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Results for “Equilibrium optimization”

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At least 217 records · Page 12Linked to original sources

PubMed1998

Context-dependent reflex control: some insights into the role of balance.

Recent research suggests that the balance requirements of a task dictate the reflexive response. However, these observations were inferred indirectly from either different tasks or different phases of the same task. This study directly tested the hypothesis of balance-dependent control during recovery from an unexpected trip. The subjects were tripped in two different support conditions: unilimb support (provided by the stance limb) or trilimb support (provided by the stance limb and both arms placed on adjacent parallel bars). The subjects exhibited anticipatory changes: they biased the body center of mass toward the support limb in the mediolateral direction and elevated the swing limb higher when there was a possibility of being tripped. The electromyographic (EMG) latencies were not influenced by the threat to equilibrium. The magnitudes of the EMG reflexive response to the trip were clearly modulated as a function of the threat to stability, not in a simple manner, but rather in a complex manner, which optimized the recovery strategy. It is evident that the overriding concern, equilibrium control during locomotion, has a dominant influence on reflex modulation.

Adaptation, Physiological↗

PubMedDate not supplied

Inverse analyses of transport of chlorinated hydrocarbons subject to sequential transformation reactions.

Chemical and biological transformations can significantly affect contaminant transport in the subsurface. To better understand such transformation reactions, an equilibrium-nonequilibrium sorption transport model, HYDRUS-1D, was modified by including inverse solutions for multiple breakthrough curves resulting from the transport of solutes undergoing sequential transformations. The inverse solutions were applied to miscible-displacement experiments involving dissolved concentrations of trichloroethylene (TCE) undergoing reduction and/or transformations in the presence of zero-valent metal porous media (i.e., iron or copper-coated iron filings) to produce ethylene. The inverse model solutions provided a reasonable description of the transport and transformation processes. Simultaneous fitting of multiple breakthrough curves of TCE and ethylene placed additional constraints on the inverse solution and improved the reliability of parameter estimates. Confidence intervals of optimized parameters were reduced significantly in comparison with those obtained by fitting TCE breakthrough curves independently. Further evidence for accurate parameter estimates was given when the parameter values agreed with previously reported values from independent batch and degradation experiments. Optimized values of the normalized degradation rates for the equilibrium (1.4 x 10(-4) to 7.2 x 10(-5) L h(-1)m(-2)) and nonequilibrium (1.2 x 10(-4) to 5.5 x 10(-5)L h(-1)m(-2)) models compared well with values (0.03 to 6.5 x 10(-5) L h(-1) m(-2)) obtained from previous studies. The estimated TCE-iron sorption coefficients (0.52 to 2.85 L kg(-1)) were also consistent with a previously reported value (1.47 L kg(-1)).

Biotransformation↗

PubMed2001

Optimal harvesting and stability for a two-species competitive system with stage structure.

In this paper, we consider a stage-structured competitive population model with two life stages, immature and mature, with a mature population of harvesting. We obtain conditions for the existence of a globally asymptotically stable positive equilibrium and a threshold of harvesting for the mature population. The optimal harvesting of the mature population is also considered.

Animals↗

PubMed2005

Isobaric vapor-liquid equilibrium for methyldichlorosilane-dimethyldichlorosilane-benzene system.

The elucidation of vapor-liquid equilibrium (VLE) of the halogenated silane was necessary for the production of silicon derivatives, especially for methylvinyldichlorosilane, due to the lack of the relevant reports. Isobaric VLE for the system methyldichlorosilane-dimethyldichlorosilane-benzene and isobaric VLE of the three binary systems were measured with a new pump-ebulliometer at the pressure of 101.325 kPa. These binary compositions of the equilibrium vapor were calculated according to the Q function of molar excess Gibbs energy by the indirect method and the resulted VLE data agreed well with the thermodynamic consistency. Moreover, the experimental data were correlated with the Wilson, NRTL, Margules and van Laar equations by means of the least-squares fit, the acquired optimal interaction parameters were fitted to experimental vapor-liquid equilibrium data for binary systems. The binary parameters of Wilson equation were also used to calculate the bubble point temperature and the vapor phase composition for the ternary mixtures without any additional adjustment. The predicted vapor-liquid equilibrium for the ternary system was in a good agreement with the experimental results. The VLE of binary and multilateral systems provided essential theory for the production of the halogenated silane.

Journal Article↗

PubMed2003

The role of packaging film permselectivity in modified atmosphere packaging.

Modified atmosphere packaging (MAP) is commercially used to increase the shelf life of packaged produce by reducing the produce respiration rate, delaying senescence, and inhibiting the growth of many spoilage organisms, ultimately increasing product shelf life. MAP systems typically optimize O(2) levels to achieve these effects while preventing anaerobic fermentation but fail to optimize CO(2) concentrations. Altering film permselectivity (i.e., beta, which is the ratio of CO(2)/O(2) permeation coefficients) could be utilized to concurrently optimize levels of both CO(2) and O(2) in MAP systems. We investigated the effect of modifying film permselectivity on the equilibrium gas composition of a model MAP produce system packaged in containers incorporating modified poly(ethylene) ionomer films with CO(2)/O(2) permselectivites between 4-5 and 0.8-1.3. To compare empirical to calculated data of the effect of permselectivity on the equilibrium gas composition of the MAP produce system, a mathematical model commonly used to optimize MAP of respiring produce was applied. The calculated gas composition agreed with observed values, using empirical respiration data from fresh cut apples as a test system and permeability data from tested and theoretical films. The results suggest that packaging films with CO(2)/O(2) permselectivities lower than those commercially available (<3) would further optimize O(2) and CO(2) concentration in MAP of respiring produce, particularly highly respiring and minimally processed produce.

Carbon Dioxide↗

PubMed1993

Natural selection on multivariate traits in age-structured populations.

The theory of selection on multivariate traits in age-structured populations has important implications for empirical and theoretical studies of life-history evolution. A model of natural selection on a set of correlated quantitative traits with age structure is derived here, using an extension of previous work on selection at a single locus. This formulation provides an equation for the change of the mean of a vector of traits that is accurate is selection is weak, and does not require the population always to be in demographic equilibrium as selection proceeds. The treatment is extended to density-dependent populations, and to equilibrium populations under frequency-dependent selection. In addition, further approximations are derived that produce evolutionary equilibria equivalent to those predicted by optimization and evolutionarily stable strategy (ESS) theory. It is shown that the conditions for equilibrium are valid even if selection is strong.

Aging↗

PubMed1999

First-pass myocardial perfusion imaging and equilibrium signal changes using the intravascular contrast agent NC100150 injection.

In this phase I clinical study, the new ultrasmall superparamagnetic iron oxide contrast agent, NC100150 Injection (Nycomed AS, Oslo, Norway, a part of Nycomed Amersham), was assessed for first-pass magnetic resonance myocardial perfusion studies and its ability to produce equilibrium signal changes, as a possible indicator of myocardial blood volume. Data were acquired in 18 healthy male volunteers at 0.5 T and 1.5 T. At both field strengths, first-pass studies using T1-weighted sequences were acquired. Long TE spin-echo echoplanar imaging (EPI) was used at 0.5 T and short TE fast low-angle shot (FLASH) imaging at 1.5 T. With both sequences, T1 effects dominated the images for low doses, and time intensity curves potentially suitable for perfusion analysis were generated. At higher doses, T2 and T2* effects were observed. At 1.5 T, these predominantly affected the blood pool signal; however, at 0.5 T the myocardial signal was also involved, reflecting the relative T2 and T2* sensitivity of the spin-echo EPI sequence as a result of the long TE and long readout window, respectively. Equilibrium changes were assessed at both field strengths using T1-weighted FLASH sequences and in addition at 1.5 T using T2*-weighted gradient-echo EPI. With the T1-weighted images at both field strengths, signal changes were observed in all subjects; however, no dose-response relationship could be shown. With the T2*-weighted EPI there was significantly lower signal (P < 0.05) with the 3 and 4 mg/kg doses than with the 2 mg/kg dose. In conclusion, NC100150 Injection is useful for first-pass myocardial perfusion using T1-weighted sequences; however, low doses in combination with short TE sequences are required to minimize sensitivity to T2* effects. Equilibrium signal changes can also be induced in the myocardium. More work is required to optimize the imaging sequences and dose of NC100150 Injection for first-pass studies and also to determine whether the equilibrium signal changes can be used to measure myocardial blood volume changes in ischemic heart disease.

Adult↗

PubMed1976

Glucocorticoid receptors: evidence for a second, non-glucocorticoid binding site.

Previous studies on cytoplasmic glucocorticoid receptors and enzyme induction led to the classification of steroids as inducers (optimal or sub-optimal), antagonists, or inactive steroids, with respect to their activity as glucocorticoids. The receptor was postulated to exist in allosteric equilibrium between two conformational states, one "active" and the other "inactive". Steroids behaved as inducers (optimal or sub-optimal), antagonists, or inactive steroids depending on their relative affinity for the active and inactive conformational state of the receptor. Another possible model would invoke multiple binding sites on a single receptor with interactions between the binding sites depending upon the particular steroid bound. To test this latter possibility, an experimental technique was developed to measure the rate of dissociation of tritiated dexamethasone ([3H]DM) or tritiated aldosterone ([3H]A) from the glucocorticoid receptor of rat liver or kidney cytosol. The dissociation of the [3H]DM-receptor at 25 C was not due to irreversible denaturation, and minimal recombination of the receptor with [3H]DM occurred. Progesterone and a number of other steroids consistently and significantly increased the dissociation rate of [3H]DM-receptor complexes in both liver and kidney cytosol. An identical effect was seen with hepatic glucocorticoid receptors labelled with [3H]A, like dexamethasone an optimal inducer. All steroids which enhanced glucocorticoid-receptor dissociation were either antagonists or sub-optimal inducers. Thus, it is postulated that glucocorticoid receptors have at least two classes of binding sites, and that occupation of the second site increases the dissociation rate of agonists from glucocorticoid receptors.

Aldosterone↗

PubMed1976

Metabolic control of respiratory neuronal activity and the accompanying changes in breathing movements of the rabbit. III. Phase shifts in respiratory neurons induced by inflation and collapse of the lung, hyperventilation, or metabolic modifiers.

Phase shifts between inspiratory-related and expiratory-related discharge patterns can be reversibly induced in respiratory neurons following volume changes of the lung, hypocapnic apnea as a result of hyperventilation, or superfusion with certain metabolic modifiers. Phase-spanning expiratory-inspiratory or inspiratory-expiratory discharges are frequently induced in those neurons which are activated either by pulmonary stretch receptors or collapse afferents. The same is true for regulatory effectors which activate key steps of the neuronal metabolism such as ADP, 3-phosphoglycerate, L-glutamine, fructose-6-phosphate and fructose-1,6-diphosphate. In contrast, inhibitory vagal inputs or superfusion with citrate, an inhibitory metabolic modifier, revert preexisting expiratory-inspiratory discharges into a phase-coupled inspiratory pattern. It is postulated that the respiratory neuronal networks represents a time-optimal control system which strives to adjust to a new equilibrium value in a minimum of time, following a given mechanical or chemical perturbation. Following the hypothesis advanced by Cohen (1974) that the phase-spanning units modulate the activity of the in-phase neurons, it is suggested that the additional recruitment of expiratory-inspiratory and inspiratory-expiratory units provides a measure of the quality of time-optimal control and hence a performance index of the system.

Animals↗

PubMedDate not supplied

Thyrotropin binding to porcine thyroid plasma membranes: kinetic and thermodynamic analyses.

Evaluation of TSH binding to plasma membranes of porcine thyroid revealed unique sensitivity to pH and temperature. Analysis of apparent equilibrium binding yielded a linear Scatchard plot at the optimal pH of 6.0, indicating one class of binding sites. At physiological pH 7.4 a curvilinear Scatchard plot was obtained, resolved by computer analysis into two classes of binding sites of different affinities and capacities. Treatment of membranes with phospholipase C resulted in a 20% decrease in the number of high affinity sites, but no change occurred in binding affinity. In contrast, low affinity sites were not altered. To evaluate the significance of the curvilinear Scatchard plot, the kinetics of association were examined. The intrinsic Kd (kd/ka) was 0.20 nM, a value essentially equivalent to that of the high affinity binding component. The 'negative cooperativity' model of hormone binding was evaluated by examining the effect of excess unlabeled TSH on dissociation rate. Dissociation of bound 125I-labeled TSH was biphasic, and was enhanced by unlabeled hormone, regardless of whether the membranes were prelabeled at pH 6.0 or 7.4. This effect was not correlated with curvilinear Scatchard plots, and therefore not proof of negative cooperativity. Binding sites for TSH were further distinguished by their sensitivity to temperature. A van't Hoff plot of temperature dependence of the apparent Kd of the high affinity site was linear from 4 to 37 degrees C. In contrast, the apparent Kd of low affinity binding did not vary with respect to temperature. These results demonstrate that there are at least two independent binding sites for TSH on porcine thyroid plasma membranes, distinguishable by their equilibrium binding properties.

Animals↗

PubMed1993

Dynamic contrast-enhanced CT of the liver: comparison of contrast medium injection rates and uniphasic and biphasic injection protocols.

To determine the effect of contrast medium injection rate and biphasic versus uniphasic bolus on hepatic parenchymal enhancement at computed tomography (CT), 105 patients were randomized into four protocols. Hepatic and aortic enhancement curves were measured for each protocol. The optimal scanning interval (the duration between onset of a desired hepatic enhancement threshold and decline of enhancement below the threshold or onset of the equilibrium phase) and contrast enhancement index (CEI) (area under the hepatic enhancement curve above a desired threshold during the nonequilibrium phase) were the primary determinants of merit. Measurements were performed at hepatic enhancement thresholds between 10 and 60 HU. At most thresholds of hepatic enhancement, optimal scanning intervals were significantly longer and CEIs were significantly higher for the biphasic protocols than for the uniphasic protocols (P < .03). A biphasic injection with a high initial flow rate produces high peak contrast enhancement but delayed onset of equilibrium and thus provides a longer optimal scanning interval.

Adult↗

PubMed2006

[Ru(TAP)3]2+-photosensitized DNA cleavage studied by atomic force microscopy and gel electrophoresis: a comparative study.

Topological modifications of plasmid DNA adsorbed on a variety of surfaces were investigated by using atomic force microscopy (AFM). On mica modified with 3-aminopropyltriethoxysilane (APS) or poly-L-lysine, the interaction between the plasmid DNA and the surface "freezes" the plasmid DNA conformation deposited from solution, and the AFM images resemble the projection of the three-dimensional conformation of the plasmid DNA in solution. Modified mica with low concentrations of Mg(2+) leads to a decrease in the interaction strength between plasmid DNA and the substrate, and the AFM images reflect the relaxed or equilibrium conformation of the adsorbed plasmid DNA. Under these optimized deposition conditions, topological modifications of plasmid DNA were produced under irradiation in the presence of [Ru(TAP)(3)](2+) (TAP = 1,4,5,8-tetraazaphenanthrene), which is a non-intercalating complex, and were followed as a function of illumination time. The observed structural changes correlate well with the conversion of the supercoiled covalently closed circular form (ccc form) into the open circular form (oc form), induced by a single-strand photocleavage. The AFM results obtained after fine-tuning of the plasmid DNA-substrate interaction compare well with those observed from gel electrophoresis, indicating that under the appropriate deposition conditions, AFM is a reliable technique to investigate irradiation-induced topological changes in plasmid DNA.

DNA, Superhelical↗

PubMed2006

Binding of gefitinib, an inhibitor of epidermal growth factor receptor-tyrosine kinase, to plasma proteins and blood cells: in vitro and in cancer patients.

Gefitinib exhibits wide inter-subject pharmacokinetic variability which may contribute to differences in treatment outcome. Unbound drug concentrations are believed to be more relevant to pharmacological and toxicological responses than total drug. Thus it is desirable to determine gefitinib binding in plasma and factors affecting this process. An equilibrium dialysis method using 96-well microdialysis plates was optimized and validated for determining the fraction unbound (fu) gefitinib in human plasma. Gefitinib binding in plasma from four different species and isolated protein solutions as well as drug partitioning in human blood cells were investigated. Unbound gefitinib plasma concentrations were measured in 21 cancer patients receiving daily oral gefitinib 250 mg or 500 mg. It was found that gefitinib was extensively bound in human rat mouse and dog plasma with mean fu values of 3.4%, 3.8%, 5.1% and 6.0% respectively. In isolated protein solutions approximately 90% and 78% of gefitinib was bound to human serum albumin (HSA) (40 mg/dL) and alpha1-acid glycoprotein (AAG) (1.4 mg/dL) with binding constants of 1.85 x 10(4) M(-1) and 1.13 x 10(5) M(-1) respectively. In whole blood 2.8% of gefitinib existed as the free drug while 79.4% and 17.8% was bound to plasma proteins and blood cells respectively. In plasma from cancer patients fu at pre-treatment varied 2.4-fold (mean 3.4 +/- 0.6%; range 2.2-5.4%) and fu was constant over the 28-days of treatment (P > 0.05). Pre-treatment AAG concentration was negatively correlated with pre-treatment fu (R2 = 0.28, P = 0.01). In conclusion gefitinib is highly protein bound (approximately 97%) in human plasma. Variable AAG concentrations observed in cancer patients may affect gefitinib fu with implications for inter-subject variation in drug toxicity and response.

Animals↗

PubMed2005

Biodegradable dextran-based microspheres for delivery of anticancer drug mitomycin C.

The purpose of this work was to develop a biodegradable microsphere (MS) system for delivering mitomycin C (MMC). Various dextran-based MS systems were investigated for their loading and release characteristics, including nonionic MS, sulfopropyl dextran microspheres (SP-MS) with low or high cross-linking density, oxidized SP-MS (Ox-MS), and hydrophobically modified SP-MS. SP-MS were chemically modified by oxidation with sodium periodate or by reaction with anhydride. The chemical structure of modified SP-MS and MMC-loaded MS (MMC-MS) were examined using Fourier transform infrared (FTIR) and solid-state nuclear magnetic resonance (NMR) spectrophotometry. Drug release was conducted at 37 degrees C in aqueous solutions of 0.15 m phosphate buffer solution. The kinetics of drug absorption and release and the stability of MMC after loading and release were determined by spectrophotometry and high-performance liquid chromatography. Ionic SP-MS exhibited a higher drug-loading rate and capacity when compared to nonionic MS, while hydrophobically modified SP-MS showed an even greater loading capacity than SP-MS. These results suggest that both ionic complexation and hydrophobic interaction were important factors in MMC loading. The Ox-MS system demonstrated higher drug-loading capacity, more fractional drug release and a longer time to reach release equilibrium as compared to other investigated MS systems. Under optimized reaction and loading conditions, MMC released from Ox-MS was found to be unaltered. This work demonstrates that the Ox-MS system is a potentially useful system for the delivery of MMC.

Absorbable Implants↗

PubMed1998

Crosslinked dextran--a new capsule material for colon targeting of drugs.

In the work presented here we have studied the application of glutaraldehyde crosslinked dextran as a capsule material for colon-specific drug delivery. A reaction mixture containing dextran, MgCl2, glutaraldehyde and polyethyleneglycol 400 in water was applied onto molding pins of nylon producing capsule caps and bodies. The capsule materials were characterized by measuring of the mechanical strength in compression and equilibrium degree of swelling. Based on these results an optimal composition for the capsule material was selected. The dextran capsules were loaded with hydrocortisone and subsequently, drug release was studied. The release was found to be about 10% during the initial 3 h in a buffer solution. Over a period of 24 h the release was about 35%. However, when the dextran capsules were challenged with a dextranase solution, simulating the arrival of the drug delivery system to the colon, the capsules quickly broke and the drug was released as a dose dump. The study shows that the dextran capsules are promising candidates for providing a colon-specific drug delivery.

Capsules↗

PubMed1989

Evolutionary optimization of the catalytic effectiveness of an enzyme.

The kinetic and thermodynamic features of reactions catalyzed by present-day enzymes appear to be the consequence of the evolution of these proteins toward maximal catalytic effectiveness. These features are identified and analyzed (in detail for one substrate-one product enzymes) by using ideas that link the energetics of the reaction catalyzed by an enzyme to the maximization of its catalytic efficiency. A catalytically optimized enzyme will have a value for the "internal" equilibrium constant (Kint, the equilibrium constant between the substrates and the products of the enzyme when all are bound productively) that depends on how close to equilibrium the enzyme maintains its reaction in vivo. Two classes are apparent. For an enzyme that operates near equilibrium, the catalytic efficiency is sensitive to the value of Kint, and the optimum value of Kint is near unity. For an enzyme that operates far from equilibrium, the catalytic efficiency is less sensitive to the value of Kint, and Kint assumes a value that ensures that the rate of the chemical transformation is equal to the rate of product release. In each of these cases, the internal thermodynamics is "dynamically matched", where the concentrations of substrate- and product-containing complexes are equal at the steady state in vivo.

Biological Evolution↗

PubMed2005

Synthesis and characterization of monomeric, oligomeric, and polymeric aluminum 8-hydroxyquinolines.

We report the synthesis and characterization of monomeric, oligomeric, and polymeric aluminum 8-hydroxyquinolines. The new structures of aluminum quinolate are contrived for expanding the application of AlQ(3) in the area of solution process by modifying AlQ(3) structure for improving solution processibility and crystallization resistance. Oligomeric aluminum 8-hydroxyquinoline (OALQ) was obtained using methylaluminoxane (MAO) and 8-hydroxyquinoline (8-HQ). Polymeric aluminum 8-hydroxyquinoline (PALQ) consists of 8-HQ and a polymeric Al-O backbone, simply prepared by stoichiometrically reacting 8-hydroxyquinoline, pentaerythritol propoxylate, and triethyl aluminum in the presence of chloroform. The absorption and emission spectra of OALQ and PALQ bear a clear resemblance to those of AlQ(3), and the molecular orbitals of OALQ and PALQ are virtually identical to those of AlQ(3). In the SEM images of AlQ(3) and OALQ, cylindrical rods of >100 microm in length and 5-10 microm in diameter for AlQ(3) and 20-100 microm in length and 1-5 microm in diameter for OALQ were observed, respectively. The size of the cylindrical rods of OALQ decreased compared with that of AlQ(3). As for the image of PALQ, an amorphous phase with bulge spots (ca. 5 microm) was observed. These microscope data correspond well to the X-ray powder pattern results. The chemical shifts (31.1, 57.0 ppm) and peak broadness of (27)Al NMR of AlQ(3) and its DFT calculation results present that mer- and fac-AlQ(3) appear in equilibrium through pentacoordinated intermediates. With the combination of DFT optimization and NMR calculation, models of OALQ and PALQ, hexa-, penta-, and tetracoordinated structures, were proposed, which exist in polymeric Al-O backbone and with inter- and intracoordination of Al-O bonds.

Journal Article↗

PubMed1999

Moisture effects on protein-excipient interactions in spray-dried powders. Nature of destabilizing effects of sucrose.

The preparation of stable solid protein formulations presents significant challenges. Ultimately, the interactions between incorporated excipients and the pharmaceutical protein determine the formulation stability. In this study, moisture was utilized to probe the interactions between a model protein, trypsinogen, and sucrose in the solid state, following spray drying. Through investigation of the physical properties of the spray-dried formulations, we attempted to elucidate the mechanisms underlying the previously observed stabilizing and destabilizing effects of the carbohydrate during spray drying. Both dynamic and equilibrium moisture uptake studies indicated the presence of an optimal protein-sugar hydrogen bonding network. At low sucrose contents, a preferential protein-sucrose hydrogen bonding interaction was dominant, resulting in protein stabilization. However, at high carbohydrate concentrations, preferential sugar-sugar interactions prevailed, resulting in a phase separation within the formulation matrix. The preferential incorporation of the sucrose molecules in a sugar-rich phase reduced the actual amount of the carbohydrate available to interact with the protein and thereby decreased the number of effective protein-sucrose contacts. As a consequence, the protein could not be effectively protected during spray drying. We hypothesize that the observed phase separation at this sucrose concentration regime originates from its exclusion from the protein in solution before spray drying, further accompanied by preferential clustering of the sucrose molecules.

Calorimetry, Differential Scanning↗