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At least 1,135 records · Page 63Linked to original sources

PubMed2004

Operation of magnetically assisted fluidized beds in microgravity and variable gravity: experiment and theory.

The conversion of solid waste into useful resources in support of long duration manned missions in space presents serious technological challenges. Several technologies, including supercritical water oxidation, microwave powered combustion and fluidized bed incineration, have been tested for the conversion of solid waste. However, none of these technologies are compatible with microgravity or hypogravity operating conditions. In this paper, we present the gradient magnetically assisted fluidized bed (G-MAFB) as a promising operating platform for fluidized bed operations in the space environment. Our experimental and theoretical work has resulted in both the development of a theoretical model based on fundamental principles for the design of the G-MAFB, and also the practical implementation of the G-MAFB in the filtration and destruction of solid biomass waste particles from liquid streams.

Biomass↗

PubMed2004

Dynamic free-surface deformations in axisymmetric liquid bridges.

Thermocapillary convection in a differentially heated liquid bridge is investigated in a two-dimensional numerical study. The deformable free surface is obtained as a solution of the coupled transport equations at fixed unit Prandtl number and aspect ratio. Only steady convection can be realized in this axisymmetric computation with either non-deformable or deformable surfaces. Dynamic free-surface deformations do not induce transitions to oscillatory convection even at large Reynolds numbers. Free surfaces are convex near the cold wall due to large surface pressures at the stagnation-point. Near the hot wall they change from concave to convex with increasing Re due to large normal viscous stresses. Large Capillary numbers cause additional surface ripples there while having little effect on the interior. Heat loss from the free surface has dramatic influence on the dynamics and must be included in theoretical models of thermocapillary instabilities.

Capillary Action↗

PubMed2005

Differential regulation of proteins by bursting calcium oscillations--a theoretical study.

Calcium in ionic form is a second messenger connecting several input signals to several target processes in the cell. The question arises how one second messenger can transmit more than one signal simultaneously (bow-tie structure of signalling). Experimental data on calcium dynamics often show patterns of successive low-peak and high-peak oscillatory phases, known as bursting. Here, we propose that bursting calcium oscillations can perform the function of simultaneous transmission of two signals at physiological calcium concentrations, for example, by selective activation of two calcium-binding proteins. This differential regulation by periodic bursting is investigated in a theoretical model. The two proteins are assumed to be activated by calcium, and one of them is assumed to be subject to biphasic regulation due to additional inhibitory binding sites. To explore which characteristics of the complex signal could be responsible for independent regulation of low-peak activated and spike activated targets, different bursting patterns of simplified square pulses are applied. Depending on the change in the bursting pattern, one protein can be gradually activated at a constant level of the other protein's activity, or the two proteins can be activated simultaneously, or one protein can be activated while the other one is deactivated simultaneously. Thus, the two proteins can be regulated virtually independently.

Animals↗

PubMed2005

Binding of chemicals to melanins re-examined: adsorption of some drugs to the surface of melanin particles.

This work presents a first attempt to study the interaction of some drugs with melanins, realistically considered as solid aggregates of primary particles. This situation appears similar to the adsorption of organic molecules onto the surface of colloidal absorbers, as active carbon, zeolites or titanium dioxide. We have applied some of the most popular theoretical models used in technological applications with the aim to give a more realistic picture of the melanin-drug interaction responsible for some observed side effects in vivo. Moreover, this approach can simplify the problem of the search of the physical parameters dominating the binding processes, by reducing the phenomenon to a simple physisorption/chemisorption, at least in a first approximation. We have studied the binding to melanin of gentamicin, methotrexate and chlorpromazine, molecules with different physico-chemical and structural characteristics. Our study demonstrates the possibility to fit experimental adsorption data with Langmuir, Freundlich, Tempkin and Dubinin-Radushkevich equations. In such a way we obtain binding parameters useful to characterize the drug-surface interaction in terms of energy and of mean affinity.

Adsorption↗

PubMed2006

Can the theory of gradient liquid chromatography be useful in solving practical problems?

Advances in the theory of gradient liquid chromatography and their practical impacts are reviewed. Theoretical models describing retention in reversed-phase, normal-phase and ion-exchange modes are compared. Main attention is focused on practically useful models described by two- or three-parameter equations fitting the experimental data in the range of mobile phase composition utilized for sample migration during gradient elution. The applications of theory for gradient method development, optimization and transfer are addressed. The origins and possibilities for overcoming possible pitfalls are discussed, including the effects of the instrumental dwell volume, uptake of mobile phase components on the column and size of the sample molecules. Special attention is focused on gradient separations of large molecules.

Biopolymers↗

PubMed2006

Onset of non-linearity in zonal elution separators: the concept of effective analyte concentration.

When an analyte injected in a zonal separation method (chromatography, capillary zone electrophoresis, field-flow fractionation) is not highly diluted in the carrier fluid, the retention ratio, R--or ratio of the cross-sectional average migration velocity of the analyte to that of the carrier fluid--depends on the local concentration, c, of the center of mass of the analyte zone, and the zone migration occurs in non-linear conditions. Because the zone broadens as it moves along the separator, R varies continuously from the inlet to the outlet of the separator. That concentration, c(eff), for which R(c(eff)) is equal to the length-averaged apparent retention ratio, R(app), is called effective concentration, and that distance, z(eff), from the separator inlet, for which c(z(eff)) is equal to c(eff), i.e. for which R(z(eff)) is equal to R(app), is called effective position. Assuming that near the onset the non-linear behavior, R(c), is a linear function, values of R(app), c(eff) and z(eff) have been computed in a wide range of operating conditions which are typical of situations encountered in capillary zone electrophoresis, liquid chromatography, or field-flow fractionation. Computations have been performed both in presence and in absence of the dispersion arising from the concentration dependence of the analyte migration rate (called thermodynamic dispersion in chromatography or electromigration dispersion in capillary zone electrophoresis). It is found that, whatever the range of analyte concentration covered from inlet to outlet of the separator, c(eff) is always close to two times the analyte concentration, c(out), at the outlet of the separator, and z(eff) between one-fourth and one-third of the separator length. As c(out) is easily determined from the peak recorded by a concentration-sensitive detector, a simple pragmatic expression is given for the estimation of c(eff). This effective concentration is the appropriate concentration to be used for comparing predictions of theoretical models of R(c) with experimental retention data. This is of particular interest for validating such models in field-flow fractionation.

Chemical Fractionation↗

PubMed2006

Marangoni instability and spontaneous non-linear oscillations produced at liquid interfaces by surfactant transfer.

The systems producing non-linear spontaneous oscillations of the interfacial tension and electric potential are considered and the available criteria for development of convective instability by the surfactant transfer through a liquid interface are discussed. The non-linear oscillations are observed by the surfactant transfer from a point-like source situated in the bulk of liquid, by the transfer of two ionic solutes through a liquid interface in two opposite directions, and by the transfer of ionic solutes through a liquid membrane. All these systems are governed by more complicated mechanisms than merely arising oscillatory convective instability. The main experimental results obtained for these three systems as well as theoretical models proposed for their explanation are discussed.

Chemistry, Physical↗

PubMed2006

Surface dilational rheology of mixed adsorption layers at liquid interfaces.

The dilational rheology for mixed solutions of surface active compounds is easy to measure with existing methods, however, there are very few attempts only to analyse the resulting data on the basis of a theoretical model. Models by Jiang et al., Joos, and by Lucassen and van den Tempel (for single compounds) are compared here and applied to experimental data. The mentioned models converge to each other after some small modifications and represent a very good scientific background for studies of the mechanical behaviour of mixed adsorption layers. As example, the experimental results for mixed solutions containing a globular protein (beta-lactoglobulin) and a non-ionic surfactant (alkyl dimethyl phosphine oxide) are analysed by the obtained dependencies, using some new proposed numerical procedures.

Adsorption↗

PubMed2005

Delay of intracortical bone remodelling following a stress change: a theoretical and experimental study.

BACKGROUND: A theoretical model and an experimental setup were specifically designed to identify and determine the delay of the cortical bone response (restricted to mineralization and demineralization) to a stress change. METHODS: The in vivo experiment considered two groups of rats: a running group and a control sedentary group. The running group rats were compelled to a running activity for 15 weeks, followed by a sedentary activity for 15 weeks. Bone density was derived from hardness measurements. The parameters of the remodelling theory, including the response delay and the remodelling rates, were determined from these experimental measurements. FINDINGS: Bone density increased significantly during the activity period, and decreased rapidly when rats returned to sedentary state. The identification of the model's parameters produced evolution curves that were within the limits of the standard deviation of the experimental data. The densification rate was lower than the resorption rate, and the densification delay was greater than bone resorption delay. INTERPRETATION: The delays determined with this macroscopic model are related to response delays due to biological internal processes in bone.

Adaptation, Physiological↗

PubMed2005

Theoretical analysis of adhering lipid vesicles with free edges.

A theoretical model for describing the adhesion of lipid vesicle with free edges is developed. For adhesion in contact potential or in finite-range potential, the total energy functional is defined as the sum of elastic free energy, the surface energy, the line tension energy and the contact potential or the long-ranged potential. The equilibrium differential equation and boundary conditions for opening-up lipid vesicles are derived through minimizing the total energy functional. Numerical solutions to these equations are obtained under the axial symmetric condition. These numerical solutions can be used to qualitatively explain the influence of the substrate on the open-up lipid vesicles.

Lipids↗

PubMed2005

Increased risk for coronary vascular disease in children with developmental coordination disorder.

PURPOSE: Physical activity (PA) is compromised in children and adolescents with Developmental Coordination Disorder (DCD). The purpose of this study was to test a theoretical model linking DCD with 2 factors associated with increased risk for coronary vascular disease: (1) cardio-respiratory fitness (CF), and (2) relative body fat (BF), through physical activity. METHODS: A cross-sectional design was implemented using 571 elementary school students (313 males, 258 females). Evaluation of body fat using bioelectric impedance, cardio-respiratory fitness using Léger 20-meter Shuttle Run, and physical activity level was determined using the Participation Questionnaire. The short-form version Bruininks-Oseretsky Test of Motor Proficiency was used to verify DCD. We tested to see how much of the association between DCD and factors associated with increased risk for coronary vascular disease (percentage body fat and cardio-respiratory fitness) using multivariate ordinary least squares regression analysis. RESULTS: Regression modeling demonstrated that DCD was associated with increased body fat and low cardio-respiratory fitness. Physical activity was a significant mediator in the DCD-CF relationship. Physical activity was also a mediating factor in the DCD-BF relationship, albeit to a lesser extent. CONCLUSIONS: Developmental coordination disorder is related with factors associated with increased risk for coronary vascular disease, including decreased cardio-respiratory fitness and increased body fat through the mediating influence of physical activity in children.

Adipose Tissue↗

PubMed2004

3-D transient electrophoretic motion of a spherical particle in a T-shaped rectangular microchannel.

This paper considers the electrophoretic motion of a spherical particle in an aqueous electrolyte solution in a T-shaped rectangular microchannel, where the size of the channel is close to that of the particle. This is a complicated transient process where the electric field, the flow field, and the particle motion are coupled together. A theoretical model was developed to investigate the influences of the applied electric potentials, the zeta potentials of the channel and the particle, and the size of the particle on the particle motion. A direct numerical simulation method using the finite element method is employed. This method employs a generalized Galerkin finite element formulation that incorporates both equations of the fluid flow and equations of the particle motion into a single variational equation where the hydrodynamic interactions are eliminated. The ALE method is used to track the surface of the particle at each time step. The numerical results show that the electric field in the T-shaped microchannel is influenced by the presence of the particle, and that the particle motion is influenced by the applied electric potentials and the zeta potentials of the channel and the particle. The path of the particle motion is dominated by the local electric field and the ratio of the zeta potential of the channel to that of the particle. The particle's velocity is also dependent on its size in a small channel.

Electrophoresis↗

PubMed2005

Anticipated socio-environmental impacts of the proposed Little Darby National Wildlife Refuge in Ohio.

Data were collected during the summer of 2000 from 146 residents living in a rural area located in central Ohio that had been identified as a possible site for a national wildlife refuge. The purpose of the study was to assess anticipated socio-economic and environmental impacts of the proposed refuge on local people and natural ecosystems. A 'vested interests' theoretical model was developed to guide the investigation. The theoretical model posits that attitudes of local people toward externally induced projects are a function of costs and benefits individuals expect to receive as a result of the development action. The theory posits that individuals who expect to receive greater benefits from the refuge will exhibit more favorable attitudes toward the proposed project. Anticipated impacts of the proposed refuge were assessed with a Likert-type attitude scale that was shown to be highly reliable. Variables measuring specific costs and benefits associated with the proposed project were used as predictive variables. Descriptive statistics demonstrated that respondents believed the proposed wildlife refuge would generate more costs than benefits for local residents. Multiple linear regression findings demonstrated that six variables were significant in explaining 65.1% of the variance in the attitudes toward the proposed project. The significant variables were as follows: perceived costs and benefits associated with the proposed project, perceived water quality within the Little Darby Creek at the time of the data collection, age of primary income earner, expected future water quality within the Little Darby Creek, expected impact of farmland transition on the household, and attitude toward private property rights. Study findings consistently support research hypotheses.

Adult↗

PubMedDate not supplied

The effect of lower limb length on the energetic cost of locomotion: implications for fossil hominins.

The consequences of the relatively short lower limbs characteristic of AL 288-1 have been widely discussed, as have the causes and consequences of the short limbs of Neanderthals. Previous studies of the effect of limb length on the energetic cost of locomotion have reported no relationship; however, limb length could have accounted for as much as 19% of the variation in cost and gone undetected (Steudel and Beattie, 1995; Steudel, 1994, 1996). Kramer (1999) and Kramer and Eck (2000) have recently used a theoretical model to predict the effect of the shorter limbs of early hominids, concluding that the shorter limbs may actually have been energetically advantageous. Here, we took an experimental approach. Twenty-one human subjects, of varying limb lengths, walked on a treadmill at 2.6, 2.8, 3.0 and 3.2m.p.h., while their expired gases were analyzed. The subjects walked for 12 minutes at each speed and their rates of oxygen consumption (VO2) over four minutes were averaged to estimate VO2. We also measured each subject's height, weight and lower limb length. Lean body mass and % fat were determined using dual-energy x-ray absorptiometry. ANCOVA with total VO2 at either speed as the dependent variable and total lean mass, % fat and lower limb length as covariates resulted in all three covariates having a significant positive effect on VO2 at p<0.01. Subjects with relatively longer lower limbs had lower locomotor costs. Thus the short lower limbs characteristic of some hominid taxa would have resulted in more costly locomotion, barring some physiological anomaly. The magnitude of this effect is substantial; Neanderthals are estimated to have had locomotor costs 30% greater than those of contemporary anatomically modern humans. By contrast the increase in lower limb length seen in H. erectus would have mitigated the increase in locomotor costs produced by the increase in body size.

Adolescent↗

PubMed2004

Identification of copper(II) binding sites in actinomycin D, a cytostatic drug--correlation of coordination with DNA damage.

Actinomycin D (AD) is a potent anticancer drug widely applied in therapy, which however exhibits very high toxicity in humans. As the character of donors present in the AD molecule seems to be very favorable for Cu(II) ions, we undertook the coordination study on the Cu(II)-AD system. Potentiometric experiments proved a formation of very stable complexes and with the use of spectroscopic methods the identification of the binding sites was made. The values of potential energy minima, provided by theoretical modeling, confirmed the feasibility of formation of the complexes in water solution. We also demonstrated a significant effect of Cu(II) ions on AD interactions with DNA. The strand-nicking activity was observed. This process could be correlated with the speciation of complex forms. We also found out that in the presence of H2O2, low levels of Cu(II)-AD complexes induce the formation of considerable amounts of linearised plasmid. In consequence, the hypothesis is proposed that the physiologically available cupric ions may participate in the drug-induced toxic effects.

Antibiotics, Antineoplastic↗

PubMed2004

Commitment and nucleation in the protein G transition state.

An accurate characterization of the transition state ensemble (TSE) is central to furthering our understanding of the protein folding reaction. We have extensively tested a recently reported method for studying a protein's TSE, utilizing phi-value data from protein engineering experiments and computational studies as restraints in all-atom Monte Carlo (MC) simulations. The validity of interpreting experimental phi-values as the fraction of native contacts made by a residue in the TSE was explored, revealing that this definition is unable to uniquely specify a TSE. The identification of protein G's second hairpin, in both pre and post-transition conformations demonstrates that high experimental phi-values do not guarantee a residue's importance in the TSE. An analysis of simulations based on structures restrained by experimental phi-values is necessary to yield this result, which is not obvious from a simplistic interpretation of individual phi-values. The TSE that we obtain corresponds to a single, specific nucleation event, characterized by six residues common to all three observed, convergent folding pathways. The same specific nucleus was independently identified from computational and experimental data, and "Conservation of Conservation" analysis in the protein G fold. When associated strictly with complete nucleus formation and concomitant chain collapse, folding is a well-defined two state event. Once the nucleus has formed, the folding reaction enters a slow relaxation process associated with side-chain packing and small, local backbone rearrangements. A detailed analysis of phi-values and their relationship to the transition state ensemble allows us to construct a unified theoretical model of protein G folding.

Models, Molecular↗

PubMed2004

Substantial energetic improvement with minimal structural perturbation in a high affinity mutant antibody.

Here, we compare an antibody with the highest known engineered affinity (K(d)=270 fM) to its high affinity wild-type (K(d)=700 pM) through thermodynamic, kinetic, structural, and theoretical analyses. The 4M5.3 anti-fluorescein single chain antibody fragment (scFv) contains 14 mutations from the wild-type 4-4-20 scFv and has a 1800-fold increase in fluorescein-binding affinity. The dissociation rate is approximately 16,000 times slower in the mutant; however, this substantial improvement is offset somewhat by the association rate, which is ninefold slower in the mutant. Enthalpic contributions to binding were found by calorimetry to predominate in the differential binding free energy. The crystal structure of the 4M5.3 mutant complexed with antigen was solved to 1.5A resolution and compared with a previously solved structure of an antigen-bound 4-4-20 Fab fragment. Strikingly, the structural comparison shows little difference between the two scFv molecules (backbone RMSD of 0.6A), despite the large difference in affinity. Shape complementarity exhibits a small improvement between the variable light chain and variable heavy chain domains within the antibody, but no significant improvement in shape complementarity of the antibody with the antigen is observed in the mutant over the wild-type. Theoretical modeling calculations show electrostatic contributions to binding account for -1.2 kcal/mol to -3.5 kcal/mol of the binding free energy change, of which -1.1 kcal/mol is directly associated with the mutated residue side-chains. The electrostatic analysis reveals several mechanistic explanations for a portion of the improvement. Collectively, these data provide an example where very high binding affinity is achieved through the cumulative effect of many small structural alterations.

Amino Acid Sequence↗

PubMed2005

Subcellular positioning of F plasmid mediated by dynamic localization of SopA and SopB.

SopA, SopB proteins and the cis-acting sopC DNA region of F plasmid are essential for partitioning of the plasmid, ensuring proper subcellular positioning of the plasmid DNA molecules. We have analyzed by immunofluorescence microscopy the subcellular localization of SopA and SopB. The majority of SopB molecules formed foci, which localized frequently with F plasmid DNA molecules. The foci increased in number in proportion to the cell length. Interestingly, beside the foci formation, SopB formed a spiral structure that was dependent on SopA, which also formed a spiral structure, independent of the presence of SopB, and these two structures partially overlapped. On the basis of these results and previous biochemical studies together with our simulations, we propose a theoretical model named "the reaction-diffusion partitioning model", using reaction-diffusion equations that explain the dynamic subcellular localization of SopA and SopB proteins and the subcellular positioning of F plasmid. We hypothesized that sister copies of plasmid DNA compete with each other for sites at which SopB multimer is at the optimum concentration. The plasmid incompatibility mediated by the Sop system might be explained clearly by this hypothesis.

Animals↗