Origin of n- and p-type conductivity in undoped α-PbO: role of defects.

First-principles calculations have been applied to study the crystallographic defects in α-PbO in order to understand an origin of n- and p-type conductivity in otherwise undoped α-PbO. It was found that deposition in an oxygen-deficient environment defined in our simulations by the Pb-rich/O-poor limit stimulates a formation of O vacancies and Pb interstitials both characterized by quite low formation energies ∼1.0 eV.

Lead monoxide α-PbO: electronic properties and point defect formation.

The electronic properties of polycrystalline lead oxide consisting of a network of single-crystalline α-PbO platelets and the formation of native point defects in the α-PbO crystal lattice are studied using first-principles calculations. The results suggest that the polycrystalline nature of α-PbO causes the formation of lattice defects (i.e.

Quantum transport anomalies in DNA containing mispairs.

The effect of mispairs on charge transport in DNA of sequence (GC)(TA)(N)(GC)(3) connected to platinum electrodes is studied using the tight-binding model. With parameters derived from an ab initio density functional result, we calculate the current versus bias voltage for DNA with and without a mispair and for different numbers of (TA) basepairs N between the single and triple (GC) basepairs. The current decays exponentially with N under low bias but reaches a minimum under high bias when a multichannel transport mechanism is established.

Sustained ferromagnetism induced by H-vacancies in graphane.

The electronic and magnetic properties of graphane flakes with H-vacancies were investigated using quantum-chemistry methods. The hybridization of the edges is found to be absolutely crucial in defining the size of the HOMO-LUMO gap, which is increased from 3.04 to 7.

Electrical current through DNA containing mismatched base pairs.

Mismatched base pairs, such as different conformations of the G.A mispair, cause only minor structural changes in the host DNA molecule, thereby making mispair recognition an arduous task. Electron transport in DNA that depends strongly on the hopping transfer integrals between the nearest base pairs, which in turn are affected by the presence of a mispair, might be an attractive approach in this regard.

Unique magnetic signatures of mismatched base pairs in DNA.

Magnetic properties of DNA containing mispairs, such as different conformations of the GA mispair, or a GT mispair inserted into the DNA chain, have been theoretically investigated. The essential ingredients for these studies, the charge transfer integrals, were evaluated from the DNA sequences containing the mispair and optimized in the solvent. We find that the magnetic susceptibilities of the host DNA chain containing a large number of Watson-Crick base pairs are significantly altered in the presence of the mispairs, and the effects depend on the choice of mispairs.

Thermodynamics of G x A mispairs in DNA: continuum electrostatic model.

An analysis of the stability of a duplex containing G x A mispairs or G x A/A x G tandem during the DNA melting has shown that the duplex stability depends on both DNA sequences and the conformations of the G x A mispairs. The thermodynamics of single pair opening for the G(anti) x A(syn) and G(anti) x A(anti) conformations adopted by a G x A mispair is found to strongly correlate with that of the canonical base pairs, while for the sheared conformation a significant difference is observed.

How the surrounding water changes the electronic and magnetic properties of DNA.

We study the influence of humidity on the transport and magnetic properties of DNA within the quantum chemistry methods. Strong influence of water molecules on these properties, observed in this study, opens up opportunities for application of DNA in molecular electronics. Interaction of the nucleobases with water molecules leads to breaking of some of the pi bonds and appearance of unbound pi electrons.

Water induced weakly bound electrons in DNA.

We have studied the effect of humidity on the electronic properties of DNA base pairs. We found that the hydrogen links of the nucleobases with water molecules lead to a shift of the pi electron density from carbon atoms to nitrogen atoms and can change the symmetry of the wave function for some nucleobases. As a result, the orbital energies are shifted which leads to a decrease in the potential barrier for the hole transfer between the G-C and A-T pairs from 0.

Influence of solvent on the energetics of hole transfer in DNA.

We have investigated the contribution of molecular environment to the exchange reactions in the DNA molecule taking into account different geometries of the reaction centers in oxidized and reduced states. We have observed the influence of the ionization potential of the donor and the acceptor on the free energy of the hole transfer reaction in the solvated DNA molecule: A decrease of the free energy occurs if IPA > or = IPD and an increase if IPA < or = IPD. The corresponding decrease of the potential barrier by 0.

Energy contribution of the solvent to the charge migration in DNA.

The authors have investigated the interactions of the reaction centers, participating in the charge transfer reaction within the DNA molecule with the phosphate backbones and the solvent molecules, and have estimated the contribution of these interactions into the charge migration in DNA. They have determined the unequal shift of the energy surfaces of the initial and final transition states of the transfer reaction along the energy axis and the dependence of the magnitude of the energy shift on the nature of the reaction centers and the surrounding environment. The nonuniform distribution of the negative charge in the DNA phosphate backbones results in an increase of the positive shift of the energy surface of the DNA base pairs in the center of the structure, where the maximum density of the negative charge is concentrated.