Ability of the DNA double helix to transport electrons is its critical feature, underlying a number of important biological and biotechnological processes. Here, we show that electron transfer (ET) from the gold electrode to the DNA-bound methylene blue (MB) mediated by the DNA base-pair pi-stack is less efficient in (dGdC)-rich duplexes compared to pure (dAdT) DNA. The ET rate constant k(s) extrapolated to the DNA surface coverage Gamma(DNA) -> 0 is 121 +/- 8 s(-1) for (dAdT)(25), being almost twofold higher than 67 3 s(-1) shown for (dGdC)(20), consistent with the electric-field-disturbed submolecular structure of the (dGdC)(20) duplex earlier shown at electrified interfaces. DNA-mediated ET occurs both to MB intercalated and thus perfectly a pi-stacked into the (dGdC)20 duplex and to MB solely groove-bound to (dAdT)(25). For both (dGdC)(20) and (dAdT)(25), ET is less efficient than ET in DNA duplexes of a mixed dA, dT, dG, dC composition. The results suggest new interpretations of the biological ET processes that may occur in dsDNA of different compositions at polarized interfaces.