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 electron transfer within DNA doped with randomly spaced electron traps. The traps were introduced by careful bromination of DNA in ice-cooled aqueous solution. The procedure is shown by NMR and GC/MS techniques to modify thymine, cytosine, and guanine 2'-deoxyribosides, transforming them 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 products that result from electron scavenging in DNA by T(OH)Br and CBr units at 77 K have been identified by ESR as the 6-hydroxy-5,6-dihydrothymin-5-yl (TOH) radical and the 5-bromocytosine sigma* radical anion, CBr-. Our quantitative estimates show that electron scavenging by T(OH)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 traps through either transmission or reflection. The yields of electron scavenging by T(OH)Br moieties have been treated quantitatively considering the scavenging process as a competition between diffusion of electrons to T(OH)Br traps and their fixation on cytosines in the form of protonated radical anions, A mean displacement of the electron from its entry point evaluated using this model is about 11 bases at 77 K. After trapping at 77 K no further migration takes place until annealing to temperatures near 150 K and above. At these temperatures electron migration is activated and migration distances are found to increase with temperature likely through a hopping mechanism.