REACTION OF CYSTEAMINE WITH INDIVIDUAL DNA-BASE RADICALS IN GAMMA-IRRADIATED NUCLEOTIDES AT LOW-TEMPERATURE

An ESR investigation of the individual DNA base radicals produced by g amma-irradiation of frozen solutions of the nucleotides TMP, dCMP, dGM P and dAMP and their reactions with cysteamine upon annealing is repor ted. The results show that water radicals in bulk ice do not lead to t he formation of DNA or cysteamine radicals. Radicals from the oxidatio n pathway which include the DNA base one electron oxidized radicals an d their successors, G(C8)OH., A(C8)OH. and thymine dimers (T-.(dl)) an d/or T(C6)OH., readily react with cysteamine to form RS' and ultimatel y RSSR(-.) Reactions of dGMP and dCMP radicals from the oxidation path way with cysteamine occur at lower temperatures than those of dAMP and TMP, suggesting hole migration. Both T(C6)H-. and C(N3)H-. react with cysteamine to form RS' and diamagnetic products, but G(C8)H-. and A(C 8)H-. do not. Subtraction of the anion radical T--. and its proton add uct T(C6)H-. from the total radical yield of TMP (with or without cyst eamine) suggests that somewhat less than half of the total TMP radical s found are a result of the oxidative pathway. Similar results are fou nd in the other nucleotides. The total spectral intensity derived fi o m the radicals from the oxidative pathway such as G(C8)OH., A(C8)OH. a nd T-.(di)/T(C6)OH. are somewhat less than that for the protonated ani on radicals. Only one non-base radical is identified, a sugar radical at the C(1)' site on the deoxyribose portion of dAMP. This species, S( A)(.), is also found to react with cysteamine or its disulfide radical anion. Analyses performed in the presence and absence of a thiol are found to allow for a clear separation of oxidative and reductive pathw ays.