Showing papers by "Katholieke Universiteit Leuven published in 1992"

Finitely correlated states on quantum spin chains

Abstract: We study a construction that yields a class of translation invariant states on quantum spin chains, characterized by the property that the correlations across any bond can be modeled on a finite-dimensional vector space. These states can be considered as generalized valence bond states, and they are dense in the set of all translation invariant states. We develop a complete theory of the ergodic decomposition of such states, including the decomposition into periodic “Neel ordered” states. The ergodic components have exponential decay of correlations. All states considered can be obtained as “local functions” of states of a special kind, so-called “purely generated states,” which are shown to be ground states for suitably chosen finite range VBS interactions. We show that all these generalized VBS models have a spectral gap. Our theory does not require symmetry of the state with respect to a local gauge group. In particular we illustrate our results with a one-parameter family of examples which are not isotropic except for one special case. This isotropic model coincides with the one-dimensional antiferromagnet, recently studied by Affleck, Kennedy, Lieb, and Tasaki.

The complete DNA sequence of yeast chromosome III.

Abstract: The entire DNA sequence of chromosome III of the yeast Saccharomyces cerevisiae has been determined. This is the first complete sequence analysis of an entire chromosome from any organism. The 315-kilobase sequence reveals 182 open reading frames for proteins longer than 100 amino acids, of which 37 correspond to known genes and 29 more show some similarity to sequences in databases. Of 55 new open reading frames analysed by gene disruption, three are essential genes; of 42 non-essential genes that were tested, 14 show some discernible effect on phenotype and the remaining 28 have no overt function.

The dead-end elimination theorem and its use in protein side-chain positioning

Abstract: The prediction of a protein's tertiary structure is still a considerable problem because the huge amount of possible conformational space¹ makes it computationally difficult. With regard to side-chain modelling, a solution has been attempted by the grouping of side-chain conformations into representative sets of rotamers²⁻⁵. Nonetheless, an exhaustive combinatorial search is still limited to carefully indentified packing units⁵⁶ containing a limited number of residues. For larger systems other strategies had to be developed, such as the Monte Carlo Procedure⁶⁷ and the genetic algorithm and clustering approach⁸. Here we present a theorem, referred to as the 'dead-end elimination' theorem, which imposes a suitable condition to identify rotamers that cannot be members of the global minimum energy conformation. Application of this theorem effectively controls the computational explosion of the rotamer combinatorial problem, thereby allowing the determination of the global minimum energy conformation of a large collection of side chains.

Structural refinement of superlattices from x-ray diffraction.

Abstract: We present a general procedure for quantitative structural refinement of superlattice structures. To analyze a wide range of superlattices, we derived a general kinematical diffraction formula that includes random, continuous, and discrete fluctuations from the average structure. We show that only the structure factor of one single layer of each material has to be averaged over the random variables and prove that this relation is equivalent to earlier, less general models. Implementing a nonlinear-fitting algorithm to fit the entire x-ray-diffraction profile, refined parameters that describe the average superlattice structure and deviations from this average are obtained. We compare the results of structural refinement to results obtained independently from other measurements. The roughness introduced artificially during growth in Mo/Ni and Nb/Cu superlattices is accurately reproduced by the refinement. The lattice parameters of Ag/Mn obtained from this refinement procedure are in very good agreement with the values obtained from independent extended x-ray-absorption fine-structure and x-ray photoelectron diffraction studies. The relative thicknesses of the layers can be accurately determined, as proved for Cu/Ni in comparison with chemical analysis, for W/Ni compared to the calibrated sputtering rate, and for Mo/Ni compared to the low-angle profile.

Improving memory performance in the aged through mnemonic training: a meta-analytic study.

Abstract: The effectiveness of memory training for the elderly was examined through a meta-analysis of pre-to-posttest gains on episodic memory tasks in healthy subjects aged 60 or above. Pre-to-posttest gains were found to be significantly larger in training groups (0.73 SD, k = 49) than in both control (0.38 SD, k = 10) and placebo (0.37 SD, k = 8) groups. Treatment gains in training groups were negatively affected by age of participants and duration of training sessions and positively affected by group treatment, pretraining, and memory-related interventions. No differences in treatment gain were obtained as a function of type of mnemonic taught nor the kind of pretraining used.

Morphological Aspects of the Resin-Dentin Interdiffusion Zone with Different Dentin Adhesive Systems

Abstract: Cross-sections of resin-dentin interfaces were etched with an argon-ion beam to make their substructure detectable by scanning electron microscopy. The dentin adhesive systems were categorized morphologically into three groups, and an attempt was made to clarify their adhesive mechanism. The first group of products removed the smear layer. The argon-ion bombardment clearly disclosed a hybrid or resin-impregnated dentin layer. It is hypothesized that conditioning with acidic or chelating agents demineralized the dentin surface-layer to a certain depth, leaving behind a collagen-rich mesh-work. Hydrophilic monomers are then believed to alter this collagen-fiber arrangement in a way that facilitates penetration of the adhesive resin, resulting in a mechanical, intermingled link between collagen and the adhesive resin. The second group preserved the smear layer. In this case, the dentinal tubules were obliterated with globular particles at their orifices and remained patent underneath these smear plugs. This type of adhesive system aims at the incorporation of the smear layer into the hydrophilic monomers, which have an affinity for the organic and/or inorganic components of the underlying dentin. Finally, a third, small group only partly dissolved the smear layer, creating a thin resin-impregnated dentin layer and a resin-impregnated smear plug. This study clearly showed that the application of recent adhesive systems induced structural changes in the dentin surface morphology, creating a retentive interface, called the inter-diffusion zone, between the deep, untouched dentin layers and the composite filling material. This resin-dentin interdiffusion zone offers bonding sites for copolymerization with the resin composite and, concurrently, might have protective potential for the pulp tissues.

Material incress manufacturing by Rapid Prototyping Techniques

Abstract: Summary The paper gives a state-of-the-art overview of so called rapid prototyping techniques, like stereolithography, selective laser sintering, ballistic particle manufacturing and others. These are new manufacturing techniques in which the part is produced by gradually growing material to the required shape. A tentative classification and nomenclature is proposed. It is shown that those new processes are ideally suited for CIM. The paper tries to compare the different processes and discuss their application and performances.

Developmental changes in heparan sulfate expression: in situ detection with mAbs.

Abstract: Two mAbs that are specific for heparan sulfate-related epitopes have been raised and used to analyze the cellular and tissular distribution of this glycosaminoglycan during development. mAb 10E4 reacts with an epitope that occurs in native heparan sulfate chains and that is destroyed by N-desulfation of the glycosaminoglycan. The antibody does not react with hyaluronate, chondroitin sulfate, or DNA, and reacts only poorly with heparin. The reactivity of proteoglycan extracts or tissue sections with the 10E4 antibody is completely abolished by heparitinase, but is only partially affected by heparinase. mAb 3G10, in contrast, reacts only with heparitinase-treated heparan sulfate chains, proteoglycans, or tissue sections. The 3G10 epitope is destroyed by treatment with mercuric acetate, which indicates that the desaturated uronate generated by the lyase is essential for the reactivity of the antibody. The 3G10 epitope is not generated by treating heparan sulfate proteoglycans with heparinase or chondroitin sulfate proteoglycans with chondroitin sulfate lyases, which indicates that the 3G10 antibody recognizes desaturated uronates that occur in specific structural contexts. The antibody 10E4 and, after heparitinase treatment, the antibody 3G10 decorate the surfaces of many cell types and the extracellular matrix in proximity of the cells, in particular, the basement membranes. The analysis of embryonic and adult tissues reveals important temporal and regional differences in the abundance of the 10E4 and 3G10 epitopes at these sites. Moreover, the staining pattern of the two antibodies is not always superimposable, which is indicative of regional differences in the exposure or structure of the tissular heparan sulfates. As a whole the results suggest that heparan sulfate abounds at sites of active morphogenesis and that the expression of this glycosaminoglycan is developmentally regulated.

Voltage- and time-dependent block of the delayed K+ current in cardiac myocytes by dofetilide.

Abstract: The delayed K+ current (ik) and its change by dofetilide was studied in single myocytes from the guinea pig and rabbit heart using the two-electrode voltage clamp technique. In rabbit myocytes, ik consisted of only one component (Kr), which developed for moderate depolarizations and with a fast time course. In guinea pig myocytes, activation consisted of a rapid and a slow component, and the latter (Ks) only became manifest for depolarizations positive to 0 mV. Ks was resistant to block by dofetilide. Kr, however, was very sensitive: Kd 3.9 x 10(-9) M, Hill coefficient 2.0 (n = 5). The effect was voltage-dependent block increasing at depolarized levels. Block development was time dependent and occurred in two phases: a first fast and voltage-dependent phase was followed by a second much slower phase (time constant of 4.4 +/- 0.48 sec (n = 11). Recovery from block was slower as the membrane potential became more negative. This resulted in the absence of a steady-state frequency-dependent effect at negative membrane potentials. It is concluded that dofetilide is an efficient blocker of the fast component of ik. The block, as well as recovery, are voltage and time dependent. Block is greater at more depolarized levels, recovery is slower at more hyperpolarized levels.

Roughness and giant magnetoresistance in Fe/Cr superlattices.

Abstract: We have performed detailed studies of the structure, magnetotransport, and magnetization of Fe/Cr superlattices as a function of systematic changes in interfacial roughness. The results clearly show that the giant magnetoresistance is enhanced by the presence of roughness. This fact indicates that interfacial roughness should be explicitly included in theoretical calculations and experimental characterization of superlattices exhibiting giant magnetoresistance.

A high-speed CMOS comparator with 8-b resolution

Abstract: A comparator consisting of a differential input stage, two regenerative flip-flops, and an S-R latch is presented. No offset cancellation is exploited, which reduces the power consumption as well as the die area and increases the comparison speed. An experimental version of the comparator has been integrated in a standard double-poly double-metal 1.5- mu m n-well process with a die area of only 140*100 mu m/sup 2/. This circuit, operating under a +2.5/-2.5-V power supply, performs comparison to a precision of 8 b with a symmetrical input dynamic range of 2.5 V (therefore +or-0.5 LSB resolution is equal to +or-4.9 mV). >

Default risk on government debt in OECD countries

Abstract: Default risk Alberto Alesina, Mark De Broeck, Alessandro Prati and Guido Tabellini Different European countries pay very different interest rates on their public debts. Will these differences disappear if and when exchange rates come to be irrevocably fixed in the European Monetary Union? Or rather will default risks in the high debt countries keep interest rates from converging? To answer this question, this paper investigates whether a perceived default risk is already priced by the market The paper compares the interest rates on public and private financial instruments denominated in the same currencies in 12 OECD countries. A strong correlation is found between the size of public indebtedness and the spread between public and private rates of return. This correlation suggests that the markets perceive a default risk on the public debt of some OECD countries. The size of this default risk is however very small.

The structure, role, and regulation of type 1 protein phosphatases.

Abstract: Type 1 protein phosphatases (PP-1) comprise a group of widely distributed enzymes that specifically dephosphorylate serine and threonine residues of certain phosphoproteins. They all contain an isoform of the same catalytic subunit, which has an extremely conserved primary structure. One of the properties of PP-1 that allows one to distinguish them from other serine/threonine protein phosphatases is their sensitivity to inhibition by two proteins, termed inhibitor 1 and inhibitor 2, or modulator. The latter protein can also form a 1:1 complex with the catalytic subunit that slowly inactivates upon incubation. This complex is reactivated in vitro by incubation with MgATP and protein kinase FA/GSK-3. In the cell the type 1 catalytic subunit is associated with noncatalytic subunits that determine the activity, the substrate specificity, and the subcellular location of the phosphatase. PP-1 plays an essential role in glycogen metabolism, calcium transport, muscle contraction, intracellular transport, protein synthesis, and cell division. The activity of PP-1 is regulated by hormones like insulin, glucagon, alpha- and beta-adrenergic agonists, glucocorticoids, and thyroid hormones.

Isolated single coronary artery: a review of 50 000 consecutive coronary angiographies

Abstract: Single coronary artery is a rare congenital anomaly of the coronary arteries where only one coronary artery arises from the aortic trunk by a single coronary ostium, supplying the entire heart. A database consisting of the angiographic reports of 50,000 consecutive coronary angiographies performed in adult patients in the University Hospital of Leuven between March 1973 and August 1991 was searched for the diagnosis of single coronary artery. All films concerned were reviewed and classified according to their anatomical type. Thirty-three cases of single coronary artery were retrieved, yielding an incidence of 0.066%. Patient characteristics and clinical data are described, with a discussion on the pathological importance of this finding.

Resonant behaviour of MHD waves on magnetic flux tubes. III : effect of equilibrium flow

Abstract: The Hollweg et al. (1990) analysis of MHD surface waves in a stationary equilibrium is extended. The conservation laws and jump conditions at Alfven and slow resonance points obtained by Sakurai et al. (1990) are generalized to include an equilibrium flow, and the assumption that the Eulerian perturbation of total pressure is constant is recovered as the special case of the conservation law for an equilibrium with straight magnetic field lines and flow along the magnetic field lines. It is shown that the conclusions formulated by Hollweg et al. are still valid for the straight cylindrical case. The effect of curvature is examined.

Computational strategies for object recognition

Abstract: This article reviews the available methods for automated identification of objects in digital images. The techniques are classified into groups according to the nature of the computational strategy used. Four classes are proposed: (1) the simplest strategies, which work on data appropriate for feature vector classification, (2) methods that match models to symbolic data structures for situations involving reliable data and complex models, (3) approaches that fit models to the photometry and are appropriate for noisy data and simple models, and (4) combinations of these strategies, which must be adopted in complex situations. Representative examples of various methods are summarized, and the classes of strategies with respect to their appropriateness for particular applications.

Special geometry, cubic polynomials and homogeneous quaternionic spaces

Abstract: The existing classification of homogeneous quaternionic spaces is not complete. We study these spaces in the context of certainN=2 supergravity theories, where dimensional reduction induces a mapping betweenspecial real, Kahler and quaternionic spaces. The geometry of the real spaces is encoded in cubic polynomials, those of the Kahler and quaternionic manifolds in homogeneous holomorphic functions of second degree. We classify all cubic polynomials that have an invariance group that acts transitively on the real manifold. The corresponding Kahler and quaternionic manifolds are then homogeneous. We find that they lead to a well-defined subset of the normal quaternionic spaces classified by Alekseevskii (and the corresponding special Kahler spaces given by Cecotti), but there is a new class of rank-3 spaces of quaternionic dimension larger than 3. We also point out that some of the rank-4 Alekseevskii spaces were not fully specified and correspond to a finite variety of inequivalent spaces. A simpler version of the equation that underlies the classification of this paper also emerges in the context ofW 3 algebras.

The risk of stroke in patients with acute myocardial infarction after thrombolytic and antithrombotic treatment

Abstract: Background. Many trials in patients with acute myocardial infarction have demonstrated that thrombolytic therapy is not associated with an excessive risk of stroke, as compared with conventional treatment. However, the incidence of various forms of stroke in patients treated with different thrombolytic and antithrombotic regimens and the associated effect of risk factors for stroke are largely unknown. Methods. Strokes occurring in patients hospitalized with acute myocardial Infarction who were enrolled in either of two large trials were analyzed. The patients were randomly assigned to receive streptokinase (1.5 million units) or recombinant tissue plasminogen activator (t-PA) (100 mg) and also randomly assigned to receive subcutaneous heparin or no heparin. Ninety-one percent of the patients also received aspirin. Results. Complete data were available on 20,768 patients. A total of 236 (1.14 percent) had strokes in the hospital; 0.36 percent had hemorrhagic strokes, 0.48 percent ischemic strokes...

Analysis of the Electrolytic Codeposition of Non‐Brownian Particles with Metals

Abstract: A model for the electrolytic codeposition of spherical particles with metals on a rotating disk electrode is presented, based on a trajectory analysis of the particle deposition, including convective mass transport, geometrical interception, and migration under specific forces, coupled to a surface immobilization reaction. A number of relevant forces were included and their effects determined. Theoretical predictions of this model are compared with experimental results for the codeposition of spherical polystyrene particles with copper during electrolysis from an acid copper sulfate solution. The influence of fluid flow velocities, particle concentration, and current density on the rate of particle deposition is illustrated. Experiments done on a rotating disk electrode allow the adhesion forces to be determined from the distribution of particles on the surface. It is shown that codeposition is governed by colloidal interactions that can, in first order, be approximated by the Derjaguin‐Landau‐Verwey‐Overbeek interactions plus an additional short range repulsion that was associated with the hydration force.

Ca2+ release induced by inositol 1,4,5-trisphosphate is a steady-state phenomenon controlled by luminal Ca2+ in permeabilized cells

Abstract: LOW concentrations of inositol 1,4,5-trisphosphate (InsP3) evoke a very rapid mobilization of intracellular Ca2+ stores in many cell types, which can be followed by a further, much slower efflux1–8. Two explanations have been suggested for this biphasic release. The first proposes that the Ca2+ stores vary in their sensitivity to InsP3, and each store releases either its entire contents or nothing2,5,7 (all-or-none release); the second proposes instead that the stores are uniformly sensitive to the effects of InsP3, but that they can release only a fraction of their Ca2+ before their sensitivity is somehow attenuated6,8–11(steady-state release). Experiments using purified InsP3 receptor molecules reconstituted into lipid vesicles have shown heterogeneity of the receptors in their response to InsP3 under conditions in which the total Ca2+ level at both sides of the receptor is held constant7. We now report that in permeabilized A7r5 smooth-muscle cells incubated in Ca2+-free medium, the amount of 45Ca2+ remaining in the stores after the rapid transient phase of release is independent of their initial Ca2+ levels, indicating that partially depleted stores are less sensitive to InsP3. Moreover, if the stores are reloaded with 40Ca2+ after the first stimulus, reapplication of the same low concentration of InsP3 will release further 45Ca2+. This recovery of InsP3 sensitivity is almost complete. Under these conditions, Ca2+ release must thus occur by a steady-state mechanism, in which the decreasing Ca2+ content of the stores slows down further release.