ELECTRON MIGRATION IN OLIGONUCLEOTIDES UPON GAMMA-IRRADIATION IN SOLUTION

Electron migration in irradiated solutions of DNA was investigated usi ng 5-bromouracil synthetically incorporated into oligonucleotides of d efined base composition as a molecular indicator of electron interacti ons. Solvated electrons interact quantitatively with 5-bromouracil, le ading to a highly reactive 5-yl radical which can abstract an adjacent hydrogen atom to yield uracil. Yields of uracil, or loss of 5-bromour acil, from irradiated oligonucleotide samples were measured using gas chromatography-mass spectrometric analysis of their trimethylsilylated acid hydrolysates. To examine the effects of base composition and DNA conformation on electron migration, a set of oligonucleotides contain ing 5-bromouracil at selected positions with three base (guanine, cyto sine, thymine or adenine) spacers (e.g. [BrU(GGG)(3)](3)) were irradia ted in their single- or double-stranded form following annealing with appropriate complementary sequences. Differences in uracil yields sugg ested that electron migration occurred to different extents in oligonu cleotides containing different base sequences. In irradiated single-st randed oligonucleotides, the yield of uracil decreased in the order A >T > > C approximate to G. However, in irradiated double-stranded olig onucleotides, the yield of uracil decreased in the order G > C approxi mate to T > A. These differences were attributed to proton-transfer re actions facilitated by base pairing in double-stranded oligonucleotide s. The distance over which the elctron would migrate was then determin ed using a series of oligonucleotides containing 5-bromouracil at sele cted positions with guanine spacers (i.e. [BrU(G)(n)](3) (n = 3, 5, 7, 9). Oligonucleotides were irradiated in their double-stranded form fo llowing annealing with the appropriate complementary sequences. Analys is of the loss of 5-bromouracil revealed that electron migration occur red efficiently over c. 3-4 guanine bases assuming that migration coul d occur as efficiently in either direction along the DNA molecule. The se data can be compared with studies reporting more extensive migratio n for electrons generated by direct ionization of DNA.