Excess electron transfer in DNA: Effect of base sequence and proton transfer

The effect of base sequence on excess electron transfer (ET) along the DNA "pi-way" is investigated in this work by use of various polynucleotide duplexes and salmon sperm DNA. Studies in frozen glassy aqueous solutions (7 M LiBr-D2O) of the duplexes polydAdT(.)polydAdT and polydIdC(.)polydIdC randomly intercalated with mitoxantrone (MX) are compared with our previously reported data on electron transfer in DNA-MX systems. The values of electron tunneling constant and ET distances at I min are found to be 0.75 +/- 0.1 and 9.4 +/- 0.5 bp for pdAdT(.)pdAdT (D2O) and 1.4 +/- 0.1 and 5.9 +/- 0.5 bp for pdIdC(.)pdIdC (D2O), vs 0.92 +/- 0.1 Angstrom(-1) and 9.5 +/- 1.0 bp for DNA (D2O) reported previously. The value for DNA lies intermediate between that for pdAdT(.)pdAdT (0.75 Angstrom(-1)) and that for pdIdC-pdIdC (1.4 Angstrom(-1)). These results suggest that deuteron transfer from I to C.(-) forming CD. significantly slows but does not stop electron transfer. Similarly, in DNA proton transfer in GC anion radical is not found to stop electron transfer. The lower value of for pdAdT(.)pdAdT is expected since proton transfer in the AT base pair is not energetically favorable. A study with DNA in glassy H2O solutions was performed. The found (0.83 +/- 0.1 Angstrom(-1)) is close to that found in C Glassy D2O solutions (0.92 +/- 0.1 Angstrom(-1)) but may suggest a modest isotope effect. Electron and hole transfer processes in frozen solutions (D2O ices) of polyA(.)polyU-MX and polyC(.)polyG-MX are also studied and compared with our previously reported data on electron and hole transfer in frozen D2O solutions of DNAMX We find electron/hole transfer in polyA(.)polyU is significantly further than in DNA and transfer distances in polyC(.)polyG are substantially less than in DNA, which confirm our results in aqueous glasses.