Showing papers by "University of Valencia published in 2000"

Coexistence of ferromagnetism and metallic conductivity in a molecule-based layered compound

Abstract: Crystal engineering--the planning and construction of crystalline supramolecular architectures from modular building blocks--permits the rational design of functional molecular materials that exhibit technologically useful behaviour such as conductivity and superconductivity, ferromagnetism and nonlinear optical properties. Because the presence of two cooperative properties in the same crystal lattice might result in new physical phenomena and novel applications, a particularly attractive goal is the design of molecular materials with two properties that are difficult or impossible to combine in a conventional inorganic solid with a continuous lattice. A promising strategy for creating this type of 'bi-functionality' targets hybrid organic/inorganic crystals comprising two functional sub-lattices exhibiting distinct properties. In this way, the organic pi-electron donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and its derivatives, which form the basis of most known molecular conductors and superconductors, have been combined with molecular magnetic anions, yielding predominantly materials with conventional semiconducting or conducting properties, but also systems that are both superconducting and paramagnetic. But interesting bulk magnetic properties fail to develop, owing to the discrete nature of the inorganic anions. Another strategy for achieving cooperative magnetism involves insertion of functional bulky cations into a polymeric magnetic anion, such as the bimetallic oxalato complex [MnIICrIII(C2O4)3]-, but only insoluble powders have been obtained in most cases. Here we report the synthesis of single crystals formed by infinite sheets of this magnetic coordination polymer interleaved with layers of conducting BEDT-TTF cations, and show that this molecule-based compound displays ferromagnetism and metallic conductivity.

Electricity prices and power derivatives: Evidence from the Nordic Power Exchange

Abstract: This paper examines the importance of the regular pattern in the behavior of electricity prices, and its implications for the purposes of derivative pricing. We analyze the Nordic Power Exchange's spot, futures, and forward prices. We conclude that the seasonal systematic pattern throughout the year, in particular, is of crucial importance in explaining the shape of the futures/forward curve. Moreover, in the context of the oneand two factor models analyzed in this paper, a simple sinusoidal functionis adequate in order capture the seasonal pattern of the features and forwardcurve directly implied by the seasonal behavior of spot electricity prices.

Stress‐controlled transcription factors, stress‐induced genes and stress tolerance in budding yeast

Abstract: The transcriptional response to environmental changes is a major topic in both basic and applied research. From a basic point of view, to understand this response includes unravelling how the stress signal is sensed and transduced to the nucleus, to identify which genes are induced under each stress condition and, finally, to establish the phenotypic consequences of this induction in stress tolerance. The possibility of using genetic approaches has made the yeast Saccharomyces cerevisiae a compelling model to study stress response at a molecular level. Moreover, this information can be used to isolate and characterise stress-related proteins in higher eukaryotes and to design strategies to increase stress resistance in organisms of industrial interest. In this review the progress made in recent years is discussed.

Disruption of Eph/ephrin signaling affects migration and proliferation in the adult subventricular zone

Abstract: The subventricular zone (SVZ) of the lateral ventricles, the largest remaining germinal zone of the adult mammalian brain, contains an extensive network of neuroblasts migrating rostrally to the olfactory bulb Little is known about the endogenous proliferation signals for SVZ neural stem cells or guidance cues along the migration pathway Here we show that the receptor tyrosine kinases EphB1-3 and EphA4 and their transmembrane ligands, ephrins-B2/3, are expressed by cells of the SVZ Electron microscopy revealed ephrin-B ligands associated with SVZ astrocytes, which function as stem cells in this germinal zone A three-day infusion of the ectodomain of either EphB2 or ephrin-B2 into the lateral ventricle disrupted migration of neuroblasts and increased cell proliferation These results suggest that Eph/ephrin signaling is involved in the migration of neuroblasts in the adult SVZ and in either direct or indirect regulation of cell proliferation

P Values for Composite Null Models

Abstract: The problem of investigating compatibility of an assumed model with the data is investigated in the situation when the assumed model has unknown parameters. The most frequently used measures of compatibility are p values, based on statistics T for which large values are deemed to indicate incompatibility of the data and the model. When the null model has unknown parameters, p values are not uniquely defined. The proposals for computing a p value in such a situation include the plug-in and similar p values on the frequentist side, and the predictive and posterior predictive p values on the Bayesian side. We propose two alternatives, the conditional predictive p value and the partial posterior predictive p value, and indicate their advantages from both Bayesian and frequentist perspectives.

Mitochondrial oxidative stress plays a key role in aging and apoptosis.

Abstract: Harman first suggested in 1972 that mitochondria might be the biological clock in aging, noting that the rate of oxygen consumption should determine the rate of accumulation of mitochondrial damage produced by free radical reactions. Later in 1980 Miquel and coworkers proposed the mitochondrial theory of cell aging. Mitochondria from postmitotic cells use O2 at a high rate, hence releasing oxygen radicals that exceed the cellular antioxidant defences. The key role of mitochondria in cell aging has been outlined by the degeneration induced in cells microinjected with mitochondria isolated from fibroblasts of old rats, especially by the inverse relationship reported between the rate of mitochondrial production of hydroperoxide and the maximum life span of species. An important change in mitochondrial lipid composition is the age-related decrease found in cardiolipin content. The concurrent enhancement of lipid peroxidation and oxidative modification of proteins in mitochondria further increases mutations and oxidative damage to mitochondrial DNA (mtDNA) in the aging process. The respiratory enzymes containing the defective mtDNA-encoded protein subunits may increase the production of reactive oxygen species, which in turn would aggravate the oxidative damage to mitochondria. Moreover, superoxide radicals produced during mitochondrial respiration react with nitric oxide inside mitochondria to yield damaging peroxynitrite. Treatment with certain antioxidants, such as sulphur-containing antioxidants, vitamins C and E, or the Ginkgo biloba extract EGb 761, protects against the age-associated oxidative damage to mtDNA and the oxidation of mitochondrial glutathione. Moreover, the EGb 761 extract also prevents changes in mitochondrial morphology and function associated with aging of the brain and liver.

Nearly zero ultraflattened dispersion in photonic crystal fibers.

Abstract: We present a procedure for achieving photonic crystal fibers with nearly zero ultraflattened group-velocity dispersion. Systematic knowledge of the special guiding properties of these fibers permits the achievement of qualitatively novel dispersion curves. Unlike the behavior of conventional fibers, this new type of dispersion behavior permits remarkably improved suppression of third-order dispersion, particularly in the low-dispersion domain.

Stratified Care vs Step Care Strategies for Migraine: The Disability in Strategies of Care (DISC) Study:A Randomized Trial

Abstract: ContextVarious guidelines recommend different strategies for selecting and sequencing acute treatments for migraine. In step care, treatment is escalated after first-line medications fail. In stratified care, initial treatment is based on measurement of the severity of illness or other factors. These strategies for migraine have not been rigorously evaluated.ObjectiveTo compare the clinical benefits of 3 strategies: stratified care, step care within attacks, and step care across attacks, among patients with migraine.Design and SettingRandomized, controlled, parallel-group clinical trial conducted by the Disability in Strategies Study group from December 1997 to March 1999 in 88 clinical centers in 13 countries.PatientsA total of 835 adult migraine patients with a Migraine Disability Assessment Scale (MIDAS) grade of II, III, or IV were analyzed as the efficacy population; the safety analysis included 930 patients.InterventionsPatients were randomly assigned to receive (1) stratified care (n = 279), in which patients with MIDAS grade II treated up to 6 attacks with aspirin, 800 to 1000 mg, plus metoclopramide, 10 mg, and patients with MIDAS grade III and IV treated up to 6 attacks with zolmitriptan, 2.5 mg; (2) step care across attacks (n = 271), in which initial treatment was with aspirin, 800 to 1000 mg, plus metoclopramide, 10 mg. Patients not responding in at least 2 of the first 3 attacks switched to zolmitriptan, 2.5 mg, to treat the remaining 3 attacks; and (3) step care within attacks (n = 285), in which initial treatment for all attacks was with aspirin, 800 to 1000 mg, plus metoclopramide, 20 mg. Patients not responding to treatment after 2 hours in each attack escalated treatment to zolmitriptan, 2.5 mg.Main Outcome MeasuresHeadache response, achieved if pain intensity was reduced from severe or moderate at baseline to mild or no pain at 2 hours; and disability time per treated attack at 4 hours for all 6 attacks, compared among the 3 groups.ResultsHeadache response at 2 hours was significantly greater across 6 attacks in the stratified care treatment group (52.7%) than in either the step care across attacks group (40.6%; P<.001) or the step care within attacks group (36.4%; P<.001). Disability time (6 attacks) was significantly lower in the stratified care group (mean area under the curve [AUC], 185.0 mm · h) than in the step care across attacks group (mean AUC, 209.4 mm · h; P<.001) or the step care within attacks group (mean AUC, 199.7 mm · h; P<.001). The incidence of adverse events was higher in the stratified care group (321 events) vs both step care groups (159 events in across-attack group; 217 in within-attack group), although most events were of mild-to-moderate intensity.ConclusionOur results indicate that as a treatment strategy, stratified care provides significantly better clinical outcomes than step care strategies within or across attacks as measured by headache response and disability time.

Abdominal muscle response during curl-ups on both stable and labile surfaces.

Abstract: Background and Purpose. With the current interest in stability training for the injured low back, the use of labile (movable) surfaces, underneath the subject, to challenge the motor control system is becoming more popular. Little is known about the modulating effects of these surfaces on muscle activity. The purpose of this study was to establish the degree of modulating influence of the type of surface (whether stable or labile) on the mechanics of the abdominal wall. In this study, the amplitude of muscle activity together with the way that the muscles coactivated due to the type of surface under the subject were of interest. Subjects. Eight men (mean age=23.3 years [SD=4.3], mean height=177.6 cm [SD=3.4], mean weight=72.6 kg [SD=8.7]) volunteered to participate in the study. All subjects were in good health and reported no incidence of acute or chronic low back injury or prolonged back pain prior to this experiment. Methods. All subjects were requested to perform 4 different curl-up exercises—1 on a stable surface and the other 3 on varying labile surfaces. Electromyographic signals were recorded from 4 different abdominal sites on the right and left sides of the body and normalized to maximal voluntary contraction (MVC) amplitudes. Results. Performing curl-up exercises on labile surfaces increased abdominal muscle activity (eg, for curl-up on a stable surface, rectus abdominis muscle activity was 21% of MVC and external oblique muscle activity was 5% of MVC; for curl-up with the upper torso on a labile ball, rectus abdominis muscle activity was 35% of MVC and external oblique muscle activity was 10% of MVC). Furthermore, it appears that increases in external oblique muscle activity were larger than those of other abdominal muscles. Conclusion and Discussion. Performing curl-ups on labile surfaces changes both the level of muscle activity and the way that the muscles coactivate to stabilize the spine and the whole body. This finding suggests a much higher demand on the motor control system, which may be desirable for specific stages in a rehabilitation program.

Computational evidence in favor of a two-state, two-mode model of the retinal chromophore photoisomerization

Abstract: In this paper we use ab initio multiconfigurational second-order perturbation theory to establish the intrinsic photoisomerization path model of retinal chromophores. This is accomplished by computing the ground state (S0) and the first two singlet excited-state (S1, S2) energies along the rigorously determined photoisomerization coordinate of the rhodopsin chromophore model 4-cis-γ-methylnona-2,4,6,8-tetraeniminium cation and the bacteriorhodopsin chromophore model all-trans-hepta-2,4,6-trieniminium cation in isolated conditions. The computed S2 and S1 energy profiles do not show any avoided crossing feature along the S1 reaction path and maintain an energy gap >20 kcal⋅mol−1. In addition, the analysis of the charge distribution shows that there is no qualitative change in the S2 and S1 electronic structure along the path. Thus, the S1 state maintains a prevalent ionic (hole–pair) character whereas the S2 state maintains a covalent (dot–dot) character. These results, together with the analysis of the S1 reaction coordinate, support a two-state, two-mode model of the photoisomerization that constitutes a substantial revision of the previously proposed models.

Explicit Algorithms for a New Time Dependent Model Based on Level Set Motion for Nonlinear Deblurring and Noise Removal

Abstract: In this paper we formulate a time dependent model to approximate the solution to the nonlinear total variation optimization problem for deblurring and noise removal introduced by Rudin and Osher [ Total variation based image restoration with free local constraints, in Proceedings IEEE Internat. Conf. Imag. Proc., IEEE Press, Piscataway, NJ, (1994), pp. 31--35] and Rudin, Osher, and Fatemi [ Phys. D, 60 (1992), pp. 259--268], respectively. Our model is based on level set motion whose steady state is quickly reached by means of an explicit procedure based on Roe's scheme [ J. Comput. Phys., 43 (1981), pp. 357--372], used in fluid dynamics. We show numerical evidence of the speed of resolution and stability of this simple explicit procedure in some representative 1D and 2D numerical examples.

Relativistic Jets from Collapsars.

Abstract: We have studied the relativistie beamed outflow proposed to occur in the collapsar model of gamma-ray bursts. A jet forms as a consequence of an assumed energy deposition of ~ 1050 - 1051 erg/s within a 30° cone around the rotation axis of the progenitor star. The generated jet flow is strongly beamed ( ≲ few degrees) and reaches the surface of the stellar progenitor (r ≈ 31010 cm) intact. At break-out the maximum Lorentz factor of the jet flow is about 33. Simulations have been performed with the GENESIS multi-dimensional relativistie hydrodynamic code.

Mitochondria, oxidative stress and aging.

Abstract: In the eighties, Miquel and Fleming suggested that mitochondria play a key role in cellular aging. Mitochondria, and specially mitochondrial DNA (mtDNA), are major targets of free radical attack. At present, it is well established that mitochondrial deficits accumulate upon aging due to oxidative damage. Thus, oxidative lesions to mtDNA accumulate with age in human and rodent tissues. Furthermore, levels of oxidative damage to mtDNA are several times higher than those of nuclear DNA. Mitochondrial size increases whereas mitochondrial membrane potential decreases with age in brain and liver.Recently, we have shown that treatment with certain antioxidants, such as sulphur-containing antioxidants, vitamins C and E or the Ginkgo biloba extract EGb 761, protects against the age-associated oxidative damage to mtDNA and oxidation of mitochondrial glutathione. Moreover, the extract EGb 761 also prevents changes in mitochondrial morphology and function associated with aging of the brain and liver. Thus, mitochondr...

Yukawa unified supersymmetric SO(10) model: Cosmology, rare decays and collider searches

Abstract: It has recently been pointed out that viable sparticle mass spectra can be generated in Yukawa unified SO(10) supersymmetric grand unified models consistent with radiative breaking of electroweak symmetry. Model solutions are obtained only if tan{beta}{approx}50, {mu}<0 and positive D-term contributions to scalar masses from SO(10) gauge symmetry breaking are used. In this paper, we attempt to systematize the parameter space regions where solutions are obtained. We go on to calculate the relic density of neutralinos as a function of parameter space. No regions of the parameter space explored were actually cosmologically excluded, and very reasonable relic densities were found in much of parameter space. Direct neutralino detection rates could exceed 1 event/kg/day fora{sup 73}Ge detector, for low values of GUT scale gaugino mass m{sub 1/2}. We also calculate the branching fraction for b{yields}s{gamma} decays, and find that it is beyond the 95% C.L. experimental limits in much, but not all, of the parameter space regions explored. For the Fermilab Tevatron collider, significant regions of parameter space can be explored via b{bar B}A and b{bar B}H searches. There also exist some limited regions of parameter space where a trilepton signal can be seen at TeV33. Finally, there exist significant regionsmore » of parameter space where direct detection of bottom squark pair production can be made, especially for large negative values of the GUT parameter A{sub 0}.« less

Leptin and leptin receptor are expressed in the human endometrium and endometrial leptin secretion is regulated by the human blastocyst.

Abstract: Embryonic implantation is a crucial event for the human reproductive function. Cytokines and paracrine molecules have been proposed as putative local regulators of this process. The leptin or the OB protein has been linked to the reproductive function and inflammatory response. In the present study, we describe for the first time the expression of leptin and leptin receptor (long form) in the secretory endometrium and that endometrial leptin secretion is regulated in vitro by the human blastocyst. Leptin and leptin receptor messenger RNA and protein were identified in secretory endometrium and in cultured endometrial epithelial cells (EECs) by RT-PCR, Western blot, and immunohistochemistry. The concentrations of immunoreactive leptin secreted by human embryos alone or cocultured with EECs were also assessed. We found that human blastocysts secrete significantly higher levels of leptin than arrested embryos. In contrast, leptin concentrations secreted by arrested embryos cocultured with EECs were significantly higher than blastocysts cocultured with EECs. These findings suggest that the human endometrium is a site for local production and a target tissue for circulating leptin. Expression of leptin and its functional receptor in the endometrium and regulation of endometrial leptin secretion by the human embryo suggests that the leptin system may be implicated in the human implantation process.

Cost of host radiation in an RNA virus.

Abstract: Although host radiation allows a parasite to expand its ecological niche, traits governing the infection of multiple host types can decrease fitness in the original or alternate host environments. Reasons for this reduction in fitness include slower replication due to added genetic material or modifications, fitness trade-offs across host environments, and weaker selection resulting from simultaneous adaptation to multiple habitats. We examined the consequences of host radiation using vesicular stomatitis virus (VSV) and mammalian host cells in tissue culture. Replicate populations of VSV were allowed to evolve for 100 generations on the original host (BHK cells), on either of two novel hosts (HeLa and MDCK cells), or in environments where the availability of novel hosts fluctuated in a predictable or random way. As expected, each experimental population showed a substantial fitness gain in its own environment, but those evolved on new hosts (constant or fluctuating) suffered reduced competitiveness on the original host. However, whereas evolution on one novel host negatively correlated with performance on the unselected novel host, adaptation in fluctuating environments led to fitness improvements in both novel habitats.

Minimizing total variation flow

Abstract: We prove existence and uniqueness of weak solutions for the minimizing Total Variation flow with initial data in L1 under Neumann boundary conditions. We prove that the H N−1 measure of the boundaries of level sets of the solution decreases with time, as one would expect. We also prove that local maxima (minima) strictly decrease (increase) their level with time. We shall also consider the Dirichlet problem which presents some particular difficulties for general initial data in L1.

Proofs produced by secondary school students learning geometry in a dynamic computer environment

Abstract: As a key objective, secondary school mathematics teachers seek to improve the proof skills of students. In this paper we present an analytic framework to describe and analyze students' answers to proof problems. We employ this framework to investigate ways in which dynamic geometry software can be used to improve students' understanding of the nature of mathematical proof and to improve their proof skills. We present the results of two case studies where secondary school students worked with Cabri-Geometre to solve geometry problems structured in a teaching unit. The teaching unit had the aims of: i) Teaching geometric concepts and properties, and ii) helping students to improve their con- ception of the nature of mathematical proof and to improve their proof skills. By applying the framework defined here, we analyze students' answers to proof problems, observe the types of justifications produced, and verify the usefulness of learning in dynamic geometry computer environments to improve students' proof skills.