REACTION OF HYDRATED ELECTRONS WITH GUANINE NUCLEOSIDES - FAST PROTONATION ON CARBON OF THE ELECTRON ADDUCT

The reactions of hydrated electrons (e(aq)-) and hydrogen atoms (H.) with guanosine, 2'-deoxyguanosine, and 1-methylguanosine (G) were studied by pulse radiolysis with optical and conductometric detection. In neutral solution, the reaction of both e(aq)- and H. with these purines leads to the same neutral radical. This is suggested to be the H adduct to the C8 position (G(C8H).) formed by H. addition and by protonation at C8 of the electron adducts, via tautomerization (k = 1.2 x 10(6) s-1 for the case of guanosine) following protonation of the radical anion at a hetero atom (k greater-than-or-equal-to 10(7) s-1). The C-protonated radical is only a weak reductant: It does not react (k less-than-or-equal-to 10(7) M-1 s-1) with the oxidant methylviologen (E = -0.44 V/NHE), but with the stronger oxidant Fe(CN)63- (E = 0.36 V/NHE) there is some reactivity (k = 5.9 x 10(8) M-1 s-1). In acidic solution G(C8H). protonates at a heteroatom to give a radical cation (pK(a) = 5.4-5.5). The same radical cation results also from the reaction of H. with G at pH 3. The C8 protonated electron adduct of 1-methylguanosine deprotonates (from the amino function at C2) with a pK(a) of 13.3. C-protonated electron adducts of the purines have so far not been detected in irradiated DNA. Since the tendency for their formation (by protonation by the water of hydration of DNA) is probably very high, the failure to detect these radicals in DNA can be taken as indication that electron transfer from the purine radical anions to pyrimidines in their neighborhood is faster than their C protonation.