Tunneling or Hopping? A Direct Electrochemical Observation of Electron Transfer in DNA

We developed an axis-mode donor-DNA-acceptor electrochemical system to distinguish whether electron transfer in DNA occurs by tunneling or hopping. In the axis-mode, rigid stem-loop DNA was designed with the redox probe Ag⁺ embedded at the axis of the strand through a C-Ag⁺-C mismatch, which was immobilized onto the electrode surface in a saturated manner. Thus, the rotation, swing, and bending of the DNA strand were restricted and then the number of Ag⁺, the distance L between Ag⁺ and the electrode, and the chemical environment could be precisely controlled. In addition, fast scan cyclic voltammetry was applied to realize the in situ redox reaction of Ag⁺, without diffusion away from the electrode and the ensuing deconstruction of the stem-loop DNA. In this case, as a direct indicator of rate, the peak Faradaic current i_p was extracted and used to fit the tunneling mechanism i ∝ exp (-βL) and the hopping mechanism i ∝ L^-. The value of β was determined to be 0.100 Å^-1, which is consistent with the range of 0.1∼1.5 Å^-1 reported previously, while η was determined to be 0.677, which is completely beyond the correct range of 1 ≤ η ≤ 2, demonstrating that electron transfer in DNA occurs by tunneling instead of hopping or that tunneling dominates. Additionally, current additivity and the irrelevance of the base sequence illustrate this point again. Thus, the possibility of independent parallel tunneling currents in DNA strands is revealed, which is helpful for recognizing the feasibility of DNA-based wires and devices.