Promoted Phase Separation Research Articles

Tuning Membrane Phase Separation Using Nonlipid Amphiphiles

Lipid phase separation may be a mechanism by which lipids participate in sorting membrane proteins and facilitate membrane-mediated biochemical signaling in cells. To provide new tools for membrane lipid phase manipulation that avoid direct effects on protein activity and lipid composition, we studied phase separation in binary and ternary lipid mixtures under the influence of three nonlipid amphiphiles, vitamin E (VE), Triton-X (TX)-100, and benzyl alcohol (BA). Mechanisms of additive-induced phase separation were elucidated using coarse-grained molecular dynamics simulations of these additives in a liquid bilayer made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-diundecanoyl-sn-glycero-phosphocholine (DUPC). From simulations, the additive's partitioning preference, changes in membrane thickness, and alterations in lipid order were quantified. Simulations showed that VE favored the DPPC phase but partitioned predominantly to the domain boundaries and lowered the tendency for domain formation, and therefore acted as a linactant. This simulated behavior was consistent with experimental observations in which VE promoted lipid mixing and dispersed domains in both gel/liquid and liquid-ordered/liquid-disordered systems. From simulation, BA partitioned predominantly to the DUPC phase, decreased lipid order there, and thinned the membrane. These actions explain why, experimentally, BA promoted phase separation in both binary and ternary lipid mixtures. In contrast, TX, a popular detergent used to isolate raft membranes in cells, exhibited equal preference for both phases, as demonstrated by simulations, but nonetheless, was a strong domain promoter in all lipid mixtures. Further analysis showed that TX increased membrane thickness of the DPPC phase to a greater extent than the DUPC phase and thus increased hydrophobic mismatch, which may explain experimental observation of phase separation in the presence of TX. In summary, these nonlipid amphiphiles provide new tools to tune domain formation in model vesicle systems and could provide the means to form or disperse membrane lipid domains in cells, in addition to the well-known methods involving cholesterol enrichment and sequestration.

New thermosets obtained from bisphenol A diglycidyl ether and hydroxyl‐ended hyperbranched polymers partially blocked with benzoyl and trimethylsilyl groups

AbstractNew epoxy thermosets with improved flexibility were prepared by chemical modification of bisphenol A diglycidyl ether (DGEBA) with hyperbranched polymers (HBPs). Hydroxyl‐ended hyperbranched polyesters were modified by blocking part of the hydroxyl groups with trimethylsilyl or benzoyl groups. The curing of mixtures of DGEBA with various proportions of two modified HBPs using ytterbium triflate as cationic initiator was investigated using differential scanning calorimetry and thermomechanical analysis. The characterization of these materials was performed using several thermal analysis techniques and their morphology was investigated using electron microscopy. High proportions of HBPs reduced the glass transition temperature and the relaxed storage modulus but barely affected gelation. The overall curing shrinkage was controlled by the content of hydroxyl groups and by the changes of HBP molecular interactions during curing. The results indicated that the relative proportion and type of terminal groups play a role in the evolution of the curing and the properties of the thermosets. Hydroxyl groups promoted the covalent incorporation of the HBP to the network via hydroxyl‐induced chain‐transfer reactions, whereas benzoyl groups promoted phase separation. Formulations containing HBP blocked with benzoyl groups showed two phases connected through covalent linkages between the HBP‐rich phase and the epoxy matrix. Copyright © 2010 Society of Chemical Industry

Effects of verbascoside, a phenylpropanoid glycoside from lemon verbena, on phospholipid model membranes

Phenylpropanoid glycosides are water-soluble compounds widely distributed, most of them deriving from medicinal herbs. Among them, verbascoside or acteoside has exhibited a wide biological activity, being free radical scavenging the most representative one. Moreover, antitumor, antimicrobial, anti-inflammatory, anti-thrombotic and wound healing properties have been previously described. Herein, the interaction of verbascoside with phospholipid membranes has been studied by means of differential scanning calorimetry, fluorescence anisotropy and dynamic light scattering. Verbascoside showed stronger affinity for negatively charged membranes composed of phosphatidylglycerol (PG) than for phosphatidylcholine (PC) membranes. This compound promoted phase separation of lipid domains in PC membranes and formed a stable lipid complex with and approximate phospholipid/verbascoside ratio of 4:1. Despite its hydrophilic character, verbascoside's caffeoyl moiety was located deep into the hydrophobic core of PC membranes and was almost inaccessible to spin probes located at different depths in PG membranes. This compound affected the ionization behavior of the PG phosphate group and most likely interacted with the vesicles surface. The presence of verbascoside decreased the particle size in PG unilamellar vesicles through the increase of the phospholipid head group area. A localization of verbascoside filling the upper region of PG bilayers close to the phospholipid/water interface is proposed. These effects on membranes may help to understand the mechanism of the biological activity of verbascoside and other similar phenylpropanoid glycosides.

Carbon-supported ternary PtSnIr catalysts for direct ethanol fuel cell

Binary PtIr, PtSn and ternary PtSnIr electrocatalysts were prepared by the Pechini–Adams modified method on carbon Vulcan XC-72, and these materials were characterized by TEM and XRD. The XRD results showed that the electrocatalysts consisted of the Pt displaced phase, suggesting the formation of solid solutions between the metals Pt/Ir and Pt/Sn. However, the increase in Sn loading promoted phase separation, with the formation of peaks typical of cubic Pt3Sn. The electrochemical investigation of these different electrode materials was carried out as a function of the electrocatalyst composition, in a 0.5 mol dm−3 H2SO4 solution, with either the presence or the absence of ethanol. Cyclic voltammetric measurements and chronoamperometric results obtained at room temperature showed that PtSn/C and PtSnIr/C displayed better electrocatalytic activity for ethanol electrooxidation compared to PtIr/C and Pt/C, mainly at low potentials. The oxidation process was also investigated by in situ infrared reflectance spectroscopy, to identify the adsorbed species. Linearly adsorbed CO and CO2 were found, indicating that the cleavage of the CC bond in the ethanol substrate occurred during the oxidation process. At 90 °C, the Pt89Sn11/C and Pt68Sn9Ir23/C electrocatalysts displayed higher current and power performances as anode materials in a direct ethanol fuel cell (DEFC).

Study on synthesis and morphology control of poly(4-vinylpyridine-co-butyl acrylate)/poly(styrene-co-butyl acrylate) composite microspheres

Monodispersed crosslinked cationic poly(4-vinylpyridine-co-butyl acrylate) [P(4VP-BA)] seed latexes were prepared by soapless emulsion polymerization, using 2,2′-azobismethyl(propionamidine)dihydrochloride (V50) as an initiator and divinylbenzene (DVB) or ethylene glycol dimethacrylate (EGDMA) as a crosslinker. The optimum condition to obtain monodispersed stable latex was investigated. It was found that the colloidal stability of the P4VP latex can be improved by adding an adequate amount of BA (BA/4VP = 1/4, w/w), and adopting a semicontinuous monomer feed mode. Subsequently, poly(4-vinylpyridine-co-butyl acrylate)/Poly(styrene-co-butyl acrylate) [P(4VP-BA)/P(ST-BA)] composite microspheres were synthesized by seeded polymerization, using the above latex as a seed and a mixture of ST and BA as the second-stage monomers. The effects of the type of crosslinker, the degree of crosslinking, and the initiators (AIBN and V50) on the morphology of final composite particles are discussed in detail. It was found that P(4VP-BA)/P(ST-BA) composite microspheres were always surrounded by a PST-rich shell when V50 was used as initiator, while sandwich-like or popcorn-like composite particles were produced when AIBN was employed. This is because the polarity of the polymer chains with AIBN fragments is lower than for the polymer with V50 fragments, hence leading to higher interfacial tension between the second-stage PST-rich polymer and the aqueous phase, and between PST-rich polymer and P4VP-rich seed polymer. As a result, the seed cannot be engulfed by the PST-rich polymer. Furthermore, the decrease of Tg of the second-stage polymer promoted phase separation between the seeds and the PST-rich polymer: sandwich-like particles formed more preferably than popcorn-like particles. It is important knowledge that various morphologies different from PST-rich core/P4VP-rich shell morphology, can be obtained only by changing the initiator, considering P4VP is much more hydrophilic than PST. The zeta potential of composite particles initiated by AIBN in seeded polymerization shifted from a positive to a negative charge. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1190–1203, 2002

The effect of carbon additives on the mesophase induction period of Athabasca bitumen

The ability of solid carbonaceous material to retard the formation of coke during thermal cracking and hydrocracking of heavy hydrocarbons is well known. In this study, we used in-situ microscopy (hot-stage) to obtain additional mechanistic information on whether fine coke and fullerene soot particles retard the growth of mesophase during thermal cracking of Athabasca bitumen, thus reducing the possibility of fouling in preheaters and furnaces. The findings from this study could also have application in other non-catalytic thermal processes such as visbreaking and coking. In the absence of additives in the Athabasca bitumen feed, the formation of mesophase occurred after 61 and 67 min (measured from room temperature) at reaction temperatures of 450°C and 440°C, respectively. The addition of solid coke (ca. 5 wt.%) from a commercial delayed coking operation shortened the mesophase formation time to almost 45–50 min under similar conditions. The coke, having surface area of only 1.65 m 2/g, resulted in enhanced bitumen fluidity and large-textured mesophase. These observations were rationalized based on the ability of delayed coker coke to release hydrocarbons into the bulk fluid during thermal cracking. Light hydrocarbons released from coke may have changed the solvating power of the liquid phase in bitumen and promoted phase separation, resulting in a shorter induction period. In contrast, adding small amounts of fine fullerene soot (ca. 1 and 5 wt.%) delayed the appearance of mesophase significantly under similar conditions. The ability of fullerene soot to physically absorb the mesophase precursors into its pore structure led to an increase in the apparent viscosity of the bulk phase, which is known to reduce mesophase size and prolong the induction period. Consistent with this, the induction period was prolonged an additional 10 min when the soot surface area was increased from 152 to 208 m 2/g. The increase in induction period is significant with respect to reaction times and suggests that these fullerene soot materials could be effective in allowing for increased severity and liquid products yield from visbreaking, with less likelihood of fouling in the preheater tubes and furnace walls.

Influence of xanthan gum on physical characteristics of heat-denatured whey protein solutions and gels

The influence of xanthan on the phase separation, optical properties and rheology of aqueous solutions and gels containing native (N) or heat-denatured (HD) whey protein isolate (WPI) was studied. In the absence of added salt, xanthan promoted phase separation in HD-WPI solutions but not in N-WPI solutions, which was attributed to the increased effective molecular weight of the heated proteins. In the presence of salt, xanthan caused a significant increase in gelation rate, shear modulus and opacity of HD-WPI solutions gelled by adding 200 mM NaCl. Rheology and turbidity measurements suggested that HD-WPI gels containing xanthan formed water-in-water emulsions consisting of xanthan-rich regions surrounded by a protein-rich aqueous phase. These results indicate that the thermodynamic incompatibility of xanthan and heat denatured whey proteins can be utilized to create products that exhibit a variety of different physicochemical characteristics.

Structural evolution and phase separation of CaO · Al2O3 · SiO2 glasses in an electric field

In the present paper, the process of phase separation in calcium aluminosilicate glasses (CAS glasses) containing TiO2 as a nucleating agent is studied. A static electric field promotes the process of phase separation even when the time of heat treatment is short. The effect of electric field on phase separation acts through the higher polarizability of Ti ions. Alkali ions, when present, will diffuse toward the cathode, which may generate different micromorphology at the parts of samples near the cathode and the anode. X-ray photoelectron spectroscopy and Raman spectra analysis confirm the conclusion that the electric field has promoted phase separation in CAS glasses.

On the origin of sphingolipid/cholesterol-rich detergent-insoluble cell membranes: physiological concentrations of cholesterol and sphingolipid induce formation of a detergent-insoluble, liquid-ordered lipid phase in model membranes.

Detergent-insoluble membrane fragments that are rich in sphingolipid and cholesterol can be isolated from both cell lysates and model membranes. We have proposed that these arise from membranes that are in the liquid-ordered phase both in vivo and in vitro [Schroeder et al. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 12130-12134]. In order to detect formation of the liquid-ordered phase while avoiding possible detergent artifacts, we have now used fluorescence quenching to examine the phase behavior of mixtures of phosphatidylcholines, sphingolipids, and cholesterol. Phase separation was found in binary mixtures of either dipalmitoylphosphatidylcholine (DPPC) or sphingomyelin (SM) and a nitroxide-labeled phosphatidylcholine (12SLPC). A DPPC- or SM-enriched solidlike gel phase coexisted with a 12SLPC-enriched liquid-disordered fluid phase at 23 degrees C. As expected, phase separation was not seen at low concentrations of DPPC or SM. Instead, only a uniform fluid phase was present. Including 33 mol % cholesterol in model membranes greatly promoted phase separation. Phase separation was seen at higher temperatures and/or at lower concentrations of DPPC or SM in the presence of cholesterol than in its absence. Mixtures of DPPC or SM and cholesterol are known to form the liquid-ordered phase. Therefore, the fact that phase separation was observed in the cholesterol-containing membranes shows that liquid-ordered and liquid-disordered phase domains coexist. At 37 degrees C, the SM-enriched liquid-ordered phase was first seen at a SM/PC ratio of close to 0.25, when SM made up 17% of the total lipid including cholesterol. (This is similar to or less than the SM concentration of the plasma membranes of mammalian cells.) Furthermore, the detergent insolubility of cholesterol-containing model membranes correlated well with the amount of liquid-ordered phase as detected by fluorescence quenching. Thus, the detergent-insoluble membranes isolated from cells are likely to exist in the liquid-ordered phase prior to detergent extraction. The promotion of liquid-ordered phase formation may be an important function of cholesterol and sphingolipids in cells and may be a major distinction between the cholesterol- and sphingolipid-rich plasma membrane and most other cellular membranes.