26467-87-8Relevant articles and documents
One-electron-reduction potentials of pyrimidine bases, nucleosides, and nucleotides in aqueous solution. Consequences for DNA redox chemistry
Steenken,Telo,Novais,Candeias
, p. 4701 - 4709 (2007/10/02)
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, ～ -1.1 V/NHE at pH 8, i.e., the effect of methylation at C5, C6, or of substitution at N1 is small, ≤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 ≥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.
