Conductance Of DNA Molecules: A Double-chain Tight-binding Model

The conductance of DNA molecules is one of the central problems of biophysics because it plays a critical role in biological systems. Also, DNA is a promising candidate which may serve as the building block of molecular electronics because of its sequence-dependent and self-assembly properties. There have been many experimental results on the conductance of DNA from different measurements for the last few years. Yet the results are still highly controversial. The experimental results cover almost all possibilities, ranged from insulating, semiconducting, Ohmic, and even induced superconductivity. Theoretically, first-principles calculations are an excellent but time consuming tool. One-dimensional tight-binding models, providing semi-experiential analytical solutions, have also been widely developed, by which one can control the physical outcomings in wide parameter ranges. Nevertheless, one- dimensional chain models deal with effective sites (base pairs) instead of a double- chain structure. The soft matter characters of DNA may suggest that double-chain tight-binding model may describe DNA's structure character better.In this thesis, a double-chain tight-binding model is developed basing on transfer matrix method. Considering both the intrastrand and interstrand nearest-neighbor hoppings of sites, electronic transmission coefficient of DNA is calculated, which depends on energy and interstrand hopping integral. Also, the effects of sequence, chain length, temperature and intrastrand hopping integral on transmission coefficient distributing are investigated. Among short periodic sequence DNA, Poly (G)-(C), Poly (A)-(T) DNA, series of G (guanine) and A (adenine) are found to have good diffusive charge tunneling. On the contrary, aperiodic genomic DNA have weak charge transfer; the transmission coefficient of DNA with short period sequence does not seem to depend on the length, yet differently, the transmission pattern of aperiodic genomic DNA sequences strongly depends on the strand length. As the number of corresponding bases increases, fewer states will present good transmission ability, because aperiodic increasing bases make more backscattering; at low temperature, the transmission spectrum presents a higher number of transmitting states, due to a breaking of level degeneracy. At higher temperature, the number of transmitting states decreases; Small intrastrand hopping integral does not seem to diminish the transmission coefficient, but shrinks the location in the three-dimension figure. And the distribution of energy is concentrated.