Phospholipid monolayer films transition

Stigter and Dill [98] studied phospholipid monolayers at the n-heptane-water interface and were able to treat the second and third virial coefficients (see Eq. XV-1) in terms of electrostatic, including dipole, interactions. At higher film pressures, Pethica and co-workers [99] observed quasi-first-order phase transitions, that is, a much flatter plateau region than shown in Fig. XV-6. 

Monolayer Films at the A/W Interface. Previous studies of phospholipid monolayers at gas-liquid interfaces have shown that it is possible to follow the first order thermodynamic phase transition of these monolayer films using the infrared reflectance techniques described in this manuscript (see e.g. ref. 6 and references cited therein). For long chain hydrocarbon molecules, it has been demonstrated that the frequencies of the antisymmetric and symmetric CH2 stretching vibrations are conformation-sensitive, and may be empirically correlated with the order (i.e. the trans-gauche character) of the hydrocarbon chains (9-11). 

Monolayers of distearoyl lecithin at hydrocarbon/water interfaces undergo temperature and fatty acid chain length dependent phase separation. In addition to these variables, it is shown here that the area and surface pressure at which phase separation begins also depend upon the structure of the hydrocarbon solvent of the hydrocarbon oil/aqueous solution interfacial system. Although the two-dimensional heats of transition for these phase separations depend little on the structure of the hydrocarbon solvent, the work of compression required to bring the monomolecular film to the state at which phase separation begins depends markedly upon the hydrocarbon solvent. Clearly any model for the behavior of phospholipid monolayers at hydrocarbon/water interfaces must account not only for the structure of the phospholipid but also for the influence of the medium in which the phospholipid hydrocarbon chains are immersed. 

It has been mentioned previously that sterols were readily incorporated into lecithin or mixed phospholipid monolayers [194—196]. The area of sterol-containing films was smaller than that calculated for the separate components, indicating some kind of interaction between the two molecules [221,222]. Sterols have also been shown to reduce the phase transitions of pure lecithin dispersions [214]. It has been suggested that cholesterol modifies the fluidity of the hydrocarbon chains of the phospholipid molecules by disrupting the crystalline chain lattice of the gel phase and by inhibiting the flexing of the chains in the dispersed liquid-crystalline phase [221—226]. 

Pethica, B. A. Phospholipid monolayers at the -heptane/water interface. Part 2. Dilute monolayers of saturated 1,2-diacyl-lecithins and -cephahns. J. Chem. Soc. Faraday Trans. 1 1976, 72, 2694-2702. (d) Yue, B. Y. T., Jackson, C. M., Taylor, J. A. G., Mingins, J., Pethica, B. A. Phospholipid monolayers at non-polar oU/water interfaces. Part 1. Phase transitions in distearoyl-lecithin films at the -heptane aqueous sodium chloride interface. J. Chem. Soc. Faraday Trans. 1 1976, 72, 2685-2693. 

It is well known that water dispersions of amphiphile molecules may undergo different phase transitions when the temperature or composition are varied [e.g. 430,431]. These phase transitions have been studied systematically for some of the systems [e.g. 432,433]. Occurrence of phase transitions in monolayers of amphiphile molecules at the air/water interface [434] and in bilayer lipid membranes [435] has also been reported. The chainmelting phase transition [430,431,434,436] found both for water dispersions and insoluble monolayers of amphiphile molecules is of special interest for biology and medicine. It was shown that foam bilayers (NBF) consist of two mutually adsorbed densely packed monolayers of amphiphile molecules which are in contact with a gas phase. Balmbra et. al. [437J and Sidorova et. al. [438] were among the first to notice the structural correspondence between foam bilayers and lamellar mesomorphic phases. In this respect it is of interest to establsih the thermal transition in amphiphile bilayers. Exerowa et. al. [384] have been the first to report such transitions in foam bilayers from phospholipids and studied them in various aspects [386,387,439-442]. This was made possible by combining the microscopic foam film with the hole-nucleation theory of stability of bilayer of Kashchiev-Exerowa [300,402,403]. Thus, the most suitable dependence for phase transitions in bilayers were established. 

IRRAS has become an important tool for studying Langmuir monolayers and LB films. Much work has been done, in particular on the acyl chain conformational order in monolayers of single chain amphiphiles and phospholipids as a function of surface pressure and on the occurrence of phase transitions . Examples of IRRAS studies on LB films can be found Reviews of the applications of IRRAS are given by Dluhy et al. and Mendelsohn et al. ). 

Are any of these structures typical of those that would be observed in a pure amphiphile The role played by the probe, which is essential to the fluorescence method, is not completely clear. It has been argued that the formation of dendritic structures in phospholipids is the result of constitutional supercooling, a mechanism that depends on the differential solubility of an impurity between two phases. This may not be the case similar patterns have been observed in LB films by surface-plasmon microscopy, for which no probe is added. The foam structures at the LE-G transition have also been attributed by some to the presence of the probe, but foams have also been observed in monolayers composed solely of a labeled amphiphile. 

Thermodynamic parameters for the mixing of dimyristoyl lecithin (DML) and dioleoyl lecithin (DOL) with cholesterol (CHOL) in monolayers at the air-water interface were obtained by using equilibrium surface vapor pressures irv, a method first proposed by Adam and Jessop. Typically, irv was measured where the condensed film is in equilibrium with surface vapor (V < 0.1 0.001 dyne/cm) at 24.5°C this exceeded the transition temperature of gel liquid crystal for both DOL and DML. Surface solutions of DOL-CHOL and DML-CHOL are completely miscible over the entire range of mole fractions at these low surface pressures, but positive deviations from ideal solution behavior were observed. Activity coefficients of the components in the condensed surface solutions were greater than 1. The results indicate that at some elevated surface pressure, phase separation may occur. In studies of equilibrium spreading pressures with saturated aqueous solutions of DML, DOL, and CHOL only the phospholipid is present in the surface film. Thus at intermediate surface pressures, under equilibrium conditions (40 > tt > 0.1 dyne/cm), surface phase separation must occur. 

Cholesterol crystallisation is thought to be the first step in the formation of gallstones in the human biliary system and the process of cholesterol nucleation remains incompletely understood. GIXD revealed a phase transition from a monolayer to a highly crystalline rectangular bilayer phase (165). The presence of the phospholipid DPPC in the cholesterol film inhibited cholesterol crystallisation [165]. AFM provided complementary information on the thickness and morphology of the cholesterol films transferred to a solid support The cholesterol monolayer thickness was 13 2 A and in the bilayer phase the presence of elongated faceted crystallites of pure cholesterol about 10 layers thick could be observed [165]. 

The systems that we study are monolayers of phospholipids and fatty acids at air/water interfaces. This interface is physically very interesting since it enables a high degree of freedom to vary many parameters. On the other hand films manipulated at the water surface are also the precursors of Langmuir-Blodgett films, i.e. mono- or multilayers of these lipids on solid support. Phase transitions in these systems, which are also of technical relevance, will be discussed at the end of the second part. 

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