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 $\pi$ in the magnetoconductance oscillations, and halving of the fundamental $h/e$ 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 Pb$_{0.92}$Eu$_{0.08}$Te 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 $2e^2/h$ 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