Interaction of a laminin derived peptide with phosphatidyl choline/phosphatidyl glycerol

The physicochemical interactions between an active peptide sequence derived from laminin and phospholipids have been studied. The main aim is to determine the suitability of this peptide fragment to be linked to liposome's with the purpose to develop targeting devices. Results indicate that this peptide is able to insert in lipid monolayers and also to form monomolecular layers at an air/water interface. Besides, miscibility studies carried out through compression isotherms of mixed monolayers [dipalmitoyl phosphatidylcholine (DPPC)/peptide], indicate a strong interaction at 60-80% DPPC molar composition. Studies carried out with lipid bilayers indicate that the interaction is restricted to the external face of the vesicles. Moreover, the presence of this peptide in the incubation media promotes a low level of carboxyfluorescein (CF) leakage and no fusion of vesicles. These results indicate that the association of this sequence to vesicles do not produce damage of the bilayer and can be used as potential targeting vector.     

New GHK hydrophobic derivatives: interaction with phospholipid bilayers

Three hydrophobic derivatives of GHK peptide containing either N-terminal hexanoyl, decanoyl or myristoyl acyl moieties were synthesized. The binding of these peptidolipids to phospholipid bilayers as well as their hemolytic activity were determined. Moreover, the influence of these peptidolipids on several physicochemical properties of liposomes was studied. Binding experiments indicate a high affinity of these peptidolipids for lipids ordered in liposomes. Nevertheless, this interaction does not promote the release of entrapped carboxyfluorescein. Experiments carried out by the asymmetric membrane method (NBD-PE/dithionite) and quenching studies (PC-pyrene/KI) indicate that this association has a protective effect suggesting that the hydrophobic moiety inserts in the external part of the bilayer and the peptide chain remains protruding from the surface hindering the entrance or the approach of reactants to it. The microviscosity of DPPC bilayers determined using TMA-DPH as fluorescent marker was not affected by the presence of peptidolipids. Besides, results indicate that myristoyl-GHK produces total hemolysis at 2.5 × 10⁻⁴ M but decanoyl and hexanoyl derivatives at 5 × 10⁻⁴ M induce only 10% of hemolysis.     

Interaction of the Hepatitis A peptide VP3(110–121) with lipid mono and bilayer models of cellular membranes

 The surface activity of HAV-VP3(110–121) peptide was studied at different concentrations in an aqueous solution. Saturation was reached at 0.62 μM concentration. The ability of the peptide to insert into monolayers of CL, SA, DPPC, DPPC/5% CL and DPPC/5% SA was also performed. Mixed mono-layers composed of this peptide and the lipid mixtures were also studied as far as the miscibility of the two components is concerned. The mixed monolayers showing small negative deviations from ideality. The values of excess free energy of mixing (ΔG ^E _M) suggest that the energy associated to the miscibility process is almost non-significant except for a 0.2 molar fraction of DPPC/SA and 0.6 molar fraction of DPPC/CL. The peptide has an expanding effect upon the monolayers but due to its amphoteric character this interaction is not dependent on the electrical charge of the lipids. In fluorescence studies, the peptide showed some degree of interaction with the lipid polar heads, but no interactions were detected with its alkylchains. This results show that after incubation with DPPC/5% CL and DPPC/5% SA liposomes the peptide remains in the outer part of the bilayers. Received: 20 January 1997 Accepted: 28 May 1997     

Fusogenic tilted peptides induce nanoscale holes in supported phosphatidylcholine bilayers

Tilted peptides are known to insert in lipid bilayers with an oblique orientation, thereby destabilizing membranes and facilitating membrane fusion processes. Here, we report the first direct visualization of the interaction of tilted peptides with lipid membranes using in situ atomic force microscopy (AFM) imaging. Phase-separated supported dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC) bilayers were prepared by fusion of small unilamellar vesicles and imaged in buffer solution, in the absence and in the presence of the simian immunodeficiency virus (SIV) peptide. The SIV peptide was shown to induce the rapid appearance of nanometer scale bilayer holes within the DPPC gel domains, while keeping the domain shape unaltered. We attribute this behavior to a local weakening and destabilization of the DPPC domains due to the oblique insertion of the peptide molecules. These results were directly correlated with the fusogenic activity of the peptide as determined using fluorescently labeled DOPC/DPPC liposomes. By contrast, the nontilted ApoE peptide did not promote liposome fusion and did not induce bilayer holes but caused slight erosion of the DPPC domains. In conclusion, this work provides the first direct evidence for the production of stable, well-defined nanoholes in lipid bilayer domains by the SIV peptide, a behavior that we have shown to be specifically related to the tilted character of the peptide. A molecular mechanism underlying spontaneous insertion of the SIV peptide within lipid bilayers and the subsequent removal of bilayer patches is proposed, and its relevance to membrane fusion processes is discussed.     

Physicochemical study of laminin-related peptides

Four peptide analogues related to the active sequence YIGSR of laminin have been synthesised. The synthesis and chemical characterisation of the peptides are described. Physicochemical properties of these peptides such as surface activity, spreadability, monolayer formation, as well as their interaction with lipid monolayers and bilayers, have been studied by using Langmuir-Blodgett films and liposomes as membrane models. In spite of their good water solubility, these peptides are able to form stable monolayers at the air/water interface and to insert into lipid monolayers. The interaction with bilayers is soft; they are not able to induce the leakage of entrapped CF nor to modify the microviscosity of bilayers in general. Thus in these models electrostatic forces apparently do not play an important role, as we expected previously according to the electrical charge of bilayers, markers and peptides.     

NSAIDs interactions with membranes: a biophysical approach

This work focuses on the interaction of four representative NSAIDs (nimesulide, indomethacin, meloxicam, and piroxicam) with different membrane models (liposomes, monolayers, and supported lipid bilayers), at different pH values, that mimic the pH conditions of normal (pH 7.4) and inflamed cells (pH 5.0). All models are composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) which is a representative phospholipid of most cellular membranes. Several biophysical techniques were employed: Fluorescence steady-state anisotropy to study the effects of NSAIDs in membrane microviscosity and thus to assess the main phase transition of DPPC, surface pressure-area isotherms to evaluate the adsorption and penetration of NSAIDs into the membrane, IRRAS to acquire structural information of DPPC monolayers upon interaction with the drugs, and AFM to study the changes in surface topography of the lipid bilayers caused by the interaction with NSAIDs. The NSAIDs show pronounced interactions with the lipid membranes at both physiological and inflammatory conditions. Liposomes, monolayers, and supported lipid bilayers experiments allow the conclusion that the pH of the medium is an essential parameter when evaluating drug-membrane interactions, because it conditions the structure of the membrane and the ionization state of NSAIDs, thereby influencing the interactions between these drugs and the lipid membranes. The applied models and techniques provided detailed information about different aspects of the drug-membrane interaction offering valuable information to understand the effect of these drugs on their target membrane-associated enzymes and their side effects at the gastrointestinal level.     

Hepatitis A Synthetic Peptide VP3(110-121) Miscibility with Dipalmitoylphosphatidylcholine, Dipalmitoylphosphatidylglycerol, and Stearylamine Monolayers

To prepare liposomes containing a synthetic hepatitis A virus antigen (HAV) [VP3(110-121)] as a vaccine, the miscibility of this peptide (with negative net charge) with a neutral lipid [dipalmitoylphosphatidylcholine (DPPC)], a negatively charged lipid [dipalmitoylphosphatidylglycerol (DPPG)], and a positively charged lipid [Stearylamine (SA)] was studied through compression isotherms of monolayers. Mixtures with DPPC and SA showed a low degree of interaction with the peptide, the composition of the monolayer being stable through compression. For DPPG-containing monolayers larger positive deviations from ideality were found, and the peptide was squeezed out from the monolayer at a DPPG/VP3(110-121) mole fraction of 0.8/0.2. All this suggests that besides hydrophobic interactions between the peptide and the lipid, electrostatic forces also play a role; thus it seems that neutral and positively charged lipids would be more suitable for preparing stable liposomes with VP3(110-121). Copyright 2000 Academic Press.     

Interfacial behavior of chroman-6 and chroman-6 palmitoyl ester and their interaction with phospholipids

The surface activity of chroman-6 (CR-6) and chroman-6 palmitoyl ester (PCR-6) and the interactions with lipid membranes, using 1,2-dipamitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) monolayers, were determined. 8-Anilino-1-naphthalenesulfonic acid titration indicates that none of these molecules was able to form aggregates in aqueous media. The presence of a palmitoyl chain in PCR-6 increases strongly the surface activity of the parent compound (CR-6), rendering a molecule to form stable monomolecular layers. The interaction of both compounds with DPPC and DOPC, measured at constant area or in a compression isotherm model, follows the same trend. Miscibility studies performed with DPPC/CR-6 or PCR-6 indicate that the energies involved are small. The presence of CR-6 and PCR-6 has a soft influence on the compressibility of the Langmuir mixed films. Differential scanning calorimetry studies indicate that CR-6 and PCR-6 modify the temperature and cooperativity of the transition from gel to liquid crystal process in DPPC vesicles.     

Fluorescence transduction of an enzyme-substrate reaction by modulation of lipid membrane structure

The emission intensity of the fluorophore nitrobenzoxadiazoledipalmitoylphosphatidylethanolamine (NBD-PE) is sensitive to local environmental structure when this species is used as a component of a phospholipid membrane. The physical and electrostatic structure of a membrane may be modulated by selective chemical reactions, and the resulting alteration in fluorescence intensity provides transduction of such selective chemical processes. One example is the reaction between the extrinsic membrane-associated enzyme acetylcholinesterase (AChE) and the substrate acetylcholine (ACh), which produces an increase in hydronium ion activity at the surface of a lipid membrane. A mechanism of transduction of the enzymatic reaction by lipid monolayer membranes was investigated by spectrofluorimetric methods and fluorescence microscopy. Mixed monolayers composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidic acid (DPPA) which contained 30 mol-% or more of DPPA and 1 mol-% of NBD-PE provided transduction of the AChEACh reaction. Reaction of micromolar concentrations of ACh with AChE-monolayer systems induced increases in fluorescence intensity of up to 50%. Direct observation of the microscopic structure of lipid monolayers on a time scale of minutes showed that the reaction did not drastically affect the distribution of coexisting microscopic phase domains that were present in the monolayers The fluorescence imaging and spectroscopic results did indicate that massive structural reorganization at a molecular level probably occurred in a period of seconds. The results are consistent with an electrostatic mechanism of perturbation of the structure of the monolayer in which local pH gradients associated with the reaction of AChE with substrate altered the extent of ionization of DPPA in the headgroup zone of the membrane.     

Influence of the saturation chain and head group charge of phospholipids in the interaction of hepatitis G virus synthetic peptides

Using the Langmuir technique, we have studied the properties at the air/water interface and the interaction of the hepatitis G virus synthetic peptide E1(53-66) and its palmitoyl derivative with membrane phospholipids. These phospholipids had different characteristics referring to the net charge and saturation of the acyl chain. The palmitoyl derivative was more stable at the air/water interface and in the kinetic at constant area measurements showed higher incorporation to the monolayer. The interaction was higher for saturated phospholipids and those with a negative net charge. When the peptides were in the subphase, they produced changes in the miscibility of mixed monolayers composed of DPPC/DPPG or DOPC/DOPG. It can be deduced from the results obtained that electrostatic interactions play a major role, but when the peptide is derivatized with the palmitoyl chain, hydrophobic interactions are added to the former ones. The interaction is also influenced by the saturation of the acyl chain.     

Atomic force microscopy studies of ganglioside GM1alpha in dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine mixed monolayers and hybrid bilayers

The membrane states of the alpha-series ganglioside GM1alpha in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) mixed monolayers and hybrid bilayers were investigated using atomic force microscopy (AFM). The AFM image for the GM1alpha/DOPC/DPPC ternary monolayers showed the formation of GM1alpha-raft in the DOPC matrix. As increase of the surface pressure, GM1alpha are condensed in DPPC-rich domains; long and slender GM1alpha-rafts are separated from the DPPC-rich domains into the DOPC matrix. The GM1alpha/DOPC/DPPC ternary monolayers were deposited on mica coated with the first layer (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine: DPPE) using the Langmuir-Schaeffer technique. The AFM image for the hybrid bilayers showed that same molecules were heterogeneously concentrated according to increase of the surface pressure to form GM1alpha-raft, DPPC-rich domain and DOPC matrix, being in agreement with the observation on the monolayer experiment. The found phenomenon implies that a binding of lectin to GM1alpha causes the increase of the surface pressure, the localization of GM1alpha and the succeeding formation of the raft as a first step of a specific signal transduction.     

The interaction of an antiparasitic peptide active against African Sleeping Sickness with cell membrane models

Zwitterionic peptides with trypanocidal activity are promising lead compounds for the treatment of African Sleeping Sickness, and have motivated research into the design of compounds capable of disrupting the protozoan membrane. In this study, we use the Langmuir monolayer technique to investigate the surface properties of an antiparasitic peptide, namely S-(2,4-dinitrophenyl)glutathione di-2-propyl ester, and its interaction with a model membrane comprising a phospholipid monolayer. The drug formed stable Langmuir monolayers, whose main feature was a phase transition accompanied by a negative surface elasticity. This was attributed to aggregation upon compression due to intermolecular bond associations of the molecules, inferred from surface pressure and surface potential isotherms, Brewster angle microscopy (BAM) images, infrared spectroscopy and dynamic elasticity measurements. When co-spread with dipalmitoyl phosphatidyl choline (DPPC), the drug affected both the surface pressure and the monolayer morphology, even at high surface pressures and with low amounts of the drug. The results were interpreted by assuming a repulsive, cooperative interaction between the drug and DPPC molecules. Such repulsive interaction and the large changes in fluidity arising from drug aggregation may be related to the disruption of the membrane, which is key for the parasite killing property.     

DOPC/DPPC单层膜中分子间的相互作用及形态特征

利用Langmuir-Blodgett(LB)技术制备了不同表面压力下的1,2-二油酸-甘油-3-磷脂酰胆碱(DOPC)/1,2-二棕榈酸甘油-3-磷脂酰胆碱(DPPC)(摩尔比为1:1)和DOPC/DPPC/Chol(摩尔比为2:2:1)单层膜, 对单层膜内分子间的相互作用进行了热力学分析, 并用荧光显微镜和原子力显微镜对其形态进行了观测.热力学分析表明, DOPC与DPPC分子在单层膜结构中相互作用为排斥力, 诱导单层膜出现相变; DOPC, DPPC与胆固醇(Chol)间的相互作用均为吸引力, 当表面压力(π)大于18 mN/m时, DPPC与胆固醇的作用力大于DOPC.荧光显微镜观测表明, DOPC/DPPC单层膜出现明显相分离现象, 富含DPPC微区成“花形”结构, 且随着表面压力的升高微区逐渐增大, “花瓣”增多; 当胆固醇加入到DOPC/DPPC体系时, 单层膜相态由液相与凝胶相共存转变为液态无序相与液态有序相共存结构, 富含DPPC的微区形状从“花形”转变成“圆形”.原子力显微镜对单层膜的表征验证了荧光显微镜的观测结果, 表明胆固醇加入到DOPC/DPPC体系中对单层膜排列具有明显的影响, 压力和溶液状态等是影响脂膜结构的重要因素.     

The interaction of a peptide with a scrambled hydrophobic/hydrophilic sequence (Pro-Asp-Ala-Asp-Ala-His-Ala-His-Ala-His-Ala-Ala-Ala-His-Gly) (PADH) with DPPC model membranes: a DSC study

Depending on their hydrophobicity, peptides can interact differently with lipid membranes inducing dramatic modifications into their host systems. In the present paper, the interaction of a synthetic peptide with a scrambled hydrophobic/hydrophilic sequence (Pro-Asp-Ala-Asp-Ala-His-Ala-His-Ala-His-Ala-Ala-Ala-His-Gly) (PADH) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) model membranes has been investigated by differential scanning calorimetry (DSC), adopting three different experimental approaches. In the first, the peptide is forced to be included into the hydrocarbon region of the lipid bilayer, by codissolving it with the lipid giving rise to mixed multilamellar vesicles–peptide systems; in the second, this system is passed through an extruder, thus producing large unilamellar vesicles–peptide systems; in the third, it is allowed to interact with the external surface of the membrane.

The whole of the DSC results obtained have shown that the incorporation of the peptide into the lipid bilayer by means of the first method induces a decrease in the enthalpy of the gel–liquid crystal transition of the membrane and a shift of the transition to the lower temperatures, thus resembling, in spite of its prevalently hydrophilic nature, the behavior of transbilayer hydrophobic peptides. The extrusion of these systems creates unilamellar vesicles free of peptides but of smaller size as evidenced by the decreased cooperativity of the transition. The peptide, added externally to the DPPC model membrane, has no effect on the phase behavior of the bilayer.

These findings suggest that the effect of the interaction of scrambled hydrophobic/hydrophilic peptides into lipid bilayers strongly affects the thermotropic behavior of the host membrane depending on the preparation method of the lipid/peptide systems. The whole of the results obtained in the present paper can be useful in approaching studies of bioactive peptides/lipids systems.     

Interaction of 3β-amino-5-cholestene with phospholipids in binary and ternary bilayer membranes

3β-Amino-5-cholestene (aminocholesterol) is a synthetic sterol whose properties in bilayer membranes have been examined. In fluid palmitoyl sphingomyelin (PSM) bilayers, aminocholesterol and cholesterol were equally effective in increasing acyl chain order, based on changes in diphenylhexatriene (DPH) anisotropy. In fluid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, aminocholesterol ordered acyl chains, but slightly less efficiently than cholesterol. Aminocholesterol eliminated the PSM and DPPC gel-to-liquid crystalline phase transition enthalpy linearly with concentration, and the enthalpy approached zero at 30 mol % sterol. Whereas cholesterol was able to increase the thermostability of ordered PSM domains in a fluid bilayer, aminocholesterol under equal conditions failed to do this, suggesting that its interaction with PSM was not as favorable as cholesterols. In ternary mixed bilayers, containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), PSM or DPPC, and cholesterol at proportions to contain a liquid-ordered phase (60:40 by mol of POPC and PSM or DPPC, and 30 mol % cholesterol), the average lifetime of trans-parinaric acid (tPA) was close to 20 ns. When cholesterol was replaced with aminocholesterol in such mixed bilayers, the average lifetime of tPA was only marginally shorter (about 18 ns). This observation, together with acyl chain ordering data, clearly shows that aminocholesterol was able to form a liquid-ordered phase with saturated PSM or DPPC. We conclude that aminocholesterol should be a good sterol replacement in model membrane systems for which a partial positive charge is deemed beneficial.     

An electrochemical study into the interaction between complement-derived peptides and DOPC mono- and bilayers

Electrochemical methods employing the hanging mercury drop electrode were used to study the interaction between variants of the complement-derived antimicrobial peptide CNY21 (CNYITELRRQH ARASHLGLAR) and dioleoyl phosphatidylcholine (DOPC) monolayers. Capacitance potential and impedance measurements showed that the CNY21 analogues investigated interact with DOPC monolayers coating the mercury drop. Increasing the peptide hydrophobicity by substituting the two histidine residues with leucine resulted in a deeper peptide penetration into the hydrophobic region of the DOPC monolayer, indicated by an increase in the dielectric constant of the lipid monolayer (Deltaepsilon = 2.0 after 15 min interaction). Increasing the peptide net charge from +3 to +5 by replacing the histidines by lysines, on the other hand, arrests the peptide in the lipid head group region. Reduction of electroactive ions (Tl+, Pb2+, Cd2+, and Eu3+) at the monolayer-coated electrode was employed to further characterize the types of defects induced by the peptides. All peptides studied permeabilize the monolayer to Tl+ to an appreciable extent, but this effect is more pronounced for the more hydrophobic peptide (CNY21L), which also allows penetration of larger ions and ions of higher valency. The results for the various ions indicate that charge repulsion rather than ion size is the determining factor for cation penetration through peptide-induced defects in the DOPC monolayer. The effects obtained for monolayers were compared to results obtained with bilayers from liposome leakage and circular dichroism studies for unilamellar DOPC vesicles, and in situ ellipsometry for supported DOPC bilayers. Trends in peptide-induced liposome leakage were similar to peptide effects on electrochemical impedance and permeability of electroactive ions for the monolayer system, demonstrating that formation of transmembrane pores alone does not constitute the mechanism of action for the peptides investigated. Instead, our results point to the importance of local packing defects in the lipid membrane in close proximity to the adsorbed peptide molecules.     

Asymmetric or symmetric bilayer formation during oblique drop impact depends on rheological properties of saturated and unsaturated lipid monolayers

Bilayer structures are formed by approaching two liquid surfaces with phospholipid monolayers, which are brought into contact by oblique drop impact on a liquid surface. Asymmetric bilayers can be produced by the coupling of drop and target monolayers. In contrast, symmetric bilayers or multilayers are formed by collapse of the compressed target monolayer. We show that under all studied conditions bilayer/multilayer synthesis takes place. The experimental conditions for the synthesis of asymmetric or symmetric bilayers are described quantitatively in terms of the surface rheological (surface elasticity and dilational viscosity) and the hydrodynamical parameters (Weber number and impact angle). The composition and mechanical properties of the phospholipid monolayers strongly influences the patterns of drop impact and the bilayer/multilayer formation. Cholesterol stiffens unsaturated phospholipid monolayers and fluidifies saturated monolayers. All monolayers form asymmetric vesicle-like structures, which are stable in the aqueous medium. Additionally, unsaturated phospholipid monolayers without cholesterol form symmetric vesicles by folding parts of the target monolayer. Sufficient presence of cholesterol in unsaturated phospholipid monolayers inhibits the folding of the target monolayer and the subsequent formation of symmetric bilayers. The rheological properties of saturated and unsaturated phospholipid monolayers and their mixtures with cholesterol are discussed. Based on drop impact results it is shown that the state of a so far undefined region in the DPPC/cholesterol phase diagram is a fluid phase.     

A monolayer study on interactions of docetaxel with model lipid membranes

Docetaxel (DCT) is an antineoplastic drug for the treatment of a wide spectrum of cancers. DCT surface properties as well as miscibility studies with l-alpha-dipalmitoyl phosphatidylcholine (DPPC), which constitutes the main component of biological membranes, are comprehensively described in this contribution. Penetration studies have revealed that when DCT is injected under DPPC monolayers compressed to different surface pressures, it penetrates into the lipid monolayer promoting an increase in the surface pressure. DCT is a surface active molecule able to decrease the surface tension of water and to form insoluble films when spread on aqueous subphases. The maximum surface pressure reached after compression of a DCT Langmuir film was 13 mN/m. Miscibility of DPPC and DCT in Langmuir films has been studied by means of thermodynamic properties as well as by Brewster angle microscopy (BAM) analysis of the mixed films at the air-water interface, concluding that DPPC and DCT are miscible and they form non-ideally mixed monolayers at the air-water interface. Helmholtz energies of mixing revealed that no phase separation occurs. In addition, Helmholtz energies of mixing become more negative with decreasing areas per molecule, which suggests that the stability of the mixed monolayers increases as the monolayers become more condensed. Compressibility values together with BAM images indicate that DCT has a fluidizing effect on DPPC monolayers.     

Interaction of antimicrobial arginine-based cationic surfactants with liposomes and lipid monolayers

The present work examines the relationship between the antimicrobial activity of novel arginine-based cationic surfactants and the physicochemical process involved in the perturbation of the cell membrane. To this end, the interaction of these surfactants with two biomembrane models, namely, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar lipid vesicles (MLVs) and monolayers of DPPC, 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] sodium salt (DPPG), and Escherichia coli total lipid extract, was investigated. For the sake of comparison, this study included two commercial antimicrobial agents, hexadecyltrimethylammonium bromide and chlorhexidine dihydrochloride. Changes in the thermotropic phase transition parameters of DPPC MLVs in the presence of the compounds were studied by differential scanning calorimetry analysis. The results show that variations in both the transition temperature (Tm) and the transition width at half-height of the heat absorption peak (deltaT1/2) were consistent with the antimicrobial activity of the compounds. Penetration kinetics and compression isotherm studies performed with DPPC, DPPG, and E. coli total lipid extract monolayers indicated that both steric hindrance effects and electrostatic forces explained the antimicrobial agent-lipid interaction. Overall, in DPPC monolayers single-chain surfactants had the highest penetration capacity, whereas gemini surfactants were the most active in DPPG systems. The compression isotherms showed an expansion of the monolayers compared with that of pure lipids, indicating an insertion of the compounds into the lipid molecules. Owing to their cationic character, they are incorporated better into the negatively charged DPPG than into zwitterionic DPPC lipid monolayers.     

Direct submolecular scale imaging of mesoscale molecular order in supported dipalmitoylphosphatidylcholine bilayers

Supported dipalmitoylphosphatidylcholine (DPPC) bilayers are widely used membrane systems in biophysical and biochemical studies. Previously, short-range positional and orientational order of lipid headgroups of supported DPPC bilayers was observed at room temperature using low deflection noise frequency modulation atomic force microscopy (FM-AFM). While this ordering was supported by X-ray diffraction studies, it conflicted with diffusion coefficient measurements of gel-phase bilayers determined from fluorescence photobleaching experiments. In this work, we have directly imaged mica-supported DPPC bilayers with submolecular resolution over scan ranges up to 146 nm using low deflection noise FM-AFM. Both orientational and positional molecular ordering were observed in the mesoscale, indicative of crystalline order. We discuss these results in relation to previous biophysical studies and propose that the mica support induces mesoscopic crystalline order of the DPPC bilayer at room temperature. This study also demonstrates the recent advance in the scan range of submolecular scale AFM imaging.