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À propos de : Excess Electron Transfer in DNA: Effect of Base Sequence and Proton Transfer        

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  • Excess Electron Transfer in DNA: Effect of Base Sequence and Proton Transfer
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  • The effect of base sequence on excess electron transfer (ET) along the DNA “π-way" is investigated in thiswork by use of various polynucleotide duplexes and salmon sperm DNA. Studies in frozen glassy aqueoussolutions (7 M LiBr−D2O) of the duplexes polydAdT·polydAdT and polydIdC·polydIdC randomly intercalatedwith mitoxantrone (MX) are compared with our previously reported data on electron transfer in DNA−MXsystems. The values of electron tunneling constant β and ET distances at 1 min are found to be 0.75 ± 0.1Å-1 and 9.4 ± 0.5 bp for pdAdT·pdAdT (D2O) and 1.4 ± 0.1 Å-1 and 5.9 ± 0.5 bp for pdIdC·pdIdC (D2O),vs 0.92 ± 0.1 Å-1 and 9.5 ± 1.0 bp for DNA (D2O) reported previously. The β value for DNA lies intermediatebetween that for pdAdT·pdAdT (0.75 Å-1) and that for pdIdC·pdIdC (1.4 Å-1). These results suggest thatdeuteron 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 β forpdAdT·pdAdT is expected since proton transfer in the AT base pair is not energetically favorable. A studywith DNA in glassy H2O solutions was performed. The β found (0.83 ± 0.1 Å-1) is close to that found inglassy D2O solutions (0.92 ± 0.1 Å-1) but may suggest a modest isotope effect. Electron and hole transferprocesses in frozen solutions (D2O ices) of polyA·polyU−MX and polyC·polyG−MX are also studied andcompared with our previously reported data on electron and hole transfer in frozen D2O solutions of DNA−MX. We find electron/hole transfer in polyA·polyU is significantly further than in DNA and transfer distancesin polyC·polyG are substantially less than in DNA, which confirm our results in aqueous glasses.
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