On the interaction of dipalmitoyl phosphatidylcholine with normal long-chain alcohols in a mixed monolayer: a thermodynamic study

Inverse electron-demand diels-alder reactions of tetrazine and norbornene at the air-water interface

The surface chemistry of the inverse electron-demand Diels-Alder (IEDDA) reaction at the air-water interface is elucidated. Tetrazine (C18-Tz) and norbornene derivatives (C16-NCA) were used as the reactants. Langmuir monolayers of C18-Tz, C16-NCA, and their binary mixtures were prepared on aqueous substrates. The surface properties were analyzed using the surface pressure (π)-molecular area (A) and surface potential (ΔV)-A isotherms, as well as fluorescence microscopy to monitor the progress of the reaction. First, to provide comparison data to evaluate the reaction on the surface, the two components were mixed in stock solutions of organic solvents for the IEDDA reaction. The Langmuir monolayer spread from the reaction solution was characterized as a function of the reaction time. In the subsequent experiments, the Langmuir monolayers were deposited onto the surface of the substrate solutions by spreading from separate stock solutions of C18-Tz and C16-NCA. The variation of the surface behavior of the monolayers with the molecular area, surface composition of the two components, compression speed of the monolayers, and the temperature was studied. We discuss the effects of the air phase in the reaction field on the reaction efficiency by comparing the results obtained from the two methods.

Phosphatidylcholine-fatty Alcohols Equilibria in Monolayers at the Air/Water Interface

Monolayers of phosphatidylcholine (PC), tetradecanol (TD), hexadecanol (HD), octadecanol (OD) and eicosanol (E) and their binary mixtures were investigated at the air/water interface. The surface tension values of pure and mixed monolayers were used to calculate π–A isotherms. The surface tension measurements were carried out at 22 °C using a Teflon trough and a Nima 9000 tensiometer. The interactions between phosphatidylcholine and fatty alcohols (tetradecanol, hexadecanol, octadecanol, eicosanol) result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants, Gibbs free energy values and areas occupied by one molecules of PC–TD, PC–HD, PC–OD and PC–E complexes. We considered the equilibrium between the individual components and the complex and established that phosphatidylcholine and fatty alcohols formed highly stable 1:1 complexes.

Studies of mixed liposomes with novel sorbyl functionalized head group lipids

Three novel polymerizable amphiphiles with a sorbyl-substituted head group were synthesized and systematically characterized. These amphiphiles are neutral in charge. None of these molecules forms vesicles by itself, presumably due to lack of amphiphilicity and/or extensive head group interaction. Therefore, mixed vesicles were formed with other fluid lipids such as DPenPC, eggPC, or DOPC. We investigated the properties of these mixtures in both vesicles and Langmuir films. The Langmuir isotherms show formation of monolayers by all three molecules. However, the isotherms for mixed monolayers suggest that two components are largely immiscible to the mixing lipid. Under polymerization conditions, mixed vesicles of these amphiphiles form oligomers, suggesting that in spite of a larger head group, they form mesophases.

Differences in surface behaviour of galactolipoids originating from different kind of wheat tissue cultivated in vitro

The aim of presented researches was to investigate the physicochemical properties of Langmuir monolayer of galactolipids extracted from two different kinds of plastids: immature embryos and inflorescences. Differences between the physicochemical properties of the plastid membranes may help to explain different physiological processes, such as plant regeneration. Surface pressure (pi) vs. molecular area (A) isotherms of the monogalactosyldiacylglycerol (MGDG)/digalactosyldiacylglycerol (DGDG) monolayers of various molar ratios were measured at 15 degrees C. Galactolipids were extracted from two different types of tissue: inflorescences and embryos. Based on the analysis of the pi-A isotherms, the properties of monolayers, such as collapse pressure (pi(coll)), limiting area (A(lim)), compressibility modulus (C(s)(-1)), excess free energy of mixing (DeltaG(EXC)) and free energy of mixing (DeltaG(MIX)), were calculated. The results show that pure MGDG and DGDG and their mixtures form liquid-expanded monolayers, independently on the kind of tissue. Galactolipids originating from inflorescences produce more compressible films at the air/water interface, with larger limiting area per molecule and lower stability against the collapse process. MGDG and DGDG are miscible and form non-ideal mixed monolayers at the air/water interface. Negative values of DeltaG(EXC) were calculated for the mixture of galactolipids originating from inflorescences, with the content of MGDG, x(MGDG)>0.6. In the case of embryos, the negative values of DeltaG(EXC) were found for x(MGDG) approximately 0.5. Therefore, the attractive interactions between MGDG and DGDG exist in the mixtures of these compositions. As it is shown by negative values of DeltaG(MIX), mixed monolayers are more stable compared with unmixed ones.

Comparing experimental and simulated pressure-area isotherms for DPPC

Although pressure-area isotherms are commonly measured for lipid monolayers, it is not always appreciated how much they can vary depending on experimental factors. Here, we compare experimental and simulated pressure-area isotherms for dipalmitoylphosphatidylcholine (DPPC) at temperatures ranging between 293.15 K and 323.15 K, and explore possible factors influencing the shape and position of the isotherms. Molecular dynamics simulations of DPPC monolayers using both coarse-grained (CG) and atomistic models yield results that are in rough agreement with some of the experimental isotherms, but with a steeper slope in the liquid-condensed region than seen experimentally and shifted to larger areas. The CG lipid model gives predictions that are very close to those of atomistic simulations, while greatly improving computational efficiency. There is much more variation among experimental isotherms than between isotherms obtained from CG simulations and from the most refined simulation available. Both atomistic and CG simulations yield liquid-condensed and liquid-expanded phase area compressibility moduli that are significantly larger than those typically measured experimentally, but compare well with some experimental values obtained under rapid compression.

Thermodynamic studies of bovine lung surfactant extract mixing with cholesterol and its palmitate derivative

Langmuir film behavior of bovine lipid extract surfactant (BLES), mixed with cholesterol (CHOL) and cholesterol palmitate (CHOLP), has been studied by surface pressure (pi)-area (A) measurements. Associative interactions, observed for both systems, were less favored at lower BLES content. The presence of unsaturated phospholipids and surfactant proteins in BLES favored the association. Miscibility of BLES was better with CHOLP than with CHOL at all compositions, indicating more compact packing of the BLES-CHOLP than of the BLES-CHOL system. The most stable mixtures were found at 30-40 mol% CHOL and at low pi and at 20-25 mol% CHOLP but at higher pi. These results suggest that BLES-CHOL miscibility is better at low pi and low CHOL concentrations, while BLES-CHOLP miscibility is better at high pi and high CHOLP concentrations.

Mixed Langmuir monolayers of gramicidin A and fluorinated alcohols

Mixed monolayers of gramicidin A (GA) and three alcohols, differing in the degree of fluorination, namely C18OH, F18OH, and F8H10OH have been investigated by means of: surface manometry (pi-A isotherms) and Brewster angle microscopy (BAM) aiming at finding appropriate molecules for incorporating gramicidin A for a biosensor design. Our results proved that only the semifluorinated alcohol is appropriate material for this purpose since it forms miscible and homogeneous monolayers with GA within the whole concentration range. The experimental results have been supported by the calculations of van der Waals energy profiles using the Insight II program. Both the hydrogenated and perfluorinated alcohols were found to aggregate at higher surface pressures, which exclude their application for gramicidin-based biosensor construction.

Modifying dipalmitoylphosphatidylcholine monolayers by n-hexadecanol and dipalmitoylglycerol

The monolayer structure of pure dipalmitoylphosphatidylcholine (DPPC) and equimolar mixtures of DPPC/n-hexadecanol (C(16)OH) and DPPC/dipalmitoylglycerol (DPG) are studied by the film balance technique and grazing incidence X-ray diffraction measurements. At 20 degrees C, the binary systems exhibit complete miscibility. In contrast to pure DPPC monolayers, a condensing effect is observed in the presence of both non-phospholipid additives; but the phase transition behavior differs. The tilt angle of the hydrocarbon chains in the DPPC/C(16)OH mixture is significantly smaller than in pure DPPC monolayers. The tilt of the chains is even further reduced in the mixed monolayer of DPPC/DPG. A comparison of the three systems reveals distinct structural features such as phase state, chain tilt, and molecular area over a wide range of surface pressures. Therefore, these monolayers provide a highly suitable model to investigate the influence of structural parameters on biological processes occurring at the membrane surface, e.g. enzymatic reactions and adsorption events.

Two-Dimensional Miscibility StudiesThe Analysis of Interaction between Long-Chain Alcohols and Semifluorinated Alkanes

This work presents the results of phase behavior studies of two-dimensional (2D) binary systems involving semifluorinated alkanes (SFAs) and fatty alcohols. Four different SFAs were selected for investigations: (i) with a short and branched perfluorinated moiety (iF3H20), (ii) with a short and normal perfluorinated chain (F4H20), (iii) with a long and branched perfluorinated fragment (iF9H20), and (iv) with a long and normal perfluorinated group (F10H20). Two alcohols were selected to mix with the above-mentioned SFAs: tetradecanol and docosanol. The measurements were based on surface pressure/area isotherms in addition to Brewster angle microscope (BAM) imaging. Dependencies of the collapse surface pressure and the compression modulus vs the monolayer composition together with the excess free energy of mixing values, complemented with BAM images, enabled us to draw some general conclusions regarding the phase behavior of the investigated mixed systems. Generally, it has been noticed that the addition of docosanol into an SFA monolayer exerts a condensing effect, contrary to tetradecanol. Moreover, SFAs with a long perfluorinated segment mix to a larger extent with alcohols as compared to their analogues having a short perfluorinated moiety. The resultant phase diagrams for all the investigated eight mixtures are presented and discussed.

Mixing behavior of 10-(perfluorohexyl)-decanol and DPPC

Fluorocarbon alcohol such as 10-(perfluorohexyl)-decanol are of interest for novel pulmonary drug delivery approaches. The purpose of this study was to investigate the mixing behavior of 10-(perfluorohexyl)-decanol with dipalmitoylphosphatidylcholine (DPPC), the major component of lung surfactant as an aid in assessing usefulness for this and other biomedical applications. The impact of 10-(perfluorohexyl)-decanol on the phase transitions of DPPC bilayers fully hydrated with a 0.15 M sodium chloride solution were studied using differential scanning calorimetry (DSC). No peak corresponding to excess alcohol was observed. The fluorinated alcohol caused DPPC peak broadening, especially below X(DPPC) < 0.95, and elimination of the pretransition of DPPC at X(DPPC) approximately 0.91. The onset of the main phase transition remains constant down to X(DPPC) approximately 0.91, suggesting limited miscibility in the gel phase. Hydration of the 10-(perfluorohexyl)-decanol-DPPC mixtures with calcium chloride (2 mM) in place of sodium chloride did not alter the macroscopic phase behavior. In addition to the thermal properties, the miscibility of 10-(perfluorohexyl)-decanol in DPPC in monolayers at the air water interface was investigated on water, sodium chloride (0.15 M), calcium chloride (2 mM) or hydrochloric acid (pH 1.9) subphases. The concentration dependence of the onset pressure of the liquid-expanded to liquid condensed phase transition of DPPC showed a slight change with increasing mole fraction on all four subphases. The surface area-mole fraction diagrams of 10-(perfluorohexyl)-decanol and DPPC on water, sodium chloride and calcium chloride showed near ideal behavior with slight negative deviations at higher surface pressure. A more significant negative deviation was observed for the hydrochloric acid subphase. Overall, both the DSC and the monolayer studies suggest that 10-(perfluorohexyl)-decanol and DPPC are partially miscible in biological mono- and bilayers. The macroscopic phase behavior 10-(perfluorohexyl)-decanol-DPPC system is significantly different from the analogous hydrocarbon system, which is attributed to a less favorable packing of the partially fluorinated hydrophobic tails in the mono- and bilayer.

Mixing of perfluorinated carboxylic acids with dipalmitoylphosphatidylcholine

Perfluorinated acids are emerging as an important class of persistent environmental pollutant, thus raising human health concerns. To understand the behavior of these compounds in biological systems, the mixing behavior of two perfluorinated acids, perfluorododecanoic and perfluorotetradecanoic acid, with dipalmitoylphosphatidylcholine (DPPC) was studied in monolayers at the air-water interface and in fully hydrated DPPC bilayers. The mixing behavior of both acids was indicative of an attractive interaction and partial miscibility with DPPC at the air-water interface. In the bilayer studies, the fluorinated acids cause peak broadening and elimination of the pretransition of DPPC. The onset temperature of the main phase transition remains constant in the presence of the fluorinated acids suggesting immiscibilities in the gel phase. Below X(DPPC) = 0.97 significant peak broadening of the main phase transition can be observed. These results suggest strong interaction between the respective acid and DPPC, and that both acids are able to partition into the lipid bilayer. However, their mixing behavior is far from ideal, thus suggesting the presence of domains or lipid aggregates with high acid concentrations which may (adversely) impact the function of biological mono- and bilayers.

A long-lived amphiphilic fluorescent probe studied in POPC air-water monolayer and solution bilayer systems

A novel amphiphilic fluorescent probe (Fluorazophore-L) with a strongly dipolar, nonionic azoalkane as headgroup and a palmitoyl tail has been synthesized and characterized. Pure Fluorazophore-L was found to be sufficiently amphiphilic to form stable air-water monolayers. An analysis of the surface pressure versus area suggests an area per molecule of about 34+/-2 A(2) at 29 mN m(-1). The partitioning into a lipid membrane model was quantified at the air-water interface by spreading 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) monolayers. Measurements with different molar fractions of Fluorazophore-L revealed a small but significant reduction of the mean area in the mixed monolayer. The excess free energy of mixing (-0.5+/-0.1 kT) indicated a weakly attractive interaction slightly above thermal energy, suggesting a good miscibility of the fluorescent probe within the lipid monolayer without major structural modifications. Spectroscopic measurements confirmed the incorporation of Fluorazophore-L into POPC vesicles. The fluorescence lifetime was very long (125+/-5 ns under air) with monoexponential fluorescence decays.