Thermodynamic analysis and AFM study of the interaction of palmitic acid with DPPE in Langmuir monolayers

Lysozyme Influence on Monolayers of Individual and Mixed Lipids

Fatty acids, cholesterol, and phospholipids are amphiphilic compounds of biological interest, which form ordered monolayers mimicking biomembranes, and can be studied with the Langmuir technique using surface pressure-area isotherms and compressibility plots. Proteins are also components of biomembranes or are present in body fluids. In this study, the influence of lysozyme on different films of a fatty acid (stearic acid or oleic acid), cholesterol, a phospholipid (dipalmitoylphosphatidylcholine, DPPC, or palmitoyloleoylphosphatidylcholine, POPC), and mixtures of them is presented using a 0.9% saline solution as subphase. Results show that the presence of lysozyme alters the lipid monolayer formation in an important way at the beginning (low surface pressures) and the middle (intermediate surface pressures) parts of the isotherm. At high surface pressures, the phospholipids DPPC and POPC and the saturated fatty acid, stearic acid, expel lysozyme from the surface, while oleic acid and cholesterol permit the presence of lysozyme on it. The mixtures of oleic acid-DPPC also expel lysozyme from the surface at high surface pressures, while mixtures of oleic acid-POPC and cholesterol-POPC permit the presence of lysozyme on it. The compressibility of the monolayer is affected in all cases, with an important reduction in the elastic modulus values and an increase in the fluidity, especially at low and intermediate surface pressures.

A Lipidated Peptide with Mitochondrial Membrane Localization in Human A549 Lung Cells: From Enhanced Cell-Penetrating Properties to Biological Activity Mechanism

Here, a lipidated peptide Pal-pHK-pKV with self-assembly properties and the ability to provoke the disruption of the mitochondrial voltage-dependent anion channel-1 protein (VDAC1)-hexokinase-II (HK-II) complex is reported. The effects of the peptide pHK (N-terminal 15-amino acid fragment of HK-II that specifically binds VDAC1) are compared to those of a designed biomimetic amphiphilic pHK-pKV conjugate (pHK coupled with a cell-penetrating peptide pKV) and Pal-pHK-pKV (a lipidated conjugate modified with a hydrophobic palmitic (Pal) alkyl chain). The Pal-pHK-pKV exhibits a stronger interaction with the membrane as compared to pHK-pKV, which is demonstrated by the Langmuir-Blodgett technique and two-photon excitation microscopy. The amphiphilic peptide derivatives are cytotoxic to the A549 cells, but Pal-pHK-pKV is more cytotoxic. The inhibitory effects of the pHK derivatives on the A549 cells growth are investigated through induced apoptosis pathway, depolarized mitochondrial membrane potential, inhibited glycolysis, and activated caspase. The results of the immunofluorescence evidence the specific mitochondrial targeting by those derivatives.

Morphological and Interaction Characteristics of Surface-Active Ionic Liquids and Palmitic Acid in Mixed Monolayers

A series of water soluble, surface-active ionic liquids (SAILs), namely, 1-alkyl-3-methyl imidazolium chlorides ([C_n -mim]Cl) and their mixtures with palmitic acid (PA) are investigated in Langmuir monolayers and Langmuir-Blodgett films. It is inferred from the surface pressure-area isotherms that C₁₆ -mim-IL mixes non-ideally with PA and stabilizes the binary mixed films. In addition, the residence of mim-IL at the water surface is enhanced as a function of the increasing alkyl side chain length. Generally, the compressional moduli values decrease upon increasing the content of the mim-ILs over a wide range of compositions. Furthermore, film relaxation measurements indicate that the IL component is selectively excluded from the mixed films upon achieving a certain target pressure. Brewster angle microscope images demonstrate minimal changes on the PA domains in the presence of either C₄ - and C₈ -mim-ILs, whereas presence of the hexadecyl counterpart results in the formation of condensed sheets. Atomic force microscopy imaging of deposited films show the formation of propeller-like aggregates when C₈ - or C₁₆ -mim-IL is present in the mixed films.

Interaction of different lipoprotein types with cholesterol at the air/water interface

Cholesterol (Chol) interacts with lipoproteins, in order to be transported through the aqueous bloodstream. High density lipoproteins (HDL) and low density lipoproteins (LDL) transport cholesterol differently, a result that may be due to a difference in their interactions with cholesterol. Here, we investigated how the lipoprotein type affects the interaction with cholesterol by using a Langmuir trough and fluorescence microscope. We studied pure monolayers of 1) Chol, 2) LDL, and 3) HDL, and mixed monolayers of 1) Chol-LDL, and 2) Chol-HDL at air/water interfaces. Images of the Chol-LDL mixed monolayer showed many small sterol domains distributed in the non-sterol molecules (e.g. phospholids, proteins and lipids) of LDL. The sterol domains that were seen in the Chol-HDL mixed monolayer were larger in size but smaller in number than those seen in the Chol-LDL mixed monolayers. These images and the excess area, excess free energy, and free energy of mixing values obtained from the thermodynamic analysis of the surface pressure-area per molecule isotherms suggested that the cholesterol phase separated more from HDL than from LDL. Cholesterol was therefore concluded to interact with LDL better than with HDL. This more favorable interaction was explained by the presence of hydrophobic interactions between cholesterol and Apo-B, the major apoprotein of LDL.

Studies on the interfacial behavior of DPPC/DPPG mixed monolayers in the presence of fluoxetine

In this study, the interfacial behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPC/DPPG) mixed monolayers with fluoxetine (FLX) in the subphase was investigated by a combination of the Langmuir-Blodgett technique and atomic force microscopy (AFM). It was found that DPPC/DPPG mixed monolayers showed different interfacial behaviors before and after addition of FLX in the subphase. The electrostatic interaction between FLX and lipids molecules destroys the homogeneity of the mixed monolayers and changes the arrangement of lipids molecules at the interface after addition of FLX in the subphase, thereby leading to an increase of compressibility and miscibility and a decrease in the stability of the mixed monolayers. The surface morphology of the mixed monolayers observed by AFM was different between without and with FLX in the subphase, indicating the penetration of FLX into the mixed monolayers. The present study has provided detailed information for further understanding the interactions of drugs with membrane lipids in other lipid monolayers.

The antibacterial activity of p-tert-butylcalix[6]arene and its effect on a membrane model: molecular dynamics and Langmuir film studies

The antibacterial activity of a calixarene derivative, p-tert-butylcalix[6]arene (Calix6), was assessed and was shown not to inhibit the growth of E. coli, S. aureus and B. subtilis bacteria. With the aim of gaining more insights into the absence of antibacterial activity of Calix6, the interaction of this derivative with DPPG, a bacterial cell membrane lipid, was studied. Langmuir monolayers were used as the model membrane. Pure DPPG and pure Calix6 monolayers, as well as binary DPPG:Calix6 mixtures were studied using surface pressure measurements, compressional modulus, Brewster angle and fluorescence microscopies, ellipsometry, polarization-modulation infrared reflection absorption spectroscopy and molecular dynamics simulations. Thermodynamic properties of the mixed monolayers were additionally calculated using thermodynamic parameters. The analysis of isotherms showed that Calix6 significantly affects the DPPG monolayers, modifying the isotherm profile and increasing the molecular area, in agreement with the molecular dynamics simulations. The presence of Calix6 in the mixed monolayers decreased the interfacial elasticity, indicating that calixarene disrupts the strong intermolecular interactions of DPPG hindering its organization into a compact arrangement. At low molar ratios of Calix6, the DPPG:Calix6 interactions are preferentially attractive, due to the interactions between the hydrophobic tails of DPPG and the tert-butyl groups of Calix6. Increasing the proportion of calixarene generates repulsive interactions. Calix6 significantly affects the hydrophobic tail organization, which was confirmed by PM-IRRAS measurements. Calix6 appears to be expelled from the mixed films at a biologically relevant surface pressure, π = 30 mN m-1, indicating a low interaction with the cell membrane model related to the absence of antibacterial activity.

Thermodynamic Characterization of Mixed Monolayers of a Novel Oxazolidine Derivative and Phospholipids

Oxazolidine derivatives (OxD) are five ring-membered compounds that contain at least one oxygen and nitrogen in their molecular structure. OxD are known due to several therapeutic activities such as anticancer and antibiotic properties. In this paper, we performed a thermodynamic analysis of the mixed films composed by dipalmitoylphosphatidylglycerol (DPPG), dipalmitoylphosphoethanolamine (DPPE), dipalmitoyl phosphatidylcholine (DPPC) or L-α phosphatidylcholine (PC) with a novel oxazolidine derivate (OxD). Relevant thermodynamic parameters such as excess areas (ΔAE), excess free energies (ΔG), and Gibbs free energy of mixing (AG^mix) were derived from the surface pressure data. The topographical analysis was performed using atomic force microscopy. Based on the calculated values of the thermodynamic parameters, we observed that the miscibility of the mixed films was directly dependent on their composition. DPPG/OxD and DPPE/OxD systems present the best-mixed character at low pressures at OxD molar fraction equivalent to 0.25.

DPPC monolayer response to non-spanning cobalt-cage metallosurfactants: Electrostatic complex formation

A novel series of amphiphilic cobalt-cage derivatives (ACCD), bearing a diaza-crown bridge and varying alkyl chains, facilitate ion transport across biomembrane models via self-aggregation. In this study, compression isotherm analyses and atomic force microscopy (AFM) were used to assess the interactions of these amphiphiles with Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC) in order to elucidate electrostatic and steric contributions to ion transport. The stability and compressibility of DPPC monolayers are disrupted by ACCD molecules with short (C₁₂) alkyl chains. These top-heavy amphiphiles (large cone angles) create voids at the interface of the hydrophobic/aqueous layer leading to monolayer expansion and packing efficiency of the aliphatic chains is disrupted. Long-tailed analogues (C₁₆, C₁₈) are cohesively integrated into DPPC monolayers due to their smaller cone angles at the interfacial region and increased hydrocarbon compatibility in the hydrophobic region. Thermodynamic data indicate the formation of electrostatic complexes between DPPC and longer-tailed amphiphiles consistent with AFM observations of aggregate structures at the corresponding concentrations.

Phase Segregation at the Liquid-Air Interface Prior to Liquid-Liquid Equilibrium

Binary systems with partial miscibility segregate into two liquid phases when their overall composition lies within the interval defined by the saturation points; out of this interval, there is one single phase, either solvent-rich or solute-rich. In most systems, in the one-phase regions, surface tension decreases with increasing solute concentration due to solute adsorption at the liquid-air interface. Therefore, the solute concentration at the surface is higher than in the bulk, leading to the hypothesis that phase segregation starts at the liquid-air interface with the formation of two surface phases, before the liquid-liquid equilibrium. This phenomenon is called surface segregation and is a step toward understanding liquid segregation at a molecular level and detailing the constitution of fluid interfaces. Surface segregation of aqueous binary systems of alkyl acetates with partial miscibility was theoretically demonstrated by means of a thermodynamic stability test based on energy minimization. Experimentally, the coexistence of two surface regions was verified through Brewster's angle microscopy. The observations were further interpreted with the aid of molecular dynamics simulations, which show the diffusion of the acetates from the bulk toward the liquid-air interface, where acetates aggregate into acetate-rich domains.