Monolayers of thiol-labeled DNA duplexes of 15, 20, and 30 basepairs were assembled on gold electrodes. Electron transfer was investigated by electrochemical impedance spectroscopy with Fe(CN)(6)(3-/4-) as a redox probe. The spectra, in the form of Nyquist plots, were analyzed with a modified Randles circuit which included an additional component in parallel, R(x), for the resistance through the DNA. For native B-DNA R(x) and R(ct), the charge transfer resistance, both increase with increasing length. M-DNA was formed by the addition of Zn(2+) at pH 8.6 and gave rise to characteristic changes in the Nyquist plots which were not observed upon addition of Mg(2+) or at pH 7.0. R(x) and R(ct) also increased with increasing duplex length for M-DNA but both were significantly lower compared to B-DNA. Therefore, electron transfer via the metal DNA film is faster than that of the native DNA film and certain metal ions can modulate the electrochemical properties of DNA monolayers. The results are consistent with an ion-assisted long-range polaron hopping mechanism for electron transfer.