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PubMed1990

Electron microscopy mapping of pBR322 DNA curvature. Comparison with theoretical models.

A map of local curvature of the pBR322 DNA has been established by electron microscopy analysis of linearized plasmid molecules. To determine their polarity these molecules are one end labelled with an avidin-ferritin-biotin complex and the images are digitized. Local curvature is calculated from two mathematical treatments of the DNA trajectory and expressed in term of a mean dinucleotide wedge angle. Eight regions of curvature are distinguished. The four main regions of curvature have a high content of phased AA runs. The experimental curvature map is compared to theoretical maps of curvature obtained from four available models for DNA curvature.

Base Composition↗
PubMed1994

Theoretical models of the ion channel structure of amyloid beta-protein.

Theoretical methods are used to develop models for the ion channel structure of the membrane-bound amyloid beta-protein. This follows recent observations that the beta-protein forms cation-selective channels in lipid bilayers in vitro. Amyloid beta-protein is the main component of the extracellular plaques in the brain that are characteristic of Alzheimer's disease. Based on the amino acid sequence and the unique environment of the membrane, the secondary structure of the 40-residue beta-protein is predicted to form a beta-hairpin followed by a helix-turn-helix motif. The channel structures were-designed as aggregates of peptide subunits in identical conformations. Three types of models were developed that are distinguished by whether the pore is formed by the beta-hairpins, the middle helices, or by the more hydrophobic C-terminal helices. The latter two types can be converted back and forth by a simple conformational change, which would explain the variable conduction states observed for a single channel. It is also demonstrated how lipid headgroups could be incorporated into the pore lining, and thus affect the ion selectivity. The atomic-scale detail of the models make them useful for designing experiments to determine the real structure of the channel, and thus further the understanding of peptide channels in general. In addition, if beta-protein-induced channel activity is found to be the cause of cell death in Alzheimer's disease, then the models may be helpful in designing counteracting drugs.

Alzheimer Disease↗
PubMed2004

Social support and psychological adjustment among Latinas with arthritis: a test of a theoretical model.

BACKGROUND: Among people coping with chronic illness, tangible social support sometimes has unintended negative consequences on the recipient's psychological health. Identity processes may help explain these effects. Individuals derive self-worth and a sense of competence by enacting social roles that are central to the self-concept. PURPOSE: This study tested a model drawing from some of these theoretical propositions. The central hypothesis was that tangible support in fulfilling a highly valued role undermines self-esteem and a sense of self-efficacy, which, in turn, affect psychological adjustment. METHODS: Structured interviews were conducted with 98 Latina women with arthritis who rated the homemaker identity as being of central importance to the self-concept. RESULTS: A path analysis indicated that, contrary to predictions, tangible housework support was related to less psychological distress. Emotional support predicted greater psychological well-being. These relationships were not mediated by self-esteem or self-efficacy. Qualitative data revealed that half of the sample expressed either ambivalent or negative feelings about receiving housework support. CONCLUSIONS: Results may reflect social and cultural norms concerning the types of support that are helpful and appropriate from specific support providers. Future research should consider the cultural meaning and normative context of the support transaction. This study contributes to scarce literatures on the mechanisms that mediate the relationship between social support and adjustment, as well as illness and psychosocial adaptation among Latina women with chronic illness.

Activities of Daily Living↗
PubMed1985

A theoretical model of a compliant arterial stenosis.

Recent clinical and experimental evidence indicates that coronary artery stenoses may rapidly change their size and shape in response to alterations in vasomotor tone and intraluminal pressure. This theoretical study models a partially compliant arterial stenosis to examine the hemodynamic impact of these alterations. In rigid vessels, a 98% reduction in luminal area would predictably produce subendocardial ischemia in the resting state. In contrast, stenoses, with part of the arterial wall normal by the underlying plaque, responded to vasoconstriction and to changes in intraluminal pressure. With part of the arterial wall normal by the plaque, both vasoconstriction and decreases in intraluminal pressure could increase the hemodynamic severity of the stenosis. Further, the more eccentrically positioned the underlying plaque was, the greater the effects of vasoconstriction and intraluminal pressure on stenotic severity. Thus the morphological configuration of the plaque and the normal wall segment in the stenosis appear to be important determinants of the hemodynamic response of the stenosis to vasoconstriction and blood pressure changes. In turn, these changes in stenotic severity can greatly influence flow through the vessel.

Arterial Occlusive Diseases↗
PubMed1987

Laryngeal modeling: theoretical, in vitro, in vivo.

Because the larynx is situated anatomically in an area which is difficult to measure and visualize, theoretical, in vitro, and in vivo models are used in laryngeal research. Vocal fold vibration was studied in anesthetized dogs, while electrically stimulating independently the superior and recurrent laryngeal nerves under conditions of constant airflow. Photoglottographic (PGG), electroglottographic (EGG), and subglottic pressure signals were obtained while stroboscopically photographing the larynx. Specific points along PGG, EGG, and subglottic pressure waveforms were correlated with laryngeal events which occurred during vibration. The canine larynx, in an experimentally produced phonatory mode, vibrates in a two mass (upper and lower margin) system and appears comparable to modal human voice production. The recorded glottographic waveforms from experimentally produced phonation in the canine are similar to signals recorded from humans. However, observed differences can be related to anatomic differences.

Animals↗
PubMed1983

Community diagnosis. A theoretical model for prevention in primary health care.

The current trend in health policies in Sweden is based on a holistic view of health and this means an increased overall responsibility for the county councils. There will be new challenges for the primary health care services, especially concerning epidemiology and health education. The community diagnosis model, which has three levels--namely the descriptive, analytical, and health action programmes--should prove to be a useful tool in this work. This article gives a general review and outlines the theoretical background of community diagnosis as a tool in broad preventive programmes at the local level. Skaraborg is the only county in Sweden that has a broad preventive programme, or health plan. This is based on a political decision taken in 1971. Experience of cooperation between primary health care and the Community Health Unit, at the county level during the seventies has provided the practical foundation for the community diagnosis model. The knowledge made available by the community diagnosis is of great value to the district physician in his daily work. When he knows more about the highly prevalent diseases (such as diabetes and hypertension) in the local community, he can better organize the treatment of these diseases in the health care centre. A knowledge of health risks, individual or environmental can, in the same way, facilitate his participation in the dialogue with members of the community and the planning of preventive measures.

Models, Theoretical↗
PubMed2000

Can induction of systemic hypotension help prevent nidus rupture complicating arteriovenous malformation embolization?: analysis of underlying mechanism achieved using a theoretical model.

BACKGROUND AND PURPOSE: Nidus rupture is a serious complication of intracranial arteriovenous malformation (AVM) embolotherapy, but its pathogenetic mechanisms are not well described. An AVM model based on electrical network analysis was used to investigate theoretically the potential role of hemodynamic perturbations for elevating the risk of nidus vessel rupture (Rrupt) after simulated AVM embolotherapy, and to assess the potential benefit of systemic hypotension for preventing rupture. METHODS: Five separate hypothetical mechanisms for nidus hemorrhage were studied: 1) intranidal rerouting of blood pressure; 2) extranidal rerouting of blood pressure; 3) occlusion of draining veins with glue; 4) delayed thrombosis of draining veins; and 5) excessively high injection pressures proximal to the nidus. Simulated occlusion of vessels or elevated injection pressures were implemented into the AVM model, and electrical circuit analysis revealed the consequent changes in intranidal flow, pressure, and Rrupt for the nidus vessels. An expression for Rrupt was derived based on the functional distribution of the critical radii of component vessels. If AVM rupture was observed (Rrupt > or = 100%) at systemic normotension (mean pressure [P] = 74 mm Hg), the theoretical embolization was repeated under systemic hypotension (minor P = 70 mm Hg, moderate P = 50 mm Hg, or profound P = 25 mm Hg) to assess the potential benefit of this maneuver in reducing hemorrhage rates. RESULTS: All five pathogenetic mechanisms under investigation were able to produce rupture of AVMs during or after embolotherapy. These different mechanisms had in common the capability of generating surges in intranidal hemodynamic parameters resulting in nidus vessel rupture. The theoretical induction of systemic hypotension during and after treatment was shown to be of significant benefit in attenuating these surges and reducing Rrupt to safer levels below 100%. CONCLUSION: The induction of systemic hypotension during and after AVM embolization would appear theoretically to be of potential use in preventing iatrogenic nidus hemorrhage. The described AVM model should serve as a useful research tool for further theoretical investigations of AVM embolotherapy and its hemodynamic sequelae.

Blood Pressure↗
PubMed1999

Theoretical model of adsorption in a templated porous material.

We introduce a theoretical description of adsorption in a templated porous material modeled as a quenched binary mixture configuration with one component removed. Using tools from equilibrium statistical mechanics, we analyze the diagrammatic structure of and derive an approximate formula for this nonequilibrium material's porosity and Henry regime selectivity. We find that templating significantly increases the porosity as well as the selectivity for the larger component of a mixture.

Journal Article↗
PubMed1990

Genesis of sigmoidal dose-response curve during defibrillation by random shock: a theoretical model based on experimental evidence for a vulnerable window during ventricular fibrillation.

The sigmoidal dose-response curve (percent success vs shock energy) suggests a probabilistic nature of defibrillation. The mechanism is still largely unknown, however, random variation in the excitable state during ventricular fibrillation (VF) is suspected. A canine defibrillation study was designed to determine whether random variation in absolute VF voltage (AVFV) (a crude marker of number of excitable cells) was related to success of defibrillation, using a DC shock successful at the 50% level. The results were: (a) transmyocardial resistance (73.4 +/- 1.4 vs 73.6 +/- 1.5 ohms) and delivered energy (6.1 +/- 1.2 vs 6.2 +/- 1.2 joules) were similar; however, (b) AVFV 2 msec prior to DC shock was greater for successful as compared to unsuccessful attempts (0.5 +/- 0.1 vs 0.3 +/- 0.0 mV, P less than 0.01). A mathematical model was subsequently developed based on fluctuation in the number of excitable cells. Variation in the state of excitability resulted in a cyclic window potentially vulnerable to defibrillation. The vulnerable window occurred at a point when the number of excitable cells was low, i.e., a higher state of total depolarization, which was in agreement with the experimental finding. For a given VF pattern, duration of the vulnerable window was regulated by the shock energy. A larger shock energy generated a wider vulnerable window and, in turn, a higher success rate. Finally, the sigmoidal dose-response curve of defibrillation was theoretically constructed by calculating the variable chances of a random DC shock occurring either in a vulnerable window or elsewhere during VF. It is concluded that a vulnerable window susceptible to defibrillation can be demonstrated in the early stages (10 sec) of VF. The mathematical model provides a theoretical basis for the vulnerable window and helps elucidate the probabilistic nature of defibrillation.

Animals↗
PubMed1988

Theoretical model for predicting rates of nitrosamine and nitrosamide formation in the human stomach.

A mathematical model has been developed to estimate the rates of formation of nitrosamines and nitrosamides in the human stomach, under a variety of physiological and environmental conditions. The model combines a detailed description of the kinetics of N-nitrosation with mass balance equations which account for gastric emptying, dilution and absorption. The simulations were based on a typical schedule of dietary inputs, and included variations in gastric pH and in the volume of the stomach contents over a 24-h period. Consideration of these transient phenomena allowed a distinction to be made between amines or amides present in the diet and in gastric or salivary secretions. A comparison of the theoretical results with available data on the nitrosation of proline suggests that the model accurately predicts gastric rates of nitrosamine formation under control conditions, and correctly represents the strong catalytic effects of thiocyanate and the inhibitory effects of ascorbic acid or ascorbate ion. The results further suggest that nitrosoproline (NPro) excretion is not an accurate index of gastric nitrosation under physiological (low-dose) conditions, even when corrections are made for dietary intake of NPro. The predicted rates of formation of N-nitrosodimethylamine (NDMA), even for a diet high in dimethylamine, were found to be a factor of approximately 10(2) to 10(3) lower than published estimates of the dietary exposure to preformed NDMA. Thus, these findings do not support the hypothesis that gastric formation of NDMA from dietary dimethylamine poses a serious additional health risk. The results are presented in a graphical and tabular form which makes it possible to readily estimate the rates of formation of other nitrosamines or nitrosamides in the stomach, under various assumed conditions.

Ascorbic Acid↗
PubMed2006

Accessing the dynamics of end-grafted flexible polymer chains by atomic force-electrochemical microscopy. Theoretical modeling of the approach curves by the elastic bounded diffusion model and Monte Carlo simulations. Evidence for compression-induced lateral chain escape.

The dynamics of a molecular layer of linear poly(ethylene glycol) (PEG) chains of molecular weight 3400, bearing at one end a ferrocene (Fc) label and thiol end-grafted at a low surface coverage onto a gold substrate, is probed using combined atomic force-electrochemical microscopy (AFM-SECM), at the scale of approximately 100 molecules. Force and current approach curves are simultaneously recorded as a force-sensing microelectrode (tip) is inserted within the approximately 10 nm thick, redox labeled, PEG chain layer. Whereas the force approach curve gives access to the structure of the compressed PEG layer, the tip-current, resulting from tip-to-substrate redox cycling of the Fc head of the chain, is controlled by chain dynamics. The elastic bounded diffusion model, which considers the motion of the Fc head as diffusion in a conformational field, complemented by Monte Carlo (MC) simulations, from which the chain conformation can be derived for any degree of confinement, allows the theoretical tip-current approach curve to be calculated. The experimental current approach curve can then be very satisfyingly reproduced by theory, down to a tip-substrate separation of approximately 2 nm, using only one adjustable parameter characterizing the chain dynamics: the effective diffusion coefficient of the chain head. At closer tip-substrate separations, an unpredicted peak is observed in the experimental current approach curve, which is shown to find its origin in a compression-induced escape of the chain from within the narrowing tip-substrate gap. MC simulations provide quantitative support for lateral chain elongation as the escape mechanism.

Aluminum Oxide↗
PubMed2002

A theoretical model for prescription of the patient-specific therapeutic activity for radioiodine therapy of Graves' disease.

A fundamental function of the thyroid is to extract iodine from the blood, synthesize it into thyroid hormones, and release it into the circulation under feedback control by pituitary-secreted hormones. This capability of the thyroid, termed as functionality, can in principle be related to the severity of hyperthyroidism in individual patients. In this paper the uptake and release of 131I by the thyroid following the administration of 131I therapy for Graves' disease has been theoretically studied. The kinetics of iodine in the thyroid and blood have been evaluated using a two-compartment model. This simplified model appears to be adequate for dosimetry purposes and allows one to correlate levels of increased thyroid functionality (hyperthyroidism) with clinically measurable kinetic parameters. An expression has been derived for the rate of change of thyroid mass following therapy; this has the same form as an empirical relationship described in an earlier work. A method is presented for calculation of the amount of radioiodine activity to be administered to individual patients in order to achieve the desired final functionality of the gland. The activity to be administered is based on measurements of 131I kinetics after the administration of a 'low-activity' (1850 kBq) tracer for treatment planning.

Graves Disease↗
PubMed2004

Organization of telomeric nucleosomes: atomic force microscopy imaging and theoretical modeling.

Telomeric chromatin has peculiar features with respect to bulk chromatin, which are not fully clarified to date. Nucleosomal arrays, reconstituted on fragments of human telomeric DNA and on tandemly repeated tetramers of 5S rDNA, have been investigated at single-molecule level by atomic force microscopy and Monte Carlo simulations. A satisfactory correlation emerges between experimental and theoretical internucleosomal distance distributions. However, in the case of telomeric nucleosomal arrays containing two nucleosomes, we found significant differences. Our results show that sequence features of DNA are significant in the basic chromatin organization, but are not the only determinant.

Animals↗
PubMed1997

Self-organization of trajectory formation. II. Theoretical model.

Most studies of movement coordination deal with temporal patterns of synchronization between components, often without regard to the actual amplitudes the components make. When such a system is required to produce a composite action that is spatially constrained, coordination persists, but its stability is modulated by spatial requirements effected, we hypothesize, through the component amplitudes. As shown experimentally in part I, when a redundant three-joint system (wrist, elbow, and shoulder) is required to trace a specified arc in space, the joint angles may be frequency- and phased-locked even as the curvature of the trajectory is manipulated. Transitions between joint coordination patterns occur at a critical curvature, accompanied by a significant reduction in wrist amplitude. Such amplitude reduction is viewed as destabilizing the existing coordinative pattern under current task constraints, thereby forcing the joints into a more stable phase relationship. This paper presents a theoretical analysis of these multijoint patterns and proposes an amplitude mechanism for the transition process. Our model uses three linearly coupled, nonlinear oscillators for the joint angles and reproduces both the observed interjoint coordination and component amplitude effects as well as the resulting trajectories of the end effector.

Arm↗
PubMed2000

Lord, now you are letting your servant depart in peace: a theoretical model, combining quality of life and years of life, which offers clues for the proper time to die with dignity.

"Lord, now you are letting your servant depart in peace according to your word" said Saint Symeon (New Testament, Luke 2, 29). But, which is this day? How can we know it? In this article we re-examine theoretically, using a "thought experiment", the ancient dilemma of "when is the proper time to die". It is a contribution to the international debate on quality of life, quality of death, and dying with dignity. Quality is defined as a variable, which incorporates every other dimension of life except time (except years of life). It is accepted that a heuristic definition of life is the product of duration and quality, and, since quality can be negative, we accept the idea that life can be negative as well. Given that, the proper time to die in dignity is when life, from being positive becomes negative. After that we examine the individual as a member and part of the society in which he belongs and we conclude that negative life can be prolonged only be consuming positive life of other members of the society; this means that negative life in a hypothetical one-member society cannot exist; a critical population size is necessary. Finally, rising to the society level, and having defined society's life as the sum of every person's life, we discovered that preserving the negative life of some people can only be possible by consuming the positive life of other members of this society or/and other society (ies) or/and of the whole biosphere. Now the question is raised whether this is ethical or not, whether it is a human right and, if so, whose?

Ethics, Medical↗
PubMed2006

Theoretical model of temperature regulation in the brain during changes in functional activity.

The balance between metabolic heat production, heat removal by blood flow, and heat conductance defines local temperature distribution in a living tissue. Disproportional local increases in blood flow as compared with oxygen consumption during functional brain activity disturb this balance, leading to temperature changes. In this article we have developed a theoretical framework that allows analysis of temperature changes during arbitrary functional brain activity. We established theoretical boundaries on temperature changes and explained how these boundaries depend on physiology (blood flow and metabolism) and external (heat exchange with the environment) experimental conditions. We show that, in regions located deep in the brain, task performance should be accompanied by temperature decreases in regions where blood flow increases (activated regions) and by temperature increases in regions where blood flow decreases (deactivated regions). The sign of temperature effect may be reversed for superficial cortex regions, where the baseline brain temperature is lower than the temperature of incoming arterial blood due to the heat exchange with the environment. Importantly, due to heat conductance, the temperature effect is not localized to the activated region but extends to a surrounding tissue at rest over the distances regulated by the temperature-shielding effect of blood flow. This temperature-shielding effect quantifies the means by which cerebral blood flow prevents "temperature perturbations" from propagating away from the perturbed regions. For small activated regions, this effect also substantially suppresses the magnitude of the temperature response, making it especially important for small animal brains.

Animals↗
PubMed2006

Theoretical modeling of the valence UV spectra of 1,2,3-triazine and uracil in solution.

Assessment of the perturbed matrix method (PMM) ability in reproducing valence UV absorption spectra is carried out on two model systems: 1,2,3-triazine in methanol solution and uracil in water solution. Results show that even using the simplest definition of the quantum center, i.e. the portion of the system explicitly treated quantum mechanically, PMM provides rather good results. This paper further confirms the possibility of using PMM as a theoretical-computational tool, complementary to other methodologies, for addressing the electronic properties in molecular systems of high complexity.

Mathematics↗