Phase Transition Temperature Of Bilayers Research Articles

Preparation and Characterization of Trastuzumab Fab-Conjugated Liposomes (Immunoliposomes).

Immunoliposomes are made by conjugating antibodies or antibody fragments on liposome surfaces. Antibody fragments Fab', single-chain Fv fragments (scFv), or new constructs such as nanobodies are commonly used instead of whole IgGs for reduced risk immunogenicity. Here we described the preparation and characterization of immunoliposome-containing trastuzumab Fabs on the surface. The targeting ligand Fab-PEG-DSPE was synthesized by site-specific coupling between the C-terminal cysteine of the Fab and the maleimide group at the distal end of a DSPE-PEG. It was incorporated into preformed liposomes at 60°C above the lipid bilayer phase transition temperature. The binding avidity of the immunoliposomes containing different Fab valencies was characterized using biolayer interferometry.

Temperature- and composition-dependent conformational transitions of amphipathic peptide–phospholipid nanodiscs

Nanodiscs are discoidal particles in which a lipid bilayer is encircled by amphipathic molecules such as proteins, peptides, or synthetic polymers. The apolipoprotein-A-I-derived peptide 18A is known to form nanodiscs in the presence of phospholipids, but the detailed mechanism of the formation and deformation of these nanodiscs in response to changes in the surrounding environment is not well understood. Here, we investigated the temperature- and composition-dependent structural changes of 18A–phosphatidylcholine complexes using fluorescence spectroscopy, dynamic light scattering, circular dichroism, static 31P NMR, and electron microscopy. We found that the nanodiscs in fast isotropic rotational motion increased in size above the gel-to-liquid-crystalline phase transition temperature of the lipid bilayers, resulting in the formation of enlarged nanodiscs and a lamellar phase. The lamellar phase was found to be oriented along the magnetic field. Further increase in temperature induced the formation of lipid vesicles. These transformations were explained using a transition model based on the migration of the peptide from the rim of the nanodiscs to the liquid-crystalline bilayer phase. The study outcomes provide a basis for understanding the design principles of discoidal nanostructures for structural biology and nanomedicine applications.

Does Sucrose Change Its Mechanism of Stabilization of Lipid Bilayers during Desiccation? Influences of Hydration and Concentration.

Sugar-membrane interactions are believed to be responsible for cell preservation during desiccation and freezing, but the molecular mechanism by which they achieve this is still not well understood. The associated decrease of the main phase transition temperature of phospholipid bilayers is explained by two opposing views on the matter: the direct sugar-phospholipid interaction at the bilayer interface (water replacement hypothesis) and an entropy-driven phase transition with sugar molecules concentrating away from the lipid interface (hydration forces explanation). Both mechanisms are supported by experiments but molecular dynamics (MD) simulations have overwhelmingly shown the occurrence of direct sugar-phospholipid interactions. We have performed MD simulations of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers at different water and sucrose contents. The behavior of sucrose was found to depend on both the sucrose and water contents: at high sucrose concentration and at low hydration, it is best described by the hydration forces explanation model, whereas at low sucrose concentration, it is consistent with the water replacement hypothesis model. These simulations reveal that at low concentration, sucrose molecules preferentially interact directly with the membrane interface while at high concentration, they preferentially accumulate in the intermembrane solution. The transition between the two modes of interaction is revealed for the first time as being governed by the saturation of the lipid bilayer interface with sucrose molecules, and this occurs more rapidly as the level of hydration decreases.

Characterization of phospholipid vesicles containing lauric acid: physicochemical basis for process and product development

Lauric acid (LAH) strongly inhibits the growth of acne-causing bacteria. LAH is essentially water-insoluble and the solubility of laurate (LA) salts are medium and temperature dependent. Hence, LAH/LA preparations are difficult to formulate. Here we fully characterized phospholipid vesicles containing up to 50 mol% LAH. Vesicles of dipalmitoylphosphatidylcholine (DPPC) containing LAH, at pHs 7.4 and 5.0, were characterized measuring size, charge, bilayer phase transition temperature (Tm) and permeability of water-soluble probes. Small angle X-ray scattering and cryotransmission electron microscopy showed multilamellar vesicles at low LAH %. Increasing LAH % had a negligible effect on particle size. An internal aqueous compartment in all vesicle's preparations, even at equimolar DPPC: LAH fractions, was demonstrated using water-soluble probes. At pH 5.0, the interaction between DPPC and LAH increased the Tm and phase transition cooperativity showing a single lipid phase formed by hydrogen-bonded DPPC: LAH complexes. At pH 7.4, vesicles containing 50 mol% LAH exhibited distinct phases, ascribed to complex formation between LAH and LA or LAH and DPPC. LAH incorporated in the vesicles minimally permeated a skin preparation at both pHs, indicating that the primary sites of LAH solubilization were the skin layers. These results provide the foundations for developing processes and products containing DPPC: LAH.

Structural characterization of cationic DODAB bilayers containing C24:1 β-glucosylceramide

The effect of 5 mol%, 9 mol%, and 16 mol% of C24:1 β-glucosylceramide (βGlcCer) on the structure of cationic DODAB bilayers was investigated by means of differential scanning calorimetry (DSC), electron spin resonance (ESR) spectroscopy and fluorescence microscopy. βGlcCer is completely miscible with DODAB at all fractions tested, since no domains were observed in fluorescence microscopy or ESR spectra. The latter showed that βGlcCer destabilized the gel phase of DODAB bilayers by decreasing the gel phase packing. As a consequence, βGlcCer induced a decrease in the phase transition temperature and cooperativity of DODAB bilayers, as seen in DSC thermograms. ESR spectra also showed that βGlcCer induced an increase in DODAB fluid phase order and/or rigidity. Despite their different structures, a similar effect of loosening the gel phase packing and turning the fluid phase more rigid/organized has also been observed when low molar fractions of cholesterol were incorporated in DODAB bilayers. The structural characterization of mixed membranes made of cationic lipids and glucosylceramides may be important for developing novel immunotherapeutic tools such as vaccine adjuvants.

Partitioning and Accumulation of Perfluoroalkyl Substances in Model Lipid Bilayers and Bacteria.

Perfluoroalkyl substances (PFAS) are ubiquitous and persistent environmental contaminants, yet knowledge of their biological effects and mechanisms of action is limited. The highest aqueous PFAS concentrations are found in areas where bacteria are relied upon for functions such as nutrient cycling and contaminant degradation, including fire-training areas, wastewater treatment plants, and landfill leachates. This research sought to elucidate one of the mechanisms of action of PFAS by studying their uptake by bacteria and partitioning into model phospholipid bilayer membranes. PFAS partitioned into bacteria as well as model membranes (phospholipid liposomes and bilayers). The extent of incorporation into model membranes and bacteria was positively correlated to the number of fluorinated carbons. Furthermore, incorporation was greater for perfluorinated sulfonates than for perfluorinated carboxylates. Changes in zeta potential were observed in liposomes but not bacteria, consistent with PFAS being incorporated into the phospholipid bilayer membrane. Complementary to these results, PFAS were also found to alter the gel-to-fluid phase transition temperature of phospholipid bilayers, demonstrating that PFAS affected lateral phospholipid interactions. This investigation compliments other studies showing that sulfonated PFAS and PFAS with more than seven fluorinated carbons have a higher potential to accumulate within biota than carboxylated and shorter-chain PFAS.

Small-Angle Neutron Scattering investigation of cholesterol-doped DMPC liposomes interacting with β-cyclodextrin

The Small-Angle Neutron Scattering technique has been applied to characterize the influence of a randomly methylated β-cyclodextrin, called RAMEB, on dimyristoylphosphatidylcholine (DMPC) liposomes doped with cholesterol. From the modelling of the experimental neutron scattering cross sections, the detailed response of the vesicle structure upon addition of increasing amounts of RAMEB up to 30 mM has been assessed. This study has been performed below and above the DMPC bilayer phase transition temperature and shows that cholesterol extraction by RAMEB is linked to a decrease of the average radius and of the aggregation number of the vesicles. This extraction takes place in a dose-dependent way until a more monodisperse population of cholesterol-free liposomes is obtained. In addition, the bilayer thickness evolution was inferred, as well as the liposome coverage by RAMEB.

Surface properties of encapsulating hydrophobic nanoparticles regulate the main phase transition temperature of lipid bilayers: A simulation study

The main phase transition temperature of a lipid membrane, which is vital for its biomedical applications such as controllable drug release, can be regulated by encapsulating hydrophobic nanoparticles into the membrane. However, the exact relationship between surface properties of the encapsulating nanoparticles and the main phase transition temperature of a lipid membrane is far from clear. In the present work, we performed coarse-grained molecular dynamics simulations to meet this end. The results show the surface roughness of nanoparticles and the density of surface-modifying molecules on the nanoparticles are responsible for the regulation. Increasing the surface roughness of the nanoparticles increases the main phase transition temperature of the lipid membrane, whereas it can be decreased in a nonlinear way via increasing the density of surface-modifying molecules on the nanoparticles. The results may provide insights for understanding recent experimental studies and promote the applications of nanoparticles in controllable drug release by regulating the main phase transition temperature of lipid vesicles. Open image in new window

Phospholipid diffusion coefficients of cushioned model membranes determined via z-scan fluorescence correlation spectroscopy.

Model cellular membranes enable the study of biological processes in a controlled environment and reduce the traditional challenges associated with live or fixed cell studies. However, model membrane systems based on the air/water or oil/solution interface do not allow for incorporation of transmembrane proteins or for the study of protein transport mechanisms. Conversely, a phospholipid bilayer deposited via the Langmuir-Blodgett/Langmuir-Schaefer method on a hydrogel layer is potentially an effective mimic of the cross section of a biological membrane and facilitates both protein incorporation and transport studies. Prior to application, however, such membranes must be fully characterized, particularly with respect to the phospholipid bilayer phase transition temperature. Here we present a detailed characterization of the phase transition temperature of the inner and outer leaflets of a chitosan supported model membrane system. Specifically, the lateral diffusion coefficient of each individual leaflet has been determined as a function of temperature. Measurements were performed utilizing z-scan fluorescence correlation spectroscopy (FCS), a technique that yields calibration-free diffusion information. Analysis via the method of Wawrezinieck and co-workers revealed that phospholipid diffusion changes from raftlike to free diffusion as the temperature is increased-an insight into the dynamic behavior of hydrogel supported membranes not previously reported.

Diffusion of hydrogen peroxide across DPPC large unilamellar liposomes

The decomposition of hydrogen peroxide catalyzed by catalase entrapped in the pool of di-palmitoylphosphatidyl choline unilamellar liposomes has been studied. The rate of the process was evaluated by following the production of oxygen as a function of time. Under the experimental conditions employed the rate of oxygen production was controlled by the diffusion of hydrogen peroxide, allowing for the estimation of the diffusion coefficient of hydrogen peroxide across the liposome bilayer.The rate of diffusion across the bilayer increases with the temperature and the presence of fluidizers (n-nonanol), according with changes in the bilayer fluidity, as sensed by 1,6-diphenyl hexatriene (DPH) fluorescence anisotropy. A peculiar aspect of the data is the fast hydrogen peroxide diffusion observed at the bilayer phase transition temperature. This fast diffusion is associated to rafts fluctuations that take place in the partially melted bilayer. These fluctuations have no effect on the microviscosity sensed by DPH.

High pressure effect on phase transition behavior of lipid bilayers

Phase behavior of lipid bilayers at high pressure is critical to biological processes. Using coarse grained molecular dynamic simulations, we report critical characteristics of dipalmitoylphosphatidylcholine bilayers with applied high pressure, and also show their phase transition by cooling bilayer patches. Our results indicate that the phase transition temperature of dipalmitoylphosphatidylcholine bilayers obviously shifts with pressure increasing in the rate of 37 °C kbar(-1), which are in agreement with experimental data. Moreover, the main phase transition is revealed to be strongly dependent on lipid area. A critical lipid area of ~0.57 nm(2) is found on the main phase transition boundary. Similar structures of acyl chains lead to the same sensitivity of phase transition temperature of different lipids to the pressure. Based on the lateral density and pressure profiles, we also discuss the different effects on bilayer structure induced by high temperature and high pressure, e.g., increasing temperature induces higher degree of interdigitation of lipid tails and thinner bilayers, and increasing pressure maintains the degree of interdigitation and bilayer thickness.

Interaction of epicatechin gallate with phospholipid membranes as revealed by solid-state NMR spectroscopy

Epicatechin gallate (ECg), a green tea polyphenol, has various physiological effects. Our previous nuclear Overhauser effect spectroscopy (NOESY) study using solution NMR spectroscopy demonstrated that ECg strongly interacts with the surface of phospholipid bilayers. However, the dynamic behavior of ECg in the phospholipid bilayers has not been clarified, especially the dynamics and molecular arrangement of the galloyl moiety, which supposedly has an important interactive role. In this study, we synthesized [13C]-ECg, in which the carbonyl carbon of the galloyl moiety was labeled by 13C isotope, and analyzed it by solid-state NMR spectroscopy. Solid-state 31P NMR analysis indicated that ECg changes the gel-to-liquid-crystalline phase transition temperature of DMPC bilayers as well as the dynamics and mobility of the phospholipids. In the solid-state 13C NMR analysis under static conditions, the carbonyl carbon signal of the [13C]-ECg exhibited an axially symmetric powder pattern. This indicates that the ECg molecules rotate about an axis tilting at a constant angle to the bilayer normal. The accurate intermolecular–interatomic distance between the labeled carbonyl carbon of [13C]-ECg and the phosphorus of the phospholipid was determined to be 5.3±0.1Å by 13C–31P rotational echo double resonance (REDOR) measurements. These results suggest that the galloyl moiety contributes to increasing the hydrophobicity of catechin molecules, and consequently to high affinity of galloyl-type catechins for phospholipid membranes, as well as to stabilization of catechin molecules in the phospholipid membranes by cation–π interaction between the galloyl ring and quaternary amine of the phospholipid head-group.

Partitioning of homologous nicotinic acid ester prodrugs (nicotinates) into dipalmitoylphosphatidylcholine (DPPC) membrane bilayers

The partitioning behavior of a series of perhydrocarbon nicotinic acid esters (nicotinates) between aqueous solution and dipalmitoylphosphatidylcholine (DPPC) membrane bilayers is investigated as a function of increasing alkyl chain length. The hydrocarbon nicotinates represent putative prodrugs, derivatives of the polar drug nicotinic acid, whose functionalization provides the hydrophobic character necessary for pulmonary delivery in a hydrophobic, fluorocarbon solvent, such as perfluorooctyl bromide. Independent techniques of differential scanning calorimetry and 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence anisotropy measurements are used to analyze the thermotropic phase behavior and lipid bilayer fluidity as a function of nicotinate concentration. At increasing concentrations of nicotinates over the DPPC mole fraction range examined ( X DPPC = 0.6–1.0), all the nicotinates (ethyl (C2H5); butyl (C4H9); hexyl (C6H13); and octyl (C8H17)) partition into the lipid bilayer at sufficient levels to eliminate the pretransition, and decrease and broaden the gel to fluid phase transition temperature. The concentration at which these effects occur is chain length-dependent; the shortest chain nicotinate, C2H5, elicits the least dramatic response. Similarly, the DPH anisotropy results demonstrate an alteration of the bilayer organization in the liposomes as a consequence of the chain length-dependent partitioning of the nicotinates into DPPC bilayers. The membrane partition coefficients (logarithm values), determined from the depressed bilayer phase transition temperatures, increase from 2.18 for C2H5 to 5.25 for C8H17. The DPPC membrane/water partitioning of the perhydrocarbon nicotinate series correlates with trends in the octanol/water partitioning of these solutes, suggesting that their incorporation into the bilayer is driven by increasing hydrophobicity.

Gauging the Effect of Impurities on Lipid Bilayer Phase Transition Temperature

We report on the gel-to-fluid phase transition behavior of unilamellar vesicles formed with 1,2-dimyristoyl-sn-phosphatidylcholine (14:0 DMPC). We have interrogated the gel-to-fluid transition temperature of these bilayer structures using the chromophore perylene incorporated in their nonpolar region. We observe a discontinuous change in the reorientation time of perylene sequestered within the bilayer at the known melting transition temperature of 14:0 DMPC, 24 degrees C. The perylene reorientation data reveal a local viscosity of 14.5 +/- 2.5 cP in the gel phase, and 8.5 +/- 1.5 cP in the fluid phase. We have also incorporated small amounts of 1,2-dimyristoleoyl-sn-glycero-3-phosphocholine (14:1 DMPC) into these unilamellar vesicles and find that the melting transition temperature for these bilayers varies in a regular manner with the amount of 14:1 DMPC present. These data demonstrate that very little "contaminant" is required to cause a substantial change in the gel-to-fluid transition temperature, even though these contaminants do not alter the viscosity of the bilayer sensed by perylene, either above or below the melting transition.

Cerebrocrast promotes the cotransport of H + and Cl − in rat liver mitochondria

Considering that cerebrocrast stimulates oligomycin-inhibited state 3 respiration simultaneously with mitochondrial transmembrane potential (Δ ψ) dissipation, the mechanism underlying the uncoupler activity of cerebrocrast was assessed by its ability to permeabilize the mitochondrial inner membrane to H + or to K + or to cotransport anions with H +. The partition coefficient of cerebrocrast in mitochondrial membrane and its ability to act as a membrane-active compound disturbing membrane lipid organization were also investigated. Cerebrocrast induced no permeabilization of mitochondrial inner membrane to H + or K +, but it was able to transport H + in association with Cl −. Cerebrocrast showed a strong incorporation into the mitochondrial membrane, with a partition coefficient (Kp m/w) of 2.7(±0.1)×10 5. Cerebrocrast also reduced, in a concentration dependent manner, the phase transition temperature, the cooperative unit size, and the enthalpy associated with the phase transition temperature of DMPC membrane bilayers. It was concluded that the uncoupler activity of cerebrocrast is due to its ability to promote the cotransport of H + with Cl − through the rat liver mitochondrial inner membrane, and that this cerebrocrast mechanism of action may be potentiated by alterations of membrane lipid organization and membrane lateral heterogeneity.

Antimicrobial activity and membrane selective interactions of a synthetic lipopeptide MSI-843

Lipopeptide MSI-843 consisting of the nonstandard amino acid ornithine (Oct–OOLLOOLOOL–NH 2) was designed with an objective towards generating non-lytic short antimicrobial peptides, which can have significant pharmaceutical applications. Octanoic acid was coupled to the N-terminus of the peptide to increase the overall hydrophobicity of the peptide. MSI-843 shows activity against bacteria and fungi at micromolar concentrations. It permeabilizes the outer membrane of Gram-negative bacterium and a model membrane mimicking bacterial inner membrane. Circular dichroism investigations demonstrate that the peptide adopts α-helical conformation upon binding to lipid membranes. Isothermal titration calorimetry studies suggest that the peptide binding to membranes results in exothermic heat of reaction, which arises from helix formation and membrane insertion of the peptide. 2H NMR of deuterated-POPC multilamellar vesicles shows the peptide-induced disorder in the hydrophobic core of bilayers. 31P NMR data indicate changes in the lipid head group orientation of POPC, POPG and Escherichia colitotal lipid bilayers upon peptide binding. Results from 31P NMR and dye leakage experiments suggest that the peptide selectively interacts with anionic bilayers at low concentrations (up to 5 mol%). Differential scanning calorimetry experiments on DiPOPE bilayers and 31P NMR data from E.coli total lipid multilamellar vesicles indicate that MSI-843 increases the fluid lamellar to inverted hexagonal phase transition temperature of bilayers by inducing positive curvature strain. Combination of all these data suggests the formation of a lipid–peptide complex resulting in a transient pore as a plausible mechanism for the membrane permeabilization and antimicrobial activity of the lipopeptide MSI-843.

Investigations of Polypeptide Rotational Diffusion in Aligned Membranes by 2H and 15N Solid-State NMR Spectroscopy

Transmembrane and in-plane oriented peptides have been prepared by solid-phase peptide synthesis, labeled with 3,3,3-2H3-alanine and 15N-leucine at two selected sites, and reconstituted into oriented phophatidylcholine membranes. Thereafter, proton-decoupled 15N and 2H solid-state NMR spectroscopy at sample orientations of the membrane normal parallel to the magnetic field direction have been used to characterize the tilt and rotational pitch angle of these peptides in some detail. In a second step the samples have been tilted by 90 degrees . In this setup the spectral line shapes are sensitive indicators of the rate of rotational diffusion. Whereas monomeric transmembrane peptides exhibit spectral averaging and well-defined resonances, larger complexes are characterized by broad spectral line shapes. In particular the deuterium line shape is sensitive to association of a few transmembrane helices. In contrast, the formation of much larger complexes affects the 15N chemical shift spectrum. The spectra indicate that in liquid crystalline membranes an amphipathic peptide of 14 amino acids exhibits fast rotational diffusion on both the 2H and 15N time scales (>10(-5) s). Extending the sequences to 26 amino acids results in pronounced changes of the 2H solid-state NMR spectrum, whereas the signal intensities of 15N solid-state NMR spectra degrade. Below the phase transition temperature of the phospholipid bilayers, motional averaging on the time scale of the 2H solid-state NMR spectrum ceases for transmembrane and in-plane oriented peptides. Furthermore at temperatures close to the phase transition the total signal intensities of the deuterium solid-state NMR spectra strongly decrease.

Effect of Variations in the Structure of a Polyleucine-Based α-Helical Transmembrane Peptide on Its Interaction with Phosphatidylethanolamine Bilayers

High-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy were used to study the interaction of a cationic α-helical transmembrane peptide, acetyl-Lys 2-Leu 24-Lys 2-amide (L 24), and members of the homologous series of zwitterionic n-saturated diacyl phosphatidylethanolamines (PEs). Analogs of L 24, in which the lysine residues were replaced by 2,3-diaminopropionic acid (acetyl-DAP 2-Leu 24-DAP 2-amide (L 24DAP)) or in which a leucine residue at each end of the polyleucine sequence was replaced by a tryptophan (Ac-K 2-W-L 22-W-K 2-amide (WL 22W)), were also studied to investigate the roles of lysine side-chain snorkeling and aromatic side-chain interactions with the interfacial region of phospholipid bilayers. The gel/liquid-crystalline phase transition temperature of the PE bilayers is altered by these peptides in a hydrophobic mismatch-independent manner, in contrast to the hydrophobic mismatch-dependent manner observed previously with zwitterionic phosphatidylcholine (PC) and anionic phosphatidylglycerol (PG) bilayers. Moreover, all three peptides reduce the phase transition temperature to a greater extent in PE bilayers than in PC and PG bilayers, indicating a greater disruption of PE gel-phase bilayer organization. Moreover, the lysine-anchored L 24 reduces the phase transition temperature, enthalpy, and the cooperativity of PE bilayers to a much greater extent than DAP-anchored L 24DAP, whereas replacement of the terminal leucines by tryptophan residues (Ac-K 2-W-L 22-W-K 2-amide) only slightly attenuates the effects of this peptide on the chain-melting phase transition of the host PE bilayers. All three peptides form very stable α-helices in PE bilayers, but small conformational changes occur in response to mismatch between peptide hydrophobic length and gel-state lipid bilayer hydrophobic thickness. These results suggest that the lysine snorkeling plays a significant role in the peptide-PE interactions and that cation- π-interactions between lysine and tryptophan residues may modulate these interactions. Altogether, these results suggest that the lipid-peptide interactions are affected not only by the hydrophobic mismatch between these peptides and the host lipid bilayer but also by the electrostatic and hydrogen-bonding interactions between the positively charged lysine residues at the termini of these peptides and the polar headgroups of PE bilayers.

Effect of variations in the structure of a polyleucine-based alpha-helical transmembrane peptide on its interaction with phosphatidylglycerol bilayers.

High-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy were used to study the interaction of a cationic alpha-helical transmembrane peptide, acetyl-Lys(2)-Leu(24)-Lys(2)-amide (L(24)), and members of the homologous series of anionic n-saturated diacyl phosphatidylglycerols (PGs). Analogues of L(24), in which the lysine residues were replaced by 2,3-diaminopropionic acid (L(24)DAP), or in which a leucine residue at each end of the polyleucine sequence was replaced by a tryptophan (WL(22)W), were also studied to investigate the roles of lysine side-chain snorkeling and aromatic side-chain interactions with the interfacial region of phospholipid bilayers. The gel/liquid-crystalline phase transition temperature of the host PG bilayers is altered by these peptides in a hydrophobic mismatch-dependent manner, as previously found with zwitterionic phosphatidylcholine (PC) bilayers. However, all three peptides reduce the phase transition temperature and enthalpy to a greater extent in anionic PG bilayers than in zwitterionic PC bilayers, with WL(22)W having the largest effect. All three peptides form very stable alpha-helices in PG bilayers, but small conformational changes are induced in response to a mismatch between peptide hydrophobic length and gel-state lipid bilayer hydrophobic thickness. Moreover, electrostatic and hydrogen-bonding interactions occur between the terminal lysine residues of L(24) and L(24)DAP and the polar headgroups of PG bilayers. However, such interactions were not observed in PG/WL(22)W bilayers, suggesting that the cation-pi interactions between the tryptophan and lysine residues predominate. These results indicate that the lipid-peptide interactions are affected not only by the hydrophobic mismatch between these peptides and the host lipid bilayer, but also by the tryptophan-modulated electrostatic and hydrogen-bonding interactions between the positively charged lysine residues at the termini of these peptides and the negatively charged polar headgroups of the PG bilayers.

Interaction of odorous lactones with phospholipids: implications in toxicity towards producing yeast cells

Some lactonic aroma compounds, that can be produced industrially by microorganisms, become toxic towards the producing cells as these compounds reach high concentrations in the culture medium. To determine the manner by which these metabolites may influence yeast physiology, the effects of four lactones (concentration range of 100 to 300 mg l−1) on the growth of Yarrowia lipolytica and on the phase behaviour of deuterated dimyristoylphosphatidylcholine (DMPC-d27) were studied. The results showed that the hydrophobic lactones decrease the phase transition temperature (Tm) of DMPC-d27 bilayers and that Tm decrease (ΔTm) was related to the inhibitory action of the lactones on yeast growth (evaluated by the lag time). These results suggest that whatever the lactone, a ΔTm of at least 2.5 °C resulted in a total growth inhibition: this implicates the lactone-phospholipids interaction in the mechanism of yeast growth inhibition. The test used in the present study may be a predicting method to assess the in vivo action of potential membrane active compounds.