Polar-group behaviour in mixed monolayers of phospholipids and fusogenic lipids.

1. The surface potentials of mixed monolayers of synthetic phospholipids with lipids that are fusogenic for hen erythrocytes were investigated. 2. At pH 5.6 and 10, but not at pH2, mixed monolayers of the fusogenic lipid, glycerol mono-oleate, with phosphatidylcholine exhibited negative deviations from the ideality rule in surface potential per molecule which were accompanied by negative deviations in mean molecular area. 3. Interactions of this type were not seen with chemically related but non-fusogenic lipids, nor were they found in mixed monolayers of any of the lipids with phosphatidylethanolamine. 4. Experiments with dihexadecyl phosphate and hexadecyltrimethyl-ammonium indicated that the complete head group of phosphatidylcholine is required for its observed behaviour with fusogenic lipids. 5. Bivalent cations (Ca2+, UO2(2+) or Zn2+) in the subphase at pH 5.6 significantly modified the behaviour of mixed monolayers of fusogenic lipids with phospholipids; there was a parallel perturbing effect of fusogenic lipids on interactions between monolayers of phospholipids and bivalent cations. 6. Possible molecular interactions of fusogenic lipids with membrane phospholipids, and the role of Ca2+, are discussed which may be relevant to cell fusion in erythrocytes induced by low-melting lipids in the presence of Ca2+.     

Interaction of melittin with glycosphingolipids and phospholipids in mixed monolayers at different temperatures. Effect of the lipid physical state

The influence of the liquid-expanded or liquid-condensed state of the lipid interface induced by changes of temperature on the lipid-protein interactions and their two-dimensional miscibility was studied for mixtures of melittin with different phospholipids (DPPC, DMPC, DOPC egg PC) and gangliosides (G_M1, G_D1a) in mixed monolayers at the air/145 mM NaCl interface. The critical amount of melittin at which a phase separation takes place in the mixed film increases as the glycosphingolipid or phospholipid is more liquid-expanded. The lipid-protein interaction increases the stability of both melittin and the lipid. The interaction of melittin with gangliosides is thermodynamically more favorable as these are more liquid-expanded. The interaction of melittin with phospholipids, on the other hand, is more favorable when the lipids are in the liquid-condensed state even if these films show lateral immiscibility at a lower proportion of protein compared to lipids in the liquid-expanded state. Hydration-dehydration effects in the polar head group region are likely to participate in these lipid-protein interactions.     

Favorable and unfavorable lateral interactions of ceramide, neutral glycosphingolipids and gangliosides in mixed monolayers

Interactions among four natural neutral sphingolipids (ceramide, glucosyl-ceramide, lactosyl-ceramide and asialo-GM1) and six gangliosides (GM3, GM2, GM1, GD3, GD1a and GT1b) were studied in binary Langmuir monolayers at the air-buffer interface in terms of their molecular packing, compressibility, dipole potential and mixing behavior. The changes of surface organization can be grouped into three sets: (a) binary films of neutral GSLs, and of the latter with ceramide, exhibit thermodynamically unfavorable mixing with mean molecular area expansions and dipole moment hyperpolarization; (b) mixed monolayers of ceramide, or of GlcCer, and gangliosides occur with thermodynamically favorable interactions leading to mean molecular area condensation and depolarisation; (c) binary mixtures of LacCer or Gg4Cer with gangliosides, and all ganglioside species among them, revealed molecular immiscibility characterized by additive mean molecular area and dipole potential, with composition-independent constant collapse pressure. These results disclose basic tendencies of GSLs to molecularly mix or demix, leading to their surface segregation, which may underlay vectorial separation of their specific biosynthetic pathways.     

The study on the interaction between phytosterols and phospholipids in model membranes

Sterols are one of the major components of cellular membranes. Although in mammalian membranes cholesterol is a predominant sterol, in the human organism plant sterols (phytosterols) can also be found. Phytosterols, especially if present in concentrations higher than normal (phytosterolemia), may strongly affect membrane properties. In this work, we studied phytosterol-phospholipid interactions in mixed Langmuir monolayers serving as model membranes. Investigated were two phytosterols, beta-sitosterol and stigmasterol and a variety of phospholipids, both phosphatidylethanolamines and phosphatidylcholines. The phospholipids had different polar heads, different length and saturation of their hydrocarbon chains. The interactions between molecules in mixed sterol/phospholipid films were characterized with the mean area per molecule (A(12)) and the excess free energy of mixing (DeltaG(Exc)). The effect of the sterols on the molecular organization of the phospholipid monolayers was analyzed based on the compression modulus values. It was found that the incorporation of the phytosterols into the phospholipid monolayers increased their condensation. The plant sterols revealed higher affinity towards phosphatidylcholines as compared to phosphatidylethanolamines. The phytosterols interacted more strongly with phospholipids possessing longer and saturated chains. Moreover, both the length and the saturation of the phosphatidylcholines influenced the stoichiometry of the most stable complexes. Our results, compared with those presented previously for cholesterol/phospholipid monolayers, allowed us to draw a conclusion that the structure of sterol (cholesterol, beta-sitosterol, stigmasterol) does not affect the stoichiometry of the most stable complexes formed with particular phospholipids, but influences their stability. Namely, the strongest interactions were found for cholesterol/phospholipids mixtures, while the weakest for mixed systems containing stigmasterol.     

Studies on mixed monolayers of phospholipids and fusogenic lipids.

1. The behaviour of mixed monolayers of 14 different lipids with preparations of erythrocyte lipids, purified natural and synthetic phospholipids, cholesterol and galactosylceramide was investigated. 2. The mean areas occupied per molecule in mixed films containing lipids that are fusogenic for hen erythrocytes were compared with those for corresponding films containing lipids that are inactive as fusogens. 3. Fusogenic lipids were found to exhibit interactions, which were not shown by non-fusogenic lipids, in mixed monolayers with several species of phospholipid, particularly those containing a choline head group. 4. Heterogeneity in the hydrophobic chains of phosphatidylcholine, their degree of unsaturation and the presence of cholesterol had little effect on the interaction of phosphatidylcholine with fusogenic lipids. 5. Fusogenic lipids showed little specific interaction with natural or synthetic preparations of phosphatidylethanolamine. 6. The possible significance of these observations in relation to the action of fusogenic lipids on biological membranes is discussed in the light of the asymmetrical distribution of phospholipids in erythrocyte membranes.     

Configuration and interactions of the polar head group in gangliosides

1. The interactions of gangliosides with Ca²⁺ and some polar-head-group requirements for establishment of particular interactions with phosphatidylcholine were studied in monolayers at the air/145mm-NaCl interface. 2. Ganglioside–Ca²⁺ interactions, as revealed by surface-potential measurements, depended on the position occupied by sialosyl residues in the oligosaccharide chain. The interactions with Ca²⁺ of the single sialosyl residue of monosialogangliosides occurred above 0.1mm-CaCl₂, whereas the interaction of the cation with additional sialosyl groups in di- or tri-sialogangliosides depended on the carbohydrate residue to which the sialosyl moiety was attached. The sialosyl residue bound in sialosyl–sialosyl linkage interacted very little with Ca²⁺. The sialosyl residue attached to the terminal galactose of the neutral tetrasaccharide chain interacted with Ca²⁺ above 1μm-CaCl₂. 3. Experiments with mixed monolayers containing dihexadecyl phosphate and hexadecyltrimethylammonium indicated that for the occurrence of interactions of polysialogangliosides with phosphatidylcholine characterized by reductions in molecular packing and surface potential both charged groups of the phospholipid and sialosyl residues with particular dipolar properties in the ganglioside are participating. 4. Possible configurations that can explain the behaviour in monolayers were inspected with space-filling molecular models. The position of the carboxylate group of sialosyl residues with respect to the interface and to the sialosyl molecular plane can explain the different orientation of the dipole-moment vector of this residue, which depends on the position to which it is linked in the oligosaccharide chain. Favoured interactions of polysialogangliosides with phosphatidylcholine may result from a configuration allowing a partial matching of two oppositely oriented electrical vectors contributed by the zwitterionic phosphocholine group and particular sialosyl groups.     

Interaction of myelin basic protein, melittin and bovine serum albumin with gangliosides, sulphatide and neutral glycosphingolipids in mixed monolayers

Some parameters that may regulate the miscibility and stability of mixed lipid-protein monolayers at the air-145 mM NaCl interface were studied employing six glycosphingolipids (acidic or neutral), three different types of proteins (soluble, extrinsic or highly amphipathic) and some phospholipids. The results obtained show that the percentage of the total area occupied by the protein at the interface is an important parameter leading to lateral phase separations; the amount and area contribution of the protein accepted in the film before the components become immiscible increase with the complexity of the polar head group of the glycosphingolipids. The interactions occur with progressive reductions of the intermolecular packing as the polar head group of the glycosphingolipid becomes more complex and this is accompanied by more negative values of the excess free energy of mixing. The lipid component seems to be the major responsible for the reduction in mean molecular area.     

Interaction of 1-anilinonaphthalene 8-sulfonic acid with interfaces containing cerebrosides, sulfatides and gangliosides

The fluorescence lifetime, quantum yield and emission spectra of 1-anilinonaphthalene 8-sulfonic acid (ANS) associated with interfaces of pure dipalmitoylphosphatidylcholine or its mixtures with phosphatidylserine, galactosylceramide, sulfatide or gangliosides GM1 and GD1a were studied at low and high ionic strength. Modification of the molecular organization of the lipid interfaces in the presence of the probe was also studied with mixed lipid monolayers. ANS has little affect on the intermolecular packing of the lipids but influences their surface potential, consistent with a location of ANS in the polar head group region of the interface. ANS senses a more polar microenvironment when associated with interfaces containing anionic glycosphingolipids at low ionic strength but, except for interfaces containing phosphatidylserine, it detects approximately the same polarity for neutral or anionic interfaces in 0.25 M NaCl.     

Interaction of chitosan with cell membrane models at the air-water interface

In this paper we employed phospholipid Langmuir monolayers as membrane models to probe interactions with chitosan. Using a combination of surface pressure--area and surface potential--area isotherms and rheological measurements with the pendent drop technique, we observed that chitosan interacts with phospholipid molecules at the air-water interface. We propose a model in which chitosan interacts with the phospholipids mainly through electrostatic interactions, but also including H-bonding and hydrophobic forces, depending on the phospholipid packing density. At large areas per molecule, chitosan in the subphase adsorbs onto the monolayer, expanding it. At small areas per molecule, chitosan is located in the subsurface. Indeed, a mixed chitosan-phospholipid monolayer can be transferred onto solid supports, even at high surface pressures. The effects of chitosan on the viscoelastic properties of phospholipid monolayers may be taken as evidence for the ability of chitosan to disrupt cell membranes.     

Calcium-ganglioside interactions and synaptic plasticity: effect of calcium on specific ganglioside/peptide (valinomycin, gramicidin A)-complexes in mixed mono- and bilayers.

A controlled exchange of calcium between the extracellular space (mM Ca2+) and the neuroplasm (microM Ca2+) is considered to be an essential prerequisite for almost every stage of neuronal activity. Our research interest is focused on those compounds, which due to their physico-chemical properties and localization within the synaptic membrane might fulfill the task as neuromodulators for functional synaptic proteins. Because of this specific binding properties towards calcium and their peculiar interactions with calcium in model systems gangliosides (amphiphilic sialic acid containing glycosphingolipids) are favorite candidates for a functional involvement in synaptic transmission of information. In this study we used monolayers to investigate the molecular packing and surface potential at the air/water interface, the interaction of gangliosides with the depsipeptide valinomycin (= monovalent ion carrier), and its influenceability by calcium. Furthermore we looked at calcium effects on the single channel conductance and mean channel life-time of the monovalent ion channel gramicidin A in mixed PC/ganglioside bilayers. In pure ganglioside monolayers the addition of 0.01 mM Ca2+ induces monolayer condensation, a rise in collapse pressure (= higher film stability), a shift of phase transition (= change of conformation), and a more negative head group potential (change of electric properties). In mixed ganglioside-valinomycin monolayers the addition of Ca2+ causes phase separation and/or aggregate formation between the ganglioside and the peptide. Single channel conductance fluctuations as well as mean channel life-time were analyzed for gramicidin A incorporated into binary mixed black lipid membranes of negatively charged gangliosides (GM1, GD1a, GT1b, GMix) and neutral lecithin (DOPC) in different molar ratios. At monovalent electrolyte concentrations up to < 250 mM CsCl the single channel conductance was significantly larger in the negatively charged mixed DOPC/ganglioside membranes than in the neutral DOPC membrane. Additionally, in the presence of gangliosides the mean channel life-time is increased. The addition of calcium (0.05 mM) induced a reduction of single channel conductance of gramicidin A in DOPC- and mixed DOPC/ganglioside membranes. These physico-chemical data in connection with new electromicroscopical evidences for a precise localization of calcium, a calcium pump (Ca(2+)-ATPase), a clustered arrangement of gangliosides in synaptic terminals, and biochemical results with regard to activatory nature of exogenous gangliosides for neuronal protein phosphorylation and ATPases, support the hypothesis of a modulatory function of gangliosides in synaptic transmission.     

1-Palmitoyl-2-pyrenedecanoyl glycerophospholipids as membrane probes: evidence for regular distribution in liquid-crystalline phosphatidylcholine bilayers

We have synthesized 1-palmitoyl-2-pyrenedecanoyl-sn-glycero derivatives of 3-phosphatidylcholine, 3-phosphatidylethanolamine, 3-phosphatidylserine, 3-phosphatidylglycerol, 3-phosphatidylinositol, and 3-phosphatidic acid and investigated their behavior in monolayers and in neat and mixed bilayers. Fluorescence spectroscopy of neat pyrene phospholipid dispersions revealed a well-defined thermotropic transition at 13.5-19 degrees C depending on the polar head group. An endotherm coinciding with this transition was observed with differential scanning calorimetry, indicating it to be due to the melting of the lipid acyl chains. For pyrenephosphatidylethanolamine, the endotherm was observed at a much higher temperature (70 degrees C). Compression isotherms obtained at an argon/water interface revealed that the pyrene moiety somewhat increases the mean molecular area of a phospholipid molecule but does not prevent the expression of head-group-dependent packing behavior. Partition of the pyrene lipids between coexisting fluid and solid phases was investigated with fluorometry and calorimetry. Both techniques indicate that these lipids prefer the fluid phase and that this preference is independent of the head group. The rates and apparent activation energies of lateral diffusion in fluid bilayers were found to be similar for most pyrene lipids, suggesting that the lateral movement of phospholipids is not critically dependent on interactions at the head-group level. Lateral distribution of the pyrene lipids in gel and fluid phosphatidylcholine bilayers was studied with the excimer technique and calorimetry. In gel-state dipalmitoylphosphatidylcholine bilayers, the pyrene lipids form clusters. These clusters, however, do not consist of pure pyrene lipid but of aggregates (compounds) of the labeled and unlabeled lipid.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of soluble and membrane proteins with monolayers of glycosphingolipids.

1. The interactions of four proteins (albumin, myelin basic protein, melittin and glycophorin) with eight neutral or acidic glycosphingolipids, including sulphatides and gangliosides, five zwitterionic or anionic phospholipids and some of their mixtures, were studied in lipid monolayers at the air/145 mM-NaCl interface. 2. In lipid-free interfaces, the surface pressure and surface potential reached by either soluble or integral membrane proteins did not reveal marked differences. 3. All the proteins studied showed interactions with each of the lipids but the maximal interactions were found for basic proteins with acidic glycosphingolipids. 4. Surface-potential measurements indicated that different dipolar organizations at the interface can be adopted by lipid-protein interactions showing the same value for surface free energy. 5. The individual surface properties of either the lipid of protein component are modified as a consequence of the lipid-protein interaction. 6. In mixed-lipid monolayers, the composition of the interface may affect the lipid-protein interactions in a non-proportional manner with respect to the relative amount of the individual lipid components.     

Evidence for the presence of a specific ganglioside GM1/valinomycin complex in mixed monolayers

The effect of a negatively charged mono-sialoglyco-sphingolipid (GM1-ganglioside) on the molecular organization and on physiochemical properties of lipid/peptide (valinomycin) systems was investigated in monolayers at the air/water interface. At a high molar fraction of GM1, the surface pressure/area isotherms of the two-component films of the system GM1/valinomycin and the isotherm of the pure ganglioside monolayer are identical concerning the space requirement of the molecules and thereby the packing of the monolayer. Using space-filling molecular models, a simple calculation gives the theoretical amount of 4.5 ganglioside molecules associated with one molecule of the depsipeptide valinomycin. The average surface potential indicates, that valinomycin, interacting with the polar head group of GM1, becomes partly embedded within the lipid interface. For GM1/eicosanol and valinomycin/eicosanol mixtures, the agreement between theory and experimental data strongly supports the model of ideal mixing without any molecular interactions between the different components. The results suggest the formation of a ganglioside/valinomycin complex with simultaneous alteration of the surface potential and molecular structure of the single components.     

A model for the packing of lipids in bilayer membranes

A number of known structural properties of mixed lipid bilayer membranes and monolayers are accounted for by a model in which lipids pack into bilayers and monolayers like building blocks, each characterized by a surface head group area and characteristic solid angle. In phospholipids above the melting transition the head group area (at a given temperature and degree of hydration) is fairly invariant while the hydrocarbon region may be liquid-like so long as the molecule is not compressed beyond its characteristic solid angle.Phosphotidylcholine and phosphotidylserine are tapered lipids, i.e. their surface head group areas are greater than their non-polar end areas; cholesterol is frayed, i.e. its polar end area is less than its non-polar end area; while phosphotidylethanolamine is almost cylindrical. The “condensing” effect of cholesterol in mixed phospholipid-cholesterol films is seen as a taper-fray accomodation. The lipid distribution in erythrocyte membranes is shown to be conductive to a stable strain-free membrane.     

A model for the packing of lipids in bilayer membranes.

A number of known structural properties of mixed lipid bilayer membranes and monolayers are accounted for by a model in which lipids pack into bilayers and monolayers like building blocks, each characterized by a surface head group area and characteristic solid angle. In phospholipids above the melting transition the head group area (at a given temperature and degree of hydration) is fairly invariant while the hydrocarbon region may be liquid-like so long as the molecule is not compressed beyond its characteristic solid angle. Phosphatidylcholine and phosphatidylserine are tapered lipids, i.e. their surface head group areas are greater than their non-polar end areas; cholesterol is frayed, i.e. its polar end area is less than its non-polar end area; while phosphatidylethanolamine is almost cylindrical. The "condensing" effect of cholesterol in mixed phospholipid-cholesterol films is seen as a taper-fray accommodation. The lipid distribution in erythrocyte membrane is shown to be conducive to a stable strain-free membrane.     

Behavior of a GPI-anchored protein in phospholipid monolayers at the air-water interface

The interaction between alkaline phosphatase (AP), a glycosylphosphatidylinositol (GPI)-anchored protein (AP-GPI), and phospholipids was monitored using Langmuir isotherms and PM-IRRAS spectroscopy. AP-GPI was injected under C16 phospholipid monolayers with either a neutral polar head (1,2-dipalmitoyl-sn-glycero-3-phosphocholine monohydrate (DPPC)) or an anionic polar head (1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS)). The increase in molecular area due to the injection of protein depended on the surface pressure and the type of phospholipid. At all surface pressures, it was highest in the case of DPPS monolayers. The surface elasticity coefficient E, determined from the pi-A diagrams, allowed to deduct that the AP-GPI-phospholipid mixtures presented a molecular arrangement less condensed than the corresponding pure phospholipid films. PM-IRRAS spectra suggested different protein-lipid interactions as a function of the nature of the lipids. AP-GPI modified the organization of the DPPS deuterated chains whereas AP-GPI affected only the polar group of DPPC at low surface pressure (8 mN/m). Different protein hydration layers between the DPPC and DPPS monolayers were suggested to explain these results. PM-IRRAS spectra of AP-GPI in the presence of lipids showed a shape similar to those collected for pure AP-GPI, indicating a similar orientation of AP-GPI in the presence or absence of phospholipids, where the active sites of the enzyme are turned outside of the membrane.     

Molecular packing of high-density and low-density lipoprotein surface lipids and apolipoprotein A-I binding

The surface pressure (pi)-molecular area (A) isotherms for monolayers of human high-density lipoprotein (HDL3) and low-density lipoprotein (LDL) phospholipids and of mixed monolayers of these phospholipids with cholesterol spread at the air-water interface were used to deduce the likely molecular packing at the surfaces of HDL3 and LDL particles. LDL phospholipids form more condensed monolayers than HDL3 phospholipids; for example, the molecular areas of LDL and HDL3 phospholipids at pi = 10 dyn/cm are 88 and 75 A2/molecule, respectively. The closer packing in the LDL phospholipids monolayer can be attributed to the higher contents of saturated phosphatidylcholines and sphingomyelin relative to HDL3. Cholesterol condenses both HDL3 and LDL phospholipid monolayers but has a greater condensing effect on the LDL phospholipid monolayer. The pi-A isotherms for mixed monolayer of HDL3 phospholipid/cholesterol and LDL phospholipid/cholesterol at stoichiometries similar to those at the surfaces of lipoprotein particles suggest that the monolayer at the surface of the LDL particle is significantly more condensed than that at the surface of the HDL3 particle. The closer lateral packing in LDL is due to at least three factors: (1) the difference in phospholipid composition; (2) the higher unesterified cholesterol content in LDL; and (3) a stronger interaction between cholesterol and LDL phospholipids relative to HDL3 phospholipids. The influence of lipid molecular packing on the affinity of human apolipoprotein A-I (apo A-I) for HDL3 and LDL surface lipids was evaluated by monitoring the adsorption of 14C-methylated apo A-I to monolayers of these lipids spread at various initial surface pressures (pi i).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of the phospholipid head group in antibiotic-phospholipid association at water-air interface

We studied the interactions of tetracycline antibiotics, TCs, with phospholipid monolayers with the two-fold aim of elucidating the mechanism of action of TCs and to provide a first step for the realization of bio-mimetic sensor for such drugs by means of the Langmuir-Blodgett technique. Preliminary surface tension studies demonstrated that surface activity of tetracycline is moderate and dependent on the pH of the subphase. We selected three phospholipids having hydrophobic chains of the same length but differing in the polar head structures, i.e. dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylethanolamine, and dipalmitoylphosphatidic acid. Surface pressure- and surface potential- area isotherms were employed to investigate the behavior of the phospholipid monolayers at the water-air interface when tetracycline was added to the aqueous subphase. Analysis of the results indicated that the electrostatic interaction is the driving force for migration of tetracycline towards the interface where localized adsorption to the head groups occurs. Nevertheless, such interactions appear to be insufficient to promote penetration of tetracycline through the hydrophobic layer.     

Mixed monolayers involving DPPC, DODAB and oleic acid and their interaction with nicotinic acid at the air-water interface

The behaviour of binary mixtures involving dipalmitoylphosphatidylcholine (DPPC), dioctadecyldimethylammonium bromide (DODAB) and oleic acid (OA) was investigated at the air-water interface by surface pressure-area (pi-A) measurements and by Brewster angle microscopy (BAM). Thermodynamic analysis indicates for the system DPPC/DODAB miscibility with strong negative deviations from the ideal behaviour, from low to high surface pressures over all the composition range. For systems DODAB/OA and DPPC/OA, thermodynamic analysis and BAM observation indicate miscibility from low to intermediate surface pressures, and phase separation in a limited range of composition at high surface pressures. The interaction of nicotinic acid (NA) with pure lipids and with selected compositions of mixed systems was investigated. Significant positive deviations of pi-A isotherms in the presence of NA indicate attractive interactions between NA and the polar groups of DPPC and DODAB. NA easily penetrates in expanded regimes while it tends to be segregated from condensed regimes in mixed monolayers.     

Spreading and polymerization behavior of diacetylenic phospholipids at the gas-water interface

A variety of polymerizable lipids derived from hexacosa-10,12-diynoic acid and hexacosa-10,12-diyne-1-ol have been synthesized and spread at the gas/water interface. The measured surface pressure/area isotherms indicate that head group charge and bulkiness have strong influences on the area occupied per molecule. In the case of zwitterionic phospholipids additional area changes are brought about by alkaline and acidic subphases, which is probably due to an alteration of head group conformation. Condensed state diacetylenic lipid monolayers in a nitrogen atmosphere are polymerizable by UV irradiation. The polymerization reaction was monitored at the gas/water interface by the area change at constant surface pressure and the change of optical density in the visible region. As already observed for vesicle polymerization, single chain amphiphiles exhibit a different absorption behavior than asymmetric double chain amphiphiles of the phosphoglycerol type. The polymerized monolayers were more densely packed and more stable than their monomeric counterparts as indicated by the smaller areas and higher pressures reached before the collapse points.