Phospholipid Monolayers Research Articles

Capturing the Liquid-Crystalline Phase Transformation: Implications for Protein Targeting to Sterol Ester-Rich Lipid Droplets.

Lipid droplets are essential organelles that store and traffic neutral lipids. The phospholipid monolayer surrounding their neutral lipid core engages with a highly dynamic proteome that changes according to cellular and metabolic conditions. Recent work has demonstrated that when the abundance of sterol esters increases above a critical concentration, such as under conditions of starvation or high LDL exposure, the lipid droplet core can undergo an amorphous to liquid-crystalline phase transformation. Herein, we study the consequences of this transformation on the physical properties of lipid droplets that are thought to regulate protein association. Using simulations of different sterol-ester concentrations, we have captured the liquid-crystalline phase transformation at the molecular level, highlighting the alignment of sterol esters in alternating orientations to form concentric layers. We demonstrate how ordering in the core permeates into the neutral lipid/phospholipid interface, changing the magnitude and nature of neutral lipid intercalation and inducing ordering in the phospholipid monolayer. Increased phospholipid packing is concomitant with altered surface properties, including smaller area per phospholipid and substantially reduced packing defects. Additionally, the ordering of sterol esters in the core causes less hydration in more ordered regions. We discuss these findings in the context of their expected consequences for preferential protein recruitment to lipid droplets under different metabolic conditions.

Microtubule motor driven interactions of lipid droplets: Specificities and opportunities.

Lipid Droplets (LDs) are evolutionarily conserved cellular organelles that store neutral lipids such as triacylglycerol and cholesterol-esters. Neutral lipids are enclosed within the limiting membrane of the LD, which is a monolayer of phospholipids and is therefore fundamentally different from the bilayer membrane enclosing most other organelles. LDs have long been viewed as a storehouse of lipids needed on demand for generating energy and membranes inside cells. Outside this classical view, we are now realizing that LDs have significant roles in protein sequestration, supply of signalling lipids, viral replication, lipoprotein production and many other functions of important physiological consequence. To execute such functions, LDs must often exchange lipids and proteins with other organelles (e.g., the ER, lysosomes, mitochondria) via physical contacts. But before such exchanges can occur, how does a micron-sized LD with limited ability to diffuse around find its cognate organelle? There is growing evidence that motor protein driven motion of LDs along microtubules may facilitate such LD-organelle interactions. We will summarize some aspects of LD motion leading to LD-organelle contacts, how these change with metabolic state and pathogen infections, and also ask how these pathways could perhaps be targeted selectively in the context of disease and drug delivery. Such a possibility arises because the binding of motor proteins to the monolayer membrane on LDs could be different from motor binding to the membrane on other cellular organelles.

Synaptotagmin rings as high-sensitivity regulators of synaptic vesicle docking and fusion

Synchronous release at neuronal synapses is accomplished by a machinery that senses calcium influx and fuses the synaptic vesicle and plasma membranes to release neurotransmitters. Previous studies suggested the calcium sensor synaptotagmin (Syt) is a facilitator of vesicle docking and both a facilitator and inhibitor of fusion. On phospholipid monolayers, the Syt C2AB domain spontaneously oligomerized into rings that are disassembled by Ca2+, suggesting Syt rings may clamp fusion as membrane-separating "washers" until Ca2+-mediated disassembly triggers fusion and release [J. Wang etal., Proc. Natl. Acad. Sci. U.S.A. 111, 13966-13971 (2014)].). Here, we combined mathematical modeling with experiment to measure the mechanical properties of Syt rings and to test this mechanism. Consistent with experimental results, the model quantitatively recapitulates observed Syt ring-induced dome and volcano shapes on phospholipid monolayers and predicts rings are stabilized by anionic phospholipid bilayers or bulk solution with ATP. The selected ring conformation is highly sensitive to membrane composition and bulk ATP levels, a property that may regulate vesicle docking and fusion in ATP-rich synaptic terminals. We find the Syt molecules hosted by a synaptic vesicle oligomerize into a halo, unbound from the vesicle, but in proximity to sufficiently phosphatidylinositol 4,5-bisphosphate (PIP2)-rich plasma membrane (PM) domains, the PM-bound trans Syt ring conformation is preferred. Thus, the Syt halo serves as landing gear for spatially directed docking at PIP2-rich sites that define the active zones of exocytotic release, positioning the Syt ring to clamp fusion and await calcium. Our results suggest the Syt ring is both a Ca2+-sensitive fusion clamp and a high-fidelity sensor for directed docking.

ORP5 and ORP8 orchestrate lipid droplet biogenesis and maintenance at ER-mitochondria contact sites.

Lipid droplets (LDs) are the primary organelles of lipid storage, buffering energy fluctuations of the cell. They store neutral lipids in their core that is surrounded by a protein-decorated phospholipid monolayer. LDs arise from the endoplasmic reticulum (ER). The ER protein seipin, localizing at ER-LD junctions, controls LD nucleation and growth. However, how LD biogenesis is spatially and temporally coordinated remains elusive. Here, we show that the lipid transfer proteins ORP5 and ORP8 control LD biogenesis at mitochondria-associated ER membrane (MAM) subdomains, enriched in phosphatidic acid. We found that ORP5/8 regulates seipin recruitment to these MAM-LD contacts, and their loss impairs LD biogenesis. Importantly, the integrity of ER-mitochondria contact sites is crucial for ORP5/8 function in regulating seipin-mediated LD biogenesis. Our study uncovers an unprecedented ORP5/8 role in orchestrating LD biogenesis and maturation at MAMs and brings novel insights into the metabolic crosstalk between mitochondria, ER, and LDs at the membrane contact sites.

Interplay between Lipid Metabolism, Lipid Droplets, and DNA Virus Infections.

Lipid droplets (LDs) are cellular organelles rich in neutral lipids such as triglycerides and cholesterol esters that are coated by a phospholipid monolayer and associated proteins. LDs are known to play important roles in the storage and availability of lipids in the cell and to serve as a source of energy reserve for the cell. However, these structures have also been related to oxidative stress, reticular stress responses, and reduced antigen presentation to T cells. Importantly, LDs are also known to modulate viral infection by participating in virus replication and assembly. Here, we review and discuss the interplay between neutral lipid metabolism and LDs in the replication cycle of different DNA viruses, identifying potentially new molecular targets for the treatment of viral infections.

A novel lipid droplets-specific fluorescent bio-probe with large Stokes shift for the marking of living cells and zebrafish

In eukaryotes, lipid droplets (LDs) composed of phospholipid monolayers and neutral lipids are crucial spherical subcellular organelles with diameters ranging from 40 nm to 100 μm. LDs play a pivotal role in various physiological activities and are closely associated with many diseases. Therefore, it is essential to develop a fluorescent probe with excellent photostability and large Stokes shift (∼100 nm) for tracking LDs. Herein, a novel lipid droplet-specific fluorescent bioprobe based on coumarin-substituted p-nitrophenyl ethylene was designed and constructed, which exhibited the desirable effects of excellent photostability, favorable resistance to ionic interference, low cytotoxicity and LDs specificity. As a proof of principle, we labeled and imaged LDs of HeLa cells by Cou-LDs successfully.

Ceramide enhances binding of LC3/GABARAP autophagy proteins to cardiolipin-containing membranes

Macroautophagy, or autophagy, is a process in which cell macromolecules, or even organelles, are engulfed in a double-membrane vesicle, the autophagosome, and directed to a lysosome. Among autophagy-related proteins, LC3/GABARAP constitute a protein family derived from yeast Atg8. They play important roles in autophagosome formation, binding future cargo organelles and promoting autophagosome growth. The involvement of specific lipids in this process is poorly understood. The present study explores the interaction of LC3/GABARAP proteins with phospholipid monolayers and bilayers based on phosphatidylcholine or on sphingomyelin. Cardiolipin is found to be essential for the protein interaction with such bilayers, as measured through gradient centrifugation experiments, while ceramide markedly increases binding. Giant unilamellar vesicles examined under confocal fluorescence microscopy reveal that ceramide segregates laterally into very rigid domains, while GABARAP binds only the more fluid regions, suggesting that the enhancing role of ceramide is exerted by the minority of ceramide molecules dispersed in the fluid phase. Although in further autophagy steps the LC3/GABARAP proteins are covalently bound to a phospholipid, this is not the case in our system, thus it is proposed that the observed ceramide effects would correspond to very early stages in the process, such as cargo recognition.

Membrane shaping proteins, lipids, and cytoskeleton: Recipe for nascent lipid droplet formation.

Lipid droplets (LDs) are ubiquitous, neutral lipid storage organelles that act as hubs of metabolic processes. LDs are structurally unique with a hydrophobic core that mainly consists of neutral lipids, sterol esters, and triglycerides, enclosed within a phospholipid monolayer. Nascent LD formation begins with the accumulation of neutral lipids in the endoplasmic reticulum (ER) bilayer. The ER membrane proteins such as seipin, LDAF1, FIT, and MCTPs are reported to play an important role in the formation of nascent LDs. As the LDs grow, they unmix from the highly charged ER membrane to form mature LDs. LD biogenesis is an exciting, emerging research area, and herein, we discuss the recent progress in our understanding of the formation of eukaryotic nascent LDs. We focus on the role of ER membrane shaping proteins such as reticulons and reticulon-like proteins, membrane lipids, and cytoskeleton proteins such as septin in the formation of nascent LDs.

The effect of chitosan/TiO2/hyaluronic acid subphase on the behaviour of 1,2-dioleoyl-sn-glycero-3-phosphocholine membrane.

The main aim of the study was to determine the effect of two polysaccharides: chitosan (Ch) and hyaluronic acid (HA), and/or titanium dioxide (TiO2) on the structure and behaviour of the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) membrane. To achieve this goal the surface pressure as a function of the area per molecule (π-A) isotherm for the phospholipid monolayer was recorded. The shape of the π-A isotherms and compression-decompression cycles, as well as the compression modulus values, were analysed in terms of biocompatibility. Besides, morphology and thickness of the phospholipid layers obtained by means of Brewster angle microscope at different π, were determined. The obtained results show that both polysaccharides Ch, HA, as well inorganic TiO2 affect slightly the structure of the DOPC monolayer but do not disrupt it. Their presence brings no typical arrangements of both the polar heads and tails of DOPC molecules at the interface.

Sar1 Affects the Localization of Perilipin 2 to Lipid Droplets.

Lipid droplets (LDs) are intracellular organelles that are ubiquitous in many types of cells. The LD core consists of triacylglycerols (TGs) surrounded by a phospholipid monolayer and surface proteins such as perilipin 2 (PLIN2). Although TGs accumulate in the phospholipid bilayer of the endoplasmic reticulum (ER) and subsequently nascent LDs buds from ER, the mechanism by which LD proteins are transported to LD particles is not fully understood. Sar1 is a GTPase known as a regulator of coat protein complex Ⅱ (COPⅡ) vesicle budding, and its role in LD formation was investigated in this study. HuH7 human hepatoma cells were infected with adenoviral particles containing genes coding GFP fused with wild-type Sar1 (Sar1 WT) or a GTPase mutant form (Sar1 H79G). When HuH7 cells were treated with oleic acid, Sar1 WT formed a ring-like structure around the LDs. The transient expression of Sar1 did not significantly alter the levels of TG and PLIN2 in the cells. However, the localization of PLIN2 to the LDs decreased in the cells expressing Sar1 H79G. Furthermore, the effects of Sar1 on PLIN2 localization to the LDs were verified by the suppression of endogenous Sar1 using the short hairpin RNA technique. In conclusion, it was found that Sar1 has some roles in the intracellular distribution of PLIN2 to LDs in liver cells.

Nondestructive Compression and Fluidization of Phospholipid Monolayers by Gaseous and Aerolized Perfluorocarbons: Promising Substances for Lung Surfactant Treatment.

We present a surface-sensitive X-ray scattering study on the influence of gaseous and aerolized perfluorocarbons (FCs) on zwitterionic and anionic phospholipid Langmuir films, which serve as a simplified model system of lung surfactants. It was found that small gaseous FC molecules like F-propane and F-butane penetrate phospholipid monolayers and accumulate between the alkyl chains and form islands. This clustering process can trigger the formation of lipid crystallites at low initial surface pressures. In contrast, the large linear FC F-octyl bromide fluidizes membranes, causing a dissolution of crystalline domains. The bicyclic FC F-decalin accumulates between the alkyl chains of 1,2-dipalmitoyl phosphatidylcholine but cannot penetrate the more densely packed 1,2-dipalmitoyl phosphatidic acid films because of its size. The effects of FCs on lung surfactants are discussed in the framework of currently proposed therapeutic methods for acute respiratory distress syndrome using FC gases, vapor, or aerosol ventilation causing monolayer fluidization effects. This study implies that the highly biocompatible and nontoxic FCs could be beneficial in the treatment of lung diseases with injured nonfunctional lung surfactants in a novel approach for ventilation.

Structure and Dynamics of Human Perilipin 3 Membrane Association

Lipid droplets (LDs) are a unique organelle that contain an oil core mainly composed of triglycerides that is surrounded by a phospholipid monolayer and coated by LD‐associated proteins called perilipins (PLINs). There are five PLINs in human that generally contain a highly conserved N‐terminal PAT domain of unclear function, followed by variable stretches of amphipathic helices previously implicated in LD association, and a C‐terminal 4‐helix bundle domain. During the process of LD biogenesis, perilipin 3 (PLIN3) is the first PLIN to be recruited to nascent LDs as they emerge from the endoplasmic reticulum. However, how PLIN3 recruitment to LDs is affected by lipid composition, and the structure and dynamics of PLIN membrane association has not been examined in detail. Here, using liposome co‐sedimentation and LD‐flotation assays we found that PLIN3 membrane association was enhanced by membrane packing defects and further promoted by the triglyceride precursors diacylglycerol (DAG) and phosphatidic acid. Hydrogen‐deuterium exchange mass spectrometry experiments identified that both the PAT domain and amphipathic helices undergo a transition upon membrane binding from intrinsically disordered to an ordered state. Using constructs containing different portions of the PAT domain and amphipathic helices, we identify a new functional PAT domain that contains all conserved residues and is strongly recruited to DAG enriched membranes. Consistent with these findings, this newly determined functional PAT domain is predicted to form a triangular helical tertiary structure by alphafold and RoseTTAfold. Taken together, this study reveals molecular details of PLIN3 recruitment to membranes and suggests a role for the PAT domain as a major driver of PLIN3 to DAG enriched membranes, which is consistent with the localization of PLIN3 on early nascent LDs.

Formation of Nascent Lipid Droplets at Specialized ER subdomains

Lipid droplets (LDs) are neutral lipid containing organelles enclosed in a single monolayer of phospholipids. LD formation begins with the accumulation of neutral lipids within the bilayer of the endoplasmic reticulum (ER) membrane. It is not known how the sites of formation of nascent LDs in the ER membrane are determined. Here we show that multiple C2 domain–containing transmembrane proteins, MCTP1 and MCTP2, are at sites of LD formation in specialized ER subdomains. We show that the transmembrane domain of these proteins is similar to a reticulon homology domain. Like reticulons, MCTPs tubulate the ER membrane and favor highly curved regions of the ER. Our data indicate that the transmembrane domains (TMD) of MCTP promote LD biogenesis by increasing LD number. MCTPs colocalize with seipin, a protein involved in LD biogenesis, but form more stable microdomains in the ER. The MCTP C2 domains bind charged lipids and regulate LD size, likely by mediating ER–LD contact sites. Together, our data indicate that MCTPs form specialized subdomains within ER tubules that regulate LD biogenesis, size, and ER–LD contacts. Interestingly, MCTP punctae are associated with other organelles as well, suggesting that these proteins may play a more general role in linking tubular ER to organelle contact sites.

Proteome analysis of nuclear Lipid‐Associated Promyelocytic Leukemia (PML) Structures (LAPS) Using Biotin Proximity Labelling

Lipid droplets (LD) are organelles composed of a phospholipid monolayer surrounding a core of and triacylglycerol and cholesterol ester. LD have a primary role in energy homeostasis and lipotoxic stress through the regulated storage and release of fatty acids. While LD are cytoplasmic, lipoprotein secreting cells harbor nuclear LD (nLD) that associate with PML, the structural protein of PML nuclear bodies (PML‐NB) that regulate DNA repair and stress responses. Knockout studies in U2OS cells indicated that the 50% of nLDs contain PML isoform II (PML‐II) and have a unique lipid and protein composition marked by the lack of canonical PML‐NB‐associated proteins, such as DAXX, SP100 and SUMO. Owing to their unique composition, we term these subnuclear lipid:protein domains, lipid‐associated PML structures (LAPS). To gain insight into LAPS function, we conducted proximity labeling and mass spectrometry (MS) studies using PML‐I and PML‐II modified by fusion to the ascorbate peroxidase APEX2 that uses biotin‐phenol to biotinylate proteins. U2OS and Huh7 cells expressing PML‐APEX2 fusion proteins were treated with and without oleate, and biotinylated proteins were isolated and analyzed by LC‐MS/MS. Using standard filtering techniques (CRAPome, SAINT, Bayesian false discovery rate) we identified several high confidence interacting proteins associated with the COPII coatomer complex, including Sec24A, Sec23IP, Sec24B, and Sec23B. These Sec23/24 proteins were associated with PML‐I and, to a lesser extent, PML‐II under oleate‐treated conditions. Sec23B was observed to associate with both PML‐NBs as well as LAPS by immunofluorescence microscopy in U2OS cells expressing PML‐II‐APEX2. The COPII pathway has previously been implicated in protein delivery to cytoplasmic LD setting a precedence for the interaction we observed with LAPS. Thus, given our preliminary data showing interaction between Sec 23/24 heterodimer, PML‐NBs and LAPS we are now pursuing the hypothesis that lipid membrane deformation and/or cargo transport activity in the nucleus by COPII proteins could play a role in lipid homeostasis.

Decreasing Phosphatidylcholine on the Lipid Droplet Surface Selectively Recruits Proteins with Amphipathic Alpha Helices

Lipid droplets are ubiquitous, intracellular fat‐storage organelles. Each lipid droplet is surrounded by a phospholipid monolayer primarily composed of two phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE). We have previously shown that lipid droplets accumulating in a rat model of alcoholic fatty liver disease have a decreased PC:PE ratio compared to healthy control animals. Moreover, altering the phospholipid composition of the lipid droplet surface changes the amount of specific lipid droplet proteins, suggesting a mechanism for fat accumulation in alcoholic fatty liver disease. Using subcellular fractionation and immunoblotting, we investigated the abundance and lipid droplet association of multiple proteins in AML12 cells cultured under conditions that alter the PC:PE ratio and mimic the lipid droplet structure in alcoholic fatty liver disease. With this approach, we measured the relative binding of proteins that interact with lipid droplets through a variety of structural motifs, including amphipathic alpha helices, lipid anchors, and hairpin loops. Our results indicate that the amphipathic alpha helix‐containing proteins perilipin 1, perilipin 2, and CIDE‐C are significantly more likely to associate with lipid droplets with decreased PC:PE ratios. This same change in PC:PE levels did not change association of rab 18, a protein that interacts with lipid droplets through a lipid anchor, and decreased lipid droplet association of a protein (DGAT2) that trafficks to lipid droplets as they bud from the ER. Our results support a model where increased binding of the amphipathic alpha helical domains of some lipid droplet proteins contributes to lipid droplet overaccumulation in alcoholic fatty liver disease. This work builds on our understanding of alcoholic fatty liver disease and the mechanisms that facilitate protein binding to lipid droplets.

Development and Characterization of Lipospheres Containing Repaglinide

Lipospheres formulation is an aqueous micro dispersion of solid water insoluble spherical micro the lipospheres are prepared of solid hydrophobic triglycerides with a monolayer of phospholipids embedded on the shell of the particle. Repaglinide is a water insoluble ant diabetic drug which belongs to the class of medications known as meglitinides. An attempt was made to develop and characterized the lipospheres in order to enhance the bioavailability of a highly permeable and a poorly soluble anti-diabetic drug by using different concentrations of bees wax, stearic acid, cetyl alcohol, tween 80 and phospholipids coat such as soybean phosphotidylcholine. The lipospheres were prepared by solvent evaporation technique and characterized for particle size, scanning electron microscopy, % entrapment efficiency, % yield, % drug content, in vitro drug release and stability study. All the formulation gave the satisfactory result in terms of particle size, entrapment efficiency and drug content. Lipospheres were substantially stable after 3 months storage at 2–8°C.

Heterogeneous Rate Constant for Amorphous Silica Nanoparticle Adsorption on Phospholipid Monolayers.

The interaction of amorphous silica nanoparticles with phospholipid monolayers and bilayers has received a great deal of interest in recent years and is of importance for assessing potential cellular toxicity of such species, whether natural or synthesized for the purpose of nanomedical drug delivery and other applications. This present communication studies the rate of silica nanoparticle adsorption on to phospholipid monolayers in order to extract a heterogeneous rate constant from the data. This rate constant relates to the initial rate of growth of an adsorbed layer of nanoparticles as SiO2 on a unit area of the monolayer surface from unit concentration in dispersion. Experiments were carried out using the system of dioleoyl phosphatidylcholine (DOPC) monolayers deposited on Pt/Hg electrodes in a flow cell. Additional studies were carried out on the interaction of soluble silica with these layers. Results show that the rate constant is effectively constant with respect to silica nanoparticle size. This is interpreted as indicating that the interaction of hydrated SiO2 molecular species with phospholipid polar groups is the molecular initiating event (MIE) defined as the initial interaction of the silica particle surface with the phospholipid layer surface promoting the adsorption of silica nanoparticles on DOPC. The conclusion is consistent with the observed significant interaction of soluble SiO2 with the DOPC layer and the established properties of the silica–water interface.

The interaction of styrene maleic acid copolymers with phospholipids in Langmuir monolayers, vesicles and nanodiscs; a structural study

HypothesisSelf-assembly of amphipathic styrene maleic acid copolymers with phospholipids in aqueous solution results in the formation of ‘nanodiscs’ containing a planar segment of phospholipid bilayer encapsulated by a polymer belt. Recently, studies have reported that lipids rapidly exchange between both nanodiscs in solution and external sources of lipids. Outstanding questions remain regarding details of polymer-lipid interactions, factors influencing lipid exchange and structural effects of such exchange processes. Here, the dynamic behaviour of nanodiscs is investigated, specifically the role of membrane charge and polymer chemistry. ExperimentsTwo model systems are investigated: fluorescently labelled phospholipid vesicles, and Langmuir monolayers of phospholipids. Using fluorescence spectroscopy and time-resolved neutron reflectometry, the membrane potential, monolayer structure and composition are monitored with respect to time upon polymer and nanodisc interactions. FindingsIn the presence of external lipids, polymer chains embed throughout lipid membranes, the extent of which is governed by the net membrane charge. Nanodiscs stabilised by three different polymers will all exchange lipids and polymer with monolayers to differing extents, related to the properties of the stabilising polymer belt. These results demonstrate the dynamic nature of nanodiscs which interact with the local environment and are likely to deposit both lipids and polymer at all stages of use.

Molecular organization of dengue fusion peptide in phospholipid monolayers revealed by tensiometry and vibrational spectroscopy

The interaction of Dengue fusion peptide (FLAg) in selected lipid Langmuir monolayers was characterized with surface pressure-area isotherms and infrared spectroscopy to investigate the role of the membrane charge and molecular organization in the peptide-lipid binding. Surface pressure-area isotherms were employed to analyze the thermodynamic and mechanical properties of the FLAg-lipid monolayer, showing that charged lipid monolayers showed different peptide adsorption patterns for an optimized peptide concentration (maximum membrane adsorption). Polarization modulation infrared reflection-absorption spectroscopy pointed out that incorporating FLAg changed the dipole orientations for the lipid polar head groups, as confirmed in PG-containing monolayers. Also, FLAg reorients the lipid film when it interacts with the phosphate and choline groups. Finally, analysis of the 310-helix bands suggests that FLAg assumes a configuration as a hairpin, an essential premise for the beginning of the membrane fusion process.

Hydrocarbon Penetration into Phospholipid Monolayers Formed at Hydrocarbon-Water Interfaces.

Phospholipid monolayers formed at oil-water interfaces are used for various biological applications. However, monolayer structures are not well understood. Herein, we investigated hydrocarbon partitioning in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine monolayers formed at hydrocarbon-water interfaces using fluorescence microscopy and pendant drop tensiometry. The monolayers strongly interacted with squalene, n-hexadecane, n-tetradecane, n-dodecane, n-decane, and n-butylcyclohexane. These alkane and alkylcyclohexane molecules remained within the monolayers during area compression. In contrast, the monolayers interacted weakly with n-pentylbenzene and n-butylbenzene. These alkylbenzenes were gradually removed from the monolayers upon area compression and were completely expelled at an area per lipid of ∼70 Å2. Surface pressure analysis indicated that the ability of hydrocarbons to penetrate the monolayers was enhanced in the order of n-butylbenzene < n-pentylbenzene < n-butylcyclohexane < n-hexadecane.