Positive Deviation (positive + deviation)

Distribution by Scientific Domains

Selected Abstracts

Self-association and cyclodextrin solubilization of drugs

Thorsteinn Loftsson
Abstract Phase-solubility diagrams are frequently used to calculate stoichiometry of drug/cyclodextrin complexes. Linear diagrams (AL -type systems) are thought to indicate that the complexes are first order with respect to cyclodextrin and first or higher order with respect to the drug. Positive deviation from linearity (AP -type systems) are thought to indicate formation of complexes that are first order with respect to the drug but second or higher order with respect to cyclodextrin. The phase solubility of several different compounds, i.e., cholesterol, ibuprofen, diflunisal, alprazolam, 17,-estradiol and diethylstilbestrol, and various charged and uncharged cyclodextrins was investigated. Phase-solubility diagrams of cholesterol in aqueous cyclodextrin solutions were all of AP type. However, the phase-solubility diagrams obtained with charged cyclodextrins could not be fitted to complexes of second or higher order with respect to cyclodextrin. The phase-solubility diagrams of ibuprofen and diflunisal were of AL type with slope greater than unity indicating formation of 2:1 drug/cyclodextrin complexes. However, Job's plots and space filling docking studies indicated that 1:1 complexes were formed. These and other observations show that stoichiometry of drug/cyclodextrin complexes cannot be derived from simple phase-solubility studies. Furthermore, the results indicate that drug/cyclodextrin complexes can self-associate to form water-soluble aggregates, which then can further solubilize the drug through non-inclusion complexation. © 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:2307,2316, 2002 [source]

Transgalactosylation by thermostable ,-glycosidases from Pyrococcus furiosus and Sulfolobus solfataricus

FEBS JOURNAL, Issue 16 2000
-glycosides during lactose conversion, Binding interactions of nucleophiles with the galactosylated enzyme intermediate make major contributions to the formation of new
The hyperthermostable ,-glycosidases from the Archaea Sulfolobus solfataricus (Ss,Gly) and Pyrococcus furiosus (CelB) hydrolyse ,-glycosides of d -glucose or d -galactose with relaxed specificities pertaining to the nature of the leaving group and the glycosidic linkage. To determine how specificity is manifested under conditions of kinetically controlled transgalactosylation, the major transfer products formed during the hydrolysis of lactose by these enzymes have been identified, and their appearance and degradation have been determined in dependence of the degree of substrate conversion. CelB and Ss,Gly show a marked preference for making new ,(1,3) and ,(1,6) glycosidic bonds by intermolecular as well as intramolecular transfer reactions. The intramolecular galactosyl transfer of CelB, relative to glycosidic-bond cleavage and release of glucose, is about 2.2 times that of Ss,Gly and yields ,- d -Galp- (1,6)- d -Glc and ,- d -Galp- (1,3)- d -Glc in a molar ratio of ,,1 : 2. The partitioning of galactosylated Ss,Gly between reaction with sugars [kNu (m,1·s,1)] and reaction with water [kwater (s,1)] is about twice that of CelB. It gives a mixture of linear ,- d -glycosides, chiefly trisaccharides at early reaction times, in which the prevailing new glycosidic bonds are ,(1,6) and ,(1,3) for the reactions catalysed by Ss,Gly and CelB, respectively. The accumulation of ,- d -Galp- (1,6)- d -Glc at the end of lactose hydrolysis reflects a 3,10-fold specificity of both enzymes for the hydrolysis of ,(1,3) over ,(1,6) linked glucosides. Galactosyl transfer from Ss,Gly or CelB to d -glucose occurs with partitioning ratios, kNu/kwater, which are seven and >,170 times those for the reactions of the galactosylated enzymes with 1-propanol and 2-propanol, respectively. Therefore, the binding interactions with nucleophiles contribute chiefly to formation of new ,-glycosides during lactose conversion. Likewise, noncovalent interactions with the glucose leaving group govern the catalytic efficiencies for the hydrolysis of lactose by both enzymes. They are almost fully expressed in the rate-limiting first-order rate constant for the galactosyl transfer from the substrate to the enzyme and lead to a positive deviation by ,,2.5 log10 units from structure,reactivity correlations based on the pKa of the leaving group. [source]

Modifications in the correlation function in poly(vinyl alcohol)/silica hybrid wet gels

Dimas R. Vollet
Small-angle X-ray scattering was used to study structural modifications in tetraethoxysilane-derived poly(vinyl alcohol) (PVA)/silica hybrids. The basic structure of the wet gels can be described as a mass-fractal structure with fractal dimension D equal to 2 and characteristic length ,, which increases with addition of PVA. Wet gels with high PVA content exhibit a positive deviation from the mass-fractal power-law scattering at low q; this deviation is associated with additional scattering due to a second large correlation distance ,, reinforced by the addition of PVA. The fraction of both contributions to the total correlation function was estimated; this is the first time that such a study has been carried out for mass-fractal structures. [source]

Thermodynamic Properties and Phase Diagram for the System MoO2,TiO2

K. Thomas Jacob
The activity of molybdenum dioxide (MoO2) in the MoO2,TiO2 solid solutions was measured at 1600 K using a solid-state cell incorporating yttria-doped thoria as the electrolyte. For two compositions, the emf was also measured as a function of temperature. The cell was designed such that the emf is directly related to the activity of MoO2 in the solid solution. The results show monotonic variation of activity with composition, suggesting a complete range of solid solutions between the end members and the occurrence of MoO2 with a tetragonal structure at 1600 K. A large positive deviation from Raoult's law was found. Excess Gibbs energy of mixing is an asymmetric function of composition and can be represented by the subregular solution model of Hardy as follows The temperature dependence of the emf for two compositions is reasonably consistent with ideal entropy of mixing. A miscibility gap is indicated at a lower temperature with the critical point characterized by Tc (K)=1560 and. Recent studies indicate that MoO2 undergoes a transition from a monoclinic to tetragonal structure at 1533 K with a transition entropy of 9.91 J·(mol·K),1. The solid solubility of TiO2 with rutile structure in MoO2 with a monoclinic structure is negligible. These features give rise to a eutectoid reaction at 1412 K. The topology of the computed phase diagram differs significantly from that suggested by Pejryd. [source]

Rheological Features and Flow-Induced Crystallization of Branched Poly[ethylene- co -(1,4-cyclohexanedimethylene terephthalate)] Copolyesters

Robert Quintana
Abstract A set of amorphous poly[ethylene- co -(1,4-cyclohexanedimethylene terephthalate)] (PECT) copolymers containing 25 and 30% of 1,4-cyclohexane dimethylene (CHDM) units and small amounts of branching agent pentaerythritol (PER) is investigated. The level of long chain branching was estimated by analyzing the positive deviation from law. Branching also produced melt elasticity enhancement which is desirable for certain processing methods. Capillary extrusion experiments at 180,°C generated flow-induced crystallization in PECT containing 25% of CHDM. Crystallization increased with the amount of PER added, which was explained by the favorable effect of branching to increase elongational rate at the entrance of the capillary. Linear and branched PECTs containing 30% of CHDM did not crystallize. [source]

Linear and nonlinear melt-state viscoelastic properties of polypropylene/organoclay nanocomposites

Ehssan Nazockdast
Rheological behavior of polypropylene (PP)/organoclay nanocomposites varying in compatibilizer (PP- g -MA) and organoclay concentration was investigated. The samples were prepared by melt intercalation method in an internal mixer. The wide angle X-ray diffraction patterns and results of rheological measurements showed that the compatibilizer had strong influence in increasing the interlayer spacing. The observed low frequency liquid-like to solid-like transition and apparent yield stress in simple shear flows, along with convergence of transient shear stress to nonzero values in stress relaxation after the cessation of flow experiments, were found to be consistent with formation of a physical network in quiescent conditions which could be easily ruptured with applying low shear rates. The values of stress overshoot strain in flow reversal experiments were independent of shear rate, organoclay, and compatibilizer content. From the results of frequency sweep experiments in different nonlinear strain amplitudes it was shown that extended Cox-Merz analogy was valid in nonlinear dynamic deformations while the shear viscosity showed positive deviation from this analogy with higher deviations at lower shear rates. Results of storage modulus recovery and flow reversal experiments at different shear rates suggested that network structure is reformed with a much slower rate compared to the rotational relaxation of organoclay platelets. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers [source]

Blends of nylon/acrylonitrile butadiene rubber: Effects of blend ratio, dynamic vulcanization and reactive compatibilization on rheology and extrudate morphology

C. Radhesh Kumar
The melt flow behavior of thermoplastic elastomers from nylon and nitrile rubber (NBR) was studied as a function of blend ratio, dynamic crosslinking, compatibilization and temperature. The morphology of the extrudates, i.e., the size, shape and distribution of the domains, was analyzed. Uncompatibilized and compatibilized blends showed pseudoplastic behavior. The viscosity of the blends showed positive deviation from a linear rule of mixtures. Compatibilization using chlorinated polyethylene (CPE) increased the melt viscosity of the blends. The addition of the compatibilizer decreased the domain size of the dispersed phase, followed by an increase after a critical concentration of the compatibilizer, where the interface was saturated. The influence of dynamic vulcanization on the rheological behavior was also studied. The extrudate morphology depended on blend ratio, compatibilization and shear rate. [source]

Rheological behavior of brominated isobutylene-co-paramethylstyrene: Effect of fillers, oil and blending with EPDM

Bhuwneesh Kumar
The melt flow properties of unfilled and filled brominated isobutylene-co-paramethylstyrene (BIMS) were measured by means of a capillary viscometer at three different temperatures (90°C, 110°C and 130°C) and four different shear rates (61, 122, 245 and 306 s,1). The effect of addition of EPDM rubber on melt flow properties of unfilled BIMS was also studied. Evaluation of the processability was done by measuring the extrudate roughness (ER) of the extrudates obtained from the MPT. The viscosity of the systems decreased with the shear rate, indicating their pseudoplastic or shear thinning nature. As expected, the viscosity of BIMS increased with the addition of fillers and decreased with the addition of oil. For the neat systems, viscosity increased with the addition of EPDM, and the blends showed a positive deviation, indicating interdiffusion of the polymer chains across the phase boundaries. The activation energy of the filled systems at constant filler loading increased with increasing filler surface area (N330 > N550 > N774, each at 30 phr loading), and filler loading (50 > 30 > 10 phr, for N330) and decreased with the addition of oil (2.5 > 5.0 > 7.5 phr, for system containing 30 phr of N330). The silica filled system showed a higher activation energy and ER than the carbon black-filled systems. With addition of N330 and N550 carbon blacks to BIMS, the extrudate roughness (ER) decreased, whereas it increased with the addition of N774 carbon black. With an increase in filler loading, ER initially increased and then decreased as compared to the neat system. For the filled systems, ER initially decreased up to 5 phr of oil, beyond which it increased. [source]

Polymeric liquid crystals: cholesteric superstructure from blends of hydroxypropylcellulose esters

Estella Bianchi
Abstract n-Butyric, iso-butyric, n-valeric and iso-valeric esters of hydroxypropylcellulose were synthesized. Their behaviors, in terms of cholesteric structure, glass (Tg) and anisotropic-isotropic (Ti) transition temperatures are compared to results in the literature. A 6,8 month time period from sample preparation has no influence on the cholesteric phase characteristics. Six binary blends may be formed from the four esters. They show only a Tg value at each composition, which suggests a good miscibility among the components. However, positive deviation of linearity of Tg vs. composition varies from one pair to another. Analysis of the absorption curves in UV-vis spectra show that three pairs form a single cholesteric helix containing both components while other three give a only nematic phase. Colors changing from red to violet are shown for blends of n-butyric and iso-butyric esters at different weight ratio. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Average protein density is a molecular-weight-dependent function

PROTEIN SCIENCE, Issue 10 2004
Hannes Fischer
Abstract The mass density of proteins is a relevant basic biophysical quantity. It is also a useful input parameter, for example, for three-dimensional structure determination by protein crystallography and studies of protein oligomers in solution by analytic ultracentrifugation. We have performed a critical analysis of published, theoretical, and experimental investigations about this issue and concluded that the average density of proteins is not a constant as often assumed. For proteins with a molecular weight below 20 kDa, the average density exhibits a positive deviation that increases for decreasing molecular weight. A simple molecular-weight-depending function is proposed that provides a more accurate estimate of the average protein density. [source]

A mass spectrometric study of Al2O3 -SiO2 melts using a Knudsen cell

Johan Björkvall
The thermodynamic activities of SiO2 in Al2O3 -SiO2 liquid slags were measured by the high-temperature Knudsen cell mass spectromeric method in the present work. The measurements were carried out in the temperature range 1863,2169,K. Tungsten crucibles were used to hold the slags. The system was calibrated using a CaF2 standard. The mass spectra obtained for pure SiO2 were in agreement with earlier data. The activities of silica, measured in the present work at 2150,K, show a slight negative deviation at very low alumina mole fractions which changed to a positive deviation at higher alumina contents. The activity values are in reasonable agreement with the assessment carried out by Hillert et al. The results were analysed on the basis of a slag model developed earlier at KTH, Stockholm. The present results are found to be compatible with the phase diagram proposed by Klug et al. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Native and nonnative conformational preferences in the urea-unfolded state of barstar

PROTEIN SCIENCE, Issue 12 2004
Neel S. Bhavesh
NMR, nuclear magnetic resonance; HSQC, hetero-nuclear single-quantum coherence Abstract The refolding of barstar from its urea-unfolded state has been studied extensively using various spectroscopic probes and real-time NMR, which provide global and residue-specific information, respectively, about the folding process. Here, a preliminary structural characterization by NMR of barstar in 8 M urea has been carried out at pH 6.5 and 25°C. Complete backbone resonance assignments of the urea-unfolded protein were obtained using the recently developed three-dimensional NMR techniques of HNN and HN(C)N. The conformational propensities of the polypeptide backbone in the presence of 8 M urea have been estimated by examining deviations of secondary chemical shifts from random coil values. For some residues that belong to helices in native barstar, 13C, and 13CO secondary shifts show positive deviations in the urea-unfolded state, indicating that these residues have propensities toward helical conformations. These residues are, however, juxtaposed by residues that display negative deviations indicative of propensities toward extended conformations. Thus, segments that are helical in native barstar are unlikely to preferentially populate the helical conformation in the unfolded state. Similarly, residues belonging to ,-strands 1 and 2 of native barstar do not appear to show any conformational preferences in the unfolded state. On the other hand, residues belonging to the ,-strand 3 segment show weak nonnative helical conformational preferences in the unfolded state, indicating that this segment may possess a weak preference for populating a helical conformation in the unfolded state. [source]

High-temperature mass spectrometric study of the vaporization processes of V2O3 and vanadium-containing slags

H. Wang
A Knudsen effusion mass spectrometric method was used to study the vaporization processes and thermodynamic properties of pure V2O3 and 14 samples of vanadium-containing slags in the CaO-MgO-Al2O3 -SiO2 system in the temperature range 1875,2625,K. The system was calibrated using gold in the liquid state as the standard. Vaporization was carried out from double tungsten effusion cells. First it was shown that, in vapor over V2O3 and the vanadium-containing slags in the temperature range 1875,2100,K, the following vapor species were present: VO2, VO, O, WO3 and WO2, with the latter two species being formed as a result of interaction with the tungsten crucibles. The temperature dependencies of the partial pressures of these vapor species were obtained over V2O3 and the slags. The ion current comparison method was used for the determination of the V2O3 activities in slags as a function of temperature with solid V2O3 as a reference state. The V2O3 activity coefficients in the slags under investigation indicated positive deviations from ideality at 1900,K and a tendency to ideal behavior at 2100,K. It was shown that the V2O3 activity as a function of the slag basicity decreased at 1900,K and 2000,K and was practically constant in the slag melts at 2100,K. The results are expected to be valuable in the optimization of slag composition in high-alloy steelmaking processes as well as for their environmental implications. Copyright © 2010 John Wiley & Sons, Ltd. [source]