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At least 703 records · Page 39Linked to original sources
PubMed2006

Kinetic and equilibrium studies of methylene blue biosorption by Posidonia oceanica (L.) fibres.

Batch biosorption experiments were carried out for the removal of methylene blue, a basic dye, from aqueous solution using raw Posidonia oceanica (L.) fibres, a marine lignocellulosic biomass. A series of assays were undertaken to assess the effect of the system variables, i.e. contact time, solution pH, biosorbent dosage and initial dye concentration. The results had showed that biosorption capacity was optimal using 6-9 solution pH range and by increasing the biosorbent concentration up to 1 g/L. The biosorption kinetics were analyzed using irreversible-first-order, reversible-first-order and pseudo-second-order and the sorption data were very well described by the pseudo-second-order model for the entire adsorption time with squared correlation coefficients equal to unity for all experimented initial dye concentrations. Besides, equilibrium data were very well represented by both Langmuir and Redlich-Peterson isotherm models followed by Freundlich, which confirm the monolayer coverage of methylene blue molecules onto P. oceanica fibres.

Adsorption↗
PubMed2005

Information asymmetry, insurance, and the decision to hospitalize.

We analyze the problem of second-best optimal health insurance in the context of a model in which patients and doctors must decide not only on an aggregate quantity of health services to use in treating various kinds of illness, but also have a choice between different kinds of providers (in particular, outpatient services rendered by primary-care physicians or inpatient services provided by hospital-based specialists). We consider well-informed patients' choices of provider when they have conventional insurance so they only pay part of the cost of their health services, as well as the equilibrium strategies of doctors and patients when there is patient-provider asymmetry; in the latter case we also analyze a managed-care insurance setup under which doctors are paid by capitation. We find that under certain plausible conditions, second-best optimal managed-care plans with supply-side incentives dominate second-best optimal conventional plans that rely on cost control through demand-side cost sharing.

Decision Making↗
PubMed1999

Enzymatic synthesis of unsaturated fatty acid glucoside esters for dermo-cosmetic applications.

Unsaturated fatty acid alpha-butylglucoside esters were prepared by enzymatic esterification of alpha-butylglucoside in nonaqueous media. Conditions were firstly optimized using oleic acid as acyl group. Synthesis was possible in several solvents but the presence of water co-product in the medium limited the reaction to a thermodynamic equilibrium corresponding to a maximal conversion yield of 62%. In pure molten substrates, the removal of water under reduced pressure enabled yields superior to 95% to be obtained. Product profiles depended on enzyme origin : whatever the support, immobilized lipase B from Candida antarctica proved to be far more regioselective for the primary hydroxyl group of glucose than immobilized lipase from Rhizomucor miehei. Results obtained could be easily transposed to the acylation of alpha-butylglucoside with a commercial mixture of unsaturated fatty acids containing more than 60% of linoleic acid. The biocatalyst could be recycled more than ten times without any significant activity loss.

Acylation↗
PubMed1995

The use of the STRs HUMTH01, HUMVWA31/A, HUMF13A1, HUMFES/FPS, HUMLPL in forensic application: validation studies and population data for Galicia (NW Spain).

The 5 tetranucleotide short tandem repeats, HUMTH01, HUMVWA31/A, HUMF13A1, HUMFES/FPS and HUMLPL were studied using different electrophoretic methods and PCR amplification conditions in order to optimize the typing conditions. A genetic population study in the population of Galicia was carried out and the allele and genotype frequencies are given. Compliance with the Hardy-Weinberg equilibrium was tested using different statistical parameters, with clear advantages resulting in favor of using the exact test (Guo-Thompson method) instead of conventional chi-square methods. Some statistical parameters of forensic interest (PD, CE, h) were also calculated. There were no mutations found in a total of 73 paternal meioses and 101 maternal meioses. Abnormal electrophoretic mobility was found in the AT-rich STR HUMF13A1 under non-denaturing conditions and, therefore, the use of denaturing conditions is absolutely necessary. No "stutter" bands were found, although double peaks in the HUMFES/FPS system were observed in some samples. The advantage of using automated sequencers with fluorescent technology is also reported.

Base Sequence↗
PubMed1994

Prediction of bone adaptation using damage accumulation.

The adaptation of bones to a change in function has been recognised for many centuries, but only recently have mathematical laws been proposed to describe it. One proposed mathematical law is based on the hypothesis that, after a change in load, the strain in the bone microstructure is regulated to a homeostatic equilibrium. Strain-adaptive remodelling has been used successfully to simulate bone adaptation around orthopaedic implants but the predictive capabilities are constrained because many empirical constants are required in the remodelling law (a reference stimulus, a zone of equilibrium stresses or 'lazy zone' and a parameter transducing macroscopic stresses to a tissue level stimulus). An alternative approach has been proposed. It is that bone adapts to attain an optimal strength by regulating the damage generated in its microstructural elements. The question is raised whether or not a mathematical law to predict the time course of bone adaptation can be derived for damage-adaptive remodelling in a similar way to the mathematical laws based on a strain stimulus. In the present study, the hypotheses required to develop damage-adaptive remodelling laws are proposed and a remodelling law to predict the time course of bone adaptation is derived. It is shown that this is an integral remodelling law which accounts naturally for the stress history to which the tissue has been exposed since formation. A simulation of the adaptive response of a bone diaphysis under a change in torsional load shows that the law gives physically reasonable predictions. The initial remodelling prediction is similar to strain-adaptive remodelling. However, in the later stages of remodelling, the predictions differ from strain-adaptive remodelling in that direct convergence to a homeostatic strain is not predicted. Instead, undershoot (in the case of a reduction in load) and overshoot (in the case of an increase in load) are predicted.

Adaptation, Physiological↗
PubMed2006

Incorporation of a monolithic column into sequential injection system for drug-protein binding studies.

A sequential injection analysis (SIA) manifold was incorporated with a monolithic strong anion-exchanger disk for on-line drug-protein interaction studies. The antibiotic ciprofloxacin (CF) was selected as a model drug compound. The separation principle was based on the strong retention of bovine serum albumin (BSA) on the monolithic strong anion-exchanger and the liberation/release of the free form of the drug. Elution of the retained BSA was easily achieved by delivering a different mobile phase via the SIA manifold. The type of functional group of the monolithic support, the breakthrough volume and the injected volumes of CF and BSA were studied and optimized. The influence of the variation of incubation time was studied in on-line binding assays. Scatchard plot was employed to obtain the number of binding sites and the equilibrium binding constants. For the off-line study of the CF-BSA binding, two binding classes were determined with constants of (3.16+/-0.21)x10(6)M(-1) and (1.27+/-0.48)x10(4)M(-1) and 6.1+/-1.3 and 17.8+/-3.9 binding sites per class, respectively. In non-equilibrium binding experiments the binding rate constant was k(1)=785 M(-1)min(-1). All measurements were monitored with fluorescence (lambda(ext)=300 nm, lambda(em)=460 nm) and spectrophotometric detection (lambda=280 nm). To evaluate the accuracy of the developed method the obtained results were compared versus ultrafiltration experiments and were found in good agreement.

Anion Exchange Resins↗
PubMed2006

SNP genotyping by multiplex amplification and microarrays assay for forensic application.

OBJECTIVE: Research on the application feasibility of SNP genotyping for forensic identification by microarrays. METHODS: Oligonucleotide microarrays which could detect 34 different SNPs were used. After hybridization and washing, the arrays were scanned and fluorescence intensities analyzed using Microarray software. Population studies on 34 SNP loci were carried out in a sample of 109 unrelated Chinese Han individuals using oligonucleotide microarrays for genotype detection. The method was also applied to cases. RESULTS: According to the results of population studies, no deviations from Hardy-Weinberg equilibrium could be found. Among the 34 loci, 3 SNPs were low informative, 4 were medium informative and 27 were high informative. The combination discrimination power (CDP) of the 31 optimal polymorphic SNPs was 0.9999999999979. The matching probability was 2.13 x 10(-12). The average exclusion probability in paternity testing for duos was 0.9609. The average exclusion probability in paternity testing for trios was 0.9970. CONCLUSION: The data and case application demonstrated that SNP typing by oligonucleotide probe microarrays was a useful technique for paternity testing and individual identification. Combined with the 28 SNPs loci distributed on HLA-DRB1 and ABO genes, the combination discrimination power (CDP) was 0.9999999999999910. The matching probability was 9.02 x 10(-15). The average exclusion probabilities in duos and in trios were 0.9894 and 0.9992, respectively. It may be concluded that the 59 SNPs loci yield the same power in forensic identification as CODIS STRs currently used.

China↗
PubMed2002

Preparative liquid chromatography.

The status of the theory and the main methods of implementation of preparative liquid chromatography are reviewed. On the theory front, the focus has recently shifted. The theory of non-linear, non-ideal chromatography has given rise to numerous models whose advantages, disadvantages and ranges of application are now well understood. Interest now resides in investigating the equilibrium thermodynamics of complex new systems, in the study of the kinetics of mass transfers in conventional chromatographic systems, and in the application of the various models of chromatography to optimize the experimental conditions. Progress in computer technology allows the use of sophisticated models, provided their parameters can be measured. This allows the detailed investigation of separations for which the mass transfer kinetics is slow such as chiral separations, the purification of basic compounds, and the extraction of recombinant proteins. On the applied front, in addition to numerous incremental improvements in reliability and economic performance, a few essential new features should be noted, i.e. the availability of instruments for simulated moving bed separations at the scale needed for preparative chiral separations, the use of expanded beds for the extraction of recombinant proteins from fermentation broths, and the attention given to improvements in the performance of packed beds. A survey of the literature dealing with practical applications and recent meetings shows that preparative chromatography is becoming a well established separation and purification method in the pharmaceutical industry.

Chromatography, Liquid↗
PubMed1995

Binding and interconversion of tetrahydrofolates at a single site in the bifunctional methylenetetrahydrofolate dehydrogenase/cyclohydrolase.

The bifunctional dehydrogenase/cyclohydrolase domain of the human NADP-dependent trifunctional methyleneH4folate dehydrogenase/methenylH4folate cyclohydrolase/formylH4folate synthetase (H4folate = tetrahydrofolate) catalyzes two sequential reactions involved in the interconversion of H4folate derivatives. We have established by equilibrium dialysis that a single H4folate-binding site exists per monomer of the dimeric domain and that the presence of nucleotides has two unexpected effects on H4folate substrate binding. Nucleotides containing a 5'-phosphate cause positive cooperativity in the binding of methyleneH4folate but not of 10-formylH4folate, and NADP increases the affinity for 10-formylH4folate by a factor of 25. The results indicate that dinucleotide preferentially binds before 10-formylH4folate in the reverse cyclohydrolase reaction, and this mechanism increases the efficiency of conversion of 10-formylH4folate to methyleneH4folate. We report new kinetic data that are also consistent with a steady-state random mechanism for this enzyme. To assess whether the enzyme functions at equilibrium in vivo, we determined the overall chemical equilibrium constant of Keq = 16 for ([10- formylH4folate][NADPH])/([methyleneH4folate][NADP]). Using this value and reported ratios of free dinucleotides and folate derivatives in vivo, we estimate that the cytosolic dehydrogenase/cyclohydrolase reactions exist near the equilibrium position. However, the NAD-dependent dehydrogenase/cyclohydrolase reactions in mitochondria are far from equilibrium and are poised toward 10-formylH4folate synthesis. The results of the binding and kinetic studies indicate that the bifunctional nature of the methyleneH4folate dehydrogenase/methenylH4folate cyclohdrolase domain is designed to optimize the overall reverse reactions in vivo.

Aminohydrolases↗
PubMed2001

Relationship between excitation energy transfer, trapping, and antenna size in photosystem II.

We present a systematic study of the effect of antenna size on energy transfer and trapping in photosystem II. Time-resolved fluorescence experiments have been used to probe a range of particles isolated from both higher plants and the cyanobacterium Synechocystis 6803. The isolated reaction center dynamics are represented by a quasi-phenomenological model that fits the extensive time-resolved data from photosystem II reaction centers and reaction center mutants. This representation of the photosystem II "trapping engine" is found to correctly predict the extent of, and time scale for, charge separation in a range of photosystem II particles of varying antenna size (8-250 chlorins). This work shows that the presence of the shallow trap and slow charge separation kinetics, observed in isolated D1/D2/cyt b559 reaction centers, are indeed retained in larger particles and that these properties are reflected in the trapping dynamics of all larger photosystem II preparations. A shallow equilibrium between the antennae and reaction center in photosystem II will certainly facilitate regulation via nonphotochemical quenching, and one possible interpretation of these findings is therefore that photosystem II is optimized for regulation rather than for efficiency.

Chlorophyll↗
PubMed2006

Aggregation of polyalanine in a hydrophobic environment.

The dimerization of polyalanine peptides in a hydrophobic environment was explored using replica exchange molecular dynamics simulations. A nonpolar solvent (cyclohexane) was used to mimic, among other hydrophobic environments, the hydrophobic interior of a membrane in which the peptides are fully embedded. Our simulations reveal that while the polyalanine monomer preferentially adopts a beta-hairpin conformation, dimeric phases exist in an equilibrium between random coil, alpha-helical, beta-sheet, and beta-hairpin states. A thermodynamic characterization of the dimeric phases reveals that electric dipole-dipole interactions and optimal side-chain packing stabilize alpha-helical conformations, while hydrogen bond interactions favor beta-sheet conformations. Possible pathways leading to the formation of alpha-helical and beta-sheet dimers are discussed.

Alanine↗
PubMed2005

Graphene nanostructures as tunable storage media for molecular hydrogen.

Many methods have been proposed for efficient storage of molecular hydrogen for fuel cell applications. However, despite intense research efforts, the twin U.S. Department of Energy goals of 6.5% mass ratio and 62 kg/m3 volume density has not been achieved either experimentally or via theoretical simulations on reversible model systems. Carbon-based materials, such as carbon nanotubes, have always been regarded as the most attractive physisorption substrates for the storage of hydrogen. Theoretical studies on various model graphitic systems, however, failed to reach the elusive goal. Here, we show that insufficiently accurate carbon-H2 interaction potentials, together with the neglect and incomplete treatment of the quantum effects in previous theoretical investigations, led to misleading conclusions for the absorption capacity. A proper account of the contribution of quantum effects to the free energy and the equilibrium constant for hydrogen adsorption suggest that the U.S. Department of Energy specification can be approached in a graphite-based physisorption system. The theoretical prediction can be realized by optimizing the structures of nano-graphite platelets (graphene), which are light-weight, cheap, chemically inert, and environmentally benign.

Journal Article↗
PubMed2002

Mutagenesis of prochlorothrix plastocyanin reveals additional features in photosystem I interactions.

Three surface residues of plastocyanin from Prochlorothrix hollandica have been modified by site-directed mutagenesis. Changes have been made in methionine 33, located in the hydrophobic patch of the copper protein, and in arginine 86 and proline 53, both located in the eastern hydrophilic area. The reactivity toward photosystem I of single mutants M33N, P53A, P53E, R86Q, R86E, and the double mutant M33N/P14L has been studied by laser flash absorption spectroscopy. All the mutations yield increased reactivity of plastocyanin toward photosystem I as compared with wild type plastocyanin, thus indicating that in Prochlorothrix electron donation to photosystem I is not optimized. The most drastic increases in the intracomplex electron transfer rate are obtained with mutants in methionine 33, whereas replacing arginine 86 only modestly affects the plastocyanin-photosystem I equilibrium constant for complex formation. Mutations at position 53 also promote major changes in the association of plastocyanin with photosystem I, yielding a change from a mechanism involving complex formation to a simpler collisional interaction. Molecular dynamics calculations indicate that mutations at position 33 promote changes in the H-bond network around the copper center. The comparative kinetic analysis of the reactivity of Prochlorothrix plastocyanin mutants toward photosystem I from other cyanobacteria reveals that mutations M33N, P53A, and P53E result in enhanced general reactivity.

Base Sequence↗
PubMed1984

Pleiotropic overdominance and the maintenance of genetic variation in polygenic characters.

A model of selection is described in which optimizing phenotypic selection is combined with pleiotropic overdominance. Thus, the role that mutation commonly plays in models of phenotypic evolution is replaced by balancing selection. Expressions are provided for the equilibrium genetic variance in phenotype and for the heterozygosity. An approximate analysis of the transient properties of the model shows that, in certain circumstances, the behavior is quite similar to that of models based on the interaction of mutation and selection.

Genes, Dominant↗
PubMed1980

Hydroxyl-ion-induced subunit dissociation of east cytoplasmic pyruvate decarboxylase. A circular dichroism study.

Cytoplasmic pyruvate decarboxylase (EC 4.1.1.1, from Saccharomyces carlsbergensis) exhibits in its circular dichroic spectrum in the 250--320-nm range a multiple two-signal band. This couplet disappears on increasing the pH up to pH 8.5. Two classes of two protons each can be quantified by these spectral changes. The first class dissociates rapidly and the apparent pK is 7.84. The thermodynamic data are delta G = 87.7 kJ mol-1, delta H = + 56.0 kJ mol-1, delta S = - 108 J mol-1 K-1, very characteristic for the deprotonation of an amino-acid side chain. The second class of the protons has the following thermodynamic data: delta G = 88.3 kJ mol-1, delta H = - 64.3 kJ mol-1, delta S = - 520 J mol-1 K-1 which, in conjunction with kinetic reasoning and in view of enzyme stoichiometry and symmetry, suggests a conformational equilibrium exposing the second two protons. Th enzyme dissociates into two dimeric subunits. This dissociation step is considered to be rate-determining for the overall process. The data are kp = 1.4 . 10(-3), delta H not equal to = + 128.3 kJ mol-1, delta S not equal = + 136 J mol-1 K-1. If there is a conformational equilibrium, the rate constant of product formation kp will be modified by a factor beta = kc/(1 + Kc) (0 < beta less than or equal to 1) where Kc is the conformational equilibrium constant. The subunit dissociation appears to be controlled by the enthalpy of activation indicating that a number of interactions, i.e. ionic, hydrogen and hydrophobic bridges, are to be broken. Optimal conditions for the preparation of the apo-enzyme are derived from the data.

Apoenzymes↗
PubMed1979

Assay and properties of 18-hydroxylation of endogenous and exogenous corticosterone in rat adrenals. Evidence for heterogeneity of 18-hydroxylase activity.

A mass fragmentographic technique for assay of 18-hydroxylation of labeled (exogenous) and unlabeled (endogenous) corticosterone in adrenal mitochondria and in reconstituted cytochrome P-450 systems has been developed. An extract of an incubation of [14-14C]corticosterone is subjected both to thin-layer radiochromatography and to mass fragmentography (as O-methyloxime-trimethylsilyl ether derivative). In the latter procedure the ions at m/e 605 and 607 (specific for the derivatives of unlabeled and labeled 18-hydroxycorticosterone, respectively), at m/e 591 and 593 (specific for the derivatives of unlabeled labeled aldosterone, respectively) and at m/e 548 and 550 (specific for the derivatives of unlabeled and labeled corticosterone, respectively) were followed through the gas-liquid chromatography. From the ratio between the peaks obtained in the mass fragmentography and from the percentage conversion of [4-14C]corticosterone obtained in the thin-layer radiochromatography, the amount of endogenous and exogenous 18-hydroxycorticosterone and aldosterone could be calculated. The effects of time, enzyme, and substrate concentration of 18-hydroxylation were studied and optimal conditions for assay were determined. Under most conditions, the ratio between labeled and unlabeled 18-hydroxylated products was about constant, indicating that labeled and unlabeled corticosterone were not in equilibrium. It was ascertained that the 18-hydroxycorticosterone and aldosterone formed in the incubations were derived from corticosterone. [4-14C]18-Hydroxydeoxycorticosterone was not converted into aldosterone or 18-hydroxycorticosterone. In vitro studies with different 18-hydroxylase inhibitors (spironolactone, canrenone, and canrenoate-K) and studies with rats pretreated with KCl in drinking fluid suggest that 18-hydroxylation of corticosterone is catalyzed by an enzyme system different from that catalyzing 18-hydroxylation of deoxycorticosterone.

Adrenal Glands↗
PubMed1998

Optimizing the stability of single-chain proteins by linker length and composition mutagenesis.

Linker length and composition were varied in libraries of single-chain Arc repressor, resulting in proteins with effective concentrations ranging over six orders of magnitude (10 microM-10 M). Linkers of 11 residues or more were required for biological activity. Equilibrium stability varied substantially with linker length, reaching a maximum for glycine-rich linkers containing 19 residues. The effects of linker length on equilibrium stability arise from significant and sometimes opposing changes in folding and unfolding kinetics. By fixing the linker length at 19 residues and varying the ratio of Ala/Gly or Ser/Gly in a 16-residue-randomized region, the effects of linker flexibility were examined. In these libraries, composition rather than sequence appears to determine stability. Maximum stability in the Ala/Gly library was observed for a protein containing 11 alanines and five glycines in the randomized region of the linker. In the Ser/Gly library, the most stable protein had seven serines and nine glycines in this region. Analysis of folding and unfolding rates suggests that alanine acts largely by accelerating folding, whereas serine acts predominantly to slow unfolding. These results demonstrate an important role for linker design in determining the stability and folding kinetics of single-chain proteins and suggest strategies for optimizing these parameters.

Gene Library↗