Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “phasing”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 379 records · Page 21Linked to original sources
PubMed2000

Titanium alkoxide complexes: condensed phase and gas phase comparisons.

The complex [Ti(2,4-dimethyl-2,4-pentanediolate)2]2 (1) has been synthesized from [Ti(OiPr)4] by transesterification with a stoichiometric amount of 2,4-dimethyl-2,4-pentanediol. We have characterized complex 1 in the solid state by single-crystal X-ray diffraction and in the gas phase by desorption chemical ionization mass spectrometry (DCI-MS). The structural and mass spectrometric data show complex 1 to be stable as a dimer in both the solid and gas phases. The retention of dimeric nuclearity in the gas phase sets complex 1 apart from other simple titanium alkoxide complexes [Ti(OiPr)4] and [Ti(OMe)4]4 that give rise to respective families of molecular ions in the DCI-MS experiment. The highest mass molecular ions for Ti alkoxide complexes in the gas phase may reveal the highest nuclearity that these complexes achieve in condensed phases. According to this interpretation the complex [Ti(OiPr)4] is principally dimeric in the gas phase and probably also in the pure liquid phase and should be represented by the formula [Ti(OiPr)4]2.

Journal Article↗
PubMed1998

Chain Mobility in Polymer Systems: On the Borderline between Solid and Melt. 2. Crystal Size Influence in Phase Transition and Sintering of Ultrahigh Molecular Weight Polyethylene via the Mobile Hexagonal Phase.

Polymorphism is a well-established phenomenon in crystalline materials and is important for pharmaceutical and polymeric materials. In our study concerning the processability of polymers, we came across an unusual observation related to polymorphism induced by pressure. The experimental observation is that polyethylene crystals transform from the stable orthorhombic crystal into a transient hexagonal phase. The occurrence of a transient hexagonal phase is shown to be dependent on the polymer crystal size; smaller crystals transform into the transient hexagonal phase at temperatures and pressures much below the thermodynamic critical point Qo, which is located at P = 3.6 kbar and T = 230 degreesC. The crystal size dependence in the phase transition was investigated by in situ X-ray studies in the unirradiated and irradiated solution-crystallized films. Since the chain mobility is rather high in the hexagonal phase, sintering has been attempted via this transient phase using ultrahigh molecular weight polyethylene (UHMW-PE) as a model system. UHMW-PE is an intractable polymer due to its high molar mass but possesses excellent abrasion resistance properties. For this reason it is used as an inlay in demanding applications such as artificial hip and knee joints. The service life of UHMW-PE in these artificial joints, however, is limited due to the poor processing characteristics notably during sintering, and often a second operation is needed to replace the UHMW-PE interface. Sintering via the transient hexagonal phase could provide a solution for this important problem which concerns an increasing number of people.

Journal Article↗
PubMed2000

Colloidal ordering from phase separation in a liquid-crystalline continuous phase

Some binary mixtures exist as a single phase at high temperatures and as two phases at lower temperatures; rapid cooling therefore induces phase separation that proceeds through the initial formation of small particles and subsequent growth and coarsening. In solid and liquid media, this process leads to growing particles with a range of sizes, which eventually separate to form a macroscopically distinct phase. Such behaviour is of particular interest in systems composed of an isotropic fluid and a liquid crystal, where the random distribution of liquid-crystal droplets in an isotropic polymer matrix may give rise to interesting electro-optical properties. Here we report that a binary mixture consisting of an isotropic fluid and a liquid crystal forming the continuous phase does not fully separate into two phases, but self-organizes into highly ordered arrays of monodisperse colloidal droplet chains. We find that the size and spatial organization of the droplets are controlled by the orientational elasticity of the liquid-crystal phase and the defects caused by droplets exceeding a critical size. We expect that our approach to forming monodisperse, spatially ordered droplets in liquid crystals will allow the controlled design of ordered composites that may have useful rheological and optical properties.

Journal Article↗
PubMed2001

An unusual phase transition to a second liquid vortex phase in the superconductor YBa2Cu3O7.

A magnetic field penetrates a superconductor through an array of 'vortices', each of which carries one quantum of flux that is surrounded by a circulating supercurrent. In this vortex state, the resistivity is determined by the dynamical properties of the vortex 'matter'. For the high-temperature copper oxide superconductors (see ref.1 for a theoretical review), the vortex phase can be a 'solid', in which the vortices are pinned, but the solid can 'melt' into a 'liquid' phase, in which their mobility gives rise to a finite resistance. (This melting phenomenon is also believed to occur in conventional superconductors, but in an experimentally inaccessible part of the phase diagram.) For the case of YBa2Cu3O7, there are indications of the existence of a critical point, at which the character of the melting changes. But neither the thermodynamic nature of the melting, nor the phase diagram in the vicinity of the critical point, has been well established. Here we report measurements of specific heat and magnetization that determine the phase diagram in this material to 26 T, well above the critical point. Our results reveal the presence of a reversible second-order transition above the critical point. An unusual feature of this transition-namely, that the high-temperature phase is the less symmetric in the sense of the Landau theory-is in accord with theoretical predictions of a transition to a second vortex-liquid phase.

Journal Article↗
PubMed2003

Exercise-induced bronchoconstriction in children: montelukast attenuates the immediate-phase and late-phase responses.

BACKGROUND: Montelukast, a leukotriene receptor antagonist, attenuates exercise-induced bronchoconstriction. We and others have shown that there is a late-phase response 3 to 8 hours after exercise in a subset of asthmatic patients. OBJECTIVE: We sought to evaluate the protective effect of montelukast on immediate-phase and late-phase responses after exercise challenges. METHODS: Twenty-two atopic asthmatic children aged 7 to 16 years with reproducible exercise-induced bronchoconstriction (minimum of 15% decrease of FEV(1) from baseline) were enrolled in this placebo-controlled crossover study. Exercise challenges were performed while breathing cold dry air, and FEV(1) measurements were taken up to 480 minutes after exercise. Patients underwent exercise challenges on a screening day and 1 week after placebo treatment. Subsequently, after a week with no treatment, pulmonary function was assessed after breathing dry cold air (control day). Finally, an exercise challenge was carried out after a week of treatment with montelukast. RESULTS: Reproducible late-phase reactions occurred in 5 of 22 patients, which correlated with the extent of the immediate response (P <.05). After 1 week of treatment with montelukast, a significant decrease of immediate responses was observed. Montelukast treatment compared with placebo was associated with a lower mean maximum decrease of FEV(1) (mean +/- SEM: 17.3% +/- 2.4% and 35.1% +/- 2.6%, respectively), decrease of the area above the curve (267.8% +/- 42.7%/min and 868.0% +/- 103.8%/min, respectively), and shorter time for recovery (6.9 +/- 1.1 minutes and 30.9 +/- 4.0 minutes, respectively; P <.05). Treatment with montelukast also abolished late-phase responses. CONCLUSION: Once daily treatment with oral montelukast attenuated the immediate-phase response and abolished the late-phase response induced by means of exercise challenge in asthmatic children.

Acetates↗
PubMed2005

Sterol structure determines the separation of phases and the curvature of the liquid-ordered phase in model membranes.

The existence of lipid rafts in biological membranes in vivo is still debated. In contrast, the formation of domains in model systems has been well documented. In giant unilamellar vesicles (GUVs) prepared from ternary mixtures of dioleoyl-phosphatidylcholine/sphingomyelin/cholesterol, a clear separation of liquid-disordered and sphingomyelin-enriched, liquid-ordered phases could be observed. This phase separation can lead to the fission of the liquid-ordered phase from the vesicle. Here we show that in cholesterol-containing GUVs, the phase separation can involve dynamic redistribution of lipids from one phase into another as a result of a cross-linking perturbation. We found that the molecular structure of a sterol used for the preparation of GUVs determines (i) its ability to induce phase separation and (ii) the curvature (positive or negative) of the formed liquid-ordered phase. As a consequence, the latter can pinch off to the outside or inside of the vesicle. Remarkably, some mixtures of sterols induce liquid-ordered domains exhibiting both positive and negative curvature, which can lead to a new type of budding behavior in GUVs. Our findings could have implications for the role of sterols in various cell-biological processes such as budding of secretory vesicles, endocytosis, or formation of multivesicular bodies.

Cholera Toxin↗
PubMed2006

Circadian phase in delayed sleep phase syndrome: predictors and temporal stability across multiple assessments.

STUDY OBJECTIVES: To assess temporal stability across multiple assessments and predictors of circadian phase in participants with delayed sleep phase syndrome (DSPS), relative to normal-sleeping matched controls. DESIGN: Circadian phase was assessed by salivary dim light melatonin onset (DLMO) during 3 laboratory visits, separated by at least 5 days--2 scheduled at the end of the week (Friday) and 1 scheduled at the end of the weekend (Sunday). PATIENTS: Eight young volunteers who met International Classification of Sleep Disorders-Revised criteria for DSPS, and 8 age- and sex-matched controls (age range 19-27 years old). INTERVENTIONS: N/A. MEASUREMENTS AND RESULTS: As expected, salivary DLMO occurred significantly later in patients with DSPS than in controls (F 10.561, p = .006). However, circadian phase did not change significantly across the 3 DLMO assessments in either group. Estimations of circadian phase were not significantly different in the assessments conducted on weekdays versus weekends. Predictors of circadian phase included time of morning light exposure (R2 = 0.777; p < .001), recent wake time (R2 = 0.701, p < .001), and self-reported chronotype (R2 = 0.320, p = .016). DLMO preceded wake time in both groups by approximately 10.75 hours. CONCLUSIONS: Across serial laboratory assessments on an ad lib sleep schedule, patients with DSPS appeared more similar to than different from normal-sleeping control subjects, except for a stable delay in circadian phase.

Adult↗
PubMed2001

Phase clustering and transition to phase synchronization in a large number of coupled nonlinear oscillators.

The transition to phase synchronization in systems consisting of a large number (N) of coupled nonlinear oscillators via the route of phase clustering (phase synchronization among subsets of oscillators) is investigated. We elucidate the mechanism for the merger of phase clusters and find an algebraic scaling between the critical coupling parameter required for phase synchronization and N. Our result implies that, in realistic situations, phase clustering may be more prevalent than full phase synchronization.

Journal Article↗
PubMed2004

Growth laws for the phase ordering dynamics of the B1 phase of a bent-core liquid crystal.

We have experimentally investigated the dynamics of the phase ordering process of bent-core molecules at isothermal conditions, forming the liquid crystalline "banana" B1 phase from the isotropic melt. In contrast to the fractal growth patterns observed for other "banana" liquid crystal phases, the B1 phase exhibits dendritic-like growth, which at short times shows similar growth structures as observed for conventional smectic bâtonnet growth. Analysis with respect to growth laws of the form L(t) approximately tn shows that the growth process follows the theoretically predicted Allen-Cahn dynamics with n=1/2 for zero difference in the free energy between the high- and the low-temperature phase, while the growth exponent approaches n-->1 for increasing supercooling. From the experimentally obtained data, we estimate the ratio between volume and curvature driven contributions to the phase ordering process as a function of supercooling and suggest a phenomenologically determined relationship of log DeltaF approximately DeltaT for the difference in free energy between the high- and the low-temperature phase as the quench depth is varied.

Journal Article↗
PubMed2006

Scaling of geometric phases close to the quantum phase transition in the XY spin chain.

We show that the geometric phase of the ground state in the XY model obeys scaling behavior in the vicinity of a quantum phase transition. In particular we find that the geometric phase is nonanalytical and its derivative with respect to the field strength diverges at the critical magnetic field. Furthermore, the universality in the critical properties of the geometric phase in a family of models is verified. In addition, since the quantum phase transition occurs at a level crossing or avoided level crossing and these level structures can be captured by the Berry curvature, the established relation between the geometric phase and quantum phase transitions is not a specific property of the XY model, but a very general result of many-body systems.

Journal Article↗
PubMed2004

X-ray studies of the phases and phase transitions of liquid crystals.

A short review is given of recent X-ray diffraction studies of the phases and phase transitions of thermotropic liquid crystals. The areas covered are twist-grain-boundary phases, antiferroelectric phases studied with resonant X-ray diffraction and smectic phases within gel structures. In all areas, X-ray diffraction has played a key role. Nonetheless, open questions remain: the nature of the smectic C variant of the twist-grain-boundary phase, the origin of antiferroelectric phases, and whether novel Bragg glass states exist for smectic A gel samples.

Journal Article↗
PubMed1995

TDSIR phasing: direct use of phase-invariant distributions in macromolecular crystallography.

A new strategy for employing three phase triples invariant estimates from Hauptman's single isomorphous replacement (SIR) and anomalous dispersion (SAS) joint probability distribution formulae is outlined which produces a single unique phase-invariant solution in the case where the positions of the heavy-atom scatterers is known. A similar but non-identical result is obtained for the phase invariants of a structure for which a molecular-replacement solution has been obtained. It is important to note that the values of the individual native/derivative phases can be determined directly from the probability distribution formulae without having to utilize the phase-invariant estimates in an active way. Elimination of the multisolution aspect of utilizing phase-invariant estimates should have important repercussions with regard to phasing macromolecular sets of derivatized data. Trial calculations based on experimentally measured 2.5 A data for three derivatives of cytochrome c550 are encouraging. The average of the three SIR maps resolves a number of structural ambiguities seen in the published multiple isomorphous replacement (MIR) map obtained from eight derivatives.

Crystallography, X-Ray↗
PubMed1993

Experience with phase extension and ab initio phase determination in macromolecular crystallography using maximum-entropy methods.

Three procedures, or 'tools', have been developed and tested for applying maximum-entropy methods to phase extension and to ab initio phase determination. The phase expander tool has been used in connection with the solution of two previously unknown macromolecular structures. An efficient algorithm for the determination of an electron-density distribution that is everywhere positive and that agrees with observed structure amplitudes (tools II and III) has been used to determine the phases of X-ray diffraction data from recombinant bovine chymosin, a protein with 323 amino-acid residues in the molecular chain, the structure of which was recently determined using replacement methods. By use of the same maximum-entropy methods, the structure amplitudes from the unknown structure of bovine heart creatine kinase, a protein with 381 amino-acid residues, have been phased ab initio to 2.7 A resolution. The phases of the centric reflections have also been confirmed by a satisfactory solution of the Patterson map of a mercury derivative. The current status of the structure interpretation is presented. This technique has also been applied to a test case where 48 centric reflections from bovine prothrombin fragment 1 data were phased ab initio and subsequently used in the determination of Patterson solutions for a heavy-atom derivative data set.

Journal Article↗
PubMed1997

Determining phases and anomalous-scattering models from the multiwavelength anomalous diffraction of native protein metal clusters. improved MAD phase error estimates and anomalous-scatterer positions.

A strategy is presented for refining anomalous scattering models and calculating protein phases directly from the Bijvoet and dispersive differences of a macromolecular multiwavelength anomalous diffraction (MAD) experiment. This procedure, incorporated in the program MADPHSREF, is especially amenable for exploiting the weak perturbations to normal scattering produced by inner-shell electronic transitions of asymmetric metal and protein ligand assemblies. The protocol accounts for more than one type of anomalous scatterer, incorporates stereochemical restraints, treats the data in local scaling groups, and partly compensates for correlated errors. Approximating maximum likelihood by averaging observation variances and covariances over all values of phase considerably improved error estimation. Probabilistic rejection of aberrant observations, re-evaluated before each refinement cycle, improved refinement convergence and accuracy compared with other less flexible rejection criteria. MADPHSREF allows the facile combination of MAD phase information with phase information from other sources. For the suifite reductase hemoprotein (SiRHP), relative weights for MAD and multiple isomorphous replacement (MIR) phases were determined by matching histograms of electron density. Accurate metal-cluster geometries and the associated errors in atomic positions can be determined from refinement against anomalous differences using normal scattering phases from a refined structure. When applied to MAD data collected on SiRHP, these methods confirmed the Fe(4)S(4) cluster asymmetry initially observed in the refined 1.6 A resolution structure and resulted in a MAD-phased, experimental, electron-density map that is of better quality than the combined MAD/MIR map originally used to determine the structure.

Journal Article↗
PubMed2002

Understanding phase maps in MRI: a new cutline phase unwrapping method.

This paper describes phase maps. A review of the phase unwrapping problem is given. Different structures, in particular fringelines, cutlines, and poles, contained within a phase map are described and their origin and behavior investigated. The problem of phase unwrapping can then be addressed with a better understanding of the source of poles or inconsistencies. This understanding, along with some assumptions about what is being encoded in the phase of a magnetic resonance image, are used to derive a new method for phase unwrapping which relies only on the phase map. The method detects cutlines and distinguishes between noise-induced poles and signal undersampling poles based on the length of the fringelines. The method was shown to be robust to noise and successful in unwrapping challenging clinical cases.

Algorithms↗
PubMed1984

Cultured human tumour cells may be arrested in all stages of the cycle during stationary phase: demonstration of quiescent cells in G1, S and G2 phase.

Six human colon carcinoma cell lines were induced to enter stationary phase of growth by nutrient deprivation and cell crowding. Growth kinetics parameters (cell number, flow cytometric analysis of DNA distribution, and labelling and mitotic indices) were measured sequentially for all lines during the various stages of in vitro growth. Our results demonstrated that a substantial fraction of cells (9-18%) were located in G2 phase when they changed from an exponential to a stationary mode of growth. Moreover, a large number of cells in stationary phase of growth had an S-phase DNA content, as determined by flow cytometry, but failed to incorporate radioactive DNA precursors (up to 15-fold difference). To substantiate these findings, cells in stationary phase of growth were induced to enter exponential growth by re-seeding in fresh medium at a lower density. Subsequently observed changes in DNA-compartment distribution, and in labelling and mitotic indices were those expected from cells that had been arrested at different stages of the cycle during their previous stationary phase. Thus, the non-proliferating quiescent state (Q), traditionally located 'somewhere' in G1 phase, appears to be composed also of cells that can be arrested at other stages of the cycle (Qs and QG2). Although the proportion of such cells is rather small, their contribution to the growth kinetics behaviour of human in vivo tumours will become apparent following 'recruiting' or 'synchronizing' clinical manoeuvres and will prevent the formation of a clear-cut wave of synchronized cells.

Cell Cycle↗
PubMed1994

Reconstitution by bone marrow grafting of the defective protective capacity at the migratory phase but not at the intestinal phase of Nippostrongylus brasiliensis infection in W/Wv mice.

After a primary infection by subcutaneous inoculation with the infective larvae (L3) of Nippostrongylus brasiliensis, the intestinal worm burden was higher and expulsion was slower in W/Wv mice than in +/+ mice. When the course of infection was segregated into the migratory and intestinal phases, protection during the migratory phase examined by the larval recovery from the lungs and that during the intestinal phase measured by worm burden after intraduodenal implantation with adult worms were both defective in W/Wv mice. The higher susceptibility of W/Wv mice during the migratory phase was normalized by bone marrow reconstitution. On the other hand, higher susceptibility of W/Wv mice during the intestinal phase, which was measured by worm burden 24 h after intraduodenal implantation of the larvae recovered from the lungs of rats, was not normalized by bone marrow grafting. Furthermore, slower expulsion seen in W/Wv mice after intraduodenal implantation with adult worms was not hastened by bone marrow reconstitution. These results indicate that the protective mechanisms against N. brasiliensis operating during the migratory phase and those during the intestinal phase were different in terms of bone marrow dependency and that non-myeloid cells utilizing c-kit ligand/receptor system to express their functions are involved in the mucosal defence against N. brasiliensis.

Animals↗
PubMed1990

The immune response to phase I and phase II Coxiella burnetii antigens as measured by western immunoblotting.

To determine the utility of Western immunoblotting in distinguishing chronic Q fever from acute Q fever, we first examined serum samples from individuals who had no antibodies to Coxiella burnetii by either the indirect immunofluorescence antibody test or by the enzyme-linked immunosorbent assay. In these subjects, the IgG fraction in low dilutions of serum (1:8, 1:16) reacted with as many as 10 proteins in phase I and phase II antigens. This number of reacting bands seen in Western blots was reduced by using serum in a dilution of 1:1024. In contrast, IgA antibodies were uncommon even at low dilutions. Likewise, IgA antibodies were infrequently observed in patients with acute Q fever. However, in chronic Q fever there were many IgA antibodies to phase I and phase II proteins. Antibodies to phase I proteins were more common than those to phase II proteins. Several antigenic protein bands were recognized only by serum from chronic Q fever patients. Three of these antigens had molecular masses of, respectively, 50 kDa, 80 kDa, and 160 kDa. Serial serum samples from patients with chronic Q fever revealed that the number of antigens recognized by the IgA fraction decreased after the initiation of antibiotic therapy. The decline was faster for antibodies to phase II proteins. We conclude that immunoblotting is useful in the diagnosis of chronic Q fever.

Acute Disease↗