Aggregate size and dielectric behaviour of phosphatidylcholine in non-polar solvents

Phospholipids at the oil/water interface: adsorption and interfacial phenomena in an electric field

Interfacial effects produced in an immiscible liquid system by the action of an external electric field have been considered. The addition of small amounts of neutral phospholipids to the nonaqueous phase has been shown to result in a marked increase in the sensitivity of the interfacial boundary to the voltage applied, which is manifested by: (i) an accelerated decrease of the interfacial tension after the two immiscible liquid phases have been brought into contact; (ii) reduced interfacial tension, by 20-30 mN/m, at the oil/water interface at field strengths of 1-10 kV/m (the interfacial tension drop in the absence of phospholipids does not exceed 5 mN/m); (iii) development of electrohydrodynamic instability at the planar dividing surface between phases; and (iv) dispersion of water into the nonaqueous phase at smaller field strengths by a factor of about 100 as compared to those normally required in the absence of phospholipids. In order to gain a deeper insight into the mechanisms of interfacial phenomena, mainly exemplified by the n-heptane/water system containing phosphatidylcholine, three major issues have been considered: (1) Kinetics of the adsorption of phospholipid at the oil/water interface from the nonaqueous phase, and effects produced by exposure to an external electric field; also, the adsorption under equilibrium conditions, and the structure of the adsorption layer formed. (2) Interactions between neutral phospholipid and inorganic or organic ions at the interfacial boundary under the voltage applied. (3) Conditions for the occurrence of electrohydrodynamic instability at the dividing surface between oil and water and the formation of a water-in-oil emulsion; also aggregation and gelation processes induced in the nonaqueous phospholipid solution bulk by the action of a weak external electric field. Throughout the present paper, an attempt has been made to relate the microscopic behaviour of phospholipids under an external electric field to macroscopically observable properties at the movable interfacial boundaries. The adsorption studies have shown that phosphatidylcholine is prone to self-organization into a liquid-crystalline state at an immiscible liquid interface. The disintegration of the interfacial lipid film thus formed by the action of a weak electric field has been explained as due to an enhanced electrohydrodynamic instability of liquid crystals. This results in the formation of either an emulsion, or a microemulsion in the nonaqueous solution bulk. The formation of a microemulsion is manifested by the appearance of an optically anisotropic gel, stable only under an external applied electric field, in the nonaqueous solution bulk.(ABSTRACT TRUNCATED AT 400 WORDS)

Phospholipid-based reverse micelles

Physicochemical investigations on the aggregation of phospholipids (mainly phosphatidylcholines) in organic solvents are reviewed and compared with the aggregation behaviour of phospholipids in aqueous medium. In particular we review the data showing that phosphatidylcholines (lecithins) form reverse micellar structures in certain apolar solvents. In these systems not only low molecular weight compounds but also catalytically active enzymes and entire cells can be solubilized. In addition, highly viscous phosphatidylcholine gels can be obtained in organic solvents upon solubilizing a critical amount of water. Generally, phospholipid-based reverse micelles can be regarded as thermodynamically stable models for inverted micellar lipid structures possibly occurring in biological membranes.

Solubilization of enzymes and nucleic acids in hydrocarbon micellar solutions

The present state of the field of biopolymers solubilized in apolar solvents via reverse micelles is reviewed. First, an extensive discussion of the physical and chemical properties of reverse micelles is presented. Particular attention is devoted to the nature of water in the water pools of reverse micelles; to the structure and shape of the micellar aggregates; and to the dynamic properties of the reverse micelles. In the second part of the paper, the mechanism of solubilization of proteins and nucleic acids in hydrocarbon reverse micelles is discussed. Spectroscopic data, mostly circular dichroism and fluorescence, are reviewed in order to clarify the conformational changes which the biopolymers undergo upon their uptake into the micellar environment and determine the location of the biopolymers inside the reverse micelles. Data from neutron scattering, light scattering, ultracentrifugation, and electron microscopy of the protein-containing micelles are reviewed and discussed with the aim of illustrating the structure of the micellar aggregates containing the biopolymer as guest molecules. The activity of enzymes and nucleic acids is discussed, with emphasis on the influence upon the chemical reactivity brought about by the micellar parameters. Finally, a brief review of the applications and potentialities of biopolymer-containing reverse micelles is presented.

Micelle formation of diacylglycerophosphocholines in organic solvents I. Effects of the solvents on Krafft points

Krafft points of diacylglycerophosphocholines (PC) were measured in alkanes-cyclohexane solutions by differential scanning calorimetry, and it was found that they were regularly increased following the increase in alkane content in the solutions and the chain length of the alkanes. From these results it was deduced that the mixing of PC with alkanes occurred in the gel state of the PC, but not in micelles at higher temperatures above the Krafft points, where micellar solutions are provided. The penetration of alkanes into gel state PC was found to be dominated by Langmuir type interaction, and the affinity of alkanes increases with increasing in chain lengths. Above the Krafft points, the micelle formation was confirmed by using the fluorescence probe technique.

Ultracentrifugation of hydrated egg lecithin in benzene solution

Estimations of molecular weights from sedimentation equilibrium experiments indicate that the degree of aggregation of egg lecithin in benzene solutions increases with increasing addition of water. The molecular weight rises monotopically from about 14,000 in the anhydrous state to about 57,000 at a water/lecithin ratio of 0.31. Sedimentation coefficients were shown to behave in the same manner. Frictional coefficients obtained from the combined results fail to indicate definite differences in asymmetry; however, they seem to indicate that the aggregates in all stages are compact in shape.

Solubility of phosphatidylcholine in chloroform. Formation of hydrogen bonding between phosphatidylcholine and chloroform

The solubility of phosphatidylcholine (PC) was studied by the spectroscopic analysis and the measurement of the solubility. The qualitative analysis of infrared absorption spectra confirmed the existence of two types of hydrogen bondings between chloroform and PC, one between chloroform and the C=O group of PC and the other between chloroform and the phosphorylcholine group of PC. The quantitative analysis of the C-D stretching vibration bands of the chloroform-d solution of PC showed that the latter hydrogen bonding mainly contributes to the solubility and that PC dissolves in chloroform to form a complex consisting of a few or more molecules of chloroform and one molecule of PC. We discussed in this report about the molecular organization of PC in chloroform solution.

Interaction of water with egg lecithin in benzene solution

Benzene solutions of purified egg lecithin, with small amounts of water added, have been examined by 60 MGz and 100 MHz NMR spectroscopy, infrared spectrophotometry and phase contrast microscopy. The transverse relaxation times of the water, N-methyl and O--H protons are dependent on water concentration. This dependence changes sharply for the water proton at a level of one water molecule per lecithin monohydrate molecule. These results do not fully agree with those reported by other workers. Four mathematical models are examined which could account for the behaviour of the water protons. Models which assume a constant transverse relaxation time for water protons above a level of one water molecule per lecithin molecule cannot predict the behaviour observed. It is sufficient to assume that water protons above this concentration have a single relaxation time which is a linear function of water concentration. The added water associates primarily with the phosphate in the lecithin head group. Above nine water molecules per lecithin monohydrate molecule free water is present in the system.