RESUMO
We study the transport through a molecular junction exhibiting interference effects. We show that these effects can still be observed in the presence of molecular vibrations if Coulomb repulsion is taken into account. In the Kondo regime, the conductance of the junction can be changed by several orders of magnitude by tuning the levels of the molecule, or displacing a contact between two atoms, from nearly perfect destructive interference to values of the order of 2e 2/h expected in Kondo systems. We also show that this large conductance change is robust for reasonable temperatures and voltages for symmetric and asymmetric tunnel couplings between the source-drain electrodes and the molecular orbitals. This is relevant for the development of quantum interference effect transistors based on molecular junctions.
RESUMO
We calculate the spectral density and occupations of a system of two capacitively coupled quantum dots, each one connected to its own pair of conducting leads, in a regime of parameters in which the total couplings to the leads for each dot Γ(i) are different. The system has been used recently to perform pseudospin spectroscopy by controlling independently the voltages of the four leads. For an odd number of electrons in the system, equal coupling to the leads Γ1 = Γ2, equal dot levels E1 = E2 and sufficiently large interdot repulsion U12 the system lies in the SU(4) symmetric point of spin and pseudospin degeneracy in the Kondo regime. In the more realistic case Γ1 ≠ Γ2, pseudospin degeneracy is broken and the symmetry is reduced to SU(2). Nevertheless, we find that the essential features of the SU(4) symmetric case are recovered by appropriately tuning the level difference δ = E2 - E1. After this tuning, the system behaves as an SU(4) Kondo one at low energies. Our results are relevant for experiments which look for signatures of SU(4) symmetry in the Kondo regime of similar systems.
RESUMO
Starting from exact eigenstates for a symmetric ring, we derive a low-energy effective generalized Anderson Hamiltonian which contains two spin doublets with opposite momenta and a singlet for the neutral molecule. For benzene, the singlet (doublets) represent the ground state of the neutral (singly charged) molecule. We calculate the non-equilibrium conductance through a benzene molecule, doped with one electron or a hole (i.e. in the Kondo regime), and connected to two conducting leads at different positions. We solve the problem using the Keldysh formalism and the non-crossing approximation. When the leads are connected in the para position (at 180°), the model is equivalent to the ordinary impurity Anderson model and its known properties are recovered. For other positions, there is a partial destructive interference in the co-tunneling processes involving the two doublets and, as a consequence, the Kondo temperature and the height and width of the central peak (for bias voltage V(b) near zero) of the differential conductance G = dI/dV(b) (where I is the current) are reduced. In addition, two peaks at finite V(b) appear. We study the position of these peaks, the temperature dependence of G and the spectral densities. Our formalism can also be applied to carbon nanotube quantum dots with intervalley mixing.
RESUMO
Leishmania mutants have contributed greatly to extend our knowledge of this parasite's biology. Here we report the use of the mariner in vitro transposition system as a source of reagents for shuttle mutagenesis and targeted disruption of Leishmania genes. The locus-specific integration was achieved by the disruption of the subtelomeric gene encoding a DNA-directed RNA polymerase III subunit (RPC2). Further inactivation of RPC2 alleles required the complementation of the intact gene, which was transfected in an episomal context. However, attempts to generate a RPC2 chromosomal null mutant resulted in genomic rearrangements that maintained copies of the intact locus in the genome. The maintenance of the RPC2 chromosomal locus in complemented mutants was not mediated by an increase in the number of copies and did not involve chromosomal translocations, which are the typical characteristics of the genomic plasticity of this parasite. Unlike the endogenous locus, the selectable marker used to disrupt RPC2 did not display a tendency to remain in its chromosomal location but was targeted into supernumerary episomal molecules.
Assuntos
Leishmania major/genética , Mutagênese/genética , Mutação/genética , RNA Polimerase III/genética , Telômero/genética , Animais , Linhagem Celular , Regulação da Expressão Gênica , Genes Essenciais/genética , Proteínas de Protozoários/genética , Proteínas de Protozoários/metabolismo , RNA Polimerase III/metabolismoAssuntos
Leishmania major/genética , Processamento Pós-Transcricional do RNA , RNA de Protozoário/metabolismo , Sequência de Aminoácidos , Animais , Sequência de Bases , Genoma de Protozoário , Leishmania major/metabolismo , Camundongos , Camundongos Endogâmicos BALB C , Dados de Sequência Molecular , RNA de Protozoário/químicaRESUMO
The 36 chromosomes of the parasite Leishmania major range in size from 200 kb to approximately 2.5 Mb and variation between homologues seems to be restricted to the telomeric and subtelomeric regions. We have isolated three cosmids carrying the telomere hexameric repeat and assigned them to the extreme location of chromosomes 3, 7 and 20. When considering the distribution of repetitive sequences, Southern analysis of the three chromosomal ends indicated the existence of at least two classes of chromosomal extremities: one of them is composed almost exclusively of unique sequences and the other is characterised by patches of both reiterated and unique sequences. We devised a transfection-based strategy that allowed the determination of a map of transcripts in each of the regions examined. Sequencing of the chromosome 20 cosmid revealed the existence of a novel class of reiterated sequence, LST-R378, and 10 ORFs drawing a map of putative genes compatible with the map of transcripts.
Assuntos
Mapeamento Cromossômico , Leishmania major/genética , Transcrição Gênica , Animais , Sequência de Bases , Cosmídeos , DNA de Protozoário/química , DNA de Protozoário/genética , Endodesoxirribonucleases , Regulação da Expressão Gênica , Biblioteca Genômica , Dados de Sequência Molecular , Mutagênese Insercional , RNA de Protozoário/genética , Sequências Repetitivas de Ácido Nucleico , Mapeamento por Restrição , Telômero/genéticaRESUMO
We have used a chromosome-specific approach to generate a 300 kb long 'contig' across Leishmania major 500 kb chromosome. Clones from a 13-hit genomic library served as templates to generate end-specific probes that were used in hybridization to a high density array of the library. The 'contig' generated contained 12 markers uniformly spaced. Three restriction endonucleases were mapped within the map extending its resolution. Map extension indicated a peculiar feature of sequence organization in subtelomeric regions where chromosome-specificity of mapping is lost. End-probes generated from clones mapping to the extremes of a 300 kb 'contig' rescued a high percentage of 2 types of clones from the genomic library, 1 of which showed positive hybridization to the hexameric telomere repeat. Fine mapping at these regions revealed that these 2 clones contained elements common to all chromosomes of the parasite. The physical map generated constitutes ready-to-use data for the study of many aspects of genome organization. Being cloned in a shuttle vector, the genomic sequences reordered in the map can be used to generate genetic information by transfection into the parasite.
Assuntos
Mapeamento Cromossômico , Leishmania major/enzimologia , Complexos Multienzimáticos/genética , Proteínas de Protozoários/genética , Tetra-Hidrofolato Desidrogenase/genética , Timidilato Sintase/genética , Animais , Leishmania major/genéticaAssuntos
Leishmania/genética , Animais , DNA de Protozoário/genética , Amplificação de Genes , Expressão Gênica , Marcação de Genes , Genoma de Protozoário , Humanos , Leishmaniose Cutânea/parasitologia , Biologia Molecular , Processamento Pós-Transcricional do RNA , Splicing de RNA , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , TransfecçãoRESUMO
Anhydrotetracycline (AHTC), one of the major toxic decomposition products of the antibiotic tetracycline, contains several potential binding sites to metal ions. The acidity constants of the ligand were calculated in aqueous medium (I = 0.1 M) at 25 and 37 degrees C. We found pKa1 = 3.23 +/- 0.08, pKa2 = 5.94 +/- 0.09, and pKa3 = 8.48 +/- 0.02 at 25 degrees C and pKa1 = 3.12 +/- 0.09, pKa2 = 5.86 +/- 0.03, and pKa3 = 8.38 +/- 0.04 at 37 degrees C. The coordination of AHTC to Cu(II) and Ni(II) ions was studied in the solid state as well as in buffered aqueous solution at pH 10.0. At this pH, the formation of the two CuL2 and CuL species was indicated (log beta 1 = 8.41 +/- 0.04 and log beta 2 = 12.55 +/- 0.05), but only the formation of the NiL complex (log beta = 5.74 +/- 0.04) was identified. Spectroscopic data confirm the previous assignment of the C11 and C12 oxygens as the coordination sites, yielding six-membered ring chelates and excluding complexation through any of the potential binding positions on ring A.