Hydrophobic lipid additives affect membrane stability and phase behavior of N-monomethyldioleoylphosphatidylethanolamine.

The rate of formation of high-curvature intermediates or disordered cubic phases in N-methyldioleoylphosphatidylethanolamine (N-methyl-DOPE) dispersions with or without additives was studied by 31P NMR spectroscopy. In N-methyl-DOPE dispersions, both the L alpha liquid-crystalline phase and the hexagonal HII phase convert into phases of high curvature giving rise to isotropic 31P NMR resonances. Addition of the bilayer destabilizers 1,2-diolein, 1,3-diolein, or eicosane lowers the threshold temperature of the isotropic phase. The isotropic threshold temperature is strongly correlated with the L alpha-HII phase transition temperature (TH). The addition of hexagonal phase promoters does not change the rate of formation of the isotropic phase at a temperature shifted by a fixed amount below TH. However, the formation of "isotropic" phases from the additive-stabilized hexagonal phase is slow compared to that observed in pure N-methyl-DOPE lipid dispersions. Membrane leakage and fusion are promoted by the dioleins and well as by eicosane, but changes in the rates of these processes do not correlate well with the extent of formation of isotropic phases. All three additives have similar effects on phase behavior and on vesicle leakage and fusion. These similarities occur despite the fact that eicosane is believed to partition differently into the membrane than diolein. In addition to the general similarities in the effects of the two diolein isomers, 1,2-diolein is somewhat more potent in promoting the hexagonal phase and in increasing rates of leakage and fusion than is 1,3-diolein.     

The polymorphic phase behavior of dielaidoylphosphatidylethanolamine. Effect of n-alkanols

The polymorphic phase behavior of dielaidoylphosphatidylethanolamine (DEPE) has been investigated using spectrophotometry and 31P nuclear magnetic resonance (NMR). It has been demonstrated that the bilayer to inverted hexagonal phase transition can be observed by spectrophotometry. The effects of the methanol, ethanol, and propanol on both the gel to liquid crystal transition and the bilayer to inverted hexagonal transition were investigated by spectrophotometry. It was shown that these alcohols shift the gel to liquid-crystalline phase transition to lower temperature, whereas the bilayer to inverted hexagonal phase transition is shifted to higher temperatures by these alcohols. The structural transition between the bilayer and inverted hexagonal phase of pure DEPE was also investigated by 31P-NMR.     

Fusion of influenza to liposomes is not inhibited by aliphatic primary alcohols

Reports of the antiviral activity of aliphatic alcohols led us to investigate the effects of aliphatic alcohols, from 10 to 20 carbons in length, on the phase transition behaviour of model phospholipids and on the fusion of influenza to liposomes. Contrary to the effects of many other antiviral agents, we find that alcohols are potent promoters of the inverted hexagonal phase. However, we also find that aliphatic alcohols have little effect on influenza fusion to liposomes. Eicosanol is the only aliphatic alcohol tested which substantially increases in fusion of influenza virus. We also find that long chain alcohols display multi-component bilayer to hexagonal phase transitions at higher mole fractions. This suggests that eicosanol may be facilitating fusion by creating defects between alcohol-rich and alcohol-poor regions of the lipid bilayer.     

Virus replication inhibitory peptide inhibits the conversion of phospholipid bilayers to the hexagonal phase

Virus replication inhibitory peptide (carbobenzoxy-D-Phe-L-PheGly) was shown to be a potent specific inhibitor of the replication of paramyxovirus and myxovirus (Richardson, Scheid and Choppin (1980), Virology105, 205–222). This peptide inhibits the membrane fusing activity of a viral glycoprotein.Many agents which promote the formation of the hexagonal phase in membranes also accelerate membrane fusion. At a mole fraction of 0.1, viral replication inhibitory peptide can raise the bilayer to hexagonal phase transition temperature of dielaidoylphosphatidylethanolamine by almost 10°. Two related peptides, carbobenzoxy-L-PheGly and carbobenzoxy-L-GlyPhe, are less potent in raising the bilayer to hexagonal phase transition temperature, with the latter peptide being the least effective of the three. This order of potency is the same as the order of potency in inhibiting viral replication. Substances which inhibit hexagonal phase formation of pure lipids may also inhibit membrane fusion.Abbreviations DEPE dielaidoylphosphatidyethanolamine - Z carbobenzoxy - DSC differential scanning calorimetry - VRIP virus replication inhibitory peptide (Z-D-Phe-L-PheGly)     

The influence of dolichol, dolichol esters, and dolichyl phosphate on phospholipid polymorphism and fluidity in model membranes

The effect of dolichols, polyprenols, dolichol esterified with fatty acids, and dolichyl phosphate on the structure and fluidity of model membranes was studied using 31P NMR, small-angle x-ray scattering, differential scanning calorimetry, and freeze-fracture electron microscopy. These studies suggest that dolichol and dolichol derivatives destabilize unsaturated phosphatidylethanolamine containing bilayer structures and promote hexagonal II phase formation; high concentrations of dolichol induce lipid structures characterized by "isotropic" 31P NMR and particulate fracture faces; dolichol, contrary to cholesterol, has no effect on the thermotropic behavior of membranes consisting of phosphatidylcholine, while dolichyl-P incorporation abolishes the transition from the gel to liquid crystalline phase in 1,2-dimyristoyl-sn-glycero-3-phosphocholine; both dolichol and dolichyl-P increase the fatty acid fluidity in phosphatidylethanolamine mixtures; the effect of dolichol on bilayer structure and fluidity is more pronounced with increasing number of isoprene residues; dolichol esters are only soluble to a limited extent in the bilayer and segregates into domains at low concentrations; the results are consistent with a localization of dolichyl-P in which the phosphate group is oriented to the water interphase. The induction of hexagonal II phase by dolichyl-P may elicit the transmembrane movement of glycosylated lipid intermediate.     

The inverted hexagonal phase is more sensitive to hydroperoxidation than the multilamellar phase in phosphatidylcholine and phosphatidylethanolamine aqueous dispersions.

The effect of phase behaviour (hexagonal II phase and lamellar phase) on the peroxidation of membrane phospholipids has been investigated in dilinoleoyl phosphatidylcholine (DLPC)/dilinoleoyl phosphatidylethanolamine (DLPE) aqueous dispersions. Peroxidation was initiated with a water-soluble radical inducer 2,2'-azobis (2-amidino-propane) dihydrochloride (AAPN). The phospholipid morphology was monitored by 31P-nuclear magnetic resonance (NMR). Phospholipid hydroperoxides (PCOOH and PEOOH) were determined by chemiluminescence high-performance liquid chromatography (CL-HPLC). In pH-induced phase transition systems, DLPE in the bilayer state was much less oxidized than in the hexagonal II state. In composition-induced phase transition systems, the formation of total hydroperoxides and the consumption of alpha-tocopherol in the hexagonal II phase were greater than in the bilayer phase. These data suggest that the hexagonal II phase is more sensitive to hydroperoxidation than the bilayer phase in phospholipid aqueous dispersions.     

Membrane composition determines pardaxin's mechanism of lipid bilayer disruption

Pardaxin is a membrane-lysing peptide originally isolated from the fish Pardachirus marmoratus. The effect of the carboxy-amide of pardaxin (P1a) on bilayers of varying composition was studied using (15)N and (31)P solid-state NMR of mechanically aligned samples and differential scanning calorimetry (DSC). (15)N NMR spectroscopy of [(15)N-Leu(19)]P1a found that the orientation of the peptide's C-terminal helix depends on membrane composition. It is located on the surface of lipid bilayers composed of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and is inserted in lipid bilayers composed of 1,2-dimyristoyl-phosphatidylcholine (DMPC). The former suggests a carpet mechanism for bilayer disruption whereas the latter is consistent with a barrel-stave mechanism. The (31)P chemical shift NMR spectra showed that the peptide significantly disrupts lipid bilayers composed solely of zwitterionic lipids, particularly bilayers composed of POPC, in agreement with a carpet mechanism. P1a caused the formation of an isotropic phase in 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) lipid bilayers. This, combined with DSC data that found P1a reduced the fluid lamellar-to-inverted hexagonal phase transition temperature at very low concentrations (1:50,000), is interpreted as the formation of a cubic phase and not micellization of the membrane. Experiments exploring the effect of P1a on lipid bilayers composed of 4:1 POPC:cholesterol, 4:1 POPE:cholesterol, 3:1 POPC:1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG), and 3:1 POPE:POPG were also conducted, and the presence of anionic lipids or cholesterol was found to reduce the peptide's ability to disrupt bilayers. Considered together, these data demonstrate that the mechanism of P1a is dependent on membrane composition.     

The interaction of coenzyme Q with phosphatidylethanolamine membranes.

Coenzyme Q (CoQ) is a component of the mitochondrial respiratory chain which carries out additional membrane functions, such as acting as an antioxidant. The location of CoQ in the membrane and the interaction with the phospholipid bilayer is still a subject of debate. The interaction of CoQ in the oxidized (ubiquinone-10) and reduced (ubiquinol-10) state with membrane model systems of 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (Ela2Gro-P-Etn) has been studied by means of differential scanning calorimetry (DSC), 31P-nuclear magnetic resonance (31P-NMR) and small angle X-ray diffraction (SAXD). Ubiquinone-10 did not visibly affect the lamellar gel to lamellar liquid-crystalline phase transition of Ela2Gro-P-Etn, but it clearly perturbed the multicomponent lamellar liquid-crystalline to lamellar gel phase transition of the phospholipid. The perturbation of both transitions was more effective in the presence of ubiquinol-10. A location of CoQ forming head to head aggregates in the center of the Ela2Gro-P-Etn bilayer with the polar rings protruding toward the phospholipid acyl chains is suggested. The formation of such aggregates are compatible with the strong hexagonal HII phase promotion ability found for CoQ. This ability was evidenced by the shifting of the lamellar to hexagonal HII phase transition to lower temperatures and by the appearance of the characteristic hexagonal HII 31P-NMR resonance and SAXD pattern at temperatures at which the pure Ela2Gro-P-Etn is still organized in extended bilayer structures. The influence of CoQ on the thermotropic properties and phase behavior of Ela2Gro-P-Etn is discussed in relation to the role of CoQ in the membrane.     

Cholesterol sulfate inhibits the fusion of Sendai virus to biological and model membranes

Cholesterol sulfate is a component of several biological membranes. In erythrocytes, cholesterol sulfate inhibits hypotonic hemolysis, while in sperm, it can decrease fertilization efficiency. We have found cholesterol sulfate to be a potent inhibitor of Sendai virus fusion to both human erythrocyte and liposomal membranes. Cholesterol sulfate also raises the bilayer to hexagonal phase transition temperature of dielaidoyl phosphatidylethanolamine as demonstrated by differential scanning calorimetry and 31P nuclear magnetic resonance spectrometry. Although hexagonal phase structures are not readily found in biological membranes, there is a correlation between the effects of membrane additives on bilayer/non-bilayer equilibria and membrane stabilization. It is proposed that the ability of cholesterol sulfate to alter the physical properties of membranes contributes to its stabilization of biological membranes and the inhibition of membrane fusion.     

Optimizing oriented planar-supported lipid samples for solid-state protein NMR

Sample orientation relative to the static magnetic field of an NMR spectrometer allows study of membrane proteins in the lipid bilayer setting. The straightforward preparation and handling of extremely thin mica substrates with consistent surface properties has prompted us to examine oriented phospholipid bilayer and hexagonal phases on mica. The spectral characteristics of oriented lipid samples formed on mica are as good as or better than those on glass. Nine solvents with varying dielectric constants were used to cast lipid films or for vesicle spreading; film characteristics were then compared, and static solid-state 31P-NMR was used to characterize the degree of orientation of the hydrated lipid species. Lipids with four headgroup chemistries were tested: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), 1,2-dioleoyl-sn-glycero-3-phosphate (DOPA), and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). Solvent affected orientation of POPG, DOPA, and DOPE, but not POPC. Film characteristics varied with solvent, with ramifications for producing homogeneous oriented lipid samples. POPC was used to optimize the amount of lipid per substrate and compare hydration methods. POPG did not orient reproducibly, whereas POPG-POPC mixtures did. DOPA showed 1-2 oriented states depending upon hydration level and deposition method. DOPE formed an oriented hexagonal phase that underwent a reversible temperature-induced phase transition to the oriented bilayer phase.     

Modulation of membrane structure by Ca2+ and dibucaine as detected by 31P NMR

The polymorphic phase behaviour of model membrane systems consisting of 20 mol% bovine brain phosphatidylserine and 80 mol% egg yolk phosphatidylethanolamine has been examined employing 31P NMR techniques. It is shown that the addition of Ca2+ to such systems can trigger isothermal bilayer to hexagonal (HII) phase transitions, and that such effects can be reversed by the subsequent incorporation of the local anaesthetic dibucaine. These results are discussed in terms of a recent model for membrane fusion (Cullis, P.R. and Hope, M.J. (1978) Nature 271, 672--674) and mechanisms of anaesthesia.     

Acid- and calcium-induced structural changes in phosphatidylethanolamine membranes stabilized by cholesteryl hemisuccinate

The membrane stabilization effect of cholesteryl hemisuccinate (CHEMS) and the sensitivity of the CHEMS-phosphatidylethanolamine membranes to protons and calcium ions were studied by differential scanning calorimetry, freeze-fracture electron microscopy, and 31P NMR. (1) At neutral pH, the addition of 8 mol % CHEMS to transesterified egg phosphatidylethanolamine (TPE) raised the lamellar-hexagonal transition temperature of TPE by 11 degrees C. Stable bilayer vesicles were formed when the incorporated CHEMS exceeded 20 mol %. (2) At a pH below 5.5, the protonation of CHEMS enhanced the formation of the hexagonal phase (HII) of TPE. At 25 mol % CHEMS the bilayer-hexagonal transition temperature was lowered by 30 degrees C at pH 4.5. (3) The endothermic acid-induced hexagonal hexagonal transition of TPE-CHEMS was suppressed at 35 mol % CHEMS. However, 31P NMR and electron microscopy indicated that a lamellar-hexagonal transition still occurred at this composition. (4) The main transition of TPE was not affected by the protonation of the incorporated CHEMS, indicating that no macroscopic phase separation occurred in TPE-CHEMS mixtures at low pH. (5) In contrast to the HII-promoting effect of H+, the neutralization of the negative charge on TPE-CHEMS by Ca2+ resulted in aggregates that remained in the lamellar structure even at the hexagonal transition temperature of TPE. It is suggested that calcium might form a complex between CHEMS in apposed bilayers. These results are related to the possible biological function of acidic cholesterol esters in biomembranes.     

Comparison of the interaction of the anti-viral chemotherapeutic agents amantadine and tromantadine with model phospholipid membranes

Amantadine and tromantadine are agents used against influenza and herpes infections, respectively. Tromantadine raises the bilayer to hexagonal phase transition temperature of synthetic phosphatidylethanolamines and is less disruptive to phospholipid packing. Tromantadine acts similar to cyclosporin A, previously demonstrated to inhibit viral-induced cell-cell fusion. We suggest the balance between the hydrophobic and hydrophilic group sizes would allow tromantadine to prevent membrane fusion more than amantadine and thus inhibit infection by viruses such as Herpes, which fuse with the plasma membrane. Study of agents which stabilize the bilayer phase of membranes may lead to efficacious inhibitors of viral infections requiring cell fusion events.Abbreviations DEPE dielaidoyl phosphatidylethanolamine - POPE 1-palmitoyl-2-oleoyl phosphatidylethanolamine - DMPC dimyristoyl phosphatidylcholine - DSC differential scanning calorimetry - PIPES piperazine-N,N-bis(2-ethanesulphonic acid) - NMR nuclear magnetic resonance - tromantadine N-1-adamantyl-N-[2-(dimethylamino)ethoxy]a(ethoxy]acetamide-hydrochloride - amantadine (1-adamantamine)-hydrochloride - HSV Herpes Simplex Virus     

Bilayer to non-bilayer transition in mixtures of phosphatidylethanolamine and phosphatidylcholine: Implications for membrane properties

The structural phases in various mixtures of soybean phosphatidylethanolamine and egg phosphatidylcholine were studied by X-ray diffraction, freeze fracture electron microscopy, and ³¹P NMR. An intermediate state between bilayer and hexagonal structures was found at a composition of 10–25 mol% of phosphatidylcholine. The intermediate state consists of closely packed multilayers, together with arrays of lipidic intramembranous particles. The arrays of lipidic intramembranous particles, possibly membrane invaginations, give rise to an anisotropic ³¹P NMR spectrum commonly accredited to a hexagonal structure. A phase diagram of this mixed system is proposed. The compositional range at which the intermediate state exists coincides with the range of maximal mitochondrial ATPase activity when these lipids are used in reconstitution experiments.     

The Effects of 1,2-Diacylglycerols on the Structural Organization of Model Membranes and Their Fusion

Abstract

The effects of synthetic and natural 1,2 diacylglycerols (DAG) on structural organization of lamellar dispersions of phosphatidylcholine (PC) or PC: phosphatidylinositol (PI) mixtures has been studied with the help of NMR spectroscopy. Asymmetric accumulation of natural DAG in two-component model membranes was achieved by their treatment with phospholipase C specific for PI. It was found that high concentrations (20 mol per cent) of synthetic DAG are needed to induce partial lipid bilayer transition from lamellar into hexagonal and/or isotropic phase. Asymmetric accumulation of natural DAG in thionphosphatidylcholine: PI bilayers treated with phospholipase C resulted in modification of bilayer packing at DAG concentrations as low as 1.5 mol per cent and at physiological temperature (37°C). With the help of fluorescence spectroscopy it was shown that formation of DAG in one of membrane layers of large mono-bilayer liposomes results in the membranes destabilization and fusion.     

Mechanism of lipid bilayer disruption by the human antimicrobial peptide,LL-37

LL-37 is an amphipathic, alpha-helical, antimicrobial peptide. (15)N chemical shift and (15)N dipolar-shift spectroscopy of site-specifically labeled LL-37 in oriented lipid bilayers indicate that the amphipathic helix is oriented parallel to the surface of the bilayer. This surface orientation is maintained in both anionic and zwitterionic bilayers and at different temperatures and peptide concentrations, ruling out a barrel-stave mechanism for bilayer disruption by LL-37. In contrast, electrostatic factors, the type of lipid, and the presence of cholesterol do affect the extent to which LL-37 perturbs the lipids in the bilayer as observed with (31)P NMR. The (31)P spectra also show that micelles or other small, rapidly tumbling membrane fragments are not formed in the presence of LL-37, excluding a detergent-like mechanism. LL-37 does increase the lamellar to inverted hexagonal phase transition temperature of both PE model lipid systems and Escherichia coli lipids, demonstrating that it induces positive curvature strain in these environments. These results support a toroidal pore mechanism of lipid bilayer disruption by LL-37.     

Interactions of bovine lactoferricin with acidic phospholipid bilayers and its antimicrobial activity as studied by solid-state NMR

Bovine lactoferricin (LfcinB) is an antimicrobial peptide released by pepsin cleavage of lactoferrin. In this work, the interaction between LfcinB and acidic phospholipid bilayers with the weight percentage of 65% dimyristoylphosphatidylglycerol (DMPG), 10% cardiolipin (CL) and 25% dimyristoylphosphatidylcholine (DMPC) was investigated as a mimic of cell membrane of Staphylococcus aureus by means of quartz crystal microbalance (QCM) and solid-state (31)P and (1)H NMR spectroscopy. Moreover, we elucidated a molecular mechanism of the antimicrobial activity of LfcinB by means of potassium ion selective electrode (ISE). It turned out that affinity of LfcinB for acidic phospholipid bilayers was higher than that for neutral phospholipid bilayers. It was also revealed that the association constant of LfcinB was larger than that of lactoferrin as a result of QCM measurements. (31)P DD-static NMR spectra indicated that LfcinB interacted with acidic phospholipid bilayers and bilayer defects were observed in the bilayer systems because isotropic peaks were clearly appeared. Gel-to-liquid crystalline phase transition temperatures (Tc) in the mixed bilayer systems were determined by measuring the temperature variation of relative intensities of acyl chains in (1)H MAS NMR spectra. Tc values of the acidic phospholipid and LfcinB-acidic phospholipid bilayer systems were 21.5 degrees C and 24.0 degrees C, respectively. To characterize the bilayer defects, potassium ion permeation across the membrane was observed by ISE measurements. The experimental results suggest that LfcinB caused pores in the acidic phospholipid bilayers. Because these pores lead the permeability across the membrane, the molecular mechanism of the antimicrobial activity could be attributed to the pore formation in the bacterial membrane induced by LfcinB.     

Promotion of hexagonal phase formation and lipid mixing by fatty acids with varying degrees of unsaturation

A series of C18 and C22 fatty acids, with varying degrees of unsaturation, were tested for their ability to alter the bilayer to hexagonal phase transition temperature of dielaidoylphosphatidylethanolamine. Lowering the pH from 7.4 to 6.4 greatly decreased the bilayer to hexagonal phase transition temperature of fatty acid-phosphatidylethanolamine mixtures. At pH 7.4, increasing unsaturation of the fatty acid generally increased their hexagonal phase-forming ability. However, oleic acid had somewhat greater hexagonal phase-forming capacity and docosahexaenoic acid somewhat less than would be expected for their degree of unsaturation. At pH 6.4 the difference among fatty acids was small and except for stearic acid, all had similar hexagonal phase forming tendencies. The fatty acids cause acid-induced fusion. There is little effect of fatty acid structure on membrane fusion.     

The polymorphic phase behaviour of phosphatidylethanolamines of natural and synthetic origin. A 31P NMR study.

1. The polymorphic phase behaviour of aqueous dispersions of phosphatidylethanolamines isolated from human erythrocytes, hen egg yolk and Escherichia coli have been investigated employing 31P NMR techniques. All species exhibit well defined, reversible bilayer to hexagonal (H11) phase transitions as the temperature is increased. The temperatures at which these transition take place (10, 25--30 and 55--60 degrees C for erythrocyte, egg yolk and E. coli phosphatidylethanolamine, respectively) are sensitive to the fatty acid composition, occurring at a temperature up to 10 degrees C above the high temperature end of the hydrocarbon phase transition as detected by differential scanning calorimetry. In some cases the bilayer to hexagonal (H11) transitions may also be detected employing calorimetric techniques. 2. The addition of equimolar concentrations of cholesterol to these naturally occurring phosphatidylethanolamines does not dramatically affect the bilayer-hexagonal (H11) transition temperature, producing changes of up to 10 degrees C. 3. 18 : 1t/18 : 1t phosphatidylethanolamine undergoes the bilayer to hexagonal (H11) phase transition as the temperature is increased through the interval 50--55 degrees C. Alternatively, hydrated 12 : 0/12 : 0 phosphatidylethanolamine remains in the bilayer phase at temperatures up to 90 degrees C (50 degrees C above the hydrocarbon phase transition temperature). 4. The presence of 100 mM NaCl or 10 mM CaCl2 in aqueous dispersions of egg yolk phosphatidylethanolamine does not alter the temperature-dependent polymorphic phase behaviour significantly. However, at 40 degrees C, increasing the p2H above 8.0 results in progressive inhibition of the hexagonal (H11) phase and the appearance of a phase possibly of cubic structure at p2H 9.0. At p2H 10.0 the bilayer phase is preferred. 5. It is suggested that in biomembranes containing phosphatidylethanolamine as a majority species (such as that of E. coli) the fatty acid composition may primarily reflect the need to maintain bilayer structure. Alternatively, it is pointed out that in mammalian membranes such as that of the erythrocyte, phosphatidylethanolamine tends to destabilize bilayer structure. The resulting possibility that transitory non-bilayer lipid configurations may occur may be directly related to many important properties of biological membranes.     

Solid-state NMR investigation of the membrane-disrupting mechanism of antimicrobial peptides MSI-78 and MSI-594 derived from magainin 2 and melittin

The mechanism of membrane interaction of two amphipathic antimicrobial peptides, MSI-78 and MSI-594, derived from magainin-2 and melittin, is presented. Both the peptides show excellent antimicrobial activity. The 8-anilinonaphthalene-1-sulfonic acid uptake experiment using Escherichia coli cells suggests that the outer membrane permeabilization is mainly due to electrostatic interactions. The interaction of MSI-78 and MSI-594 with lipid membranes was studied using 31P and 2H solid-state NMR, circular dichroism, and differential scanning calorimetry techniques. The binding of MSI-78 and MSI-594 to the lipid membrane is associated with a random coil to alpha-helix structural transition. MSI-78 and MSI-594 also induce the release of entrapped dye from POPC/POPG (3:1) vesicles. Measurement of the phase-transition temperature of peptide-DiPoPE dispersions shows that both MSI-78 and MSI-594 repress the lamellar-to-inverted hexagonal phase transition by inducing positive curvature strain. 15N NMR data suggest that both the peptides are oriented nearly perpendicular to the bilayer normal, which infers that the peptides most likely do not function via a barrel-stave mechanism of membrane-disruption. Data obtained from 31P NMR measurements using peptide-incorporated POPC and POPG oriented lamellar bilayers show a disorder in the orientation of lipids up to a peptide/lipid ratio of 1:20, and the formation of nonbilayer structures at peptide/lipid ratio>1:8. 2H-NMR experiments with selectively deuterated lipids reveal peptide-induced disorder in the methylene units of the lipid acyl chains. These results are discussed in light of lipid-peptide interactions leading to the disruption of membrane via either a carpet or a toroidal-type mechanism.