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Results for “Models, Theoretical”

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At least 451 records · Page 25Linked to original sources

PubMed1999

Evaluation of the impedance technique for cryosurgery in a theoretical model of the head.

Bioimpedance is a noninvasive technique that produces information on the electrical characteristics of tissue inside the body from currents injected and electrical potentials measured on the surface of the body. Because freezing causes a large increase in tissue electrical impedance we thought that it may also cause significant changes in the surface electrical potential making the bioimpedance technique suitable for noninvasive monitoring and imaging of cryosurgery. To evaluate the feasibility of the bioimpedance technique in cryosurgery we examined, as a case study, a theoretical model for the electrical potentials during brain cryosurgery. A three-dimensional spherical model was used to calculate the change in the electrical potential distribution in the head as a function of the current source location and the size of the frozen lesion in the brain. The numerical calculations were executed using the finite volume method and the iterative successive over relaxation method. The results demonstrate that, indeed, freezing inside the head produces measurable changes in the electrical potential on the outer surface-the scalp.

Brain↗

PubMedDate not supplied

Theoretical models for studying the effect of antepartum stress on the family.

Three models are proposed to predict the effect of antepartum stress on the family. These models consider external and internal family environments and individual characteristics as they influence an individual family member's health status, dyadic relationships within the family, and the family's functioning as a unit. Research to date validates the complex interrelationships between antepartum stress, social support, self-esteem, sense of mastery, anxiety, depression, and their effect on health status, dyadic relationships, and family functioning. The theoretical models presented identify relationships between these important variables that can guide much-needed research on the effects of antepartum stress on the childbearing family.

Anxiety↗

PubMed1994

A theoretical model of phase transitions in the human brain.

An experiment using a multisensor SQUID (superconducting quantum interference device) array was performed by Kelso and colleagues (1992) which combined information from three different sources: perception, motor response, and brain signals. When an acoustic stimulus frequency is changed systematically, a spontaneous transition in coordination occurs at a critical frequency in both motor behavior and brain signals. Qualitatively analogous transitions are known for physical and biological systems such as changes in the coordination of human hand movements (Kelso 1981, 1984). In this paper we develop a theoretical model based on methods from the interdisciplinary field of synergetics (Haken 1983, 1987) and nonlinear oscillator theory that reproduces the main experimental features very well and suggests a formulation of a fundamental biophysical coupling.

Acoustic Stimulation↗

PubMedDate not supplied

Theoretical model of psychotherapy: Eastern Asian-Islamic women with mental illness.

The Muslim immigrant population is increasing in the United States. To provide appropriate psychotherapy for this group, especially Islamic women, treatment professionals must have a deeper understanding of the culture of Eastern Islamic women. Islam is the lifestyle of this population and it influences their thinking, the relationships among themselves and with others, and all other activities of daily life. The holistic approach of the Eastern Islamic population is incompatible with the individualistic approach of Western psychotherapy in treating Islamic women. We explore a theoretical model of psychotherapy for Eastern Asian-Islamic women suffering from mental illness (MI) to develop an effective and appropriate therapy. Health care providers, specifically those dealing with MI patients, will gain insights from the suggested psychotherapeutic model and its relevance to Islamic concepts and practices.

Adult↗

PubMed2005

Formation of amorphous alloys by ion beam mixing and its multiscale theoretical modeling in the equilibrium immiscible Sc-W system.

Unique amorphous alloys are synthesized at the compositions of 25 and 40 atom % of W by ion beam mixing in the equilibrium immiscible Sc-W system characterized by a positive heat of formation of +14 kJ/mol. In thermodynamic modeling, a Gibbs free energy diagram is constructed based on Miedema's theory, and the diagram predicts a glass-forming range of the Sc-W system to be within 12-58 atom % of W. To develop an atomistic model, ab initio calculations are first conducted to assist the construction of an n-body Sc-W potential under the embedded atom method. The proven realistic potential is applied in molecular dynamic simulations to study the crystal-to-amorphous transition in the Sc-W solid solutions, thus determining the glass-forming ability of the system to be within 15-50 atom % of W. Apparently, both theoretical predicted glass-forming ranges cover the experimentally measured one, showing an excellent agreement. We report, in this paper, the experimental results from ion beam mixing and the multiscale theoretical modeling concerning the amorphous alloy formation in the Sc-W system together with a brief discussion of the structural transition mechanism.

Journal Article↗

PubMed2005

A theoretical model of common process factors in youth and family therapy.

Recently there has been an increasing emphasis in the youth and family mental health treatment literature on the use of empirically supported treatments (ESTs). In contrast there has been scant attention paid to more universal aspects of the therapy process that may have even greater impact upon therapy outcomes. It is likely that the success of the techniques proposed by ESTs may depend on the presence of common process factors. In this article, the authors explore the status of common process factors research in the youth and family therapy literature, and propose a theoretical model linking specific therapeutic relationship variables and treatment outcomes for children and adolescents. This model is intended to guide synthesis of the empirical evidence for common process factors in youth and family treatment and to stimulate future research on common process factors.

Adolescent↗

PubMed2025

Structure-informed theoretical modeling defines principles governing avidity in bivalent protein interactions.

In signaling cascades, where domain-motif interactions tend to interact with relatively low affinity (allowing for reversibility), signaling proteins often encode multiple domains or motifs, which present the possibility of avidity - drastically increasing the interaction strength and duration as a result of multivalent binding. However, given the large combinatorial space, predicting and validating multivalent interactions that interact with avidity is a challenge. Here, we integrate mechanistic modeling, structure-based analysis, and experimental approaches as a framework for defining the conditions under which avidity plays a role. We explore the tandem SH2 domain family of interactions with bisphosphorylated partners as a multivalent archetype, which encompasses key secondary messengers in tyrosine kinase signaling networks. While certain multivalent interactions have been shown to be necessary in immune receptor recruitment of partners, bivalent recruitment of tandem SH2 domains more broadly is poorly understood. Theoretical modeling suggests that maximum avidity occurs with closely spaced or flexibly linked phosphotyrosine sites, combined with moderate monovalent affinities - exactly around the innate range of SH2 domain affinity. Surprisingly, despite sequence diversity, structure-based analysis showed remarkably conserved three-dimensional spacing between SH2 domains across all tandem SH2 families, which we corroborate experimentally, suggesting evolutionary optimization for avidity interactions. The combination of structure-based analysis of domain spacing with available monovalent experimental data appears to be sufficiently accurate to predict and rank order high affinity interactions of tandem SH2 domain recruitment to the EGFR C-terminal tail. These approaches lay the groundwork for larger utility in multivalent prediction and testing to help better understand protein interactions that drive cell signaling.

BLI↗

PubMed1986

Alpha-helix-to-random-coil transitions of two-chain, coiled coils: a theoretical model for the "pretransition" in cysteine-190-cross-linked tropomyosin.

The thermal unfolding curve for alpha alpha tropomyosin in which the two chains are cross-linked at cysteine-190 shows two striking features that distinguish it from that of its counterpart for non-cross-linked molecules: a "pretransition" at 25-50 degrees C and a shift in the principal transition to higher temperature, but with the same steepness. Previously, the pretransition was explained by postulating that the cross-link produces local strains, yielding a pinched "bubble" of chain-separated random coil about C-190, whereas the rest of the coiled coil remains intact. Results from both enzymatic digestion kinetics and equilibrium calorimetric studies have been interpreted as consistent with the existence of such a bubble. To test this idea further, a theoretical model is devised whereby various physical features can be imposed and the resulting helix content and other properties calculated from the statistical mechanical theory of the helix-coil transition. Short-range interactions employed are the geometric mean values of those in alpha-tropomyosin. The helix-helix interaction free energy is also like that in alpha-tropomyosin, including its nonuniformity; i.e., it is made larger in the amino half of the molecule. Local strain is introduced by setting the helix-helix interaction to zero in a region about the cross-link. The results show that, alone, neither local strain nor nonuniformity serves to mimic the experiments. In concert, however, they reproduce all the main experimental features, if the strain is extensive (approximately 29 residues) and somewhat dissymmetric. Theoretical helix probability profiles, however, show that no bubble of unfolded chains forms about the cross-link. Instead, in the pretransition, residues unfold from the weakly interacting end (residue 284) in to, but not through, the cross-link at C-190. The theory also indicates that the augmented stability for the principal transition occurs largely as a result of loop entropy. The same strain and nonuniformity are then employed to explore the effects of other possible cross-link positions. The thermal curves are shown to depend markedly on cross-link location. The curves are discussed in terms of loop entropy, which has drastic, long-range effects. Under appropriate circumstances it can produce, in the coiled-coil model, a thermal transition that is essentially all or none.

Cysteine↗

PubMed1991

Non-homogeneous kinetics in the competition of single-stranded calf-thymus DNA and low-molecular weight scavengers for OH radicals: a comparison of experimental data and theoretical models.

The yield of single-strand breaks induced by 60Co gamma-radiation in single-stranded calf-thymus DNA has been measured in aqueous, N2O/O2 saturated DNA solutions as a function of the concentration of added OH scavengers. Single-strand break yields have also been determined, relative to the yield of OH radicals, using pulse radiolysis. Essentially the same dependence on scavenging capacity was obtained under both irradiation conditions. At the highest scavenging capacity used (approximately 10(9) s-1), about eight times more strand breaks are formed than would be predicted from homogeneous competition kinetics. The experimental data are compared with values obtained from theoretical models which are based on non-homogeneous kinetics, and include the effects of spurs and of the direct action of radiation. The comparison shows that the observed dependence of the single-strand break yield on the scavenging capacity can be quantitatively accounted for without the need of empirical adjustment of parameters using a DNA model where the macromolecules are approximated by structureless cylinders of radius Rc = 0.80 nm which on their surface react with OH radicals at a diffusion-controlled rate. The factors determining the scavenger dependence of the single-strand break yield are discussed. The increase on going to higher scavenging capacities in the apparent rate constant of the reaction of OH with DNA is rationalized in terms of the mean diffusion length of OH radicals in solution. Estimates are given for the fractions of strand breaks due to randomized OH radicals, OH radicals from spurs, and the direct radiation effect.

Animals↗

PubMed1985

A theoretical model of phase transitions in human hand movements.

Earlier experimental studies by one of us (Kelso, 1981a, 1984) have shown that abrupt phase transitions occur in human hand movements under the influence of scalar changes in cycling frequency. Beyond a critical frequency the originally prepared out-of-phase, antisymmetric mode is replaced by a symmetrical, in-phase mode involving simultaneous activation of homologous muscle groups. Qualitatively, these phase transitions are analogous to gait shifts in animal locomotion as well as phenomena common to other physical and biological systems in which new "modes" or spatiotemporal patterns arise when the system is parametrically scaled beyond its equilibrium state (Haken, 1983). In this paper a theoretical model, using concepts central to the interdisciplinary field of synergetics and nonlinear oscillator theory, is developed, which reproduces (among other features) the dramatic change in coordinative pattern observed between the hands.

Hand↗

PubMed2006

Theoretical model for the pulse dynamics in a long granular chain.

When a long line of initially stationary and unstressed touching particles is hit on one end by another particle, dynamical self-organized impulsive waves will be generated and propagating through the granular chain. In this paper we develop a continuous function to represent the initial impulse wave. Then the partial differential wave equations for chains of cylindrical granules, which can take such function as one of its solutions, are constructed and exactly coincided with the wave equation obtained by using Taylor series expansion. The properties of the impulse wave propagating, such as the attenuation of the wave and the forward momentum transfer are studied in detail based on our theoretical model, and the analytical solutions are supported by numerical simulations.

Journal Article↗

PubMed1987

Characterization of anti-cerebroside sulfate antisera using a theoretical model to analyse liposome immune lysis data.

Antisera to the acidic glycolipid cerebroside sulfate (sulfogalactosyl ceramide) were raised in rabbits by several different methods. Reactivity with cerebroside sulfate was detected from complement-mediated lysis of liposomes composed of phosphatidylcholine/cholesterol/cerebroside sulfate and containing the spin label tempocholine chloride as a marker substance. Both cholesterol rich particles and lipid bilayer liposomes containing phosphatidylcholine and cholesterol were effective carriers for cerebroside sulfate, in combination with methylated bovine serum albumin for intravenous immunization, and with Freunds complete adjuvant for subcutaneous immunization. The antisera raised by the different methods were characterized with respect to their cross reactivity with other lipids and the relative concn of specific antibodies and their affinities for cerebroside sulfate using a theoretical model developed earlier [Vistnes A. I. (1984) J. Immun. Meth. 68, 251] for analysis of data from immune lysis of liposomes. Differences in these properties, both of which can affect antibody titer, could be detected for antisera raised by different methods and obtained at different times after immunization. Some of the antisera also reacted non-specifically to varying degrees with other anionic lipids indicating that the anti-cerebroside sulfate antibodies could bind non-specifically to anionic lipids by electrostatic interactions. This suggested that basic amino acid residues may be an important part of the antibody receptor binding site for the glycolipid head group. An important implication of this result is that antibodies raised against anionic glycolipids should be tested for non-specific binding to anionic phospholipids.

Animals↗

PubMed1985

Regulatory proteins (inhibitors or activators) affect estimates of Mr of enzymes and receptors by radiation inactivation. A theoretical model.

The radiation-inactivation method allows the determination of the Mr of enzymes and receptors by monitoring the decay of biological activity as a function of absorbed dose. The presence of regulatory or effector proteins (inhibitors or activators) associated with an enzyme or receptor, or released in the preparation after tissue homogenization, may affect the decay of biological activity. How the activity is affected, however, will depend on the type of inhibition (competitive or non-competitive), the inhibitor or activator concentration, the dissociation constant of the enzyme-effector system, and the effector Mr relative to that of the enzyme. Since little is known on how effector proteins influence radiation inactivation of enzymes and receptors, we have considered a theoretical model in an effort to provide a framework for the interpretation of experimentally obtained data. Our model predicts that competitive and non-competitive inhibitors of enzymes could be distinguished by analysing irradiated samples with various substrate concentrations. Inhibitors will decrease whereas activators will increase the apparent target size of enzymes or receptors.

Binding, Competitive↗

PubMed1986

Psycho-social aspects of youth unemployment: an interpretative theoretical model.

By utilizing representative samples of short-term (n = 33), long term (n = 14) unemployed adolescents and YTS trainees (n = 49) in North-East Scotland, the present study attempted to identify psycho-social variables involved in the individual adolescent's ability to cope with unemployment. The research was built around a series of semi-structured interviews with all subjects. Results suggested a variety of apparent contradictions--family support vs. parental pressure; informal community-based education as helpful vs. school education as irrelevant; high aspirations as reinforcing or frustrating; peer groups as supportive or socially constraining; time structure as welcome or monotonous and restrictive; self-esteem being sapped or maintained aggressively and defensively high. From the data a theoretical model is offered which attempts to resolve the paradoxes by interpreting the experience of unemployment for young people in terms of positive and negative "trade-offs".

Adolescent↗

PubMed2006

AFM imaging and theoretical modeling studies of sequence-dependent nucleosome positioning.

Telomeric chromatin has different features with respect to bulk chromatin, since nucleosomal repeat along the chain is unusually short. We studied the role of telomeric DNA sequences on nucleosomal spacing in a model system. Nucleosomal arrays, assembled on a 1500-bp-long human telomeric DNA and on a DNA fragment containing 8 copies of the 601 strong nucleosome positioning sequence, have been studied at the single molecule level, by atomic force microscopy imaging. Random nucleosome positioning was found in the case of human telomeric DNA. On the contrary, nucleosome positioning on 601 DNA is characterized by preferential positions of nucleosome dyad axis each 200 bp. The AFM-derived nucleosome organization is in satisfactory agreement with that predicted by theoretical modeling, based on sequence-dependent DNA curvature and flexibility. The reported results show that DNA sequence has a main role, not only in mononucleosome thermodynamic stability, but also in the organization of nucleosomal arrays.

Base Sequence↗

PubMed1997

A theoretical model for the Gla-TSR-EGF-1 region of the anticoagulant cofactor protein S: from biostructural pathology to species-specific cofactor activity.

Protein S (PS), which functions as a species-specific anticoagulant cofactor to activated protein C (APC), is a mosaic protein that interacts with the phospholipid membrane via its gamma-carboxyglutamate-rich (Gla) module. This module is followed by the thrombin-sensitive region (TSR), sensitive to thrombin cleavage, four epidermal growth factor (EGF)-like modules and a last region referred to as the sex hormone binding globulin (SHBG) domain. Of these, the TSR and the first EGF-like regions have been shown to be important for the species-specific interaction with APC. Difficulties in crystallising PS have so far hindered its study at the atomic level. Here, we report theoretical models for the Gla and EGF-1 modules of human PS constructed using prothrombin and factor X experimental structures. The TSR was built interactively. Analysis of the model linked with the large body of biochemical literature on PS and related proteins leads to suggestions that (i) the TSR stabilises the calcium-loaded Gla module through hydrophobic and ionic interactions and its conformation depends on the presence of the Gla module; (ii) the TSR does not form a calcium binding site but is protected from thrombin cleavage in the calcium-loaded form owing to short secondary structure elements and close contact with the Gla module; (iii) the PS missense mutations in this region are consistent with the structural data, except in one case which needs further investigation; and (iv) the two PS 'faces' involving regions of residues Arg49-Gln52-Lys97 (TSR-EGF-1) and Thr103-Pro106 (EGF-1) may be involved in species-specific interactions with APC as they are richer in nonconservative substitution when comparing human and bovine protein S. This preliminary model helps to plan future experiments and the resulting data will be used to further validate and optimise the present structure.

1-Carboxyglutamic Acid↗

PubMed2006

Appraisal antecedents of shame and guilt: support for a theoretical model.

Four studies used experimental and correlational methods to test predictions about the antecedents of shame and guilt derived from an appraisal-based model of self-conscious emotions (Tracy & Robins, 2004). Results were consistent with the predicted relations between appraisals (i.e., causal attributions) and emotions. Specifically, (a) internal attributions were positively related to both shame and guilt; (b) the chronic tendency to make external attributions was positively related to the tendency to experience shame; and (c) internal, stable, uncontrollable attributions for failure were positively related to shame, whereas internal, unstable, controllable attributions for failure were positively related to guilt. Emotions and attributions were assessed using a variety of methods, so converging results across studies indicate the robustness of the findings and provide support for the theoretical model.

Affect↗

PubMed2000

Proto-metastatic core and comet effect: new theoretical model of the mechanism of metastasis generation.

One important aspect of cancer is its capacity to generate metastasis. The mechanism by which one or several neoplastic cells escape from the primitive tumor to start the metastatic process is still a matter of discussion, hence the need to reconsider under which circumstances and conditions 'lethal escape' is produced. Thus we introduce two concepts: proto-metastatic core and comet effect. We maintain that only one group of cells (proto-metastatic core) could escape passively from the tumor focus when they enter the bloodstream together with other detritus. The proto-metastatic core disintegrates through friction with the blood (comet effect). There are two mechanism of metastasis. In the first, only one cell leaves the vessel joined to white blood cells as it moves through an inflamed area. The second, from the proto-metastatic core, could form venous thrombus. We believe that this new theoretical model of the etiopathology of cancer will contribute to future therapies.

Humans↗