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At least 343 records · Page 19Linked to original sources
PubMed2000

Cryobiology of rat embryos II: A theoretical model for the development of interrupted slow freezing procedures.

Current mammalian embryo cryopreservation protocols typically employ an interrupted slow freezing (ISF) procedure. In general, ISF consists of initial slow cooling, which raises the extracellular solute concentration, and results in cell dehydration. Permeating cryoprotective agents (CPAs), such as dimethyl sulfoxide (DMSO), are typically included in the medium to protect the cells against high solute concentrations. As this ISF procedure continues, slow cooling is terminated at an intermediate temperature (T(p)), followed by plunging into liquid nitrogen (LN(2)). If the slow cooling step allowed a critical concentration ([CPA](c)) of CPA to be reached within the cell, the CPA will interact with the remaining intracellular water during rapid cooling, resulting in the majority of the intracellular solution becoming vitrified and preventing damaging intracellular ice formation (IIF). This study presents a theoretical model to develop efficient ISF procedures, on the basis of previously developed data for the rat zygote. The model was used to select values of initial CPA concentrations and slow cooling rates (from initial estimated ranges of 0 to 4 molal DMSO and 0 to 2.5 degrees C/min cooling rates) that would allow the intracellular solute concentration to exceed the critical concentration. The optimal combination was then determined from this range based on minimizing the duration of slow cooling.

Algorithms↗
PubMed2004

Development of a theoretical model for tracing facilitators and barriers in adaptive implementation of innovative practices in dementia care.

The implementation of innovations in the care sector that have been proven effective is not always easy. The implementation of innovative care programs frequently requires adaptive implementation, modified to suit local and regional circumstances. So far there has been very little research into the strategies and factors that facilitate or impede adaptive implementation processes. In the context of a study of the conditions for successful nationwide implementation of meeting centers for people with dementia and their carers, we developed a theoretical model to trace facilitators and barriers in adaptive implementation. We proceeded on the basis of a literature study, previous experiences with setting up meeting centers, and consulting with experts. The theoretical model we constructed distinguishes different phases of implementation, and describes activities and factors that can influence implementation on the micro-, meso- and macro-level for each phase. In addition,the model distinguishes characteristics of the innovation and other preconditions that may facilitate or impede implementation during the entire process. If the model is satisfactory for the study of the implementation of meeting centers, perhaps it can also be used to study other types of care innovations that would benefit from adaptive implementation.

Adaptation, Psychological↗
PubMed1991

Theoretical models of phospholipid phase transitions.

A brief review is given of the theoretical models and calculational methods used to study the main phase transition in phospholipid membranes with particular emphasis on recent progress. The models are discussed in terms of the mechanical variables and the various interactions which are necessary to describe the phase transition. Theoretical results are presented for the transitional properties and how the phase equilibria are affected by "impurities" such as cholesterol, proteins and anaesthetics.

Cholesterol↗
PubMed1985

Theoretical models for determining 222Rn and 220Rn progeny levels in Canadian underground U mines--a comparison with experimental data.

Use has been made of several theoretical models to predict radiation levels in underground U mines. The models used are the Evans model, the Thomas-Epps mine model, the isolated mine model, the mine tunnel with no air flow and the mine tunnel with air flow (Beckman and Holub). Calculations based on the above models have been extended to include 220Rn gas and its progeny, a common occurrence in some Canadian U mines. Theoretical predictions include 222Rn and 220Rn progenies working levels and concentrations, as well as some ratios of great practical interest. The main differences between the models are pointed out and comparison is made with experimental data gathered during the last 3 yr in several Canadian underground U mines. In general terms, the mine models reported in the literature and presented here are neither satisfactory nor clearly distinguishable for practical application on the basis of the experimental data so far collected. The reason for such lack of agreement lies mainly in the unrealistic assumptions on which the models are based. Compounded is the gross oversimplification of quite complex dynamic situations encountered in actual practice. Deficiencies inherent to the models are noted and suggestions to improve the applicability of mine models to practical situations are indicated.

Air Pollutants↗
PubMed2006

Theoretical model of infrared spectra of hydrogen bonds in molecular crystals and its application to interpretation of infrared spectra of 1-methylthymine.

A theoretical model for vibrational interactions in the hydrogen bonds in molecular crystals with four molecules forming two centrosymmetric dimers in the unit cell is presented. The model takes into account anharmonic-type couplings between the high-frequency N-H(D) and the low-frequency N...O stretching vibrations in each hydrogen bond, resonance interactions (Davydov coupling) between equivalent hydrogen bonds in each dimer, resonance interdimer interactions within a unit cell, and Fermi resonance between the N-H(D) stretching fundamental and the first overtone of the N-H(D) in-plane bending vibrations. The vibrational Hamiltonian, selection rules, and expressions for the integral properties of an absorption spectrum are derived. The model is used for theoretical simulation of the NH stretching bands of 1-methylthymine and its ND derivative at 300 K. The effect of deuteration is successfully reproduced by our model. Infrared, far-infrared, Raman, and low-frequency Raman spectra of 1-methylthymine and its deuterated derivative have been measured. Experimental geometry and frequencies are compared with the results of density functional theory calculations performed at the B3LYP6-311++G**, B3LYP/cc-pVTZ, B3PW916-311++G**, and B3PW91/cc-pVTZ levels.

Journal Article↗
PubMed1998

Integration of the analytical and alphabetical ICD10 in a coding help system. Proposal of a theoretical model for the ICD representation.

UNLABELLED: In French hospitals, medical diagnosis coding with the ICD10 is commonly performed and the use of effective tools would help coders in their task. AIM OF THIS WORK: To ameliorate an existing coding help system. This system, which already consists of the ICD10 analytical index, would be increased with the terms of the alphabetical index that includes lexical variants and additional terms as well. The addition of the second volume of the ICD would allow the coding of more terms and would lessen documentary silence. METHODS: The first step of this work was a careful study of a theoretical model of the ICD content. Then the alphabetical index file was submitted to a lexical analysis, and it was automatically transformed to be integrated into the existing coding help system. RESULTS: Compromise had to be made between a theoretical model and between what could be obtained in practice by an automatic processing of the file. Finally the alphabetical index was added to the initial thesaurus, which represents 42,000 terms and 4,000 additional words. Links between words and codes were also considerably increased, which has enhanced the searching possibilities of the tool and lessen documentary silence. Conversely the research time has been increased. CONCLUSION: Difficulties have to be encountered when trying to turn a manual tool into an automatic research tool.

Abstracting and Indexing↗
PubMed1993

CO(2) laser with modulated losses: Theoretical models and experiments in the chaotic regime.

We compare two different theoretical models for a CO(2) laser, namely, the two- and four-level model, and show that the second one traces with much better accuracy the experimentally observed chaotic dynamics when the cavity losses are sinusoidally modulated. Even though the two-level model provides a qualitative explanation of the chaotic dynamics, only the four-level one assures a quantitative fitting. We also show that, at the onset of chaos, the chaotic dynamics is low dimensional and can be described in terms of a noninvertible one-dimensional map.

Journal Article↗
PubMed1989

Theoretical and computational investigations of nonlinear wave propagations in arteries. (I)--A theoretical model of nonlinear pulse wave propagations.

In this paper, a new theoretical model of nonlinear wave propagations in arteries with surrounding tissues was put forward. The equations of motion for the blood vessels and their peripheral tissues as a system have been derived. These equations were expressed in terms of the stresses of the vessel wall and fluid, and the geometry of the blood vessel. They can be used to solve numerically the problems for the propagations of nonlinear pulse waves in arteries together with the momentum and continuity equations of incompressible-viscous flow, as well as the constitutive equations of fluid and vessel wall. The numerical solutions can involve pressure, velocities and flowrate of the blood flow, as well as displacements, velocities and stresses of the vessel wall. These physical variables of propagations of pulse waves in arteries are all of significance physiologically and clinically.

Arteries↗
PubMed2003

Prefission constriction of Golgi tubular carriers driven by local lipid metabolism: a theoretical model.

Membrane transport within mammalian cells is mediated by small vesicular as well as large pleiomorphic transport carriers (TCs). A major step in the formation of TCs is the creation and subsequent narrowing of a membrane neck connecting the emerging carrier with the initial membrane. In the case of small vesicular TCs, neck formation may be directly induced by the coat proteins that cover the emerging vesicle. However, the mechanism underlying the creation and narrowing of a membrane neck in the generation of large TCs remains unknown. We present a theoretical model for neck formation based on the elastic model of membranes. Our calculations suggest a lipid-driven mechanism with a central role for diacylglycerol (DAG). The model is applied to a well-characterized in vitro system that reconstitutes TC formation from the Golgi complex, namely the pearling and fission of Golgi tubules induced by CtBP/BARS, a protein that catalyzes the conversion of lysophosphatidic acid into phosphatidic acid. In view of the importance of a PA-DAG cycle in the formation of Golgi TCs, we assume that the newly formed phosphatidic acid undergoes rapid dephosphorylation into DAG. DAG possesses a unique molecular shape characterized by an extremely large negative spontaneous curvature, and it redistributes rapidly between the membrane monolayers and along the membrane surface. Coupling between local membrane curvature and local lipid composition results, by mutual enhancement, in constrictions of the tubule into membrane necks, and a related inhomogeneous lateral partitioning of DAG. Our theoretical model predicts the exact dimensions of the constrictions observed in the pearling Golgi tubules. Moreover, the model is able to explain membrane neck formation by physiologically relevant mole fractions of DAG.

Biological Transport↗
PubMed1976

Covalent assembly of mouse immunoglobulin G subclasses in vitro: application of a theoretical model for interchain disulfide bond formation.

The pathways and kinetics of interchain disulfide bond formation have been determined in vitro for purified myeloma proteins representing the three major subclasses of mouse immunoglobulin G(IgG) using the reoxidation system described previously (Petersen, J.G.L. & Dorrington, K.J. (1974) J. Biol. Chem. 249, 5633-5641). Mixtures of oxidized and reduced glutathione were added to act as a disulfide interchange catalyst. The pathways of covalent assembly observed in vitro were qualitatively and quantitatively similar to those followed by the various subclasses in vivo. HH and HHL were the principle covalent intermediates seen with IgG1 (MOPC 31C) and IgG2a (MOPC 173 and clone 19). With IgG2b( MPC 11C), HL, HH and HHL were all prominant intermediates. The time courses of reoxidation were simulated using a theoretical model based on second-order reaction kinetics (Percy, J.R., Percy, M.E. & Dorrington, K.J. (1974) J. Biol. Chem. 250, 2398-2400). Two distinct phases were apparent in the reoxidation sequence. The first, which lasted for the initial 5-15 min, did not confirm to the theoretical model. The second phase could be accounted for by the model and represented the remainder of the covalent assembly process. The physico-chemical basis for this biphasic phenomenon was explored. Sedimentation velocity studies showed that noncovalent association was incomplete at the beginning of the reoxidation step for all proteins except IgG2b (MOPC 11C). No dissociation was apparent in the reduced and alkylated proteins at pH 5 in the absence of prior exposure to acid conditions. Thus, exposure to acid appears to affect the affinity between the subunits in the native proteins. Transfer of the proteins from pH 5 to pH 8.2 (the pH at which reoxidation proceeds) is accompanied by the generation of an absorption difference spectrum over an 8-10 min period. These data suggest that a pH-dependent conformational relaxation process may influence the early stages of reoxidation.

Binding Sites↗
PubMedDate not supplied

[Adolescent mother experiencing child care: a theoretic model].

The study aimed to comprehend the meaning for the adolescent mother of experiencing care with her child and the construction of a theoretic model that is representative of this experience. The strategies employed were participant observations and a semi-structured interview. Eight adolescent mothers who had become mothers between 15 and 19 years old took part in the study. The Symbolic Interactionism was adopted as a theoretic referential and the Grounded [corrected] Theory as a methodological reference. The comparative analysis of data revealed that for the adolescent mother the experience of taking care of her child is driven by experiencing positive feelings, which provide the development of action and interaction strategies, in the sense of searching for resources to take care of her child the best way as possible.

Adolescent↗
PubMed1981

Phosphatidylcholine bilayers. A theoretical model which describes the main and the lower transitions.

We present a theoretical model which describes both the main and the lower phase transition in phosphatidylcholine bilayers. The main transition involves a melting of the hydrocarbon chains while the lower transition is seen as a nematic to isotropic transition involving entire lipid molecules (which are rod shaped when projected onto the bilayer plane). This latter transition is consistent with experimental data which suggest the presence of long-axis rotation for temperatures below the main melting transition. The model is extended to mixtures of phosphatidylcholines and compared with experimental data.

Lipid Bilayers↗
PubMed2001

A theoretical model for calculation of the detective quantum efficiency in granular scintillators.

A theoretical model has been developed for calculating the detective quantum efficiency (DQE) of scintillators, by taking into account the internal structure of granular scintillators often used in medical imaging detectors. Scintillators were considered to consist of N elementary thin layers containing spherical scintillating grains of equal size. Grains were assumed to be regularly distributed within each thin layer, the thickness of the latter being equal to the grain diameter. Values of the X-ray absorption and X-ray attenuation coefficients, of the intrinsic X-ray to light conversion efficiency and of the optical scattering and absorption coefficients were used as input data to the model. Optical scattering and optical absorption coefficients were determined by fitting the model to experimental luminescence data. The model was employed to calculate the detective quantum efficiency of La2O2S:Tb, Y2O2S:Tb, Y2O2S:Eu, ZnSCdS:Ag, ZnSCdS:Au,Cu scintillators. Results of the calculations were found close to values published in previous studies.

Journal Article↗
PubMedDate not supplied

"Single sample concept": theoretical model for a combinatorial approach to solid-state inorganic materials.

A theoretical model for a "single sample concept" (SSC) applied to the combinatorial chemistry of solid-state inorganic compounds is presented. The SSC is performed by reacting N starting materials (randomly mixed) in a single sample of approximately 1 cm(3). Combinatorial calculations demonstrate that the number of reasonably estimated phases to be found in the space of N components grouped into compounds (e.g. oxides, sulfides, or halogenides) containing up to q < or = 6 metallic elements is smaller than combinations set up by starting conditions for local reactions within the bulk of a single ceramic sample. Recently, the SSC proved to work in the case of synthesizing libraries of 3d metal oxides, from which magnetic particulate matter was extracted by a magnetic separation technique. The SSC may be applied in a first screening cycle followed by 2D approaches of combinatorial chemistry.

Combinatorial Chemistry Techniques↗
PubMed2007

Ethnic pride and self-control related to protective and risk factors: test of the theoretical model for the strong African American families program.

OBJECTIVE: To test a theoretical model of how ethnic pride and self-control are related to risk and protective factors. DESIGN: A community sample of 670 African American youth (mean age = 11.2 years) were interviewed in households. MAIN OUTCOME MEASURES: Measures of cigarette smoking, alcohol use, and sexual behavior (lifetime to past month). RESULTS: Structural modeling analyses indicated parenting was related to self-control and self-esteem, and racial socialization was related to ethnic pride. Self-control and self-esteem variables were related to levels of deviance-prone attitudes and to perceptions of engagers in, or abstainers from, substance use and sexual behavior. The proximal factors (behavioral willingness, resistance efficacy, and peer behavior) had substantial relations to the criterion variables. Participant gender and parental education also had several paths in the model. Results were generally similar for the 2 outcome behaviors. CONCLUSIONS: In this population, self-esteem and self-control are related to parenting approaches and have pathways to attitudes and social perceptions that are significant factors for predisposing to, or protecting against, early involvement in substance use and sexual behavior.

Adolescent↗
PubMed1990

Theoretical models for the conformations and the protonation of triacetonamine.

In this paper we propose theoretical models for the conformations of triacetonamine and protonated triacetonamine (Vincubine, an anticancer chemotherapeutic agent) developed by quantum and molecular mechanics techniques. We discuss the theoretical factors which are involved in the stabilization of the conformations calculated by the MNDO, MM2 and COPEANE methods and show the relative percent abundance of each molecular shape. Graphic representations of the conformers are depicted.

Antineoplastic Agents↗
PubMed1979

A theoretical model for layered visual processing.

A theoretical general model of layered computation in the retina is presented. Each layer is functional in the sense that it may correspond or not to an anatomical layer. It is formed by computing elements which can perform in principle any non-linear arbitrary function on a three-dimensional input space. This space consists of 2 spatial dimensions, plus time. The function performed by each computing element of a layer is simplified for the cases of invariance, space or time linearity and for time independence. Two illustrations are also presented. The first is a model of the simple ganglion cells in cat's retina. The second, a model of the group 2 ganglion cell of the frog's retina.

Animals↗