Electronic properties of DNA: structural and chemical influence on the quest for high conductance and charge transfer

TL;DR

This paper investigates how structural and chemical factors influence DNA's electronic properties, aiming to identify conditions under which DNA can act as an efficient molecular wire for charge transfer.

Contribution

It provides a detailed analysis of structural and environmental effects on DNA conductivity, including the roles of water, counterions, and vibrational motions, with theoretical modeling and comparison to experimental data.

Findings

Water and counter ions can reduce activation gaps in conductivity.

Weak conductance of A-DNA explained by orbital interactions and interference.

Estimated charge transfer rates align with experimental picosecond dynamics data.

Abstract

Motivated by the wide ranging experimental results on the conductivity of DNA, we have investigated extraordinary configurations and chemical environments in which DNA might become a true molecular wire, perticularly from enhanced electronic overlaps or from small activation energies. In particular, we examine A- vs B-DNA, the ribbon-like structures proposed to arise from molecular stretching, the potential role of counterions in hole doping the DNA orbitals, the possibility of backbone conduction, and the effects of water. We find that small activation gaps observed in conductivity experiments may arise in the presence of water and counter ions. We further discuss the role of harmonic vibration and twisting motion on electron tight binding matrix elements using ab initio density functional theory and model Koster-Slater theory calculations. We find that partial cancellation between pp-sigma and pp-pi interaction of Pz orbitals on adjacent base pairs, along with destructive interference of phase factors are needed to explain the weak conductance of A-DNA. Our results lead also to a physical interpretation of the angular dependence of inter-base pair tight binding matrix elements. Furthermore, we estimate Franck-Condon factors, reorganization energies and nuclear frequencies essential for charge transfer rates, and find our estimated hole transfer rates between base pairs to be in excellent agreement with recent picosecond dynamics data.