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At least 523 records · Page 29Linked to original sources
PubMed1991

Mathematical modelling of competitive labelled-ligand assay systems. Theoretical re-evaluation of optimum assay conditions and precision data for some experimentally established radioimmunoassay systems.

A mathematical theory of competitive labelled-ligand assays was developed with the intention of theoretically re-evaluating the optimal assay conditions and precision data of assay systems established by experiment. Our theory is based upon the assumptions of a simple bimolecular reaction mechanism, homogeneous reactants, as well as kinetically indistinguishable labelled and non-labelled ligands. The general case of two-step (non-equilibrium) assay was considered including the one-step (equilibrium) assay as a special case. The solution of the system of corresponding kinetic differential equations was used to mathematically construct standard curves. Furthermore, intraassay precision profiles and indices as well as detection limits were calculated considering solely the pipetting error, epsilon, as a source of experimental error. A procedure was outlined to mathematically determine the optimal incubation conditions for any assay system targeted to a given analyte concentration, P, at which the standard deviation of assay results is to be minimized. Estimates of both the content of binding sites and the equilibrium constant, K, of the specific binding agent are necessary, and these can be derived from Scatchard plots. For six RIA systems, of which three were one-step and three were two-step assays, experimental assay conditions and precision data were compared with theoretical predictions. Experimentally determined antibody binding site concentrations agreed fairly well with those independently evaluated by mathematical optimization. Mean precision indices, defined as constituting an average over the complete precision profile, were found to be within the theoretically predicted range, i.e. two- to threefold the pipetting error.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding Sites, Antibody↗
PubMed1986

Comparison of acute hemodynamic response to dobutamine and intravenous MDL 17,043 (enoximone) in severe congestive heart failure secondary to ischemic cardiomyopathy or idiopathic dilated cardiomyopathy.

The acute hemodynamic response to intravenous dobutamine administration was compared with intravenous MDL 17,043 administration in 8 patients with severe, chronic congestive heart failure. Simultaneous radionuclide angiography was performed with gated equilibrium blood pool imaging to derive left ventricular volumes and ejection fraction during serial hemodynamic measurements. Six patients had an optimal dobutamine dose of 10 micrograms/kg/min; 2 others were compared at a dose of 7.5 micrograms/kg/min; comparisons with MDL 17,043 were after a 1.5-mg/kg bolus dose in all 8 patients. Dobutamine and MDL 17,043 caused significant and similar increases in cardiac index and stroke volume index. Dobutamine significantly increased heart rate and MDL 17,043 did not. MDL 17,043 significantly decreased pulmonary artery wedge, mean pulmonary artery and right atrial pressures; dobutamine did not. Dobutamine increased end-diastolic volume in 4 patients, with little concomitant decrease in wedge pressure; MDL 17,043 caused no change or a decrease in left ventricular end-diastolic volume in 5 patients, but consistently decreased wedge pressure in all. Thus, the left ventricular pressure-volume curve was displaced downward to a more favorable position after MDL 17,043 but not after dobutamine. In patients with chronic congestive heart failure, acute myocardial performance was more optimally influenced by MDL 17,043 than dobutamine administration.

Adult↗
PubMed1988

Epstein-Barr virus nuclear antigen type 1 binding: electron microscopy.

Epstein-Barr virus (EBV) nuclear antigen type-1 (EBNA-1) was extracted and purified from Raji cells by chromatography on DNA-Sepharose and Blue-dextran Sepharose. Its complexes with plasmid pM765-10 derived from EBV (strain M-ABA) DNA were visualized by electron microscopy. The criteria of specificity were as follows: (1) preferential binding of EBNA-1 to the ori-P region of pM765-10; (2) specific enlargement of EBV DNA/EBNA-1 complexes with anti-EBNA-1 (IR-3) IgG antibody; and (3) resistance of the resulting EBV DNA/EBNA-1/anti-EBNA-1 antibody complexes to treatment with 1.5 M NaCl. The optimal conditions for the formation of EBV DNA/EBNA-1 complexes were 50 to 150 mM NaCl and pH 6.0. A balanced equilibrium of EBNA-1 and pM765-10 was necessary to achieve both a high yield and specificity of EBV DNA/EBNA-1 complexes.

Antigens, Viral↗
PubMed1993

International migration, economic policy and human capital accumulation: a simulation study.

"This paper examines the economic policy implications of international migration and human capital accumulation within a dynamic general equilibrium model. Each country produces by means of physical and human capital of two types (skilled and unskilled labour). Along optimal growth paths in a world of diverging population growth rates immigration can only be beneficial when the free rider effect (i.e., not paying for training costs) exceeds the capital dilution effect of an increase in population growth. Under quite general conditions the optimal immigration rate is zero."

Demography↗
PubMed2005

High mutation rates, bottlenecks, and robustness of RNA viral quasispecies.

Population bottlenecks are stochastic events that strongly condition the structure and evolution of natural populations. Their effects are readily observable in highly heterogeneous populations, such as RNA viruses, since bottlenecks cause a fast accumulation of mutations. Considering that most mutations are deleterious, it was predicted that the frequent application of bottlenecks would yield a population unable to replicate. However, in vitro as well as in vivo systems evolving through bottlenecks present a remarkable resistance to extinction. This observation reveals the robustness of RNA viruses and points to the existence of internal mechanisms which must confer a high degree of adaptability to fast mutating populations. In this contribution, we review experimental observations regarding the survival of RNA viruses, both in laboratory experiments and in natural populations. By means of a simple theoretical model of evolution which incorporates strong reductions of the population size, we explore the relationship between the number of replication rounds that a single founder particle undergoes before the next bottleneck is applied, and the mutation rate in a particular environment. Our numerical results reveal that the mutation rate has evolved in a concerted way with the degree of optimization achieved by the population originated from the founder particle. We hypothesize that this mechanism generates a mutation-selection equilibrium in natural populations that maximizes adaptability while maintaining their structure.

Base Sequence↗
PubMed2004

Silica adsorbent prepared from spent diatomaceous earth and its application to removal of dye from aqueous solution.

The objective of this work is to study the activation regeneration of spent diatomaceous earth (SDE) for the preparation of silica adsorbents using thermal regeneration and acid/alkaline activation methods. Under the experimental conditions investigated, it was found that the alkaline activation method carried out by sodium hydroxide under controlled conditions is significantly superior to other heat and activation methods. The porosities of solids thus obtained are over 0.2, indicating that they are basically mesoporous. The optimal porous material thus prepared was used as a mineral adsorbent for methylene blue at 25 degrees C. The adsorption equilibrium revealed that the silica adsorbent can take up over 50 mg/g at relatively low concentrations in aqueous medium from the fittings of Langmuir and Freundlich isotherms with high correlations. On the other hand, the adsorption kinetic of methylene blue under various adsorbent dosages can be well described with a pseudo-second-order reaction model.

Journal Article↗
PubMed1994

Use of ionic detergents for enterovirus recovery from waste water.

Virus recovery from poliovirus-seeded waste water was attempted after treatment of the samples with ionic detergents in the presence of acoustic energy. Viral titers strongly fluctuated depending upon concentration and chemical structure of the detergent as well as upon the dissolved organic content of the aqueous samples. Supplementation of the cellular monolayers with an additional amount of cell culture medium or with a nonionic detergent in a subcytotoxic concentration 48 h after inoculation partly induced cytopathic effects in silent dilution steps. Since, in the presence of the same detergent, viral recovery rates varied with the type of waste water, titer-conditioning activity of detergents was suggested to depend upon their effective critical micellization concentration. Especially for virus recovery from concentrated waste water, treatment with strongly disruptive detergents such as sodium dodecylsulfate revealed to be much more efficient than with the less hydrophilic sodium N-laurylsarcosine, sodium glycodeoxycholate or lauryldimethylamine-oxide, whereas the opposite seemed to be the case if virus recovery from filtrate samples were to be optimized. The mechanisms by which viral particles become infectious for the cell appear to be triggered by general principles of equilibrium thermodynamics, which means that interactions between viral surface proteins and cellular receptor molecules seem to reflect the tendency of these reagents to assume the energetically most favored orientation.

Animals↗
PubMed2005

Peptide conformational equilibria computed via a single-stage shifting protocol.

We study the conformational equilibria of two peptides using a novel statistical mechanics approach designed for calculating free energy differences between highly dissimilar conformational states. Our results elucidate the contrasting roles of entropy in implicitly solvated leucine dipeptide and decaglycine. The method extends earlier work by Voter and overcomes the notorious "overlap" problem in free energy computations by constructing a mathematically equivalent calculation with high conformational similarity. The approach requires only equilibrium simulations of the two states of interest, without the need for sampling transition states. We discuss possible extensions and optimizations of the approach.

Peptides↗
PubMed1997

On the degree of solute mixing in liver models of drug elimination.

One of the fundamental differences between various liver models regards the underlying assumptions on the intrahepatic mixing process. A model-independent method for the evaluation of the departure from the perfectly mixed system is proposed which is based on an application of the relative entropy concept to hepatic transit time distributions of intravascular markers. This approach provides a measure of the distance between two probability distributions. Available data measured in isolated perfused livers indicate that sinusoidal solute mixing is nearly optimal. The suggestion of maximum mixedness in the liver may explain the discrepancy between the apparent validity of the venous equilibrium model and the physiological irrelevance of the underlying well-stirred assumption. In terms of the dispersion model the results are in accordance with the model equation obtained for mixed boundary conditions.

Entropy↗
PubMed2005

Vibronic coupling in the excited cationic states of ethylene: simulation of the photoelectron spectrum between 12 and 18 eV.

The effect of vibronic coupling on structure and spectroscopy is investigated in the excited cationic states of ethylene. It is found from equation of motion coupled cluster singles and doubles method for ionization potential electronic structure calculations in a triple-zeta plus double polarization basis set that ethylene in its third (B (2)A(g)) and fourth (C (2)B(2u)) ionized states does not have a stable minimum-energy geometry. The potential-energy surfaces of these states are energetically distinct and well separated at the ground-state geometry of ethylene, but in a geometry optimization as the structure of the ion relaxes, these surfaces end up in conical intersections and finally in the stable equilibrium geometry of the second ionized state (A (2)B(3g)). The topology of the potential-energy surfaces can be clearly understood using a vibronic model Hamiltonian. Furthermore, by diagonalizing this model Hamiltonian, the photoelectron spectrum of ethylene corresponding to the second, third, and fourth ionized states (12-18 eV) is simulated. Spectra from vibronic simulations including up to quartic coupling constants and using various normal-mode basis sets are compared to those from vertical Franck-Condon simulations to understand the importance of vibronic coupling and nonadiabatic effects and to examine the influence of individual normal modes on the spectrum.

Journal Article↗
PubMed1953

Mitochondria in the flight muscles of insects. II. Effects of the medium on the size form, and organization of isolated sarcosomes.

1. The sarcosomes of Drosophila and the blowfly, Phormia, are dense, spherical, homogeneous bodies when isolated from flight muscle and promptly examined under the phase contrast, oil immersion objective. 2. Their average diameter in newly emerged flies is about 1 micro. This value increases rapidly during the 1st week of adult life and then becomes constant at approximately 2.5 micro. At each age the variation in sarcosome diameter conforms approximately to a normal distribution. 3. The degree to which isolated sarcosomes retain their initial size and organization is remarkably conditioned by the composition and the hydrogen ion concentration of the medium in which they are teased and suspended. In suboptimal media three major categories of change were encountered: (1) swelling, with or without compaction of the contents (as in distilled water and salt solutions); (2) shrinkage to rod-like, pleomorphic forms (as in blood serum); and (3) fuzzy degeneration (as in sugar solutions). 4. The membrane that surrounds each sarcosome becomes plainly visible in swollen sarcosomes. A continuation of swelling is accompanied by the escape of the sarcosomal contents, the vacated membrane persisting as a spherical, optically empty ghost. 5. Sarcosomes appear to behave like osmometers when suspended in various aqueous solutions. Solutes which penetrate the membrane show only transient effects in preventing the osmotic entry of water. 6. Under this analysis we find the membrane to be more or less freely permeable to the ions of sodium, potassium, calcium, magnesium, chloride, and phosphate, to non-electrolytes smaller than hexoses, to phosphorylated hexoses, and to several intermediates of the citric acid cycle. 7. The sarcosomal membrane appears to be less permeable to non-electrolytes larger than pentoses, provided that such molecules are not phosphorylated. 8. The membrane shows a higher permeability to ATP than to ADP. The significance of this observation is considered with respect to the ADP-ATP shuttle between sarcosomes and muscle fibrils. 9. Simple solutions of electrolytes or non-electrolytes cause more or less conspicuous changes in the microscopic appearance of sarcosomes. Prolonged preservation was achieved only in more complicated media containing protein. It is concluded that the Donnan equilibrium is the source of the principal osmotic forces regulating the movement of water through the sarcosomal membrane. 10. The optimal medium for the preservation of isolated sarcosomes was an intracellular Ringer solution containing 2.5 per cent crystalline bovine albumin in 0.16 M potassium phosphate buffer at pH 7.0.

Animals↗
PubMed2003

Rational-driver approximation in car-following theory.

The problem of a car following a lead car driven with constant velocity is considered. To derive the governing equations for the following car dynamics a cost functional is constructed. This functional ranks the outcomes of different driving strategies, which applies to fairly general properties of the driver behavior. Assuming rational-driver behavior, the existence of the Nash equilibrium is proved. Rational driving is defined by supposing that a driver corrects continuously the car motion to follow the optimal path minimizing the cost functional. The corresponding car-following dynamics is described quite generally by a boundary value problem based on the obtained extremal equations. Linearization of these equations around the stationary state results in a generalization of the widely used optimal velocity model. Under certain conditions (the "dense traffic" limit) the rational car dynamics comprises two stages, fast and slow. During the fast stage a driver eliminates the velocity difference between the cars, the subsequent slow stage optimizes the headway. In the dense traffic limit an effective Hamiltonian description is constructed. This allows a more detailed nonlinear analysis. Finally, the differences between rational and bounded rational driver behavior are discussed. The latter, in particular, justifies some basic assumptions used recently by the authors to construct a car-following model lying beyond the frameworks of rationality.

Journal Article↗
PubMed1990

Characterization of 3'-phosphoadenosine 5'-phospho[35S]sulfate transport carrier from rat brain microsomes.

3'-Phosphoadenosine 5'-phospho[35S]sulfate [( 35S]PAPS) specific binding properties of rat brain tissue were studied. [35S]PAPS specific binding was optimal at pH 5.8 in either Tris-maleate or potassium phosphate buffers. Association was maximal at low temperature, reaching equilibrium in 20 min. Dissociation was rapid, with a dissociation time of 80 s. Scatchard analysis of [35S]PAPS specific binding was consistent with a single site having a KD of 0.46 +/- 0.06 microM and a Bmax of 20.8 +/- 2.0 pmol/mg of protein. Low concentrations of Triton X-100 (0.025%) were effective in increasing the number of binding sites to a Bmax of 44.5 +/- 4.6 pmol/mg of protein without affecting the affinity. [35S]PAPS specific binding was enriched in crude synaptic membranes (P2) and microsomes (P3). Regional distribution of [35S]PAPS specific binding was quite homogeneous in all brain structures studied. The pharmacological profile of [35S]PAPS specific binding in rat brain microsomes was consistent with a membrane protein having a high selectivity for the 3'-O-phosphoryl group substitution on the ribose moiety. Thus, 3'-phosphoadenosine 5'-phosphate was more potent than 2'-phosphoadenosine 5'-phosphate in competing for [35S]PAPS specific binding. Adenosine 5'-phosphosulfate was a good inhibitor of [35S]PAPS specific binding. ATP and ADP were also good displacers. Dipyridamole, a highly selective marker for adenosine uptake sites, was ineffective. 4,4-Diisothiocyanostilbene-2,2-disulfonic acid, the chloride transporter inhibitor, showed an IC50 of 36 +/- 5.1 microM for inhibition of [35S]PAPS specific binding. 2,6-Dichloro-4-nitrophenol had a low selectivity in competing for the [35S]PAPS binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenine Nucleotides↗
PubMed1980

The place of oestriol therapy after menopause.

The therapeutic principles covering sex hormone replacement therapy after menopause have undergone profound modification in recent years. Initial optimism regarding benefits, proven and unproven, was followed by deep pessimism because of potential serious adverse effects. Some degree of equilibrium has resulted from the application of risk-benefit and cost-effectiveness formulae to such hormone replacement regimens. Cost-effectiveness analysis in particular has highlighted the fact that different hormones or hormone combinations can markedly affect the therapeutic outcome. The purpose of the present paper is to examine the place of oestriol therapy after menopause based on such risk-benefit analyses. Oestriol, it would appear, has the potential for reduced risk but similar benefit to alternative oestrogen or oestrogen-progestogen combinations. The potential risks and benefits of long-term oestrogen therapy are therefore surveyed from the general standpoint of all oestrogens and the specific role of oestriol alone.

Aged↗
PubMed2005

Optimized method for dissolved hydrogen sampling in groundwater.

Dissolved hydrogen concentrations are used to characterize redox conditions of contaminated aquifers. The currently accepted and recommended bubble strip method for hydrogen sampling (Wiedemeier et al., 1998) requires relatively long sampling times and immediate field analysis. In this study we present methods for optimized sampling and for sample storage. The bubble strip sampling method was examined for various flow rates, bubble sizes (headspace volume in the sampling bulb) and two different H2 concentrations. The results were compared to a theoretical equilibration model. Turbulent flow in the sampling bulb was optimized for gas transfer by reducing the inlet diameter. Extraction with a 5 mL headspace volume and flow rates higher than 100 mL/min resulted in 95-100% equilibrium within 10-15 min. In order to investigate the storage of samples from the gas sampling bulb gas samples were kept in headspace vials for varying periods. Hydrogen samples (4.5 ppmv, corresponding to 3.5 nM in liquid phase) could be stored up to 48 h and 72 h with a recovery rate of 100.1+/-2.6% and 94.6+/-3.2%, respectively. These results are promising and prove the possibility of storage for 2-3 days before laboratory analysis. The optimized method was tested at a field site contaminated with chlorinated solvents. Duplicate gas samples were stored in headspace vials and analyzed after 24 h. Concentrations were measured in the range of 2.5-8.0 nM corresponding to known concentrations in reduced aquifers.

Environmental Monitoring↗
PubMed2000

Optimizing the classical heat engine

A pair of systems at different temperatures is a classic environment for a heat engine, which produces work during the relaxation to a common equilibrium. It is generally believed that a direct interaction between the two systems will always decrease the amount of the obtainable work, due to inevitable dissipation. Here a situation is reported where, in some time window, work can be gained due to the direct coupling, while dissipation is relevant only for much larger times. Thus, the amount of extracted work increases, at the cost of a change of the final state.

Journal Article↗
PubMed1996

Antibody characteristics for a continuous response fiber optic immunosensor for theophylline.

A self-contained fiber optic immunosensor was developed to measure continuously theophylline concentrations. The analytical signal was derived from the non-radiative energy transfer quench of fluorescence following binding of a fluorescence donor, B-phycoerythrin labeled theophylline, to an energy acceptor, Texas Red labeled antibody. Increases in free theophylline analyte concentrations resulted in a shift in antibody binding equilibrium between labeled and unlabeled theophylline that elicited a proportional increase in the fluorescence. The selection criteria for a monoclonal antibody as a molecular recognition element, and the optimization of labeling conditions to maximize the dynamic range and minimize sensor response time are described. Under one or more Texas Red labeling conditions, five antibody clones exhibited significant quenching when mixed with labeled analyte and also demonstrated 95% or greater reversible binding to labeled analyte. Two clones failed to exhibit fluorescence quenching when mixed with labeled analyte. The response time of the indicator chemistry system was dependent on the dissociation rate constant of the antibody. The equilibrium response time of intact sensors was limited by analyte diffusion across the containment membrane.

Antigen-Antibody Reactions↗