51 research outputs found
Pharmacophore elements of the TIPP class of delta opioid receptor antagonists
A series of tri-and tetrapeptides sharing the amino-terminal dipeptide unit Tyr-Tic, found in the high-affinity delta opioid receptor antagonist Tyr-Tic-Phe-Phe (TIPP), was prepared and evaluated in receptor binding assays to explore the role(s) of the phenylalanine residues in positions 3 and 4. It was found that aromaticity of residues 3 and 4 is not required for high affinity, a lipophilic side chain in either location being sufficient, as evidenced by the high delta receptor binding affinities observed for the tetrapeptide Tyr-Tic-Ala-Leu and the tripeptide Tyr-Tic-Leu. These results support the suggestion of Temussi et al. [Biochem. Biophys. Res. Commun., 198 (1994) 933] that the aromatic side chain of the Tic residue corresponds to the aromatic side chain found in residues 3 or 4 in other delta-selective peptide series.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43172/1/10989_2004_Article_BF00126275.pd
Microscopic theory for quantum mirages in quantum corrals
Scanning tunneling microscopy permits to image the Kondo resonance of a
single magnetic atom adsorbed on a metallic surface. When the magnetic impurity
is placed at the focus of an elliptical quantum corral, a Kondo resonance has
been recently observed both on top of the impurity and on top of the focus
where no magnetic impurity is present. This projection of the Kondo resonance
to a remote point on the surface is referred to as quantum mirage. We present a
quantum mechanical theory for the quantum mirage inside an ideal quantum corral
and predict that the mirage will occur in corrals with shapes other than
elliptical
Nonequilibrium Transport through a Kondo Dot in a Magnetic Field: Perturbation Theory
Using nonequilibrium perturbation theory, we investigate the nonlinear
transport through a quantum dot in the Kondo regime in the presence of a
magnetic field. We calculate the leading logarithmic corrections to the local
magnetization and the differential conductance, which are characteristic of the
Kondo effect out of equilibrium. By solving a quantum Boltzmann equation, we
determine the nonequilibrium magnetization on the dot and show that the
application of both a finite bias voltage and a magnetic field induces a novel
structure of logarithmic corrections not present in equilibrium. These
corrections lead to more pronounced features in the conductance, and their form
calls for a modification of the perturbative renormalization group.Comment: 16 pages, 7 figure
Enhancement of the Two-channel Kondo Effect in Single-Electron boxes
The charging of a quantum box, coupled to a lead by tunneling through a
single resonant level, is studied near the degeneracy points of the Coulomb
blockade. Combining Wilson's numerical renormalization-group method with
perturbative scaling approaches, the corresponding low-energy Hamiltonian is
solved for arbitrary temperatures, gate voltages, tunneling rates, and energies
of the impurity level. Similar to the case of a weak tunnel barrier, the shape
of the charge step is governed at low temperatures by the non-Fermi-liquid
fixed point of the two-channel Kondo effect. However, the associated Kondo
temperature TK is strongly modified. Most notably, TK is proportional to the
width of the level if the transmission through the impurity is close to unity
at the Fermi energy, and is no longer exponentially small in one over the
tunneling matrix element. Focusing on a particle-hole symmetric level, the
two-channel Kondo effect is found to be robust against the inclusion of an
on-site repulsion on the level. For a large on-site repulsion and a large
asymmetry in the tunneling rates to box and to the lead, there is a sequence of
Kondo effects: first the local magnetic moment that forms on the level
undergoes single-channel screening, followed by two-channel overscreening of
the charge fluctuations inside the box.Comment: 21 pages, 19 figure
Deterministic delivery of externally cold and precisely positioned single molecular ions
We present the preparation and deterministic delivery of a selectable number
of externally cold molecular ions. A laser cooled ensemble of Mg^+ ions
subsequently confined in several linear Paul traps inter-connected via a
quadrupole guide serves as a cold bath for a single or up to a few hundred
molecular ions. Sympathetic cooling embeds the molecular ions in the
crystalline structure. MgH^+ ions, that serve as a model system for a large
variety of other possible molecular ions, are cooled down close to the Doppler
limit and are positioned with an accuracy of one micrometer. After the
production process, severely compromising the vacuum conditions, the molecular
ion is efficiently transfered into nearly background-free environment. The
transfer of a molecular ion between different traps as well as the control of
the molecular ions in the traps is demonstrated. Schemes, optimized for the
transfer of a specific number of ions, are realized and their efficiencies are
evaluated. This versatile source applicable for broad charge-to-mass ratios of
externally cold and precisely positioned molecular ions can serve as a
container-free target preparation device well suited for diffraction or
spectroscopic measurements on individual molecular ions at high repetition
rates (kHz).Comment: 11 pages, 8 figure
Non-equilibrium Kondo effect in asymmetrically coupled quantum dot
The quantum dot asymmetrically coupled to the external leads has been
analysed theoretically by means of the equation of motion (EOM) technique and
the non-crossing approximation (NCA). The system has been described by the
single impurity Anderson model. To calculate the conductance across the device
the non-equilibrium Green's function technique has been used. The obtained
results show the importance of the asymmetry of the coupling for the appearance
of the Kondo peak at nonzero voltages and qualitatively explain recent
experiments.Comment: 7 pages, 6 figures, Physical Review B (accepted for publication
A novel cyclic enkephalin analogue with potent opioid antagonist activity
10.1016/j.bmcl.2004.06.077Bioorganic and Medicinal Chemistry Letters14184731-4733BMCL
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