Pure Lipid Monolayers Research Articles

Aβ Amyloid Fibers Drastically Alter the Topography and Mechanical Properties of Lipid Membranes.

Alzheimer's disease (AD) is related to the fibrillation of the Aβ peptides at neuronal membranes, a process that depends on the lipid composition and may impart different physical states to the membrane. In the present work, we study the properties of the Aβ peptide when mixed with a zwitterionic lipid (DMPC), using the Langmuir monolayer technique as an approach to control membrane physical conditions. First, we build on previous characterizations of pure Aβ monolayers and observe that, in addition to high shear, these films present a pronounced compressional hysteresis. When Aβ is assembled with DMPC in a binary film, the resulting membranes become heterogeneous, with a peptide-enriched phase distributed in a network-like pattern, and they exhibit a lateral transition that depends on the Aβ content. At lower peptide proportions, the films segregate into two well-defined phases: one consisting of lipids and another enriched with peptides. The reflectivity of these phases differs from that obtained for pure Aβ films. Thus, the formed fibers effectively cover most of the interface area and remain stable at higher pressures (from 20 to 30 mN m-1 depending on Aβ content) compared to pure peptide films (17 mN m-1). Furthermore, such structures induce a compressional hysteresis in the film, similar to that of pure peptide films (which is nonexistent in the pure lipid monolayer), even at low peptide proportions. We claim that the mechanical properties at the interface are governed by the size of the fibril-like structures. Based on the low molar fractions and surface packing at which these phenomena were observed, we postulate that as a consequence of peptide intermolecular interactions, Aβ may have drastic effects on the molecular arrangement and mechanical properties of a lipid membrane.

Studied on the dynamic adsorption process of Lycium barbarum polysaccharide in the POPC/DPPC monolayers.

In this study, the interaction between Lycium barbarum polysaccharide (LBP) and unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or saturated 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was explored using the Langmuir films technique and atomic force microscopy (AFM). Comparing the pure lipid monolayer with the mixed monolayers, the π-A isotherms of the mixed monolayers shifted to larger molecular areas when LBP was added to the subphase. The compression modulus showed that the compressibility of the monolayer films decreased with the addition of LBP. Adsorption curves revealed that the variation in the surface pressure of LBP with POPC was larger than that with DPPC. This phenomenon was verified by the AFM images and the number of each lipid molecule combining with polysaccharide molecules in the mixed monolayer (Ap value), indicating that hydrophobic interactions between LBP and POPC are stronger than those of DPPC. These findings lay the foundation for exploring the pharmacological mechanism of LBP as an in vivo therapeutic.

Nonlinear propagation of two dimensional sound waves observed at lipid interfaces

Experimental results are presented on the acoustic propagation of mechanical perturbations in a lipid monolayer along the air-water interface. The interface was excited by a piezo-cantilever, and propagating impulses were measured optically using Forster Resonance Energy Transfer (FRET). The velocity of propagation varied from 0.1 to 1 m/s depending on the compressibility of the interface. Near a nonlinearity in the state diagram of the interface, for example, near a phase transition of the lipids, impulses propagated only when the initial mechanical impulse was greater than a certain threshold. In fact, the impulse then propagated as a solitary shock wave causing local adiabatic phase transition of the lipid molecules along the way. Although the phenomenon has been observed here in pure lipid monolayers, which represent a well-documented model for biological membranes, the origin of the observed phenomenon lies in the conservation of the entropy of the interface, determined by the change in state of the interface during the impulse. Given that the state diagrams of biological membranes have nonlinearities near physiological conditions, nonlinear sound waves are expected to be fundamentally involved in inter and intra cellular communication. Indeed, the observed acoustic phenomenon is characteristically similar to nerve impulses.

Orientational Properties of DOPC/SM/Cholesterol Mixtures: A PM-IRRAS Study

Sphingomyelins (SM) and phosphatidylcholines (PC) are major lipid classes in the external plasma membrane leaflet of mammalian cells. A preferential interaction between SM and cholesterol (Cho) in both cell and model membranes has been proposed as central for the formation of Cho- and SM-rich domains in membranes. In this context, the relevance of the SM hydrophobic moiety on its interaction with Cho for domain stabilization has been investigated by our group (1-2). We report here on the effects of sphingomyelin structure on the orientational and conformational properties of monolayers of pure lipids and of two ternary lipid mixtures (DOPC/16:0SM/Cho and DOPC/24:1SM/Cho), which are relevant as mammalian cell membrane models.

CoenzymeQ10 localizations in model membranes. A Langmuir monolayer study

The interaction of coenzyme Q10 (CoQ10) in a monolayer of 1,2-dipalmitoyl-sn-glysero-3-phospho-L-choline (DPPC), in a monolayer of 1,2-dierucoyl-sn-glysero-3-phospho-L-choline (DEPC), in a monolayer of 1-palmitoyl-2-oleoyl-sn-glysero-3-phospho-L-serine (POPS) and in a monolayer of total lipid extract from pig brain (PB) has been investigated by using the Langmuir monolayer technique. Surface pressure (π)-mean molecular area (mma) isotherms have been measured for pure lipid monolayers and lipid monolayers with 0.5, 1.0, 2.0, 5.0 and 10.0 mol% CoQ10 concentrations. At the biological concentration (1.0-3.0 mol%) of CoQ10, intercalation of CoQ10 occurs in the lipid acyl chains of DPPC, POPS and PB monolayers. Above the biological concentration of CoQ10, the CoQ10 molecule induces domain formation in the monolayers of DPPC, POPS and PB lipids. The DEPC monolayer behavior deviates from the other lipids in this study. At 2.0 mol% the CoQ10 promotes very dense lipid packing, and the CoQ10 molecule is located parallel to the DEPC acyl chains at all concentrations.

Stability and softening of a lipid monolayer in the presence of a pain-killer drug

The aim of this study is to investigate the interaction of a drug (Piroxicam, 4-hydroxy-2-methyl-N-(2-pyridinyl)-2H-1,2-benzothiazine-3-carboxamide 1,1 dioxide) with a lipid (DMPC) monolayer used as a membrane-mime in terms of drug-induced changes in stability and compressibility with variation in temperature, surface-pressure, drug-dose and ionic states of the monolayers. Drug-induced fluidization is noticed in the π−A isotherms through increase in phase-transition pressure at constant temperature. The long-term dynamics of the lipid-monolayer is characterized by algebraic decays in surface-energy E with time t, E∼t−p1,2,3, with an initial decay exponent p1 that changes to p2 after ∼1000s, and, at high surface pressures and/or drug-dose, to a third exponent p3 after ∼3500s, suggesting structural reorganizations in the monolayer. With increasing drug–lipid ratio (D/L), p1 shows a decrease ending at an almost constant value after 0.05, p2 shows an almost negligible lowering while p3 shows a monotonic and considerable increase. The reorganization is summarized by proposing two mechanisms: (a) ‘charging–discharging’ where drug-molecules sitting parallel to the interface increase headgroup separations and (b) ‘discharging–charging’ where drug-molecules sitting roughly perpendicular to the interface bring headgroups closer. Drug-induced softening of lipid-monolayers is characterized by the compressibilites of pure and mixed lipid monolayers. Compressibility-change (i.e., compressibility difference between drug/lipid and pure lipid monolayer) with pressure is maximum in the LE–LC transition zone and compressibility-change with drug-dose reveals an optimum dose of drug for maximum increase in compressibility. Molecular dynamics simulation shows that the ordering in the different parts of the lipid chains is changed to different extents in the presence of drugs with maximum change near the headgroups and again points to an optimum dose for maximum disorder.

Conformational analyses of bacillomycin D, a natural antimicrobial lipopeptide, alone or in interaction with lipid monolayers at the air–water interface

Bacillomycin D is a natural antimicrobial lipopeptide belonging to the iturin family. It is produced by Bacillus subtilis strains. Bacillomycin D is characterized by its strong antifungal and hemolytic properties, due to its interaction with the plasma membrane of sensitive cells. Until now, only few limited analyses were conducted to understand the biological activities of bacillomycin D at the molecular level. Our purpose was to analyze the conformation of bacillomycin D using IR spectroscopy and to model its interactions with cytoplasmic membranes using Langmuir interfacial monolayers. Our findings indicate that bacillomycin D contains turns and allow to model its three-dimensional structure. Bacillomycin D formed a monolayer film at the air–water interface and kept its turn conformation, as shown by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). To identify the membrane lipid target of bacillomycin D, its interactions with pure lipid monolayers were analyzed and an original behavior of the lipopeptide toward cholesterol-containing monolayers was shown. This original behavior was lost when bacillomycin D was interacting with pure cholesteryl acetate monolayers, suggesting the involvement of the alcohol group of cholesterol in the lipopeptide–cholesterol interaction.

Penetration of the Antimicrobial Peptide Dicynthaurin into Phospholipid Monolayers at the Liquid–Air Interface

This work focuses on the adsorption kinetics of dicynthaurin with lipid monolayers, the effect of peptide adsorption on the structure of the lipid condensed chain lattice, peptide orientation and secondary structure in the adsorbed state. The studies with DPPG as model system revealed strong adsorption and massive incorporation of the peptide into the monolayer. Infrared reflection absorption spectroscopy (IRRAS) experiments showed that the secondary structure of the peptide is maintained upon adsorption. Specular X-ray reflectivity showed the destabilization of the condensed phase of the pure lipid monolayer and revealed a tilted orientation of the long axis of the peptide helix of about 40 degrees from the surface normal. Incorporation of the peptide was found to be pressure dependent, and at high pressure a "squeeze-out" was observed; however, the peptide remained localized to the interface, as suggested by infrared data. These findings were supported by optical fluorescence microscopy measurements which showed the squeeze-out of the peptide on water, but not under physiological conditions. The results suggest that dicynthaurin is able to adsorb to the phosphatidylglycerol-rich inner cytoplasmic membrane of bacteria and alter membrane integrity. To identify and interact with membrane motifs that are characteristic of microbes, but which are absent in eukaryotic cells, might be an intrinsic ability of peptide antibiotics.

Study of Adsorption and Penetration of E2(279−298) Peptide into Langmuir Phospholipid Monolayers

The peptide corresponding to the sequence (279-298) of the Hepatitis G virus (HGV/GBV-C) E2 protein was synthesized, and surface activity measurements, pi-A compression isotherms, and penetration of E2(279-298) into phospholipid monolayers spread at the air-water interface were carried out on water and phosphate buffer subphases. The results obtained indicated that the pure E2(279-298) Langmuir monolayer exhibited a looser packing on saline-buffered than on pure water subphase and suggest that the increase in subphase ionic strength stabilizes the peptide monolayer. To better understand the topography of the monolayer, Brewster angle microscopy (BAM) images of pure peptide monolayers were obtained. Penetration of the peptide into the pure lipid monolayers of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) and into mixtures of dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DMPC/DMPG) at various initial surface pressures was investigated to determine the ability of these lipid monolayers to host the peptide. The higher penetration of peptide into phospholipids is attained when the monolayers are in the liquid expanded state, and the greater interaction is observed with DMPC. Furthermore, the penetration of the peptide dissolved in the subphase into these various lipid monolayers was investigated to understand the interactions between the peptide and the lipid at the air-water interface. The results obtained showed that the lipid acyl chain length is an important parameter to be taken into consideration in the study of peptide-lipid interactions.

Effects of lipid chain length on molecular interactions between paclitaxel and phospholipid within model biomembranes

Molecular interactions between an anticancer drug, paclitaxel, and phosphatidylcholine (PC) of various chain lengths were investigated in the present work by the Langmuir film balance technique and differential scanning calorimetry (DSC). Both the lipid monolayer at the air–water interface and lipid bilayer vesicles (liposomes) were employed as model biological cell membranes. Measurement and analysis of the surface pressure versus molecular area curves of the mixed monolayers of phospholipids and paclitaxel under various molar ratio showed that phospholipids and paclitaxel formed a nonideal miscible system at the interface. Paclitaxel exerted an area-condensing effect on the lipid monolayer at small molecular surface areas and an area-expanding effect at large molecular areas, which could be explained by the intermolecular forces and geometric accommodation between the two components. Paclitaxel and phospholipids could form thermodynamically stable monolayer systems: the stability increased with the chain length in the order DMPC (C14:0)>DPPC (C16:0)>DSPC (C18:0). Investigation of paclitaxel penetration into the pure lipid monolayer showed that DMPC had a higher ability to incorporate paclitaxel and the critical surface pressure for paclitaxel penetration also increased with the chain length in the order DMPC>DPPC>DSPC. A similar trend was testified by DSC studies on vesicles of the mixed paclitaxel/phospholipids bilayer. Paclitaxel showed the greatest interaction with DMPC while little interaction could be measured in the paclitaxel/DSPC liposomes. Paclitaxel caused broadening of the main phase transition without significant change at the peak melting temperature of the phospholipid bilayers, which demonstrated that paclitaxel was localized in the outer hydrophobic cooperative zone of the bilayer. The interaction between paclitaxel and phospholipid was nonspecific and the dominant factor in this interaction was the van der Waals force or hydrophobic force. As the result of the lower net van der Waals interaction between hydrocarbon chains for the shorter acyl chains, paclitaxel interacted more readily with phospholipids of shorter chain length, which also increased the bilayer intermolecular spacing.

Investigations of the Orientation of a Membrane Peptide by Sum Frequency Spectroscopy

Sum frequency spectra of gramicidin A at the air/water interface in partially deuterated dimyristoylphosphatildycholine monolayers were collected as a function of surface pressure and mole fraction of the peptide. Reorganization of the lipid and peptide molecules were followed between 5 and 35 mN/m. Selective deuteration of the lipid enabled independent observation of the ordering of the lipid tail and headgroup regions. The results from pure lipid monolayer studies revealed that both portions of the amphiphile rearranged roughly in tandem as a function of pressure. When the membrane peptide was added to the monolayer, changes in its orientation were noted at mole fractions below 0.5, but not above this value. Specifically, two sets of data for the peptide could be obtained. One was for the aliphatic residues and the other for the Trp residues as each gave a distinct sum frequency signal in the CH stretch range. The aliphatic residues, which are dispersed throughout gramicidin, revealed that the whole peptide could facilely track changes in lipid orientation at 0.1 mole fraction but did so less readily at 0.4 mole fraction. On the other hand, the Trp residues, which all reside near the C-terminus and interact strongly with the lipid headgroups, showed a greater propensity to track lipid reorientation at the higher mole fraction. These results indicated that changes in the C-terminus probably precede realignment in other parts of the membrane peptide at higher peptide concentration. Furthermore, peptide-peptide interactions probably inhibit gramicidin A reorientation.

A study of mixed phospholipid/β-casein monolayers at the water|air surface

The compression/expansion properties of monolayers of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC) and distearoylphosphatidylcholine (DSPC) mixed with β-casein at the water|air interface are investigated by the film balance technique. An increase of the area per lipid molecule at constant surface pressure was found as the result of the incorporation of protein molecules into the lipid monolayers. When the films are compressed to a sufficiently high surface pressure the isotherms of the mixed protein–lipid monolayers coincide with those of pure lipid monolayers; i.e. the protein is “squeezed out”. Quantitative recording of consecutive cyclic compressions/expansions reveals that the processes in the mixed monolayers are reversible. The analysis suggests phase separation between protein and lipid.

Structure and orientation of lung surfactant SP-C and L-alpha-dipalmitoylphosphatidylcholine in aqueous monolayers

SP-C, a pulmonary surfactant-specific protein, aids the spreading of the main surfactant phospholipid L-alpha-dipalmitoylphosphatidylcholine (DPPC) across air/water interfaces, a process that has possible implications for in vivo function. To understand the molecular mechanism of this process, we have used external infrared reflection-absorption spectroscopy (IRRAS) to determine DPPC acyl chain conformation and orientation as well as SP-C secondary structure and helix tilt angle in mixed DPPC/SP-C monolayers in situ at the air/water interface. The SP-C helix tilt angle changed from approximately 24 degrees to the interface normal in lipid bilayers to approximately 70 degrees in the mixed monolayer films, whereas the acyl chain tilt angle of DPPC decreased from approximately 26 degrees in pure lipid monolayers (comparable to bilayers) to approximately 10 degrees in the mixed monolayer films. The protein acts as a "hydrophobic lever" by maximizing its interactions with the lipid acyl chains while simultaneously permitting the lipids to remain conformationally ordered. In addition to providing a reasonable molecular mechanism for protein-aided spreading of ordered lipids, these measurements constitute the first quantitative determination of SP-C orientation in Langmuir films, a paradigm widely used to simulate processes at the air/alveolar interface.

Pulmonary surfactant protein SP-C and phosphatidylcholines in bilayers and monolayers

Pulmonary surfactant lowers surface tension in the lung alveoli, reduces the work of breathing and prevents alveolar collapse at low lung volumes. The material consists of about 90% lipids, of which dipalmitoylphosphatidylcholine (DPPC) is the major component, and at least four proteins, SP-A, SP-B, SP-C, and SP-D. SP-B and SP-C, are very hydrophobic and are considered to help in the process of transferring the lipid from bilayers, the form in which it is secreted into a monolayer at the air-water interface. SP-C in saturated phosphatidylcholines in bilayers and monolayers has been studied by 2H nuclear magnetic resonance spectroscopy, high sensitivity differential scanning calorimetry, and epifluorescence of monolayers under compression. The gel to liquid-crystalline phase transition was broadened by SP-C. The T 1 relaxation times of the acyl chains were essentially unaffected by the protein. The T 2e of the chains were decreased in the liquid crystal and their temperature dependence was altered in the gel. Initial studies indicate that the first moments of the 2H NMR spectra of the head group of DPPC were only slightly affected by SP-C. SP-C affected the distribution of condensed and fluid domains in monolayers in the liquid expanded/liquid condensed region by producing more, smaller, condensed domains in the two phase region than seen for pure lipid monolayers. The results are consistent with SP-C being embedded, at least in part, in the acyl chains of the lipid, and that severe packing dislocations in PC bilayers are not produced by the protein.

A semi-empirical conformational analysis of the interaction of n-alkanols with dipalmitoylphosphatidylcholine

The interaction of a homologous series of saturated aliphatic n-alkanols (containing 1–13 carbon atoms) with dipalmitoylphosphatidylcholine was studied by a semi-empirical conformational analysis. The minimal conformational energy of the isolated molecule at the hydrocarbon-water interface was calculated as the sum of the contributions resulting from the Van der Waals, torsional, electrostatic and transfer energies. From the conformers of minimal energies were calculated the hydrophilic-hydrophobic balance, the distance between hydrophilic and hydrophobic centres and the energies of interaction between homologous alkanols and between alkanols and lipids. Using these parameters, different modes of conformation, orientation and interaction of n-alkanols and dipalmitoylphosphatidylcholine were described. For methanol, ethanol and n-propanol, the gauche conformers were the most probable interacting only with the lipid polar heads. Only all- trans conformers were obtained for alkanols with longer acyl chains. n-Butanol to n-octanol form clusters in the lipid matrix. Longer n-alkanols are randomly distributed in the lipid layer. However, due to the increase in hydrocarbon chain-length, n-nonanol and higher alkanols have an interaction energy identical or superior to that found in a pure lipid monolayer, leading to a more ordered alkanol-lipid organization.

Covalent linkage of a synthetic peptide to a fluorescent phospholipid and its incorporation into supported phospholipid monolayers

A number of fluorescent peptide-lipid conjugates have been synthesized. Peptides with ten or eleven amino acids are linked through a single lysine residue to the headgroup of phosphatidylethanolamine, fluorescently labelled on one acyl chain, using homobifunctional disuccinimidyl crosslinking reagents. Peptide-lipids can be further derivatized with the hapten dinitrophenyl. Purified peptide-lipids have been incorporated into dimyristoylphosphatidylcholine monolayers at the interface of air and phosphate-buffered saline, at concentrations of up to 11 mol%. For equal average molecular areas, monolayers containing peptide-lipids have higher surface pressures than pure lipid monolayers; for equal surface pressures, peptide-lipid monolayers are have higher average molecular areas than pure lipid monolayers. When the peptide-lipid monolayers are transferred to hydrophobic glass slides, the fluorescence appears uniformly distributed. Fluorescence recovery after photobleaching measurements indicate that peptide-lipids diffuse in the monolayer with coefficient 1.5 · 10 −9 cm 2/s, which is much smaller than that of typical lipids in fluid membranes. In addition, the diffusion coefficient of peptide-lipids decreases with increasing peptide-lipid concentration. We conclude that the peptide portion of the peptide-lipid associates with the lipid monolayer and/or that peptide-lipids oligomerize.

Lipid-protein interactions in monolayers

We restricted ourselves to a few examples of the different methodological aspects of the investigation of lipid-protein interactions in monolayer assemblies. Experiments with monolayers have the unique advantage that the arrangement and packing of the molecules can be easily measured and controlled. The first section is devoted to proteins which do not degrade lipids. Soluble proteins are generally injected in the subphase below a preformed lipid monolayer and measurements are performed either at constant surface area or at constant surface pressure. These experiments can give information on the penetration capacity into the interface, its lipid specificity with a direct access to the area of the protein segment interacting with lipids. Reconstitution of functional enzymatic complexes can be achieved as well as the determination of the orientation of the protein at the interface. Most intrinsic membrane proteins are insoluble in water. In the absence of detergent they aggregate and display no affinity for lipid interfaces. These proteins can be spread from an organic solvent solution but with the risk of being denatured. In order to circumvent this difficulty a method for spreading an aqueous suspension of lipoproteins or natural membrane vesicles was developed. This spreading method allows the formation of lipoprotein films retaining biological activities and native membrane constituents. Formation of functional transmembrane complexes in planar bilayers from such lipoprotein films is the most fascinating application of this spreading technique. In the second section, we reviewed the use of pure or mixed lipid monolayers as substrates for lipolytic enzymes. Either long-chain lipids were used and their surface density was not controlled; or short-chain lipids were applied, again without control of surface density; or short-chain lipids were used at constant surface density. Recently a method was developed to study the hydrolysis of long chain lipids with control of surface density. The monolayer technique allows us to study accurately the influence of surface pressure and protein cofactors on the hydrolysis velocity and lag time in lipolysis. Two types of mixed lipid monolayers can be formed at the air/water interface: either by spreading, from a volatile organic solvent, a mixture of water insoluble lipids or by injecting a detergent solution into the water subphase covered with a preformed pure lipid monolayer. These techniques are ideally suited for the study of the mode of action of lipolytic enzymes on controlled mixed interfaces.

Interaction of the acetylcholine (nicotinic) receptor protein from Torpedo marmorata electric organ with monolayers of pure lipids.

Membrane fragments rich in cholinergic (nicotinic) receptor protein were purified from the electric organ of Torpedo marmorata. Their lipid composition is essentially characterized by the prominence of cholesterol, phosphatidylethanolamine and phosphatidylcholine, long-chain fatty acyl constituents, and the absence of sphingomyelin. Solubilised receptor was purified from these fragments and the concentration of sodium cholate lowered by dialysis to 0.01% (w/v). When this preparation was injected under a lipid monolayer, an increase of surface pressure developed, which was not observed with the detergent alone nor in the absence of lipid film. When covalently radiolabelled receptor preparations were injected at a constant surface pressure the radioactivity recovered with the film was proportional to the increase in area. It is concluded that the pressure or area increases are due to the penetration of the cholinergic receptor protein into the lipid film. Incorporation experiments into films formed from various pure lipids showed that the protein interacts more readily with cholesterol than with ergosterol, phosphatidylcholine, or other phospholipids. Its affinity is also higher for long-chain phosphatidylcholines than for short-chain ones. The degree of unsaturation and fluidity of the 3-sn-phosphatidylcholine (lecithin) films are of secondary importance. Parallel experiments with covalently and non-covalently labelled receptor preparations showed that part of the protein recovered with the film lost its alpha-toxin binding ability during the penetration. Similar data were obtained with the receptor purified from Electrophorus electricus electric organ.

Electrochemical determination of structure and interactions in spread lipid monolayers

The structure of spread monolayers of phosphatidyl choline (egg)-PC and of phosphatidyl serine (bovine spinal cord) PS in the pure state and when interacting with polypeptides were investigated by a.c. polarography and by cyclic voltammetry with the aid of D.M.E. contacting the monolayer from the gaseous (N 2) phase. The pure lipid monolayers produced around the electrocapillary maximum (ecm) a flat low-capa-citance region ((≈1.6 μF/cm 2) until high adsorption-desorption capacitance peaks were obtained at high positive or negative polarization.. The relaxation time for the formation of the monolayer from the disrupted state, obtained by the cyclic voltammetry at different sweep rates and by measuring the decay of current after a potential step, varied between 0.5 and 1 ms. The access of polar groups to the mercury electrode surface was investigated by introducing the electroactive dinitrophenyl label into a fraction (≈5%) of the polar groups. From the shift in the reduction potential towards negative potentials when the monolayer was compressed, we estimated the frequency of fluctuations allowing access of polar groups to the electrode surface through the hydrocarbon region. On this basis of energetic considerations, we suggested fluctuating indentation in the planar layer. Synthetic basic polypeptides and the oligopeptide hormones, oxytocin, vasopressin and desamino-oxytocin, shift back the reduction potentials of the probe in compressed PS monolayers, indicating enhancement of the access of the polar groups to the electrode surface by the

Some models for lipid bilayer and biomembrane phase transitions

Biological membranes consist to a large extent of lipid bilayers, some of which exhibit fairly sharp phase transitions near body temperatures. We describe here a model for the main observed phase transition in pure lipid monolayers and bilayers. The model contains approximate treatments of hard−core repulsions and weak attractive forces between molecules. We believe that the model accounts qualitatively for many of the experimental properties that the model accounts qualitatively for many of the experimental properties of of monolayers and bilayers. Improvements which should enhance quantitative agreement are also discussed.