273 research outputs found
Conversion of the Kunitz-type module of collagen VI into a highly active trypsin inhibitor by site-directed mutagenesis.
The recombinant Kunitz protease inhibitor module (domain C5) of human collagen α3(VI) chain was previously shown to lack inhibitory activity for proteases with trypsin-like specificity and some other proteases. We have now prepared mutants in the binding loop region including the P1′ site (D2889 → A), the P2′ site (F2890 → R) and the P3 site (T2886 → P) and in a more remote region (W2907 → V) either as individual substitutions or combinations of them. These mutants were analyzed for their kinetics of binding to trypsin by surface plasmon resonance and for their capacity to inhibit various proteases. Single substitutions (D → A, T → P, W → V) showed an effect only for D → A which bound to trypsin with Kd= 0.25 μM. A 25–100-fold increase in affinity was observed for the double mutants T → P/D → A and F → R/D → A and approached the affinity of aprotinin (Kd≈0.01 nM) in two different triple mutants. These affinities correlated well with the inhibitory capacities of the mutants for trypsin in the cleavage of a large protein and a small peptide substrate. A similar but not completely identical improvement in inhibitory capacity was also observed for leucocyte elastase but not for thrombin. These data could be interpreted in terms of steric interferences or lack of hydrogen bonding of a few critical residues based on three-dimensional structures available for the C5 domain
Regular networks of Luttinger liquids
We consider arrays of Luttinger liquids, where each node is described by a
unitary scattering matrix. In the limit of small electron-electron interaction,
we study the evolution of these scattering matrices as the high-energy single
particle states are gradually integrated out. Interestingly, we obtain the same
renormalization group equations as those derived by Lal, Rao, and Sen, for a
system composed of a single node coupled to several semi-infinite 1D wires. The
main difference between the single node geometry and a regular lattice is that
in the latter case, the single particle spectrum is organized into periodic
energy bands, so that the renormalization procedure has to stop when the last
totally occupied band has been eliminated. We therefore predict a strongly
renormalized Luttinger liquid behavior for generic filling factors, which
should exhibit power-law suppression of the conductivity at low temperatures
E_{F}/(k_{F}a) >
1. Some fully insulating ground-states are expected only for a discrete set of
integer filling factors for the electronic system. A detailed discussion of the
scattering matrix flow and its implication for the low energy band structure is
given on the example of a square lattice.Comment: 16 pages, 7 figure
Atom lithography with two-dimensional optical masks
With a two-dimensional (2D) optical mask, nanoscale patterns are created for
the first time in an atom lithography process using metastable helium atoms.
The internal energy of the atoms is used to locally damage a hydrofobic resist
layer, which is removed in a wet etching process. Experiments have been
performed with several polarizations for the optical mask, resulting in
different intensity patterns, and corresponding nanoscale structures. The
results for a linear polarized light field show an array of holes with a
diameter of 260 nm, in agreement with a computed pattern. With a circularly
polarized light field a line pattern is observed with a spacing of 766 nm.
Simulations taking into account many possible experimental imperfections can
not explain this pattern.Comment: 5 pages, 4 figure
Theory of Incompressible States in a Narrow Channel
We report on the properties of a system of interacting electrons in a narrow
channel in the quantum Hall effect regime. It is shown that an increase in the
strength of the Coulomb interaction causes abrupt changes in the width of the
charge-density profile of translationally invariant states. We derive a phase
diagram which includes many of the stable odd-denominator states as well as a
novel fractional quantum Hall state at lowest half-filled Landau level. The
collective mode evaluated at the half-filled case is strikingly similar to that
for an odd-denominator fractional quantum Hall state.Comment: 4 pages, REVTEX, and 4 .ps file
Finite Size Analysis of Luttinger Liquids with a source of 2k_f Scattering
Numerical analysis of the spectrum of large finite size Luttinger liquids
(g<1) in the presence of a single source of 2k_f scattering has been made
possible thanks to an effective integration of high degrees of freedom.
Presence of irrelevant operators and their manifestation in transport are
issues treated independently. We confirm the existence of two irrelevant
operators: particle hopping and charge oscillations, with regions of dominance
separated by g=1/2. Temperature dependence of conductance is shown to be
dominated by hopping alone. Frequency dependence is affected by both irrelevant
operators.Comment: 4 pages, LaTex (RevTex), 3 PostScript figures appende
Observation of Quantum Asymmetry in an Aharonov-Bohm Ring
We have investigated the Aharonov-Bohm effect in a one-dimensional
GaAs/GaAlAs ring at low magnetic fields. The oscillatory magnetoconductance of
these systems are for the first time systematically studied as a function of
density. We observe phase-shifts of in the magnetoconductance
oscillations, and halving of the fundamental period, as the density is
varied. Theoretically we find agreement with the experiment, by introducing an
asymmetry between the two arms of the ring.Comment: 4 pages RevTex including 3 figures, submitted to Phys. Rev.
Characterization of one-dimensional quantum channels in InAs/AlSb
We report the magnetoresistance characteristics of one-dimensional electrons
confined in a single InAs quantum well sandwiched between AlSb barriers. As a
result of a novel nanofabrication scheme that utilizes a 3nm-shallow wet
chemical etching to define the electrostatic lateral confinement, the system is
found to possess three important properties: specular boundary scattering, a
strong lateral confinement potential, and a conducting channel width that is
approximately the lithography width. Ballistic transport phenomena, including
the quenching of the Hall resistance, the last Hall plateau, and a strong
negative bend resistance, are observed at 4K in cross junctions with sharp
corners. In a ring geometry, we have observed Aharonov-Bohm interference that
exhibits characteristics different from those of the GaAs counterpart due to
the ballistic nature of electron transport and the narrowness of the conducting
channel width.Comment: pdf-file, 8 figures, to be published in Phys. Rev.
Probing the potential landscape inside a two-dimensional electron-gas
We report direct observations of the scattering potentials in a
two-dimensional electron-gas using electron-beam diffaction-experiments. The
diffracting objects are local density-fluctuations caused by the spatial and
charge-state distribution of the donors in the GaAs-(Al,Ga)As heterostructures.
The scatterers can be manipulated externally by sample illumination, or by
cooling the sample down under depleted conditions.Comment: 4 pages, 4 figure
Disorder suppression and precise conductance quantization in constrictions of PbTe quantum wells
Conductance quantization was measured in submicron constrictions of PbTe,
patterned into narrow,12 nm wide quantum wells deposited between
PbEuTe barriers. Because the quantum confinement imposed by
the barriers is much stronger than the lateral one, the one-dimensional
electron energy level structure is very similar to that usually met in
constrictions of AlGaAs/GaAs heterostructures. However, in contrast to any
other system studied so far, we observe precise conductance quantization in
units, {\it despite of significant amount of charged defects in the
vicinity of the constriction}. We show that such extraordinary results is a
consequence of the paraelectric properties of PbTe, namely, the suppression of
long-range tails of the Coulomb potentials due to the huge dielectric constant.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
Diffraction of complex molecules by structures made of light
We demonstrate that structures made of light can be used to coherently
control the motion of complex molecules. In particular, we show diffraction of
the fullerenes C60 and C70 at a thin grating based on a standing light wave. We
prove experimentally that the principles of this effect, well known from atom
optics, can be successfully extended to massive and large molecules which are
internally in a thermodynamic mixed state and which do not exhibit narrow
optical resonances. Our results will be important for the observation of
quantum interference with even larger and more complex objects.Comment: 4 pages, 3 figure
- …