ESR study of electron transfer reactions between gamma-irradiated pyrimidines, adriamycin and oxygen.

Solid pyrimidine nucleic acid bases (cytosine, thymine, and uracil) were gamma-irradiated (50 KGy) and dissolved in deaerated solutions of adriamycin in water and dimethylsulfoxide (DMSO). Analogous experiments using unirradiated pyrimidines as controls were also performed. In water only gamma-irradiated cytosine showed a reaction with the adriamycin yielding a single ESR peak (g = 2.0033) consistent with the adriamycin semiquinone radical. Since the unirradiated cytosine gave no reaction, the result suggests an electron transfer from cytosine radicals (generated by gamma-radiolysis) to adriamycin. In DMSO the three gamma-irradiated and unirradiated pyrimidines reacted with adriamycin yielding the adriamycin semiquinone radical observed by ESR. These results suggest that in DMSO an electron is transferred to adriamycin from the pyrimidine radicals and from the parent pyrimidine molecules. However, the process is on the order of 10(5) times more efficient for the pyrimidine radicals. Superoxide radicals (O2-.) were formed following addition of oxygen to the deaerated DMSO solutions containing adriamycin semiquinone radicals. O2-. was spin trapped using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The results show a possible reaction sequence in which an electron transferred to adriamycin, by pyrimidine radicals and parent pyrimidine molecules, is subsequently transferred to dissolved oxygen.