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PubMed1988

Thermotropic lipid phase separation in the human immunodeficiency virus.

The presence of thermodependent lipid domains in the envelope of the human immunodeficiency virus (HIV) was studied. HIV was propagated in Hut-78 cells and purified by differential-gradient centrifugation. Since the virus was highly infectious in cell culture and Western blots of detergent-inactivated HIV showed envelope proteins when exposed to sera containing anti-HIV antibodies, this viral preparation was not deficient in 'spike' or 'knob' particles. Electron spin resonance (ESR) studies of intact HIV labeled with 5-nitroxide stearate (5-NS) indicated that a temperature-dependent lipid phase separation occurs with a high onset at approx. 42 degrees C and a low onset at approx. 15 degrees C. Cooling below 42 degrees C induces 5-NS clustering. Similar phase separations with high onsets at approx. 37-38 degrees C were previously identified in 5-NS labeled human erythrocytes (cholesterol/phospholipid (C/P) molar ratio = 0.90) and cholesterol-loaded (C/P = 0.85-0.98) rat liver plasma membranes. These were attributed to a temperature-sensitive redistribution of endogenous lipid components such that 5-NS is excluded from cholesterol-rich domains and tends to reside in cholesterol-poor domains at low temperatures. Since HIV has a lipid envelope with a similarly high C/P of 0.88 (Aloia et al. (1988) Proc. Natl. Acad. Sci. USA 85, 900-904), cholesterol-rich and cholesterol-poor domains also probably exist in HIV at physiologic temperatures. The reduced stability and infectivity of HIV noted on heating above 42 degrees C may be due, in part, to the abolition of these thermodependent domains.

Acquired Immunodeficiency Syndrome↗

PubMed2004

Surfactant free fabrication of polymeric nanoparticles by combined liquid-liquid phase separation and solvent/nonsolvent mixing technology.

It is generally agreed that, in most cases, surfactants are required to obtain stable polymeric nanoparticle dispersions. Here, we report a method which can be used to produce surfactant free yet stable polymeric nanoparticle dispersions. This method is based on explored mechanism of selective solvation of nanoparticles and EPD (electron pair donor)/EPA (electron pair acceptor) complexes formed among solvent and nonsolvent molecules. Using polyimide P84 (copolyimide 3,3(') 4,4(')-benzophenone tetracarboxylic dianhydride and 80% methylphenylene diamine+20% methylene dianiline) as the model polymer, this mechanism was realized through a combined liquid-liquid phase separation and solvent/nonsolvent mixing technology. Surfactant-free polyimide nanoparticles (<100 nm) were produced. Experimental details and principles of this technology were given based on the ternary diffusion, the liquid-liquid phase separation and the advanced nucleation and growth theory. Two types of methods [denoted as the forward titration method and the backward titration (BT) method] were examined. It was found that the BT method is extremely helpful to prepare polyimide nanoparticles (<100 nm). As another important aspect, explored stabilization mechanism of the resultant nanoparticle dispersions was supported by the comparative experiments, implying that selective solvation of nanoparticles and EPD/EPA complexes may play key roles in stabilization.

Benzophenones↗

PubMed1977

Lateral phase separations and structural integrity of the inner membrane of rat-liver mitochondria. Effect of compression. Implications in the centrifugation of these organelles.

When maintained in the vicinity of the lower transition temperature of their membrane lipids, rat-liver mitochondria undergo lysis as shown by the release of malate dehydrogenase, (an enzyme located within the mitochondrial matrix), in the surrounding medium. Structural changes take place in the membranes of mitochondria subjected to increasing pressure at 0 degrees C, when the pressure reaches 750 kg/cm2. Freeze-fracture electron microscopy shows the appearance of smooth areas devoid of particles in fracture faces of mitochondrial membranes, together with zones, where aggregated particles can be seen. Concurrently, a suppression of the malate dehydrogenase structure-linked latency is observed. These structural changes can be prevented by increasing the temperature at which compression is performed. The freeze-etching observations suggest that lateral phase separations occur in mitochondrial membranes subjected to high pressure. This can be explained by supposing that pressure promotes the gel-phase appearance in a lipid system and raises the transition temperature since the transition liquid crystal lead to gel is accompanied by a decrease in volume. The deterioration of mitochondria subjected to high pressure is interpreted as a result of the lateral phase separation induced by compression in the membranes. These results are discussed with respect to our interpretation of the damaging effects that hydrostatic pressure, generated by centrifugation, exerts on rat-liver mitochondria.

Animals↗

PubMed1975

Synthesis of a new phosphatidylserine spin-label and calcium-induced lateral phase separation in phosphatidylserine-phosphatidylcholine membranes.

A new phosphatidylserine spin label with nitroxide stearate attached at the 2 position has been synthesized by the reaction of spin-labeled CDP-diglyceride with L-serine under the catalytic action of phosphatidylserine synthetase. Some structural properties of pure phosphatidylserine (PS) and binary PS-phosphatidylcholine (PC) membranes were studied with the spin label. PS membrane became solidified on lowering solution pH, 50% solidification being attained at pH 3.5. The membrane was also solidified by addition of Ca-2+. The effect of Ba-2+,Sr-2+, and Mg-2+ was smaller than that of Ca-2+. The calcium-induced lateral phase separation in the binary membrane was studied from the side of the calcium-receiving lipid. The results confirmed and extended our previous conclusion drawn with PC spin label. The phase diagram of the binary membrane in the presence of Ca-2+ was determined. Not all PS molecules were aggregated to form the solid patches but some remained dissolved in the fluid PC matrix. The fluid PS fraction was larger for the membranes containing more PC. The membrane with 10% PS still had a significant fraction of solid phase. The rate of calcium-induced aggregation was greatly dependent on the PS content. The aggregation was almost complete within 5 min in the membrane containing 67% PS, while it was still proceeding after several hours in the membrane with 20% PS. The rate-limiting step was suggested to be in the formation of "stable" nuclei consisting of larger aggregates. The possible biological significance of the ionotropic phase separation was discussed whereby a transient density fluctuation was emphasized.

Barium↗

PubMed2006

Pair-correlation functions and phase separation in a two-component point Yukawa fluid.

We investigate the structure of a binary mixture of particles interacting via purely repulsive point Yukawa pair potentials with a common inverse screening length lambda. Using the hypernetted chain closure to the Ornstein-Zernike equations, we find that for a system with "ideal" (Berthelot mixing rule) pair-potential parameters for the interaction between unlike species, the asymptotic decay of the total correlation functions crosses over from monotonic to damped oscillatory on increasing the fluid total density at fixed composition. This gives rise to a Kirkwood line in the phase diagram. We also consider a "nonideal" system, in which the Berthelot mixing rule is multiplied by a factor (1+delta). For any delta>0 the system exhibits fluid-fluid phase separation and remarkably the ultimate decay of the correlation functions is now monotonic for all (mixture) state points. Only in the limit of vanishing concentration of either species does one find oscillatory decay extending to r=infinity. In the nonideal case the simple random-phase approximation provides a good description of the phase separation and the accompanying Lifshitz line.

Journal Article↗

PubMed2000

Crystallization and phase separation in nonadditive binary hard-sphere mixtures

We calculate for the first time the full phase diagram of an asymmetric nonadditivehard-sphere mixture. The nonadditivity strongly affects the crystallization and the fluid-fluid phase separation. The global topology of the phase diagram is controlled by an effective size ratio y(eff), while the fluid-solid coexistence scales with the depth of the effective potential well.

Journal Article↗

PubMed1988

Acetylcholinesterase from Apis mellifera head. Evidence for amphiphilic and hydrophilic forms characterized by Triton X-114 phase separation.

The polymorphism of bee acetylcholinesterase was studied by sucrose-gradient-sedimentation analysis and non-denaturing electrophoretic analysis of fresh extracts. Lubrol-containing extracts exhibited only one form, which sedimented at 5 S when analysed on high-salt Lubrol-containing gradients and 6 S when analysed on low-salt Lubrol-containing gradients. The 5 S/6 S form aggregated upon removal of the detergent when sedimented on detergent-free gradients and was recovered in the detergent phase after Triton X-114 phase separation. Thus the 5 S/6 S enzyme corresponds to an amphiphilic acetylcholinesterase form. In detergent-free extracts three forms, whose apparent sedimentation coefficients are 14 S, 11 S and 7 S, were observed when sedimentations were performed on detergent-free gradients. Sedimentation analyses on detergent-containing gradients showed only a 5 S peak in high-salt detergent-free extracts and a 6 S peak, with a shoulder at about 7 S, in low-salt detergent-free extracts. Electrophoretic analysis in the presence of detergent demonstrated that the 14 S and 11 S peaks corresponded to aggregates of the 5 S/6 S form, whereas the 7 S peak corresponded to a hydrophilic acetylcholinesterase form which was recovered in the aqueous phase following Triton X-114 phase separation. The 5 S/6 S amphiphilic form could be converted into a 7.1 S hydrophilic form by phosphatidylinositol-specific phospholipase C digestion.

Acetylcholinesterase↗

PubMed2005

Spontaneous "phase separation" of patterned binary alkanethiol mixtures.

This article describes novel phase-separation behavior by a binary mixture of alkanethiols when deposited onto a gold surface using micro- and nanodeposition tools, such as microcontact printing (muCP) and dip-pen nanolithography (DPN). This behavior is significantly different than that observed in the bulk. We demonstrate this behavior using three model compounds: 16-mercaptohexadecanoic acid (MHA), 1-octadecanethiol (ODT), and CF3(CF2)11(CH2)2SH (PFT). The identity of the resulting segregated structure is confirmed by lateral force microscopy (LFM) and by selective metal-organic coordination chemistry. Importantly, this phenomenon can be exploited to print sub-100 nm wide alkanethiol lines via conventional muCP and to form sub-15 nm features using DPN, which is below the ultimate resolution of both these techniques. We also demonstrate that these nano-patterned materials can serve as templates for constructing more complex architectures.

Journal Article↗

PubMed2000

Analysis of compression-induced chiral phase separation in Langmuir monolayers.

We analyze the compression-induced chiral phase separation (CPS) in Langmuir films, taking into account the elastic theory of liquid crystals and the mixing energy of the two constituent enantiomers. The difference between the Selinger-Wang-Bruinsma-Knobler theory [J. V. Selinger et al., Phys. Rev. Lett. 70, 1139 (1993)] and our treatment is that we do not introduce the concentration-square-gradient term in the free energy, but alternatively take into account a line tension at CPS boundaries. Our model predicts that a two-domain pattern with opposite chiralities is energy minimized, but a multistripe pattern with two alternate constant chiralities is also possible, though metastable. This offers a tentative explanation for the CPS pattern consisting of homogeneously oriented stripes with diverse widths observed by Eckhardt et al. [Nature (London) 362, 614 (1993)].

Journal Article↗

PubMed2006

Self-propulsion of nematic drops: novel phase separation dynamics in impurity-doped nematogens.

We report a novel phase separation dynamics, mediated by self-propelled motion of the nucleated drops, in a mixture of a nematogen and an isotropic dopant. We show that surface flow, induced by the gradient in the concentration of the dopant expelled by the growing drops, provides the driving force for the propulsion of nematic droplets. While the liquid crystal-isotropic transition is used here to demonstrate the phenomenon, self-propulsion should be observable in many other systems in which the dynamics of a conserved order parameter is coupled to a nonconserved order parameter.

Journal Article↗

PubMed2003

Effect of confinement on phase-separation processes in a polymer blend observed by laser scanning confocal microscopy.

Structure self-assembling in the late stage spinodal decomposition of a polymer blend at its critical composition has been explored by laser-scanning confocal microscopy with particular emphasis on the effects of confinement (dimensionality) and preferential wetting of solid surface by one of the constituent polymers. A mixture of deuterated polybutadiene and polybutadiene (PB) with relatively narrow thickness (D congruent with 55 microm) was observed in three dimensions over the entire thickness. Formation of a wetting layer was clearly observed near the glass surface, while a bicontinuous structure evolved in the middle of the specimen. Global as well as local features of the phase-separating structures were quantified by several structural parameters, e.g., characteristic length Lambda(m)(t), structure factor S(q), interfacial area per unit volume Sigma(t), probability densities of interfacial curvatures P(H,K;t), etc. (t is a phase-separation time). From the time evolution of these structural parameters, a deviation from the self-similar growth of a bicontinuous structure was found to occur at a transition time, t(tr), at which a scaled thickness, D/Lambda(m), approached unity. The breakdown of the self-similar growth was most sensitively observed by the local characteristics, i.e., Sigma(t) and P(H,K;t). On the other hand, the global characteristic, Lambda(m)(t), did not provide useful insight into the effects of dimensionality. It turned out that the bicontinuous structure, initially growing with dynamical self-similarity, eventually transformed into a "columnlike" structure (at t congruent with t(tr)) in which cylindrical PB-rich domains bridge the upper and lower PB wetting layers.

Journal Article↗

PubMed2005

Atomistic simulations of spinodal phase separation preceding polymer crystallization.

Many polymeric materials crystallize when cooled below their melting temperature. Although progress has been made in our understanding of the crystallization process through both experimental and theoretical efforts, these studies have focused mainly on the crystal nucleation and growth mechanism, where critical nuclei are formed from a metastable state during the first stages of crystallization, leading ultimately to the growth of crystal domains. Attention has also been given to the structure during the precrystallization (induction period). A pretransition state occurring before crystallization has been characterized as an unstable phase separation initiated by density and orientational fluctuations. These fluctuations are caused by an increase in the average length of rigid trans segments along the polymer backbone during the induction period. These observations are consistent with the theory proposed in ref. 14 on the isotropic-to-nematic transition of polymer liquid crystals, that is, the parallel ordering of polymers is caused by an increase in chain rigidity. Here we use large-scale computer simulations to investigate melts of polymers in the early ordering stages (induction period) before crystallization. In the ordered domains we identify growing dense regions similar to smectic liquid crystals. Our simulations reveal a 'coexistence period' in the ordering before crystallization, where nucleation and growth mechanisms coexist with a phase-separation mechanism.

Crystallization↗

PubMed1988

Effects of platelet activating factor on calcium-lipid interactions and lateral phase separations in phospholipid vesicles.

Recent studies localizing the inflammatory mediator, platelet activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), to the membranes of stimulated neutrophils, raise the possibility that PAF may, in addition to its activities as a mediator, alter the physical properties of membranes. This, and the increasing evidence that calcium-lipid interactions may have central importance in membrane organizational structure and in functions of cell homeostasis and stimulus-response coupling, prompted us to study the effects of PAF on calcium-lipid interactions in lipid vesicles. Using fluorescence polarization of dansylated probes located in the glycerol portion of the membrane bilayer, PAF (at a concentration as low as 1 mol%) was shown to reduce membrane rigidification significantly during calcium-induced lateral phase separations. This effect of PAF was structurally dependent on both the 1-position alkyl linkage and the 2-position acetyl group as shown by studies of related lipid analogs. Furthermore, using a self-quenching probe, it was shown that inhibition of lateral phase separation did not account for this reduction in the calcium-induced membrane rigidification attributed to PAF. Data suggest that PAF at low concentrations may alter phospholipid head packing and, thereby, change membrane surface features during calcium-lipid interactions, effects which may ultimately explain some of its biological actions.

Animals↗

PubMed2002

Two dimensional reversed-phase-reversed-phase separations isomeric separations incorporating C18 and carbon clad zirconia stationary phases.

Informational theory and a geometric approach to factor analysis were employed to evaluate the degree of orthogonality of a two-dimensional reversed-phase-reversed-phase chromatographic system. The system incorporated a C18 column as one dimension and a carbon clad zirconia column as the second dimension. In order to study the resolving power of this system, the separation of a sample matrix containing an artificial mix of 32 isomers (structural and diastereoisomers) was evaluated. Using this system, between 25 and 28 of the 32 isomers could be separated, depending on the mobile phase combinations--with resolution that could not possibly be achieved in a single one dimensional separation. The results from this study indicate that in order to fully evaluate the resolving power of a 2D system multiple methods of analysis are most appropriate. This becomes increasingly important when the sample contains components that are very closely related and the retention of solutes is clustered in one quadrant of the 2D space. Ultimately, the usefulness of the 2D separation is determined by the goals of analyst.

Carbon↗

PubMed1999

Noise-induced phase separation: mean-field results.

We present a study of a phase-separation process induced by the presence of spatially correlated multiplicative noise. We develop a mean-field approach suitable for conserved-order-parameter systems and use it to obtain the phase diagram of the model. Mean-field results are compared with numerical simulations of the complete model in two dimensions. Additionally, a comparison between the noise-driven dynamics of conserved and nonconserved systems is made at the level of the mean-field approximation.

Journal Article↗

PubMed1988

Ca2+-induced lateral phase separation in ternary mixtures of phosphatidic acid, phosphatidylcholine, and phosphatidylethanolamine inferred by calorimetry.

Phase transition characteristics of ternary mixtures of dipalmitoylphosphatidic acid, dipalmitoylphosphatidylcholine, and phosphatidylethanolamine (dilauroyl-, dimyristoyl-, or dipalmitoyl-phosphatidylethanolamine) were examined by differential scanning calorimetry at various concentrations of calcium ions. In the absence of calcium ion, these ternary mixtures showed a broad phase transition, which suggested a high miscibility of these components. Addition of a low concentration of calcium ions showed a tendency to induce separation of the transition into a major one and a small one. As the concentration of calcium ions increased, the separation became more distinct and the transition enthalpy of the major transition decreased. At a Ca2+/dipalmitoylphosphatidic acid ratio (mol/mol) of 1.5, the major transition became similar to the transition of dipalmitoylphosphatidylcholine and the phosphatidylethanolamine binary mixture. On the other hand, in a binary mixture dipalmitoylphosphatidic acid and dipalmitoylphosphatidylcholine, the Ca2+-induced phase separation was distinct even at the lowest concentration of calcium ions used in the present experiment. The results indicate that a high concentration of calcium ion is required for inducing complete phase separation of the transition event in the ternary mixture because of its high miscibility. It is suggested that the phase separation revealed by spin-labeled phospholipid in ternary mixtures at a low Ca2+ concentration might be a phase separation in a local domain.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2000

Isoelectric focusing nonporous RP HPLC: a two-dimensional liquid-phase separation method for mapping of cellular proteins with identification using MALDI-TOF mass spectrometry.

A novel two-dimensional liquid-phase separation method was developed that is capable of resolving large numbers of cellular proteins. The proteins are separated by pI using isoelectric focusing in the first dimension and by hydrophobicity using nonporous reversed-phase HPLC in the second dimension (IEF-NP RP HPLC). Proteins were mapped using original software in order to create a protein pattern analogous to that of the 2-D PAGE image. RP HPLC peaks are represented by bands of different intensity in the 2-D image, according to the intensity of the peaks eluting from the HPLC. Each peak was collected as the eluent of the HPLC separation in the liquid phase. The proteins collected were identified using proteolytic enzymes, MALDI-TOF MS and MSFit database searching. Using IEF-NP RP HPLC, approximately 700 bands were resolved in a pI range from 3.2 to 9.5 and 38 different proteins with molecular weights ranging from 12,000 to 75,000 were identified. In comparison to a 2-D gel separation of the same human erythroleukemia cell line lysate, the IEF-NP RP HPLC produced improved resolution of low mass and basic proteins. In addition, the proteins remained in the liquid phase throughout the separation, thus making the entire procedure highly amenable to automation and high throughput. It is demonstrated that IEF-NP RP HPLC provides a viable alternative to the 2-D gel separation method for the screening of protein profiles.

Chromatography, High Pressure Liquid↗

PubMed1998

Gas-phase separations of protease digests.

A mixture of peptides from a complete tryptic digest of ubiquitin has been analyzed by ion mobility/time-of-flight mass spectrometry techniques. All components of the mixture were electrosprayed and ions were separated in the gas phase based on differences in their mobilities through helium before being dispersed into a time-of-flight mass spectrometer for mass-to-charge analysis. The data show that ions separate into families primarily according to differences in their charge states and, to a lesser extent, differences in conformation. This approach reduces spectral congestion typically associated with electrosprayed mixtures and provides charge assignments for mass-to-charge ratio data. Gas-phase separations of ions appear to provide a new physical basis for characterizing components of biological mixtures.

Endopeptidases↗