Electron transport by molecular motion of redox-DNA strands:: Unexpectedly slow rotational dynamics of 20-mer ds-DNA chains end-grafted onto surfaces via C6 linkers

The dynamics of electron transport within molecular layers of 3'-ferrocenylated 20-mer oligonucleotide, 5'-thiol end-grafted onto gold electrode surfaces via a six-carbon (C6) linker, is studied by cyclic voltammetry. Single-stranded Fc-DNA layers are observed to behave as diffusionless systems reflecting the rapid dynamics of the ssDNA strand. Following hybridization, the Fc-dsDNA-C-6 layers give rise to a characteristic cyclic voltammetry behavior evidencing that the Fc head is animated by a purely diffusional motion, which is ascribed to free rotation of the rigid DNA duplex around its C-6 anchoring linker. A model, describing the motion of the Fc head as resulting from hinge motion of the DNA duplex, is developed allowing the motional dynamics of the Fc-dsDNA-C-6 chains to be quantified in terms of an apparent rotational diffusion coefficient, Dr. The value found for Dr is similar to 3-4 orders of magnitude slower than expected for free rotation of dsDNA in solution, pointing to a drastic motion-slowing role of the anchoring surface. Accessibility of the Fc head for the electron transfer at the electrode is also shown to modulate the apparent dsDNA dynamics. The dynamics of Fc-dsDNA-C-6 is found to be insensitive to the presence of a single mismatch in the middle of the strand, confirming that charge transport by dsDNA conduction (DNA CT) is not present for the systems studied here. However, electron transport by free hinge motion of the dsDNA chain is shown to be fast enough to, a priori, compete favorably with DNA CT.