O. Conde

Interactions between membrane sterols and phospholipids in model mammalian and fungi cellular membranes — A Langmuir monolayer study

This paper is aimed at investigating sterol/phospholipid interactions in the exact proportion that occurs in fungi/mammalian cells. We have performed a thorough analysis of surface pressure (pi)-area (A) isotherms with the Langmuir monolayer technique, complemented with Brewster angle microscopy (BAM) images. The following mixtures were analysed: cholesterol (Chol)-dipalmitoyl phosphatidylcholine (DPPC), Chol-dioleoyl phosphatidylcholine (DOPC), ergosterol (Erg)-DPPC, and Erg-DOPC. For each system, two different concentrations of the sterols were used, 13 and 30%, corresponding to the range of concentration found in various natural membranes. The obtained results show the existence of attractive interactions between phospholipids and cholesterol. Mixtures with ergosterol behave quite differently, i.e. either the interactions are repulsive (mixtures with DPPC) or the system is ideal (mixtures with DOPC). The obtained results have implications in the polyene antibiotics mode of action, i.e. the polyenes may interact easier with ergosterol, present in fungi cells, as compared to cholesterol--the main sterol of the mammalian cellular membranes.

Interactions of amphotericin B with saturated and unsaturated phosphatidylcholines at the air/water interface

Abstract Three synthetic phospholipids, differing in the degree of saturation of phosphatidylcholine (PC) acyl chains, namely: dipalmitoyl phosphatidylcholine (DPPC), dioleoyl phosphatidylcholine (DOPC) and palmitoyl-oleoyl phosphatidyl choline (POPC) were mixed with the antimycotic polyene antibiotic, amphotericin B (AmB), and investigated as monolayers at the air/water interface. The mixed films were spread on water (pH 6) at room temperature, and the surface pressure–area ( π / A ) isotherms were recorded upon compression. The interactions were examined by analysing the phase transitions of the monolayers and calculating the films compressibility. The obtained results indicate the existence of stronger interaction between AmB and the saturated phospholipid as compared with unsaturated ones, and reveal the influence of apolar structure of PC chains on the stoichiometry of AmB–PC complexes.

Study of the interaction of cholesterol with potentially hypocholesterolemic substances in a monolayer form

Abstract The behaviour of mixed monolayers of cholesterol (chol) and stigmastanil phosporylcholine (SPC) at the air–water interface was studied and compared with that of the cholesterol–arachidonic acid (Ara) system in order to examine the molecular interactions between the film components. The structural characteristics of the mixed films was determined from the π – A isotherms recorded on an automated Langmuir-type film balance. The studies confirm that the mixed films are homogeneous and their components exhibit negative deviations from ideality as the result of mutual interactions at the interface. The magnitude of these interactions depends on the surface pressure region, temperature, as well as the system composition. Values of the thermodynamic parameters such as Δ G exc , Δ S exc and Δ H exc provide evidence for the presence of interactions, which are stronger in the chol–SPC system than in the chol–Ara mixed films.

Compression—expansion curves of cyclosporin A monolayers on substrates of various ionic strengths

Abstract At pH 6, monolayers of cyclosporin A (CyA) had a limiting area of 314.5 A2 per molecule; when the monolayer collapsed (at a surface pressure of 30 mN m−1), its area was 165 A2 per molecule, i.e. 15 A2 per amino acid, a value typical of the area occupied by each amino acid in densely packed protein monolayers. During compression to collapse, the surface pressure increased steadily, the surface pressure—area (π-A) curves exhibiting no flat plateau. These findings suggest that collapse involved the expulsion of segments of CyA molecules from the interface, although strong intermolecular attraction must have prevented total immersion in the subphase. The hysteresis exhibited when successive compression—expansion cycles were carried out suggests that the return of the expelled CyA segments to the interface is a slow equilibrium process. When spread on 3 M NaCl, CyA monolayers collapsed only when a surface pressure of 64 mN m−1 and a molecular area of 40 A2 per molecule were reached, and the π-A curves exhibited a flat plateau 100 A2 per molecule long at a surface pressure of 36 mN m−1. The plateau is interpreted as reflecting a change in the orientation of the CyA peptide ring from parallel to perpendicular to the surface.

Influence of pH and temperature on poly(isobutyl cyanoacrylate) monolayers

Surface pressure-area (π-A) isotherms of monolayers of poly(isobutyl cyanoacrylate), a polymer used in the preparation of nanoparticle drug carriers, were recorded on subphases of pH 2, 6 and 10 at temperatures between 10 and 35°C. The isotherms exhibit a plateau region at a surface pressure that depends on pH (Π(pH 6)<Π(pH2)<<Π(pH 10)) and also decreases slightly with increasing temperature. Since application of the Clausius-Clapeyron equation to the plateau yields small positive ΔH and ΔS values, this region is interpreted as reflecting reorientation of the polar groups of the polymer under compression; this interpretation appears to be supported by the hysteresis behaviour of the monolayer

Monolayers of neutral and charged polyamino-acids spread on silicic acid substrates

Surface pressure-area isotherms (π-A) of poly(L-alanine) (PA), poly(L-glutamic acid) (PG), and poly(L-arginine) (PArg) monolayers spread on substrates of varous pH values containing silicic acid were recorded and compared with those obtained on silica-free substrates. The compression curves of PA showed a pH-independent plateau region which was assumed to correspond to a monolayer to bilayer transition. The inflection observed in the π-A curves of PG was attributed to the formation of loops made up of the terminal heads of the polymer and lying under the monolayer. Polysilicic acid interacted with the PArg films at pH 6; such films were more rigid and stable and had larger specific areas than those obtained in the absence of silica, which prompts the occurrence of an ionic interaction. The other two polyamino-acids did not interact with silicic acid at any pH value, so we may rule out the formation of hydrogen bonds between the silanol groups of polysilicic acid and the peptide or carboxyl groups of these amino acids.

Thermodynamics of the compression of poly(isobutyl cyanoacrylate) monolayers at acid, neutral and basic pH

Poly(isobutyl cyanoacrylate) (PIBCA) monolayers, unlike those of most substances, respond to increased temperature with a decrease in area per monomer unit and a fall in the surface pressures at which surface phase transition and collapse occur. These effects are associated with an increase in entropy during isothermal compression of the film, and are attributed to the rise in temperature causing loss of solvation water, rather than to increased solubility of PIBCA.

A study of the behaviour of mixed monolayers of amphotericin B and dipalmitoyl phosphatidic acid based on hysteresis experiments

Amphotericin B (AmB) monolayers consist of three distinct regions: a liquid expanded phase at low surface pressures, a liquid condensed phase at high surface pressures and a transitional phase that gives a plateau at 10 mN/m in the π -A curves. The presence of such a plateau is ascribed to a change in the orientation of AmB molecules from a horizontal to a vertical position relative to the air/water interface. Dipalmitoyl phosphatidic acid (DPPA) gives a condensed mono-layer, the π-A curves for which exhibit a slope change at 25 mN/m that has been ascribed to an orientational change in the nonpolar chains from a tilted to a vertical position relative to the interface. An equimolar mixture of AmB and DPPA exhibits a surface behaviour in-between those of the pure components, which includes a phase transition at 20 mN/m that suggests the existence of interactions between the components; such interactions result in deviations from the ideal behaviour reflected in plots of the mean molecular area against the mole fractions of the components. On spreading separately on the interface, the two components are initially immiscible, but become miscible gradually with time. Hysteresis experiments reveal that AmB is desorbed into the subphase by effect of its solubility, being decreased by DPPA; this is attributed to interactions between the polar heads of the two components causing AmB molecules to be retained on the surface and giving rise to positive deviations from the ideal behaviour as a result.