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Results for “Models, Theoretical”

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At least 415 records · Page 23Linked to original sources

PubMed2004

Theoretical model of internal rotation in monosubstituted derivatives of furfural.

The present work explores the effect of substitution in all free positions of furfural on conformational preferences of formyl group by using ab-initio calculations at the MP2/6-31G(p,d) level of theory. Theoretical modeling was made in vacuo. The selected substituents were -CH(3), NH(2), NO(2) and F groups in 3, 4, 5 and ipso carbonyl positions. Geometries of all derivatives were analyzed and it is ascertained that substitution has not important consequences on furan ring geometry. Differences of energy between OO-cis and trans conformers and energy barriers between them are described and extreme cases are explained. Interesting features appear in the cases of -NH(2) and -NO(2) groups, and particularly when the 3 and ipso carbonyl positions are substituted. Variations in energy barriers are correlated with variations in C2-C6 distances for the transition states and planar forms. Substitution effect on Mülliken charges are analyzed and related with internal rotation energy barriers and differences between conformers.

Journal Article↗

PubMed1984

Theoretical models in mechanics of the left ventricle.

Different rheological concepts and theoretical studies have been recently presented using models of myocardial mechanics. Complex analysis of the mechanical behavior of the left ventricular wall have been developed in order to estimate the local stresses and deformations that occur during the heart cycle as well as the ventricular stroke volume and pressure. Theoretical models have taken into account non-linear and viscoelastic passive properties of the myocardium tissue, when subjected to large deformations, through given strain energy functions or stress-strain relations. Different prolate spheroid geometries have been considered for such thick shell cardiac structure. During the active state of the contraction, the rheological behavior of the fibers has been described using different muscle models and relationships between fiber tension and strain, and activation degree. A forthcoming approach for bridging the gap between the knowledge of the muscle fiber microrheological properties and the study of the mechanical behavior of the entire ventricle, consists in including anisotropic and inhomogeneous effects through fiber direction field.

Animals↗

PubMed2001

Theoretical model for myocardial trabeculation.

During the morphogenetic process of myocardial trabeculation, most of the cardiac jelly of the initially smooth-walled heart is replaced by sponge-like muscle. The mechanisms that drive and regulate this important process are poorly understood. Using a theoretical model, we examined the possible role that cytoskeletal contraction plays during the initial stages of trabeculation. The myocardium is modeled as a thin viscoelastic membrane consisting of contractile (stress) fibers embedded in an isotropic incompressible matrix, with the interaction of myocardial cells and cardiac jelly fibers providing long-range mechanical effects. The stress fibers are assumed to behave like smooth muscle and to normally operate on the descending limb of their stress-stretch curve. Mechanical instability due to the effectively negative stiffness then leads to the creation of pattern. As a first approximation, computations were carried out for a flat rectangular membrane with stress fibers aligned along a single direction. The computed deformation patterns depend strongly on the magnitude and anisotropy of the long-range effects. Given plausible assumptions about the mechanical properties of the embryonic heart, the model predicts trabecular patterns similar to those observed in the embryo, including the development of circumferential ridges and relatively thin regions ("holes") in the trabecular sheets.

Animals↗

PubMed2006

Theoretical model of the three-dimensional structure of a disease resistance gene homolog encoding resistance protein in Vigna mungo.

Plant disease resistance (R) genes, the key players of innate immunity system in plants encode 'R' proteins. 'R' protein recognizes product of avirulance gene from the pathogen and activate downstream signaling responses leading to disease resistance. No three dimensional (3D) structural information of any 'R' proteins is available as yet. We have reported a 'R' gene homolog, the 'VMYR1', encoding 'R' protein in Vigna mungo. Here, we describe the homology modeling of the 'VMYR1' protein. The model was created by using the 3D structure of an ATP-binding cassette transporter protein from Vibrio cholerae as a template. The strategy for homology modeling was based on the high structural conservation in the superfamily of P-loop containing nucleoside triphosphate hydrolase in which target and template proteins belong. This is the first report of theoretical model structure of any 'R' proteins.

ATP-Binding Cassette Transporters↗

PubMed1980

Theoretical model for glomerular filtration of charged solutes.

Previous experimental studies have revealed that for a given molecular size, the glomerular capillary wall restricts the passage of polyanions more than that of neutral macromolecules, and restricts transport of the latter more than that of polycations. We have developed a theoretical model of this charge-selectivity based on the following principal assumptions: a) all ions (tracer macromolecule and univalent cations and anions) obey a modified Nernst-Planck flux expression, including terms for convection and size-selective retardation; b) the capillary wall has a homogeneous distribution of fixed negative charges; and c) Donnan equilibria exist at the surfaces of this "membrane." To allow specific application of the model we measured electrophoretic mobilities of narrow fractions of dextran sulfate (DS) and diethylaminoethyl dextran (DEAE), and used these to estimate effective molecular charge as a function of molecular size. Based on these measurements and fractional clearance data for DS and DEAE reported previously for the normal Munich-Wistar rat, the capillary wall has an apparent fixed charge concentration of 120--170 meq/liter. The effects of this membrane charge on the filtration of water and on the transcapillary electrical potential difference are also discussed.

Animals↗

PubMed1991

The effect of supplier inducement on Norwegian dental services; some empirical findings based on a theoretical model.

The aim of the present study was to ascertain whether the amount of dental care demanded and the amount of dental care received in Norway were influenced by supplier inducement. There is a fixed price schedule for dental care in Norway, which establishes the framework for the theoretical model on which the empirical studies were based. The model has been developed by Birch (1988), and it describes how the utilisation of dental services may be influenced both by supplier inducement and reduced shadow prices (access costs) when the population to dentist ratio decreases. The sample of 1200 individuals was representative of the non-institutionalised Norwegian population aged 20 years and over. Variables measuring access costs, rationing of care, oral health, unmet need, family income and socio-demographic characteristics of the population were included as control variables. Supplier inducement effects were found on dentist-initiated visits, and on the cost per visit. It was concluded that dentists in areas of excess supply were able to maintain their workload by increasing the demand for and the utilisation of their services. Also, that the existing maldistribution of dentists between the northern and southern parts of Norway is not likely to be altered by allowing market mechanisms to operate on the dental care market.

Adult↗

PubMed1990

Mechanism of ventricular extrasystoles with fixed coupling. A theoretical model derived from the concept of longitudinal dissociation in the reentrant pathway of extrasystoles.

An electrocardiogram taken from a 29-year-old man with old myocardial infarction is presented as an exemplary case of ventricular extrasystoles with fixed coupling. To explain the mechanism of ventricular extrasystoles with fixed coupling, a theoretical model is derived from the concept of longitudinal dissociation in the reentrant pathway. In the model, functional longitudinal dissociation divides the reentrant pathway into dual pathways F and S. When a sinus impulse is blocked in pathway F and passes only through pathway S, it becomes a manifest reentrant extrasystole because of marked conduction delay in pathway S. When the sinus rate does not exceed a certain value, such an impulse always becomes a manifest extrasystole with fixed coupling. Part of the impulse passing through pathway S enters pathway. F retrogradely. In some cases, thereafter, it reenters pathway S and initiates ventricular reentrant tachycardia. When, on the other hand, a sinus impulse passes through both of pathways F and S, it becomes a concealed reentrant extrasystole because of insufficient conduction delay in the pathways.

Adult↗

PubMed1998

Escape of autocrine ligands into extracellular medium: experimental test of theoretical model predictions.

We have developed an experimental system for testing mathematical model predictions concerning escape of autocrine ligands into the extracellular bulk medium. This system employs anti-receptor blocking antibodies against the epidermal growth factor receptor (EGFR)/transforming growth factor alpha (TGFalpha) receptor/ligand pair. TGFalpha was expressed under the control of a tetracycline-repressed promoter, together with a constitutively expressed human EGFR in B82 mouse fibroblast cells. This expression system allowed us to vary TGFalpha synthesis rates over a roughly 300-fold range by adjusting tetracycline concentration. TGFalpha accumulation in the extracellular bulk medium was then measured as a function of cell density, TGFalpha synthesis rate, and anti-EGFR blocking antibody concentration. Consistent with model predictions, amounts of ligand in the medium on a per cell basis were found to diminish as cell density was increased but with reduced dependence on cell density at higher ligand synthesis rates. Similarly consistent with model predictions, higher ligand synthesis rates also decreased the effect of anti-receptor blocking antibodies. Our investigation has established that we can successfully analyze and understand autocrine ligand secretion behavior from the basis of our theoretical model.

Animals↗

PubMed1995

Bone strain gage data and theoretical models of functional adaptation.

The in vivo implantation of strain gages on the surface of bones has proven to be a very useful technique for studying the relationship between in vivo loading and bone growth and adaptation. However, data from such experiments have yet to be well incorporated within the context of theoretical models of bone adaptation. Methods for analyzing bone rosette strain gage recordings within the framework of strain energy density-based computational modeling/remodeling theories are presented. A new strain energy density based parameter, energy equivalent strain, is proposed as a single scalar measure of cyclic strain magnitudes and the concept of a daily strain stimulus is also introduced. As an illustrative example, the approach is applied to analyze previously reported in vivo data from the anteromedial human tibia (Lanyon et al., 1975, Acta orthop. Scand. 46, 256-268).

Adaptation, Physiological↗

PubMed2006

Comparison study of 2D images of temperature fluctuations during sawtooth oscillation with theoretical models.

High temporal and spatial resolution two-dimensional (2D) images of electron temperature fluctuations were employed to study the sawtooth oscillation in the Toroidal Experiment for Technically Oriented Research tokamak plasmas. The 2D images are directly compared with the expected 2D patterns of the plasma pressure (or electron temperature) from various theoretical models. The observed experimental 2D images are only partially in agreement with the expected patterns from each model: The image of the initial reconnection process is similar to that of the ballooning mode model. The intermediate and final stages of the reconnection process resemble those of the full reconnection model. The time evolution of the images of the hot spot or island is partially consistent to those from the full reconnection model but is not consistent with those from the quasi-interchange model.

Journal Article↗

PubMed2006

Theoretical models on the Cu2O2 torture track: mechanistic implications for oxytyrosinase and small-molecule analogues.

Accurately describing the relative energetics of alternative bis(mu-oxo) and mu-eta2:eta2 peroxo isomers of Cu2O2 cores supported by 0, 2, 4, and 6 ammonia ligands is remarkably challenging for a wide variety of theoretical models, primarily owing to the difficulty of maintaining a balanced description of rapidly changing dynamical and nondynamical electron correlation effects and a varying degree of biradical character along the isomerization coordinate. The completely renormalized coupled-cluster level of theory including triple excitations and extremely efficient pure density functional levels of theory quantitatively agree with one another and also agree qualitatively with experimental results for Cu2O2 cores supported by analogous but larger ligands. Standard coupled-cluster methods, such as CCSD(T), are in most cases considerably less accurate and exhibit poor convergence in predicted relative energies. Hybrid density functionals significantly underestimate the stability of the bis(mu-oxo) form, with the magnitude of the error being directly proportional to the percentage Hartree-Fock exchange in the functional. Single-root CASPT2 multireference second-order perturbation theory, by contrast, significantly overestimates the stability of bis(mu-oxo) isomers. Implications of these results for modeling the mechanism of C-H bond activation by supported Cu2O2 cores, like that found in the active site of oxytyrosinase, are discussed.

Binding Sites↗

PubMed2005

Theoretical models of selection and mutation on quantitative traits.

Empirical studies of quantitative genetic variation have revealed robust patterns that are observed both across traits and across species. However, these patterns have no compelling explanation, and some of the observations even appear to be mutually incompatible. We review and extend a major class of theoretical models, 'mutation-selection models', that have been proposed to explain quantitative genetic variation. We also briefly review an alternative class of 'balancing selection models'. We consider to what extent the models are compatible with the general observations, and argue that a key issue is understanding and modelling pleiotropy. We discuss some of the thorny issues that arise when formulating models that describe many traits simultaneously.

Evolution, Molecular↗

PubMed2002

Theoretical model of internally cooled interstitial ultrasound applicators for thermal therapy.

Interstitial ultrasound applicators for high-temperature thermal therapy are currently being developed for treating cancerous and benign disease. Internally cooled, direct-coupled (ICDC) applicators, composed of a segmented array of cylindrical ultrasound transducers, have demonstrated capabilities of producing controllable and conformal heating distributions along the applicator length and angular orientation. In this study, 2D transient acoustic and biothermal models of ICDC applicators were developed using a mixed implicit and explicit finite difference solution with variable node spacing in cylindrical coordinates for enhanced speed, stability and accuracy. The model incorporates dynamic behaviour of acoustic parameters and blood perfusion as a function of temperature and thermal dose. Acoustic intensity distributions were modelled as a composite of measured and theoretical intensity distributions. The shape and time evolution of temperature contours and thermal lesions for 90 degrees, 200 and 360 degrees angularly directional applicators and multi-transducer applicators were modelled for heating durations between 1 and 5 min. Model parameters were selected to match previously reported ex vivo and in vivo studies of 2.2 mm diameter ICDC devices in thigh muscle and liver (15-30 W cm(-2) applied power density, 0.5-5 min treatment times, 2.8-3.6 cm diameter thermal lesions). The temperatures and lethal thermal dose (600 EM43 degrees C) contours calculated using the models were in excellent agreement with temperatures and thermal lesion dimensions (visible coagulation) determined experimentally. The differences between maximum radial depths of coagulation calculated using the r-z and r-theta models were small, less than approximately 2 mm for 10-15 mm lesions. There was a strong correlation between the calculated 50 degrees C contour and the radial, angular and axial lesion dimensions obtained for 3-5 min heating protocols. The models developed in this study have significant application in design studies and potential future use in treatment planning of ICDC interstitial ultrasound thermal therapy.

Acoustics↗

PubMed2001

A theoretical model of the effect of continuum damage on a bone adaptation model.

Throughout life, bone is continuously turning over by the well-regulated processes of bone formation and resorption. Everyday activities damage bone, and this damage is normally repaired in a continuous remodelling process. When an imbalance in this remodelling process occurs, bones may become more susceptible to fracture. This paper is devoted to a theoretical modelling of the competition between damage and internal remodelling in bones. The general theory of adaptive damaged-elastic materials proposed here as a model for the physiological process of damaged-bone remodelling follows the general framework of continuum thermodynamics where new damaged-bone remodelling law and associated thermodynamical restrictions are stated, and specialized to the case of small strain in isothermal processes. An attempt is also made to derive: (a) the damage force (adaptive damage energy release rate ) which controls the microcracks propagation and arrest, and (b) the damage rule by introducing damage thresholds and loading/unloading conditions.

Adaptation, Physiological↗

PubMed2006

Effects of mechanosensitive ion channels on ventricular electrophysiology: experimental and theoretical models.

The heart is an electrically driven mechanical pump, somewhat like an electric motor. Interestingly, like an electric motor in 'dynamo mode', the heart can also convert mechanical stimuli into electrical signals. This feedback from cardiac mechanics to electrical activity involves mechanosensitive ion channels, whose properties and pathophysiological relevance are reviewed in the context of experimental and theoretical modelling of ventricular beat-by-beat electromechanical function.

Animals↗

PubMed2005

Generalized entanglement: a new theoretical model for understanding the effects of complementary and alternative medicine.

BACKGROUND AND PROBLEM: A main problem for the acceptance of many methods belonging to the broad spectrum of complementary and alternative medicine (CAM) is that there is no conceivable theoretical bridge between the mainstream biomedical model and CAM theories and practice. Although empirical evidence is one important side of the coin of credibility, theoretical feasibility is the other. History of science teaches that no amount of empirical evidence will convince sceptics and followers of more conventional paradigms as long as there is no good theoretical model to make empirical findings plausible. METHODS AND SOLUTION: I therefore propose to broaden the spectrum of theoretical concepts beyond the reigning local-causalist model toward a non-local model that encompasses effects as encountered in CAM. Such a model can be derived from a generalized and weaker version of quantum theory recently developed and published by my colleagues and I as weak quantum theory (WQT). This theoretical model predicts nonlocal correlations analogous to Einstein-Podolsky-Rosen (EPR)-like correlations in quantum mechanics proper. The discerning moment, though, is that these nonlocal correlations within WQT are not EPR correlations postulated to extend into the classical world, but a broader, generalized version of entanglement not dependent on the strict quantum nature of the system under question. WQT predicts entanglement between elements of a system if two variables or observables are complementary: one describing a global and one the local aspects of the system. Entanglement then ensues between those local elements of a system that are complementary to the global description or observable of that system. DISCUSSION AND CONCLUSIONS: This paper explores this rather abstract and general notion and expands it into more concrete examples. It is at the moment a purely explanatory structure, which, however, lends itself to exact empirical testing due to rather precise predictions, which will be developed. Because this structure of generalized entanglement is ubiquitous and also operative in conventional medicine, and because it is derived from one of the strongest theories that science has developed so far, it would constitute a theoretical bridge between the different medical and scientific traditions.

Complementary Therapies↗

PubMed1984

Theoretical models and computer simulations of neural learning systems.

It has been generally assumed for a long time that learning is accomplished in the central nervous system (CNS) by modifying strengths of ties between neurons. Various mechanisms may contribute to this process, but it is not known which are the specific mechanisms, and what are the rules by which they operate. Theoretical models, which are based on that general assumption are introduced. The purpose of the models is to suggest plausible ways by which learned information may be stored in the neural network, and be retrieved when it is needed. The networks in the models consist of four basic subunits, in accordance with identified units in the CNS: sensing, response, feeling, and control, plus association areas. The suggested operation rules are based on established operation rules of individual neurons, and assumed rules when neurons in groups are considered. Computer simulations are done, to check the consistency of the models, and to illustrate how they work. They simulate how an hypothetical kitten learns part of its environment, and show how relevant information may be stored and retrieved in its neuronal network. The suggested mechanisms could be examined in experiments, albeit not easy ones to conduct.

Animals↗

PubMedDate not supplied

Mental health and work: meanings and limits of theoretical models.

This text is a revision of the most important theoretical approaches that fundament the studies about Mental Health and Work, pointing to the basic concepts, categories of analyses and respective limits. The objective is to underline some conceptions about the theme, in the attempt to contribute with a way to face the problems, questions and difficulties imposed by the construction of this object of study. Thus, the text is based on the critical review of theoretical paradigms that predominate in studies about Mental Health and Work. Finally, it indicates some important theoretical questions about the relation between mental health and work.

Humans↗