Contrasting behavior of zwitterionic and cationic polymers bound to anionic liposomes

Zwitterionic polymers were prepared by quaternizing polyvinylpyridine (DP = 1100) with bromoacids (Br(CH2)nCOOH, where n = 1, 2, 3, and 5). The resulting polymers were then added to unilamellar liposomes composed of egg lecithin or dipalmitoylphosphatidylcholine admixed with 20 mol % of cardiolipin (a phospholipid with two negative charges). These systems were compared (along with polyethylvinylpyridinium chloride, a polycation) by light scattering, electrophoretic mobility, fluorescence, and high-sensitivity differential scanning calorimetry. The external zwitterionic polymers induce no flip-flop of cardiolipin from the inner leaflet to the outer leaflet as does the polycation. Aside from this similarity, the four zwitterionic polymers all behave differently from each other toward the anionic liposomes: (a) For n = 1, there is no detectable interaction between the polymer and the liposomes. (b) For n = 2, electrostatic attraction induces polymer-liposome association (reversed by the addition of NaCl) that maintains the original negative charge on the liposome. Aggregation of the liposomes accompanies polymer adsorption. (c) For n = 3, electrostatic binding also occurs along with aggregation. However, the binding is so strong that NaCl is unable to induce polymer/liposome dissociation. (d) For n = 5, there is polymer binding and NaCl-promoted dissociation but no substantial aggregation. These differences among the closely related polymers are discussed and analyzed in molecular terms.     

Torsion-induced phase transitions in fluids confined between chemically decorated substrates

In this paper we investigate the phase behavior of a "simple" fluid confined to a chemically heterogeneous slit pore of nanoscopic width s(z) by means of Monte Carlo simulations in the grand canonical ensemble. The fluid-substrate interaction is purely repulsive except for elliptic regions of semiaxes A and B attracting fluid molecules. On account of the interplay between confinement (i.e., s(z)) and chemical decoration, three fluid phases are thermodynamically permissible, namely, gaslike and liquidlike phases and a "bridge phase" where the molecules are preferentially adsorbed by the attractive elliptic patterns and span the gap between the opposite substrate surfaces. Because of their lack of cylindrical symmetry, bridge phases can be exposed to a torsional strain 0相似文献   

Thermodynamics and phase transitions in a fluid confined by a harmonic trap

We study a fluid of interacting atoms confined by a three-dimensional anisotropic harmonic potential, similar to those produced by the magnetic traps used to confine cold atoms. We show that instead of the usual thermodynamic variables pressure and volume, no longer existing in this case, there appear "new" variables: the volume is replaced by (the inverse cube of) the geometric average of the oscillator frequencies of the trap, and the hydrostatic pressure is replaced by an intensive variable, conjugate to the previous one, and responsible for the mechanical equilibrium of the fluid in the trap. We discuss the origin and physical meaning of these new variables. With the aid of molecular dynamics simulations we show the emergence of novel liquid, vapor and solid-like phases in a classical fluid. In particular, we calculate the liquid-vapor-like coexistence curve and show evidence for the appearance of a critical point. These phase transitions should be observable in fluids of not-so-cold alkaline atoms.     

Stability of the normal, zwitterionic neutral and anionic forms of aspartic acid in gas phase and aqueous media

Aspartic acid has a place of special importance among amino acids in view of its property to racemize from the - to the -form in living and nonliving systems which can be used to determine age. Molecular geometries of two isomeric normal neutral forms (called A and B forms), isomeric zwitterionic neutral forms (called A(Z) and B(Z) forms) and anions of these forms of aspartic acid were optimized at the RHF/6-31+G^* level. The geometries of hydrogen bonded complexes of each of the above species with one, two and three water molecules were also optimized at the same level. All these systems were solvated in bulk water using the polarized continuum model (PCM) of the self-consistent reaction field (SCRF) theory at the RHF/6-31+G^* level using the gas phase optimized geometries. It is found that in gas phase, the normal neutral A and B forms of aspartic acid would coexist while the zwitterionic A(Z) and B(Z) forms of the same would be absent. On the other hand, in bulk aqueous media, the normal neutral A and B forms of aspartic acid would not occur while the zwitterionic A(Z) and B(Z) forms would be present. Further, the abundance of the A(Z) form would be much more than that of the B(Z) form in bulk water. If the solution pH is adjusted appropriately so that a monoanion is formed, the anion of aspartic acid obtained from the neutral zwitterionic form by deprotonating O₈ would occur dominantly.     

Shear rheology of anionic and zwitterionic modified polyacrylamides

Two groups of copolymers were synthesized from high molecular weight polyacrylamides. One group of copolymers consisted of sulfonated, anionic copolymers (PAM-S) of acrylamide with the sodium salt of 2-acryloamido-2-methyl-1-propane sulfonic acid, and the other consisted of zwitterionic copolymers (PAM-Z) of acrylamide with a sulfobetaine methacrylate monomer. The shear rheology of aqueous solutions of the copolymers and their mixtures was studied experimentally. Solutions of both copolymers exhibit shear thinning behavior in the range of concentrations explored. Solutions of mixtures of two copolymers (PAM-Z and PAM-S) exhibited a slight viscosity synergy at high relative contents of PAM-S. Addition of a relatively high concentration of an electrolyte (0.3 M NaCl) induces decreases in viscosity due to coil contraction and eliminates the synergy of the mixtures. Mixtures of the zwitterionic copolymer and a cationic surfactant, cetyl trimethylammonium p-toluene sulfonate (CTAT), were also studied. These solutions exhibit a strong synergistic effect at low-shear rates when the surfactant forms wormlike micelles. In addition, oscillatory shear measurements demonstrate that PAM-Z/CTAT mixtures are significantly more elastic than CTAT solutions, which indicates that PAM-Z is effective in promoting micelle entanglements, as reflected by the increase in relaxation time with PAM-Z content.     

两亲性分子聚集体的相变及其协同性

两亲性分子聚集体是一类重要的软物质,它们有着丰富而复杂的相行为.本文主要从两个方面综述了作者所在的研究组在两亲性分子聚集体相变研究方面的工作进展.(1)磷脂相关体系相变热力学:归纳了多种小分子(二甲基亚砜、甘油、海藻糖、尿素等)对于磷脂体系相行为的调控,比较并讨论了固醇类分子和葡萄糖神经酰胺分子诱导磷脂分子形成液态有序相的能力,还介绍了计算机模拟磷脂相行为的工作进展.(2)两亲性分子聚集体相变的协同性:先介绍了相变协同性(即分子头部、尾部、界面等基团在相变过程中的一致性)问题的提出,然后通过双十八烷基二甲基溴化铵分子和硬脂酰溶血卵磷脂两个体系的研究实例,说明两亲性分子聚集体相变过程中存在着头尾不一致的现象.对这个问题的研究,将为我们打开挑战相态转变的一系列重大问题(如相变动力学、相态多型性、相态稳定性以及相变可逆性等)的新窗口.     

Nuclear magnetic relaxation and phase transitions of diethylene glycol in Vycor porous glasses

The times of longitudinal T ₁ and transverse T ₂ magnetic relaxation of protons of diethylene glycol in the bulk phase and in Vycor porous glasses with average pore radii of 4, 11, and 32 nm are measured by the pulse NMR method in the 172–350 K temperature range. It is found that, for all samples of porous glasses, the crystallization of diethylene glycol is not observed if its content corresponds to the monolayer surface filling. The minimum content of diethylene glycol, which makes it possible to cause its crystallization in porous glass, is determined. By analyzing the dependence of the characteristics of a component with T _2b = 20−40 μs in the transverse magnetization decay on the content of diethylene glycol in porous glasses, the volume of nanopores comparable in size with diethylene glycol molecule is estimated.     

Kinetic study in water-ethylene glycol cationic, zwitterionic, nonionic, and anionic micellar solutions

The spontaneous hydrolysis of phenyl chloroformate was studied in water-ethylene glycol, EG, cationic, zwitterionic, nonionic, and anionic micellar solutions, the surfactants being tetradecyltrimethylammonium bromide, tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, tricosaoxyethylene glycol ether, and sodium dodecyl sulfate. The dependence of the observed rate constant on surfactant concentration as well as on the percentage by weight of EG, varying from 0 to 50 wt %, was investigated. Information about changes in the critical micelle concentrations, in the micellar ionization degrees (for ionic surfactants), in the aggregation numbers, and in the polarity of the interfacial region of the micelles upon changing the weight percent of EG was obtained through conductivity, surface tension, spectroscopic, and fluorescence measurements. A simple pseudophase model was adequate to rationalize the kinetic data. Micellar medium effects were explained by considering charge-charge interactions and polarity, ionic strength, and water content in the micellar interfacial region. The acceleration of the reaction produced by an increase in the amount of EG present in the mixture was explained on the basis of the substantial decrease in the equilibrium binding constant of phenyl chloroformate molecules to the micelles, resulting in the contribution of the reaction taking place in the bulk water-EG phase being more important. The weight percent of EG did not substantially influence the rate constant in the micellar pseudophase.     

Vesicle stability in aqueous mixtures of zwitterionic/anionic surfactants

It was studied that the influences of the aging, temperature, addition of the polymer and cosolvent on the stability of the vesicles spontaneously formed in the mixtures of zwitterionic surfactant (dodecyl carboxyl betaine, C₁₂BE) and double-tailed anionic surfactant (sodium bis(2-ethylhexyl) sulfosuccinate, AOT) under the inducement of salt by means of freeze-fracture and negative-staining transmission electron microscopy (TEM), dynamic light scattering (DLS) and turbidity measurements. It is found that the vesicles can exist over a long period of aging (about 300 days) at room temperature, show good stability after a heating–cooling cycle of 90–25 °C and a freeze–thaw cycle of −10 to 25 °C, respectively, and may be transformed from spherical vesicles to tubelike structures induced by high temperature 90 °C. Under the effect of (PEO)₁₃(PPO)₃₀(PEO)₁₃ (L64), the transition from unilameller vesicles to large multivesicular vesicles. The presence of ethanol may decrease the stability of vesicles, resulting in the fusion among vesicles to form large vesicles. The excessive amount of ethanol may destroy the vesicles, and the order of ability of destroying vesicles was obtained to be C₅H₁₁OH > C₄H₉OH > C₃H₇OH > C₂H₅OH > CH₃OH.     

Solid/solid phase transitions in confined thin films: a zero temperature approach

We report a density functional theory study of confinement induced solid/solid phase transitions in a thin film (modeled as methane) at T=0. The solid film is confined by two graphite surfaces represented by a mean-field potential. As the wall separation is varied the structure of the confined film changes, which influences its density and the solvation force. Using the directly accessible grand canonical potential density we determine the stable phases and calculate the exact location of the phase transitions. We observe a series of phases having square and triangular symmetry. At low wall separations we find zig-zag buckling and an asymmetric buckled phase, whose structure is consistent with the strongest buckling instability of a triangular monolayer predicted by Chou and Nelson [Phys. Rev. E 48, 4611 (1993)] but, to our knowledge, has not been observed as a stable phase before. We find that the two-dimensional order parameters Psi(4) (square symmetry) and Psi(6) (triangular symmetry) show unphysical behavior in the transition region between square and triangular symmetry. Thus, in the present model they fail to predict the right location of the phase transitions.     

Trifluoperazine effects on anionic and zwitterionic micelles: a study by small angle X-ray scattering

In this work small angle X-ray scattering (SAXS) studies on the interaction of the phenothiazine trifluoperazine (TFP, 2-10 mM), a cationic drug, with micelles of the zwitterionic surfactant 3-(N-hexadecyl-N,N-dimethylammonium) propane sulfonate (HPS, 30 mM) and the anionic surfactant sodium dodecyl sulfate (SDS, 40 mM) at pH 4.0, 7.0, and 9.0 are reported. The data were analyzed through the modeling of the micellar form factor and interference function, as well as by means of the distance distribution function p(r). For anionic micelles (SDS), the results evidence a micellar shape transformation from prolate ellipsoid to cylinder accompanied by micellar growth and surface charge screening as the molar ratio TFP:SDS increases in the complex for all values of pH. Small ellipsoids with axial ratio nu=1.5+/-0.1 (long dimension of 60 A) grow and reassemble into cylinder-like aggregates upon 5 mM drug incorporation (1 TFP:8 SDS monomers) with a decrease of the micelle surface charge. At 10 mM TFP:40 mM SDS cylindrical micelles are totally screened with an axial ratio nu approximately 4 (long dimension approximately 140 A at pH 7.0 and 9.0). However, at pH 4.0, where the drug is partially diprotonated, 10 mM TFP incorporation gives rise to a huge increase in micellar size, resulting in micelles at least 400 A long, without altering the intramicellar core. For zwitterionic micelles (HPS), the results have shown that the aggregates also resemble small prolate ellipsoids with averaged axial ratio approximately nu=1.6+/-0.1. Under TFP addition, both the paraffinic radius and the micellar size show a slight decrease, giving evidence that the micellar hydrophobic core may be affected by phenothiazine incorporation rather than that observed for the SDS/TFP comicelle. Therefore, our results demonstrate that the axial ratio and shape evolution of the surfactant:TFP complex are both dependent on surfactant surface-charge and drug:surfactant molar ratio. The results are compared with those recently obtained for another phenothiazine drug, chlorpromazine (CPZ), in SDS and HPS micelles (Caetano, Gelamo, Tabak, and Itri, J. Colloid Interface Science 248 (2002) 149).     

Spontaneous vesicle formation in aqueous solution of zwitterionic and anionic surfactant mixture

Vesicles form spontaneously in the aqueous mixtures of dodecyl sulfonate betaine (DSB) and sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT (AOT)) at certain mixing ratios, which has been demonstrated by microcalorimeter, negative-staining transmission electronic microscopy (TEM) and quasi-elastic light scattering (QELS) methods. The addition of NaCl will expand the range of vesicle formation, and monodispersed vesicles are obtained in the solution from the salinity of 0.03 to 0.09 M at the mixing molar ratio of 7/3 (DSB/AOT, mol/mol), with the polydispersity of the system lower than 0.1. To learn more about the structural change in the mixture, ultrasonic was employed finally. Meanwhile analysis was made from the viewpoint of molecular geometry structure.     

Salt-assisted and salt-suppressed sol-gel transitions of methylcellulose in water

The effects of various salts on the sol-gel transition of aqueous methylcellulose (MC) solutions have been studied systematically by means of a micro differential scanning calorimeter. It was found that the heating process was endothermic while the cooling process was exothermic for both MC solutions with and without salts. The addition of salts did not change the patterns of gelation and degelation of MC. However, the salts could shift the sol-gel transition and the gel-sol transition to lower or higher temperatures from a pure MC solution, depending on the salt type. These opposite effects were termed the salt-assisted and salt-suppressed sol-gel transitions. Either the salt-assisted transition or the salt-suppressed sol-gel transition was a function of salt concentration. In addition, each salt was found to have its own concentration limit for producing a stable aqueous solution of MC at a given concentration of MC, which was related to the anionic charge density of the salt. Cations were proved to have weaker effects than anions. The "salt-out strength", defined as the salt effect per mole of anion, was obtained for each anion studied. The thermodynamic mechanisms involved in the salt-assisted and salt-suppressed sol-gel transitions are discussed.     

Thermally stimulated current (TSC) study of the Tg and Tll transitions in anionic polystyrenes

The thermally stimulated current (TSC) technique has been used to investigate three anionic polystyrenes of M?_n 17,000, 71,700, and 1.55 × 10⁶, i.e., M < M_c, M > M_c, and M ? M_c, where M_c is the entanglement molecular weight. A current maximum near T_g designated T_Mg, has relaxation times which follow an Arrhenius equation. A second current maximum at T > T_g appears to be the T_ll process and is designated T_Mll. Relaxation times for it follow a Vogel equation. T_Mg and T_Mll vary with molecular weight, increasing below M_c and leveling off above M_c at a temperature of about 170°C. Values of T_Mg and T_Mll are compared with values of T_g and T_ll obtained from torsional braid analysis, which involves melt flow; and with differential-scanning-calorimetric values on fused films, where there is no transport of polymer. It is concluded from such cross-comparisons that TSC, at least for polystyrene, is a quasistatic test which may involve microscopic viscosity. Macroscopic viscosity does not play a role. The ratio T_Mll/T_Mg is in the range 1.10–1.16, similar to T_ll/T_g values by other methods. A few comments about T_ll in atactic poly(methyl methacrylate) by the TSC method are given.     

Encapsulation of the ferritin protein in sol-gel derived silica glasses

A significant recent development in sol-gel science has been the encapsulation of biomolecules such as proteins and enzymes in optically transparent silica glasses. This paper reports on the encapsulation of an iron (Fe) storage protein, ferritin, to develop a magnetic silica glass. Native ferritin, which has a nanometer-sized microcrystalline Fe oxide core, was encapsulated in optically transparent silica glasses using the sol-gel process. Fe could be released from ferritin but could not be reconstituted into apoferritin when the protein was trapped in the pores of the glass. Transmission electron microscopy of ferritin-doped aged silica gels indicated that crystallinity of the Fe oxide core was retained upon sol-gel encapsulation. Magnetic measurements on ferritin-doped silica gels indicated the material to be paramagnetic, but not superparamagnetic.     

Structure and characteristics of the complexes between polyampholites and anionic liposomes

Polyampholites are synthesized by the alkylation of poly-4-vinylpyridine with ω-bromocarboxylic acids, and their interaction with the negatively charged bilayer lipid vesicles (liposomes) is studied. In the above polymers, quaternized pyridine units are zwitterion (betaine) groups, in which cationic and anionic groups are linked by the -(CH₂) _n -bridges. Via the methods of fluorescence, laser scattering, and DSC, the length of the ethylene spacer in the betaine group is shown to control the ability of the polymer to interact with anionic liposomes and induce structural rearrangements in the liposomal membrane. At n = 1, polybetaine is not linked to anionic liposomes. At n = 2, polybetaine is sorbed on the membrane, but it causes no dramatic structural rearrangements in the bilayer. At n = 3, the adsorption of polybetaine triggers the lateral segregation of lipids in the outer membrane layer. At n = 5, adsorption of polymer is accompanied by the lateral segregation and flip-flop of lipid molecules; as a result, all anionic membrane lipids are involved in the microphase separation. This evidence is of evident interest for the controlled design of polymers and related complexes and conjugates for biomedical applications.     

Solubility limits and phase diagrams for fatty acids in anionic (SLES) and zwitterionic (CAPB) micellar surfactant solutions

The limiting solubility of fatty acids in micellar solutions of the anionic surfactant sodium laurylethersulfate (SLES) and the zwitterionic surfactant cocamidopropyl betaine (CAPB) is experimentally determined. Saturated straight-chain fatty acids with n=10, 12, 14, 16, and 18 carbon atoms were investigated at working temperatures of 25, 30, 35, and 40°C. The rise of the fatty acid molar fraction in the micelles is accompanied by an increase in the equilibrium concentration of acid monomers in the aqueous phase. Theoretically, the solubility limit is explained with the precipitation of fatty acid crystallites when the monomer concentration reaches the solubility limit of the acid in pure water. In agreement with theory, the experiment shows that the solubility limit is proportional to the surfactant concentration. For ideal mixtures, the plot of the log of solubility limit vs. the chainlength, n, must be a straight line, which is fulfilled for n=14, 16, and 18. For the fatty acids of shorter chains, n=10 and 12, a deviation from linearity is observed, which is interpreted as non-ideal mixing due to a mismatch between the chainlengths of the surfactant and acid. The data analysis yields the solubilization energy and the interaction parameter for the fatty acid molecules in surfactant micelles. By using the determined parameter values, phase diagrams of the investigated mixed solutions are constructed. The four inter-domain boundary lines intersect in a quadruple point, whose coordinates have been determined. The results can be applied for the interpretation and prediction of the solubility, and phase behavior of medium- and long-chain fatty acids and other amphiphiles that are solubilizable in micellar surfactant solutions, as well as for determining the critical micellization concentration (CMC) of the respective mixed solution.     

AC calorimetric study of phase transitions in phosphatidylcholine-cholesterol systems

Using an ac calorimetric method, detailed behaviour of the heat capacity in dipalmitoyl-phosphatidylcholine-cholesterol system was studied in the cholesterol concentration less than 5 mol%. It was revealed that the heat capacity near the main transition was composed of at least four anomalies, i.e., multipeak took place in the heat capacity. This fact indicates that a simple theory explaining coexistence of two phases in two component systems does not work in the multipeak region. Then, relation between the multipeak heat capacity and the change of the ripple structure with the cholesterol concentration should be taken into account, when we consider thermodynamical behaviour of the systems.

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Zusammenfassung Mittels AC-Kalorimetrie wurde bei Cholesterol-Konzentrationen von weniger als 5 mol% das Verhalten der Wärmekapazität im System Dipalmitoylphosphatidylcholin-Cholesterol untersucht. Es wurde gezeigt, daß sich die Wärmekapazität in der Nähe der Hauptumwandlung aus mindestens vier Anomalien zusammensetzt, d.h. bei der Wärmekapazität kann ein Multipeak beobachtet werden. Diese Tatsache zeigt, daß eine einfache Theorie, welche die Koexistenz zweier Phasen in einem Zweikomponenten-System erklärt, für die Multipeakregion nicht geeignet ist. Weiterhin sollte bei Überlegungen zum thermodynamischen Verhalten von Systemen eine Beziehung zwischen der Multipeak-Wärmekapazität bzw. der Welligkeitsstruktur und der Cholesterol-Konzentration berücksichtigt werden.

     

Calorimetric study of phase transitions in a liquid-crystal-based microemulsion

A lyotropic inverse micelle phase composed of water, thermotropic liquid-crystal octylcyanobiphenyl (8CB), and surfactant (DDAB) was studied by using high-resolution calorimetry on several mixtures with 3%, 8%, and 15% micelle concentration. Calorimetric results show strong depression of the isotropic to nematic (I-N) phase-transition temperature. Broad heat-capacity anomalies show the existence of a wide coexistence range of isotropic, nematic, and smectic-A phases, which mimics the behavior of a new nearly stable thermodynamic phase. An observation of the rather sharp almost bulklike nematic to smectic-A (N-A) transition at low-temperatures indicates that our heat capacity results are consistent with the phase separation scenario in which significant number of micelles is expelled during I-N conversion leaving almost pure nematic phase at lower temperatures. It was found that micelles get almost completely remixed on heating the mixture back to the isotropic phase.     

Spectroscopic study of phase transitions in natural calcite mineral

The process and the formation of new minerals upon heating the carbonate rocks containing clay minerals, together with calcite are determined with thermal analysis, X-ray diffraction, infrared and Raman spectroscopy. The calcite-calcium oxide phase transition sequence was followed up to 947 degrees C in naturally occurring limestone samples. The spectral variations of the internal modes of the carbonate trigonal (nu(1), nu(2), nu(3) and nu(4)) were used to probe the structural phase transitions. The calcium oxide phase (which on reaction with atmospheric water forms portlandite) with an onset temperature of around 950 degrees C was also characterized by the appearance of the infrared mode around 450 cm(-1). The minerals, which were formed upon heating the calcite, were calcium oxide and wollastonite.