COMPETITIVE ELECTRON SCAVENGING BY CHEMICALLY-MODIFIED PYRIMIDINE-BASES IN BROMINE-DOPED DNA - RELATIVE EFFICIENCIES AND RELEVANCE TO INTRASTRAND ELECTRON MIGRATION DISTANCES

ESR spectroscopy at low temperatures is employed to investigate electr on transfer within DNA doped with randomly spaced electron traps. The traps were introduced by careful bromination of DNA in ice-cooled aque ous solution. The procedure is shown by NMR and GC/MS techniques to mo dify thymine, cytosine, and guanine 2'-deoxyribosides, transforming th em into 5-bromo-6-hydroxy-5,6-dihydrothymine, T(OH)Br, 5-bromocytosine , CBr, and 8-bromoguanine, GBr, derivatives. The bromination products formed in molar ratio close to T(OH)Br/CBr/GBr = 0.2:1:0.23 and serve as internal electron scavengers on gamma-irradiation. Paramagnetic pro ducts that result from electron scavenging in DNA by T(OH)Br and CBr u nits at 77 K have been identified by ESR as the 6-hydroxy-5,6-dihydrot hymin-5-yl (TOH) radical and the 5-bromocytosine sigma radical anion, CBr-. Our quantitative estimates show that electron scavenging by T(O H)Br in bromine-doped DNA is over an order of magnitude more efficient than the more abundant CBr traps. This indicates that there is a high probability the electron survives encounters with the planar CBr trap s through either transmission or reflection. The yields of electron sc avenging by T(OH)Br moieties have been treated quantitatively consider ing the scavenging process as a competition between diffusion of elect rons to T(OH)Br traps and their fixation on cytosines in the form of p rotonated radical anions, A mean displacement of the electron from its entry point evaluated using this model is about 11 bases at 77 K. Aft er trapping at 77 K no further migration takes place until annealing t o temperatures near 150 K and above. At these temperatures electron mi gration is activated and migration distances are found to increase wit h temperature likely through a hopping mechanism.