Long-range electron transfer mediated by G-quadruplex wires attached to gold electrodes via phosphorothioated dA tags

G-quadruplex (G4) DNA wires are perspective highly-conductive building blocks for self-assembling bioelectronic nanocircuits. Here, we interrogated electrochemically G4-mediated electron transfer (ET) reaction between the gold electrodes and redox indicator methylene blue (MB) intercalated into ten-tetrade G4 DNA tethered to the gold electrode via four phosphorothioated adenosine tags. The efficiency of DNA-mediated ET evaluated by kinetic analysis of MB redox transformations was characterized by the k_s of 155 s⁻¹. This value was well below 10⁸ s⁻¹ per ten tetrades determined by the current-voltage measurements in the absence of any redox active species, primarily. Due to the complex 2e⁻/1H⁺ electrochemistry of MB that apparently gated the overall ET reaction. However, it was superior to 74 s⁻¹ found for MB intercalated into (dGdC)₂₀ duplexes and close to 184 s⁻¹ estimated for MB groove-bound to (dAdT)₂₅. The ET-mediating efficiency of highly hydrated G4 wires was thus superior to the electronically-corrupted and water-poor dGdC duplexes and approaching that of dAdT-rich DNA. We show that the electrical properties of G4 DNA wires assessed electrochemically with MB as a redox probe differ essentially from those measured in the absence of redox indicators, indicating that the electrochemical reaction of the redox probe intercalating/interacting with the G4 DNA wires limits the overall kinetics of ET through the wire.