| Desoxyribo-Nucleic-Acid (DNA) is a complicated biomolecule, which encodes the fundamental nature of living species. DNA has a double helix structure, and the backboned of the double helix is composed of phosphates and sugars joined by covalent bond, on which four type bases (Adenine-A, Guanine-G, Cytosine-C, and Thymine-T) paired together through hydrogen bonds according to complementary pairs law (A with T, and G with C).DNA is the most important molecule of bioorganic system. Its structure contains information that determines heredity, cell division, growth and biosynthesis of protein. Interference with gene function and prevention of transcription and translation can kill invading microorganisms or tumor cells. In recent years, the charge migration along the duplex DNA stack is attracting considerable interest for the biochemical reasons in life science, as well as for the possible applications in molecular electronics. A number of experiments have showed that, the product of electron transfer will lead to DNA mutation; light-induced electron transfer will damage DNA, or repair it to cure tumors. For example, under the irradiation of the ionization or ultraviolet radiation, an electron in DNA will move along DNA before being trapped by other atoms. So it is thought that DNA may be a quick channel for electron transfer which is independent of distance. Meanwhile DNA has two special characters: recognition, structuring and self-assembly properties.On the other hand, the progress of the electronic industry in the past few decades was based on the delivery of smaller and smaller devices and denser integrated circuits, which ensured the attainment of more and more powerful computers. However, such a fast growth is compromised by the intrinsic limitations of the conventional technology. Electronic circuits are currently fabricated with complementary metal-oxide-semiconductor (CMOS) transistors. Higher transistor density on a single chip means faster circuit performance. The trend towards higher integration is restricted by the limitations of the current lithography technologies, by heat dissipation and by capacitive coupling between different components. Moreover, the down-scaling of individual devices to the nanometer range collides with fundamental physical laws. In fact, in conventional silicon-based electronic devices the information is carried by mobile electrons within a band of allowed energies according to the semiconductor band structure. However, when the dimensions shrink to the nanometer scale, and bands turn into discrete energy levels, then quantum correlation effects induce localization. It is presupposed that the conventional electronic industry will go to its physical limitation in 2010. In order to pursue the miniaturization of integrated circuits further, a novel technology, which would exploit the pure quantum mechanical effects that rule at the nanometer scale, is therefore demanded. 40 years ago, quantum physicist, and Nobelist Feynman put forward a new idea to design circuit, under which the circuit is designed from bottom to top. This physical idea is regarded as the origin of molecular electronics and nanotechnology. The basic idea of molecular electronics is to use individual molecules as wires, switches, rectifiers and memories. Another conceptual idea that is advanced by molecular electronics is the switch from a top-bottom approach, where the devices are extracted from a single large-scale building block, to a bottom-up approach in which the whole system is composed of small basic building blocks with recognition, structuring and self-assembly properties. The great advantage of molecular electronics in the frame of the continued device miniaturization is the intrinsic nanoscale size of the molecular building blocks that are used in the bottom-up approach, as well as the fact that they may be synthesized in parallel in huge quantities and at low cost. In the past 20 years, molecular electronics has procured much material advancement, and the advancement made by molecular electronics is appraised as the first of ten technology advancements by Science in 2001.Different candidates for molecular devices are currently the subject of highly interdisciplinary investigation efforts, including small organic polymers, large bio-molecules, nanotubes and fullerenes. DNA has two special characters: recognition, structuring and self-assembly properties. If the charge can transport in DNA molecule, it should be a well prospective candidates for molecular electronic devices. However, at present the mechanism of charge transport in DNA molecule is controversial. The experiments of charge transport in DNA molecule have shown that DNA molecule may be conductor, semiconductor or insulator. So how to interpret these controversial experiment phenomena and ravel the mechanism of charge transport in DNA molecule has important significance for using DNA as a molecular device and understanding the mechanism of DNA mutation and repairing of the damaged DNA.DNA is a soft biomolecule. The charge transport in DNA is affected by many factors, such as the molecular configuration, the sequence of base pairs, temperature, humidity, solution and impurity in the molecule. The effect of these factors on the electronic structure of DNA and the charge transport of DNA has been extensively studied. These theory works partly represent the diversity of charge transport in DNA. However, at present there is currently no unanimous understanding of its electrical behavior and of the mechanisms that might control charge mobility through its structure. There are more works to understand the electrical behavior and the mechanisms of charge transport in DNA.Through widely investigations we found that the density of itinerant electrons is variable, not fixed. In theory the itinerant electrons of DNA is the unpairedπ-electrons which come from SP and SP2 orbit hybridization as atoms combine with bond. The sugar and four type bases in DNA are all heterocyclic structures made of C, O, N, and H atoms, and the condition of these orbit hybridization is complex, then the number of itinerant electrons is difficult to determine. On the other hand, the double helix DNA molecule with low symmetry has a lot of hanging bonds, and the unpaired electrons of these hanging bonds may come into theπ-electrons system. Then the density of itinerant electrons in DNA molecule is not fixed, but depends upon the component and structure of DNA molecule, the environment condition also has an important effect on the density of itinerant electrons. This viewpoint is proved by experiment also. For example, Yoo et al. have found that poly(dA)-poly(dT) behaves as an n-type semiconductor, whereas poly(dG)-poly(dC) behaves as a p-type semiconductor by the gate-voltage dependent transport measurements. This suggests that the density of itinerant electrons is different in poly(dA)-poly(dT) from that in poly(dG)-poly(dC). Based on above reasons we put forward the idea that the density of itinerant electrons in DNA molecule is variable.The structure of DNA is complex. Several models have been suggested to describe DNA molecule, such as, the simple one dimensional tight binding model, the fish bone model, the ladder model, and three dimensionals tight binding model.In this paper, the effects of the density of itinerant electrons on the electronic structure of DNA and the charge transport in DNA are studied with one dimensional tight binding model and three dimensionals tight binding model respectively. The effect of structural fluctuations on the electronic structure of DNA and the charge transport in DNA are studied with one dimensional tight binding model and three dimensionals tight binding model respectively also. The differences between two models are compared. The primary contents and results are given as follows:1. Effect of the density of itinerant electrons on the electronic structure of DNA and the charge transport in DNA with one dimensional tight binding model1.1 DNA is a soft biomolecule with strong electron-latttice interaction. The electronic structure of DNA depends on the density of itinerant electrons, vice versa. The electronic structure directly affects the behavior of charge transport in DNA. One dimensional tight binding model with electron-lattice interaction is adopted to study the effect of the density of itinerant electrons on the electronic structure of DNA and the charge transport in DNA. It is found that the energy gap (the difference between the lowest unoccupied molecular orbit (LUMO) and the highest occupied molecular orbit (HOMO)) changes with the density of itinerant electrons, and the resistivity of DNA changes with the density of itinerant electrons too. DNA may show the behavior of conductor, semiconductor, or insulator with different density of itinerant electrons.1.2 The sequence of base pairs has an important effect on the charge transport in DNA. The electronic structure and the charge transport of three different periodic sequences DNA molecule (periodic sequence, quasiperiodic sequence, and aperiodic sequence) are studied. The calculations show that the dependences of electronic structure and the behaviors of charge transport on the density of itinerant electrons are different for three different periodic sequences DNA molecule. The energy gap and the resistivity of the periodic sequence DNA (Poly (dG)-Poly (dC)) are symmetrical about the half-filling state. At the half-filling state, due to the instability of Peierls, the energy band of Poly(dG)-Poly(dC) is split into two sub-bands: the occupied valence band and the unoccupied conduction band, with a band gap of about 1 eV, and theresistivity of Poly(dG)-Poly(dC) is on the order of 10-2Ωcm. Departing from thehalf-filling state, a few localized energy levels appear in the energy band gap. These states will have important effects on the conductivity of Poly(dG)-Poly(dC). Since these states are localized and not favorable for charge transport, the resistivities of Poly(dG)-Poly(dC) are large in this region. When the density of itinerantπ-electrons departs over 20% from the half-filling state, many energy levels appear in the energy band gap. The electronic states of these levels become delocalized and have large transmission coefficients, and the resistivity decreased. The aperiodic sequence DNA molecule has no symmetry, and the resistivity is larger than Poly(dG)-Poly(dC). The resistivity of quasiperiodic sequence DNA molecule is between the periodic sequence and the aperiodic sequence.1.3 The dependences of the resistivity on the length of DNA molecule chain are different for three different periodic sequences DNA molecule. The resistivity of periodic sequence decreases with the length increasing, and tends towards a steady value when the length is long enough. The resistivity of quasiperiodic sequence fluctuates with the length increasing, and shows a long range correlation effect. The resistivity of aperiodic sequence increases with the length increasing.2. Effect of lattice site fluctuations on the electronic structure of DNA and the charge transport in DNA with one dimensional tight binding modelDue to the softness of DNA, many environment factors, such as temperature, humidity, solution and impurity in the molecule may cause the changes of molecular structure and electronic structure, and then affect its behavior of charge transport. In order to simplify the problem, the effect of all these factors on the charge transport is treated as the lattice site fluctuations and a uniform distribution is adopted to simulate the lattice site fluctuations. The effect of lattice site fluctuations on the electronic structure and charge transport of DNA are studied with one dimensional tight binding model.2.1 It was found that at half filling state of itinerant electrons the energy band gap of a periodic sequence DNA molecule decreases with increasing of the lattice site fluctuations, meanwhile the electronic states of DNA tend to localizing. One hand the decreasing of band gap is favor of charge transport in DNA, on the other hand the localizing of electronic states goes against charge transport in DNA. When the fluctuations are small, the band gap of periodic sequence DNA decreases, and the electron states is still extended, the DNA molecule has good conductance. The reason is that small lattice site fluctuations can counteract the dimerizing of the lattices, and the electrons may gain energy from the lattice fluctuations, and easily get rid of the fetters of lattice sites. When the fluctuations are large, although the band gap decreases, but the electronic states become localized heavily, and the resistivity becomes large.2.2 The resistivity of periodic sequence DNA first decreases and then increases with increasing of lattice site fluctuations. This is consistent with the qualitative analysis.3. The effect of the density of itinerant electrons on the electronic structure of DNA and the charge transport in DNA with three dimensionals tight binding model3.1 One dimensional tight binding model ignores the double helix structure of DNA, however, experiments have proved that the helix structure of DNA has important effects on the charge transport in DNA. Referencing D.Hennig's work, we suggest a three dimensionals tight binding model to describe DNA molecule. Comparing with one dimensional tight binding model, in the frame work of three dimensionals model, one hand the movement of lattice sites affects the electron transition integral by the electron-lattice interaction, on the other hand affects the on-site energy; in the frame work of one dimensional model, the movement of lattice sites only affects the electron transition integral by the electron-lattice interaction. It was found that in the frame work of three dimensionals model the effect of the movement of lattice sites on the electron transition integral is small, and the movement of lattice sites mainly changes the on-site energy, and then affects the electronic structure and charge transport in DNA.3.2 Differing from one dimensional tight binding model, in the frame work of three dimensionals tight binding model, the conductivity of PoIy(dG)-Poly(dC) takes place parity phase-transition with the varying of the density of itinerant electrons as the density of itinerant electrons departure from the half filling state over 20%. Namely, when HOMO is odd number energy level, the electronic structure and the resistivity of Poly(dG)-Poly(dC) are analogous with metal conductor; when HOMO is even number energy level, the electronic structure and the resistivity of Poly(dG)-Poly(dC) are analogous with semiconductor.4. The effect of temperature on the charge transport in Poly(dG)-Poly(dC) withthree dimensionals tight binding modelIn the frame of three dimensionals tight binding model, considering the effect of temperature on the charge transport in periodic sequence DNA, the effect of hydrogen bond length fluctuations and helix angle fluctuations on the charge transport in Poly(dG)-Poly(dC) are studied, the transmission coefficient, the localization length of electronic states, direct measured quantity current-voltage character (I-V) and the conductivity of Poly(dG)-Poly(dC) are calculated.4.1 The helix angle fluctuations have small effect on the charge transport in PoIy(dG)-Poly(dC), the hydrogen bond length fluctuations have important effect on the charge transport in Poly(dG)-Poly(dC). Namely, temperature mainly through the hydrogen bond length fluctuations to affect the charge transport in DNA.4.2 The conductivity of Poly(dG)-Poly(dC) decreases with temperature increasing. At low temperature range, the conductivity decreases rapidly, at high temperature range, the resistivity decreases slowly. We conjecture that the mechanism of charge transport in Poly(dG)-Poly(dC) transits from band resonant transport mechanism to more hopping transport mechanism with temperature increasing.
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