ELECTRON ADDUCTS OF ADENINE NUCLEOSIDES AND NUCLEOTIDES IN AQUEOUS-SOLUTION - PROTONATION AT 2 CARBON SITES (C2 AND C8) AND INTRAMOLECULAR AND INTERMOLECULAR CATALYSIS BY PHOSPHATE

The electron adducts formed by reaction in aqueous solution of the hydrated electron, e(aq)BAR, with adenine nucleosides and nucleotides (abbreviated A) are protonated by water on the nitrogens [to give A(NH)*] and on carbon 8 of the ring [to yield A(C8H)*]. In contrast to N-protonation, C8-protonation (rate constant 3.6 x 10(6) M-1 s-1 in the case of adenosine) is irreversible. A(NH)* is converted into A(C8H)* by catalysts such as phosphate (for adenosine at pH 7 rate constant = 2 x 10(6) M-1 s-1) or acetate, and there is also an uncatalyzed, "spontaneous" conversion (k = 1 x 10(4) s-1). Reaction of A(NH)* with H+ also leads to protonation on carbon, not at C8 but at C2, to yield the radical A(C2H)*. In aqeous solution, this radical is a metastable species as reflected by the fact that it can be converted into A(C8H)* by catalysis by phosphate (rate constant 6.1 x 10(5) M-1 s-1 at pH 5). With the 5'-nucleotides, an intramolecular phosphate catalysis is observed (k = 4 x 10(4) s-1). A(NH)*, A(C2H)*, and A(C8H)* differ strongly with respect to their ability to reduce oxidants such as methylviologen (MV2+) and can thereby be distinguished from one another. A(NH)* is a potent reductant [k(MV2+) = 3.0 x 10(9) M-1 s-1], A(C2H)* a fair one [k(MV2+) = 5.0 x 10(8) M-1 s-1], and A(C8H)* has no reducing properties. Comparison of the tendency of the electron adduct of the adenine moiety to be (irreversibly) protonated on carbon with that of the electron adduct of thymine leads to the extrapolation that in DNA adenine should be more readily protonated than thymine.