C and not T being the most easily reduced base in DNA. A further consequence is that lack of neutralization by intrapair proton transfer of T.- enables the irreversible extra-pair protonation on C6 of the radical anion to take place.