No abstract

A series of semifluorinated n-alkanes (SFAs) of the general formula F(CF 2 ) m (CH 2 ) n H, (in short FmHn) where m ) 10 and n ) 6-20, have been synthesized and employed for Langmuir monolayer characterization. Surface pressure and electric surface potential measurements were obtained under a variety of experimental conditions such as spreading volume, subphase temperature, and compression speed. The Langmuir monolayer experiments have been complemented with Brewster angle microscopy results which enabled both direct visualization of the monolayers structure and estimation of the monolayer thickness at different stages of compression. Our results show that these "nonclassical" film-forming materials, which are completely hydrophobic in nature and do not possess any polar group in their structure, are capable of monolayer formation at the air/water interface. It has been observed that with the increase in the molecule's length, its stability at the free water surface increases. The negative sign of the measured surface potential, ∆V, proves that SFA molecules are oriented at the air/water interface with their perfluorinated parts directed toward the air. The effective dipole moments reach the value of -0.65 ( 0.1 D at the minimum for all stable SFAs. The analysis of the direction of the molecular dipole moment in respect to the main axis enabled us to estimate that the minimum effective dipole moment is achieved for a molecule oriented at the angle of about 35°to the surface normal. The relative intensity measurements allow one to conclude that film molecules are tilted in respect to the surface normal at the vicinity of collapse.

Mixtures of cholesterol with stearic (STA), oleic (OA), and linoleic (LA) acids spread as monolayers at the air/water interface were used as model systems to examine the hypocholesterolemic effect of fatty acids. Miscibility and interactions between the components of the cholesterol/fatty acid systems were studied basing on the analysis of surface pressure/area isotherms completed with Brewster angle microscopy images. In monolayers, STA and cholesterol were found to be immiscible. In contrast, OA and LA were found to form miscible, but nonideal mixed monolayers with cholesterol. They exhibit negative deviations from ideality in the surface pressure/area plots. This reflects close-packing arrangements between bulky cholesterol molecule and the hydrocarbon chains of unsaturated fatty acids. The analysis of the excess free energies of mixing shows that the maximum negative value of ΔG exc appears at about X chol = 0.5−0.7. Thus, the formation of the most stable 1:1 and 2:1 complexes between cholesterol and an unsaturated fatty acid molecule may account for the hypocholesterolemic effect of the acids in human organism by complexing free cholesterol, thereby hindering its deposition on artery walls.

To cite this version:J. Miñones, S. Pais, O. Conde, P. Dynarowicz-Łątka. Interactions between membrane sterols and phospholipids in model mammalian and fungi cellular membranes -A Langmuir monolayer study. Biophysical Chemistry, Elsevier, 2009, 140 (1-3) This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. A C C E P T E D

π−A isotherms of mixed monolayers composed of cholesterol and amphotericin B (AmB) spread on aqueous buffers of various pH and temperatures show the existence of interactions between the two components, which are more pronounced when the mole fraction of AmB is 0.7. As a consequence of the interactions, the excess areas and excess free energies of mixing are negative at low surface pressures and positive at high surface pressures. These results suggest that negative deviations of the additivity rule are due to the formation of a hydrogen-bonded AmB−cholesterol complex in which AmB molecules are oriented horizontally at the interface and cholesterol molecules lie vertically. The positive excess areas of mixing at high surface pressures could be due to AmB being less desorbed in the substrate by composition-dependent van der Waals interactions between the apolar moieties of the components, both oriented in this situation in a vertical position at the A/W interface.

Using the monolayer technique to study the surface behaviour of systems consisting of amphotericin B (AmB) and various sterols, the components were found to interact with each other. The interactions observed are accounted for by postulating that, at low surface pressures, AmB and different sterols form mixed films where the former lies parallel and the latter normal to the air-water interface in such a way that the polar groups in both components establish hydrogen bonds that lead to the formation of an AmB-sterol 'complex' of 2:1 stoichiometry at the interface. At high surface pressures, AmB molecules rearrange themselves normal to the interface; this gives rise to the Van der Waals interactions between non-polar chains of both components that vary with the nature and composition of the system. The occurrence of these hydrophobic interactions prevents the desorption of AmB into the subphase, which is consistent with the positive excess areas of mixing obtained under these surface pressure conditions. Among the four sterols studied, ergosterol exhibits the strongest interaction with AmB and beta-sitosterol the weakest. Cholesterol and stigmasterol show intermediate behaviour.

The surface behavior of two zwitterionic phospholipids: 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-[16-fluoropalmitoyl-phosphatidylcholine] (F-DPPC), has been investigated at the air-water interface at the temperature range from 10 to 30 °C. Surface pressure-area isotherms, BAM images and thickness-time curves were obtained for monolayers made from these pure phospholipids and from their mixtures.The comparative study of the behavior of both phospholipid monolayers with temperature showed some differences as the disappearance of the liquid expanded (LE)-liquid condensed (LC) phase transition at low temperatures for the DPPC but not for F-DPPC, because the F-DPPC monolayer is more expanded and more resistant to changes of temperature. On the other hand, film elasticity (C(s)(-1)) values calculated for both phospholipids show that the film condensation diminishes when the temperature increases, in accordance with the results obtained from surface pressure measurements.BAM images for F-DPPC monolayers recorded at different surface pressures and temperatures show the existence of numerous ovoid-like domains when the LE-LC phase transition is reached. However, in the LE and LC phases, homogeneous images were obtained. Time evolution of relative thickness along the compression of F-DPPC and DPPC monolayers shows similar behavior of both phospholipids, except at low temperatures.For DPPC/F-DPPC mixed systems, the plots of the mean molecular area as a function of F-DPPC mole fraction (X(F-DPPC)) indicated that, whatever the surface pressure, the experimental results match the theoretical values calculated from the additivity rule, a typical behavior for ideal mixed monolayers made of miscible components. This conclusion is confirmed from the values calculated for the free energy of excess (ΔG(exc)) of this system, which are practically zero, whatever the composition of the mixtures and the surface pressure at which ΔG(exc) values were calculated.

The effect of different amounts of lauryl gallate (LG) on properties of the model membranes of phosphatidylcholines (PC), differing in the presence of double bonds in the hydrocarbon chains, and phosphatidylglycerol (PG) was described in terms of phase behaviour of mixtures, interactions between both components, monolayers stability and their organization. The Langmuir monolayer technique was used to monitor the surface thermodynamics (i.e. the excess area and excess Gibbs energy of mixing) on the basis of surface pressure-area per molecule (π-A) isotherms. Simultaneously, morphology of the studied monolayers was visualized by the Brewster angle microscopy (BAM). This allowed evaluating the kind and magnitude of interactions which influence on the phase behaviour and structural properties of the monolayers. The obtained results can be helpful to reveal the mechanism of phospholipid antioxidant protection and important pharmacological (antimicrobial) role of lauryl gallate for production of effective therapeutic substances.