Elementary steps for charge transport in DNA: thermal activation vs. tunneling

Using stacks of Watson–Crick base pairs as an important example of multichromophoric molecular assemblies, we studied charge migration in DNA with special emphasis on the mechanism of hole hopping between neighboring guanines (G) connected by the adenine–thymine (AT) bridge. The tight-binding model proposed for this elementary step shows that for short AT bridges, hole transfer between two G bases proceeds via quantum mechanical tunneling. By contrast, hopping over long bridges requires thermal activation. The condition for crossover between tunneling and thermal activation near room temperature is specified and applies to the analysis of experimental data. We show that thermal activation dominates, if the bridge between two G bases contains more than three AT pairs. Our theoretical findings predict that the replacement of AT base pairs by GC pairs increases the efficiency of hole transport only in the case of short base pair sequences. For long sequences, however, the opposite effect is expected.