Miscibility and phase behavior of DPPG and perfluorocarboxylic acids at the air–water interface

Headgroup-Specific Interaction of Biological Lipid Monolayer/Water Interface with Perfluorinated Persistent Organic Pollutant (f-POP): As Observed with Interface-Selective Vibrational Spectroscopy

Perfluoro compounds are widely used in various manufacturing processes, which leads to their bioaccumulation and subsequent adverse effects on human health. Using interface-selective vibrational spectroscopy (heterodyne-detected vibrational sum frequency generation (HD-VSFG)), we have elucidated the molecular mechanism of the perturbation of lipid monolayers on the water surface using a prototype perfluorinated persistent organic pollutant, perfluoroheptanoic acid (PFHA). PFHA disrupts the well-ordered all-trans conformation of a cationic lipid (1,2-dipalmitoyl-3-trimethylammonium propane (DPTAP)) monolayer and reduces the interfacial electric field at the lipid/water interface. In contrast, the hydrophobic packing of an anionic lipid (1,2-dipalmitoyl-sn-glycero-3-phospoglycerol (DPPG)) monolayer remains largely unaffected in the presence of PFHA, though the interfacial electric field is reduced. For a zwitterionic lipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC))/water interface, both alkyl chain ordering and interfacial electric field are fairly perturbed by PFHA. Lipid headgroup-specific interaction of PFHA and the repulsive interaction of oleophobic fluoroalkyl chain with the lipid alkyl chains govern these distinct perturbations of the lipid monolayers on the water surface.

Destabilizing effect of perfluorodecanoic acid on simple membrane models

Perfluoroalkyl acids (PFA) are amphiphilic surfactants widely used in industry with several commercial applications. An important feature of these compounds is their non-biodegradability and their tendency to bio-accumulate in the environment, which has led to these compounds being considered among the most persistent pollutants worldwide. Many studies have provided evidence of their toxic effect on humans and wildlife. For this reason, more and more efforts have been made to better understand the effect of these compounds on living organisms. The aim of the present study is to understand how the electrostatic interactions and film compactness of biological membrane models modulate their interaction with PFA, more specifically with perfluorodecanoic acid (PFD). Langmuir isotherms and Brewster angle microscopy (BAM) are used to evaluate the effect of PFD on lipid membrane models (air/water monolayers and vesicles), analyzing the behavior of PFD : lipid mixtures. The lipids used in this study are distearoyl phosphatidic acid (DSPA), dilauroyl phosphatidic acid (DLPA) and distearoyl phosphatidylethanolamine (DSPE). PFD induces an increase in the mean molecular area per lipid in monolayers, mainly at lower surface pressures. BAM images demonstrate that PFD mixes with DLPA, inducing a decrease in gray level, while it forms a non-miscible mixture with DSPA, segregating PFD domains. Insertion studies of PFD within monolayers and dynamic light scattering experiments demonstrate that PFD can penetrate into monolayers and bilayers above 30 mN m-1, which is the lateral pressure value accepted for a cellular bilayer.

The interfacial electrostatic potential modulates the insertion of cell-penetrating peptides into lipid bilayers

Cell-penetrating peptides (CPP) are short sequences of cationic amino-acids that show a surprising ability to traverse lipid bilayers. CPP are considered to be some of the most effective vectors to introduce membrane-impermeable cargos into cells, but the molecular basis of the membrane translocation mechanisms and its dependence on relevant membrane physicochemical properties have yet to be fully determined. In this paper we resort to Molecular Dynamics simulations and experiments to investigate how the electrostatic potential across the lipid/water interface affects the insertion of hydrophilic and amphipathic CPP into two-dimensional lipid structures. Simulations are used to quantify the effect of the transmembrane potential on the free-energy profile associated with the transfer of the CPP across a neutral lipid bilayer. It is found that the electrostatic bias has a relatively small effect on the binding of the peptides to the membrane surface, but that it significantly lowers the permeation barrier. A charge compensation mechanism, arising from the segregation of counter-ions while the peptide traverses the membrane, determines the shape and symmetry of the free-energy curves and underlines relevant mechanistic considerations. Langmuir monolayer experiments performed with a variety of amphiphiles model the incorporation of the CPP into the external membrane leaflet. It is shown that the dipole potential of the monolayer controls the extent of penetration of the CPP into the lipid aggregate, to a greater degree than its surface charge.

Investigation of Surface Behavior of DPPC and Curcumin in Langmuir Monolayers at the Air-Water Interface

Langmuir monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a mixture of DPPC with curcumin (CUR) have been investigated at the air-water interface through a combination of surface pressure measurements and atomic force microscopy (AFM) observation. By analyzing the correlation data of mean molecular areas, the compressibility coefficient, and other thermodynamic parameters, we obtained that the interaction between the two components perhaps was mainly governed by the hydrogen bonding between the amino group of DPPC and the hydroxyl groups of CUR. CUR markedly affected the surface compressibility, the thermodynamic stability, and the thermodynamic phase behaviors of mixed monolayers. The interaction between CUR and DPPC was sensitive to the components and the physical states of mixed monolayers under compression. Two-dimensional phase diagrams and interaction energies indicated that DPPC and CUR molecules were miscible in mixed monolayers. AFM images results were in agreement with these analyses results of experimental data. This study will encourage us to further research the application of CUR in the biomedical field.

Phase-separated surfactant monolayers: Exploiting immiscibility of fluorocarbons and hydrocarbons to pattern interfaces

The mutual immiscibility of hydrogenated and fluorinated surfactants at interfaces frequently leads to phase-separation, which provides a useful and flexible method for patterning air-water and solid-air interfaces. In this article, we review recent advances in the use of hydrogenated-fluorinated surfactant mixtures to achieve interfacial patterning. For even relatively simple systems comprised of binary mixed monolayers of hydrogenated and perfluorinated fatty acids, a diverse range of film morphologies can be generated at the air-water interface and successfully transferred onto solid substrates. Systematic investigations reported over the past several years have allowed for correlation between the chemical structure of the film constituents with the gross film morphology and underlying crystalline structure of the films. Early thermodynamic models based on the interplay between dipole-dipole repulsion forces between charged headgroups balanced by line tension between phases that were formulated to describe phase-behavior in simple phospholipid monolayer systems have proven highly useful to describe morphologies for the immiscible surfactant blends. Beyond simple binary fatty acid mixtures, highly-structured films have also been reported in mixed phospholipid systems, which have found important application in controlling the physical, compositional and performance properties of lung surfactant mixtures, as well as in semifluorinated alkane monolayers which form unique, hemimicellar structures at both liquid and solid interfaces. We also describe advances in using these approaches to pattern photopolymerizable, luminescent surfactants, which have found extensive use in colorimetric and fluorometric sensing devices. The long-term outlook for this field, with an emphasis on potential applications and future research directions are discussed.

Influence of film composition on the morphology, mechanical properties, and surfactant recovery of phase-separated phospholipid-perfluorinated fatty acid mixed monolayers

Monolayer surfactant films composed of a mixture of phospholipids and perfluorinated (or partially fluorinated) surfactants are of potential utility for applications in pulmonary lung surfactant-based therapies. As a simple, minimal model of such a lung surfactant system, binary mixed monolayer films composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and perfluorooctadecanoic acid (C18F) prepared on a simplified lung fluid mimic subphase (pH 7.4, 150 mM NaCl) have been characterized in terms of mixing thermodynamics and compressibility (measured through π–A compression isotherms), film morphology (via atomic force, fluorescence, and Brewster angle microscopy), as well as spreading rate and hysteresis response to repeated expansion–contraction cycles for a variety of compositions of mixed films. Under all mixing conditions, films and their components were found to be completely immiscible and phase-separated, though there were significant changes in the aforementioned film properties as a function of composition. Of particular note was the existence of a maximum in the extent of immiscibility (characterized by ΔG(ex)(π) values) and enhanced surfactant recovery during hysteresis experiments at χ(C18F) ≥ 0.30. The latter was attributed to the relatively rapid respreading rate of the perfluorinated amphiphile in comparison with DPPC alone at the air–water interface, which enhances the performance of this mixture as a potential pulmonary lung surfactant. Further, monolayer film structure could be tracked dynamically as a function of compression at the air–water interface via Brewster angle microscopy, with the C18F component being preferentially squeezed out of the film with compression, but returning rapidly upon re-expansion. In general, addition of C18F to DPPC monolayers resulted in improvements to mechanical, structural, and respreading properties of the film, indicating the potential value of these compounds as additives to pulmonary lung surfactant formulations.

Interaction of melittin peptides with perfluorocarbon nanoemulsion particles

Melittin, an antimicrobial peptide, forms pores in biological membranes and triggers cell death. Therefore, it has potential as an anticancer therapy. However, until recently, the therapeutic application of melittin has been impractical because a suitable platform for delivery was not available. Recently, we showed that phospholipid-stabilized perfluorooctyl bromide based nanoemulsion particles (PFOB-NEPs) were resistant to destruction by melittin and enabled specific delivery of melittin to tumor cells, killing them and reducing tumor growth. Earlier, prior work also showed that melittin adsorbed onto the stabilizing phospholipid monolayer of PFOB-NEP but did not disrupt the phospholipid monolayer or produce "cracking" of the PFOB-NEPs. The present work identifies the important structural motifs for melittin binding to PFOB-NEPs through a series of atomistic molecular dynamics simulations. The conformational ensemble of melittin bound to PFOB-NEP lipid monolayer was compared to structure from a control simulation of melittin bound to a lipid bilayer to identify several differences in melittin-lipid interactions between the two systems. First, melittin was deeply buried in the hydrophobic tail region of bilayer, while its depth was attenuated in the PFOB-NEP monolayer. Second, a helical conformation was the major secondary structure in the bilayer, but the fraction of helix was reduced in the PFOB-NEP. Finally, the overall pattern for the direct interaction of melittin with surrounding lipids was similar between liposome and PFOB-NEP, but the level of interaction was slightly decreased in the PFOB-NEP. These results suggest that melittin interacts with the monolayer of PFOB-NEP in a way that is similar way to its interaction with bilayers but that deeper penetration into the hydrophobic interior is inhibited.