A Langmuir and AFM study on interfacial behavior of binary monolayer of hexadecanol/DPPE at the air-water interface

Filling the gaps: Introducing plasticizers into π-conjugated OPE-NH2 Langmuir layers for defect-free anisotropic interfaces and membranes towards unidirectional mass, charge, or energy transfer

The construction of ultrathin membranes from linearly aligned π-electron systems is advantageous for targeted energy, charge, or mass transfer. The Langmuir-Blodgett (LB) technique enables the creation of such membranes, especially with amphiphilic π-electron systems. However, these systems often aggregate, forming rigid Langmuir monolayers with defects or holes. In this study we introduce plasticizers to effectively address this issue. To create anisotropic membranes, we used an oligo(phenylene ethynylene) derivative (OPE-NH2) as an linear amphiphile and bisphenol A di-tert-butyl ester (BPAE) as a plasticizer. We analyzed surface pressure (mean molecular area) (Π(mma)) isotherms and characterized Langmuir monolayers with Brewster Angle Microscopy (BAM), to determine the optimal miscibility of OPE-NH2 with BPAE. Detailed analysis of hole areas filled was performed through image binarization. We identified an optimal BPAE concentration of 4 mol-% in the OPE-NH2 Langmuir monolayer. Our BAM image evaluation via binarization determined the difference between the mean molecular areas of close-packed Langmuir domains and those quantified via the Π(mma) isotherm. This study presents an automated method for BAM image analysis and a new approach for fabricating defect-free anisotropic molecular monolayers of π-conjugated amphiphiles.

Structural Flexibility of Proteins Dramatically Alters Membrane Stability─A Novel Aspect of Lipid-Protein Interaction

Protein isoforms are structural variants with changes in the overall flexibility predominantly at the tertiary level. For membrane associated proteins, such structural flexibility or rigidity affects membrane stability by playing modulatory roles in lipid-protein interaction. Herein, we investigate the protein chain flexibility mediated changes in the mechanistic behavior of phospholipid model membranes in the presence of two well-known isoforms, erythroid (ER) and nonerythroid (NER) spectrin. We show dramatic alterations of membrane elasticity and stability induced by spectrin in the Langmuir monolayers of phosphatidylocholine (PC) and phosphatidylethanolamine (PE) by a combination of isobaric relaxation, surface pressure-area isotherm, X-ray scattering, and microscopy measurements. The NER spectrin drives all monolayers to possess an approximately equal stability, and that required 25-fold increase and 5-fold decrease of stability in PC and PE monolayers, respectively. The untilting transition of the PC membrane in the presence of NER spectrin observed in X-ray measurements can explain better membrane packing and stability.

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.

Biophysical approaches for exploring lipopeptide-lipid interactions

In recent years, lipopeptides (LPs) have attracted a lot of attention in the pharmaceutical industry due to their broad-spectrum of antimicrobial activity against a variety of pathogens and their unique mode of action. This class of compounds has enormous potential for application as an alternative to conventional antibiotics and for pest control. Understanding how LPs work from a structural and biophysical standpoint through investigating their interaction with cell membranes is crucial for the rational design of these biomolecules. Various analytical techniques have been developed for studying intramolecular interactions with high resolution. However, these tools have been barely exploited in lipopeptide-lipid interactions studies. These biophysical approaches would give precise insight on these interactions. Here, we reviewed these state-of-the-art analytical techniques. Knowledge at this level is indispensable for understanding LPs activity and particularly their potential specificity, which is relevant information for safe application. Additionally, the principle of each analytical technique is presented and the information acquired is discussed. The key challenges, such as the selection of the membrane model are also been briefly reviewed.

The wetting properties of Langmuir-Blodgett and Langmuir-Schaefer films formed by DPPC and POSS compounds

The possibility of modification of surface wettability is especially desirable in implantology. This effect is achieved by coating a given material with thin films containing nanoparticles of different chemical properties. In recent years, much interest has been paid to supported phospholipid bilayers (SPBs), because they can be exploited in novel biotechnological devices such as biosensors and mimetic membrane-coated implants. In view of the above, we decided to study the modification of wetting properties of phospholipid layer by two types of polyhedral oligomeric silsesquioxanes (POSS) with different functional groups attached to the silica open-cage. The POSS and phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) were vertically (Langmuir-Blodgett; LB) and horizontally (Langmuir-Schaefer; LS) deposited on quartz substrates to form a thin layer structure. The advancing contact angles on the modified surface coated with thin films were measured. The surface free energy (SFE) of DPPC, POSS and their mixed DPPC/POSS films was estimated by using Owens-Wendt-Rabel-Käelbe (OWRK) method. It was shown that the chemical structure of POSS used as a modifier influence the wetting properties of modified quartz surface. Incorporation fluoroalkyl-POSS into DPPC monolayer leads to obtaining a more hydrophobic film, while the addition of polyethylene glycol-POSS creates a more hydrophilic film. The transfer of the film with a more condensed structure led to a more hydrophobic material. The deposition technique (horizontal or vertical) had a particular impact on the modification of wettability of quartz surface coated with monocomponent fluoroalkyl-POSS film, whereas for the modification with mixed DPPC/POSS systems the choice of transfer method was not so significant.

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.