Electrochemical Monitoring of the Reversible Folding of Surface-Immobilized DNA i-Motifs.

Two cytosine (C) rich DNA sequences folding in i-motif upon protonation of C at low pH have been immobilized at gold electrodes to study the impact of the electrode|electrolyte interface on the stability of the noncanonical DNA secondary structure. The effects of the molecular composition and environment on the melting and folding of the structures immobilized at the gold surface have been compared to the properties of the DNA strands in solution. The DNA folding into i-motif upon protonation, both at the surface and in solution, results in a significant variation of the charge density which is monitored electrochemically through the electrostatic interactions between the DNA strand and the electroactive hexaammineruthenium(III). This method is shown to be sufficiently sensitive to distinguish hemiprotonated folded state and single strand unfolded state of i-motif. The pH of melting has been determined for both sequences in the bulk and at the gold|electrolyte interface. The results evidence a stabilizing effect of the interface on i-motif structure, whereby the pH of melting is higher for the sequences immobilized at the surface. The reversibility and precision of the electrochemical model described here allows a clear and simple characterization of DNA structures and does not require any labeling of the sequence.