M.N. Jones

The interaction of sodium dodecyl sulfate with polyethylene oxide

Abstract The interaction of sodium dodecyl sulfate with polyethylene oxide has been investigated by measurements of conductance, surface tension, and viscosity. The plots of surface tension and conductance as a function of the sodium dodecyl sulfate concentration show two transition points. These results are interpreted in terms of a polymer-surfactant complex or micelle formed by binding between the surfactant ions and the polymer.

The mechanism of phospholipid adsorption from vesicle dispersions onto glass surfaces

Abstract The adsorption of phospholipid onto glass beads from aqueous dispersions of phospholipid vesicles formed from L-α-dipalmitoylphosphatidylcholine (DPPC) and its mixtures with phosphatidylinositol (PI) has been investigated. The temperature dependence of adsorption of DPPC from vesicles is characterised by marked changes in both the adsorption constant and limiting adsorption in the region of the gel to liquid-crystalline phase transition. Below the phase transition temperature the area per molecule of adsorbed lipid corresponds to more than monolayer adsorption while above the phase transition temperature the area per molecule exceeds that in a close-packed monolayer. For vesicles consisting of 50 wt% DPPC and PI the adsorption constant is independent of temperature. The time course for the adsorption of lipid shows a dependence on the surface roughness of the glass beads consistent with initial adsorption of vesicles, followed by vesicle disruption resulting in an increase in particle size in the dispersed phase. Contact angle measurements were made on lipid monolayers on glass made by the Langmuir-Blodgett technique and by deposition from vesicles. The contact angles were independent of the method of lipid deposition and decreased from 44° for DPPC to 8° for PI. It is suggested that a proportion of the lipid molecules in the adsorbed layers are oriented with their head groups into the aqueous phase; this orientation being increasingly favoured for PI rich layers.

The electrophoretic properties and aggregation behaviour of dipalmitoylphosphatidylcholine—phosphatidylinositol vesicles in aqueous media

Abstract Mixed lipid vesicles have been prepared from dipalmitoylphosphatidylcholine (DPPC)—phosphatidylinositol (PI) mixtures. The rate of aggregation of sonicated vesicles by Ca 2+ and Mg 2+ ions is dependent on the DPPC: PI ratio. For vesicles of composition DPPC: PI, 75: 25 wt% the rate goes through a maximum as a function of divalent ion concentration in the range 0–25 m M . Microelectrophoresis measurements have been made on multilamellar vesicles of composition DPPC:PI, 75: 25 wt% as a function of Ca 2+ and Mg 2+ ion concentration. The multilamellar vesicles are negatively charged in the presence of divalent ions at low concentrations but have zero points of charge for Ca 2+ and Mg 2+ ions of approximately 2 and 15 m M , respectively. The electrophoretic data has been used to derive a Gibbs energy of binding of Ca 2+ ions to the vesicle surface of −27.9 kJ mol −1 at 25°C and a specific Gibbs energy of interaction between Ca 2+ ions and PI of −64.7 kJ mol −1 .

Vesicle—vesicle interaction and forces between bilayers in phospholipid systems incorporating phosphatidylinositol

Sonicated vesicles have been prepared from mixtures of dipalmitoylphosphatidylcholine (DPPC) and phosphatidylinositol (PI) covering a range of composition. The effect of temperature on the rates of aggregation of the vesicles on addition of calcium and magnesium ions has been investigated. Apart from pure PI vesicles the rates of aggregation decrease dramatically as the temperature approaches the gel-to-liquid crystalline phase transition temperature of DPPC. Low angle X-ray analysis of lamellar phases of DPPC-PI (75:25 wt%) in the presence of Ca2+ ions shows that between 35 and 45°C the repeat distance goes from 136 to 68 A. It is suggested that below the chain-melting temperature of DPPC Ca2+ ions induce lateral phase separation of PI giving a lamellar repeat distance corresponding to the thickness of two bilayers. The net repulsive pressure between DPPC-PI bilayers has been measured by a vapour pressure technique as a function of temperature. At close apposition (<15 A) the pressure is characteristic of hydration repulsion and increases with temperature. The repulsive force between the bilayers, lateral pressure and compressibility of the bilayers have also been determined. On progressively removing water from between the bilayers 5–14% of the work done goes into bilayer deformation, the remaining 86–95% being required to bring the bilayers together.

The measurement of phospholipid adsorption from liposomal dispersions onto glass surfaces

Abstract Three different techniques have been used to measure the adsorption of L-α-dipalmitoylphosphatidylcholine (DPPC) onto glass surfaces in contact with aqueous dispersions of unilamellar or multilamellar DPPC liposomes. Using columns of porous or solid glass beads it is shown that the phospholipid adsorbed forms an expanded monolayer. Continuous circulation of liposomal dispersions through porous glass columns resulted in retention of lipid by entrapment. A batch technique of measuring adsorption showed that equilibrium was achieved rapidly (within 1 h) and gave apparently Langmuirian isotherms. The average limiting area per molecule at 25°C was found to be 95 A 2 , consistent with the formation of an expanded monolayer. At 50°C the limiting area per molecule was 115 A 2 .

The electrophoretic properties of dipalmitoylphosphatidylcholine—phosphatidylinositol vesicles in the presence of lanthanum ions

Abstract Large multilamellar vesicles have been prepared from dipalmitoylphosphatidylcholine (DPPC)—phosphatidylinositol (PI) mixtures over the complete range of composition. The electrophoretic mobilities of the vesicles have been measured in a buffer at pH 6.6 and as a function of lanthanum ion concentration over a range covering the zero point of charge. The zeta potentials of the vesicles increase with PI content and the degree of ionisation of the PI molecules lies in the range 0.16 to 0.23. The zeta potentials have been interpreted in terms of La 3+ ion binding using a Gouy—Chapman—Stern model as proposed by Ottewill and co-workers. Apparent and intrinsic binding constants and the number of binding sites have been determined. The Gibbs energy of ion adsorption (Δ G 0 ads ) has been found to obey the linear relationship: Δ G 0 ads (kJ mol −1 ) = −(66.9 (± 1.7) x PI + 41.2 (± 0.8) x DPPC ) which demonstrates that each phospholipid molecule binds La 3+ ions independently.

The interaction of lanthanum ions with dipalmitoylphosphatidylcholine—phosphatidylinositol vesicles

Abstract Sonicated and multilamellar dipalmitoylphosphatidylcholine (DPPC)—phosphatidylinositol (PI) vesicles have been prepared with different DPPC/PI weight ratios. The interaction of the sonicated vesicles with lanthanum (La 3+ ) ions has been studied by absorbance measurements at 25°C. Lanthanum ions do not fuse DPPC—PI vesicles (or phosphatidylserine vesicles) so that the absorbance changes are interpreted in terms of vesicle aggregation. The rates of aggregation pass through maxima as a function of La 3+ ion concentration. The concentration of La 3+ ions needed to reach the maxima increases with the PI content of the vesicles. The results are consistent with the formation of multivesicular aggregates. Microelectrophoresis measurements on multilamellar vesicles as a function of La 3+ ion concentration show that the vesicles adsorb La 3+ ions. Upon addition of La 3+ , the zero point of charge is located at higher La 3+ concentrations as the vesicle PI content is increased. The electrophoretic data have been analysed on the basis of the Stern model of the electrical double layer. In water (pH 6.6) at 25°C the intrinsic Gibbs energy for La 3+ ion binding to vesicles containing 25 and 50 wt% PI (Δ G 0 ads ) is found to be −61 kJ mol −1 . Differential scanning calorimetry has been used to determine the effect of La 3+ ions on the enthalpy (Δ H ) and the gel-to-liquid crystalline transition temperature ( T c ) in dispersions of multilamellar vesicles. At low molar ratios of La 3+ to PI, minima were observed in both Δ H and T c . The results are interpreted in terms of the withdrawal of DPPC molecules from participation in chain-melting and suggest a disruption of the cooperative behaviour of DPPC in the immediate environment of PI molecules on binding La 3+ ions.

Mixed monolayers of phosphatidylinositol and dipalmitoylphosphatidylcholine and their interaction with liposomes

Surface pressure-area isotherms for phosphatidylinositol (PI) and its mixtures with dipalmitoylphosphatidylcholine (DPPC) at the aqueous-air interface have been obtained on substrates of two ionic strengths (0.013 M and 0.117 M) at pH 6.6 and 25°C. Excess Gibbs energies of mixing in the monolayers have been calculated as a function of composition and surface pressure. At the lower ionic strength at low PI content, the monolayers show negative deviations from ideality whereas at the higher ionic strength deviations are positive and the existence of the liquid to close-packed phase transition of DPPC indicates lateral phase separation. The interaction of PI, DPPC and PI/DPPC sonicated liposomes with monolayers of the same corresponding composition has been studied at constant surface area. It is found that the transfer of lipid from liposome to monolayer at a given surface pressure is greater for the PI and PI/DPPC liposome—monolayer combinations where the lipids are in the liquid-crystalline state than for the gel-state DPPC system. Only a very small fraction of liposomal lipid is available for transfer to the monolayer (< 0.2%), consistent with the transfer of lipid from the outer layer of the liposomal bilayer with retention of the integrity of the liposomes. Addition of calcium ions does not markedly change the penetration of lipid into the monolayer but relatively high calcium concentrations (25 mM) increase the rate at which the surface pressure increases on interaction with liposomes.

The aggregation of block copolypeptides in aqueous solution

Abstract A series of block copolypeptides of composition (sodium glutamate)m (leucine)n with number average molecular weights in the range 7 × 103 to 11 × 103 and m:n ratios in the range 1.7 to 3.0 have been synthesized. These materials have been shown by light scattering and ultracentrifugation to form very stable aggregates in aqueous solution. The aggregates have weight average molecular weights in the range 3.7 × 105 to 10 × 105 depending on the ionic strength. The absence of dissymmetry of scattered light shows that the aggregates have a very compact structure. Circular dichroism spectra show that at low pH both the glutamate and leucine residues are in an α-helical conformation but on raising the PH the glutamate and part of the leucine blocks undergo a conformation change. NMR spectroscopy shows that the leucine residues in the aggregates are in an extremely rigid state, probably microcrystalline. The aggregates are not dissociated under conditions which would denature globular proteins.

Thermodynamics of the interaction between n-dodecyltrimethylammonium bromide and catalases

Abstract The thermodynamic parameters for the interaction of the cationic detergent n -dode-cyltrimethylammonium bromide (DTAB) with bovine liver catalase at pH 3.2, 6.4 and 10.0 and with Aspergillus niger catalase at pH 10.0 have been measured at 25° C by a combination of equilibrium dialysis to determine the Gibbs energies of detergent binding and microcalorimetry to determine the enthalpies of interaction. The binding data have been analysed in terms of two theoretical models (the Hill equation and the Wyman binding potential concept), the calorimetric data show that DTAB interacts either athermally or endothermically with the catalases. The results have been compared with similar data for the interaction of the anionic detergent sodium n -dodecylsulphate (SDS) with catalases. For both detergents the interactions are dominated by the increase in entropy on detergent binding. For SDS the binding entropy is larger in acid solution when the detergent and protein are of opposite charge whereas for DTAB the binding entropy is larger in alkaline solution.