Direct evidence of induction of interdigitated gel structure in large unilamellar vesicles of dipalmitoylphosphatidylcholine by ethanol: studies by excimer method and high-resolution electron cryomicroscopy

The pH-sensitive action of cholesterol-conjugated peptide inhibitors of influenza virus

Influenza viruses are major human pathogens, responsible for respiratory diseases affecting millions of people worldwide, with high morbidity and significant mortality. Infections by influenza can be controlled by vaccines and antiviral drugs. However, this virus is constantly under mutations, limiting the effectiveness of these clinical antiviral strategies. It is therefore urgent to develop new ones. Influenza hemagglutinin (HA) is involved in receptor binding and promotes the pH-dependent fusion of viral and cell endocytic membranes. HA-targeted peptides may emerge as a novel antiviral option to block this viral entry step. In this study, we evaluated three HA-derived (lipo)peptides using fluorescence spectroscopy. Peptide membrane interaction assays were performed at neutral and acidic pH to better resemble the natural conditions in which influenza fusion occurs. We found that peptide affinity towards membranes decreases upon the acidification of the environment. Therefore, the released peptides would be able to bind their complementary domain and interfere with the six-helix bundle formation necessary for viral fusion, and thus for the infection of the target cell. Our results provide new insight into molecular interactions between HA-derived peptides and cell membranes, which may contribute to the development of new influenza virus inhibitors.

Altered ISGylation drives aberrant macrophage-dependent immune responses during SARS-CoV-2 infection

Ubiquitin-like protein ISG15 (interferon-stimulated gene 15) (ISG15) is a ubiquitin-like modifier induced during infections and involved in host defense mechanisms. Not surprisingly, many viruses encode deISGylating activities to antagonize its effect. Here we show that infection by Zika, SARS-CoV-2 and influenza viruses induce ISG15-modifying enzymes. While influenza and Zika viruses induce ISGylation, SARS-CoV-2 triggers deISGylation instead to generate free ISG15. The ratio of free versus conjugated ISG15 driven by the papain-like protease (PLpro) enzyme of SARS-CoV-2 correlates with macrophage polarization toward a pro-inflammatory phenotype and attenuated antigen presentation. In vitro characterization of purified wild-type and mutant PLpro revealed its strong deISGylating over deubiquitylating activity. Quantitative proteomic analyses of PLpro substrates and secretome from SARS-CoV-2-infected macrophages revealed several glycolytic enzymes previously implicated in the expression of inflammatory genes and pro-inflammatory cytokines, respectively. Collectively, our results indicate that altered free versus conjugated ISG15 dysregulates macrophage responses and probably contributes to the cytokine storms triggered by SARS-CoV-2.

Combining 25-Hydroxycholesterol with an HIV Fusion Inhibitor Peptide: Interaction with Biomembrane Model Systems and Human Blood Cells

The fusion between the viral and the target cell membrane is a crucial step in the life cycle of enveloped viruses. The blocking of this process is a well-known therapeutic approach that led to the development of the fusion inhibitor peptide enfuvirtide, clinically used against human immunodeficiency virus (HIV) type 1. Despite this significant advance on viral treatment, the appearance of resistance has limited its clinical use. Such a limitation has led to the development of other fusion inhibitor peptides, such as C34, that present the same structural domain as enfuvirtide (heptad repeat sequence) but have different functional domains (pocket-binding domain in the case of C34 and lipid-binding domain in the case of enfuvirtide). Recently, the antiviral properties of 25-hydroxycholesterol were demonstrated, which boosted the interest in this oxysterol. The combination of two distinct antiviral molecules, C34 and 25-hydroxycholesterol, may help to suppress the emergence of resistant viruses. In this work, we characterized the interaction of the C34-25-hydroxycholesterol conjugate with biomembrane model systems and human blood cells. Lipid vesicles and monolayers with defined lipid compositions were used as biomembrane model systems. The conjugate interacts preferentially with membranes rich in sphingomyelin (a lipid enriched in lipid rafts) and presents a poor partition to membranes composed solely of phosphatidylcholine and cholesterol. We hypothesize that cholesterol causes a repulsive effect that is overcome in the presence of sphingomyelin. Importantly, the peptide shows a preference for human peripheral blood mononuclear cells relative to erythrocytes, which shows its potential to target CD4⁺ cells. Antiviral activity results against different wild-type and drug-resistant HIV strains further demonstrated the potential of C34-HC as a good candidate for future studies.

A model of lipid rearrangements during pore formation in the DPPC lipid bilayer

CONTEXT

The molecular bases of pore formation in the lipid bilayer remain unclear, as do the exact characteristics of their sizes and distributions. To understand this process, numerous studies have been performed on model lipid membranes including cell-sized giant unilamellar vesicles (GUV). The effect of an electric field on DPPC GUV depends on the lipid membrane state: in the liquid crystalline phase the created pores have a cylinder-like shape, whereas in the gel phase a crack has been observed.

OBJECTIVE

The aim of the study was to investigate the geometry of pores created in a lipid bilayer in gel and liquid crystalline phases in reference to literature experimental data.

METHODS

A mathematical model of the pore in a DPPC lipid bilayer developed based on the law of conservation of mass and the assumption of constant volume of lipid molecules, independent of their conformation, allows for analysis of pore shape and accompanying molecular rearrangements.

RESULTS

The membrane area occupied by the pore of a cylinder-like shape is greater than the membrane area occupied by lipid molecules creating the pore structure (before pore appearance). Creation of such pores requires more space, which can be achieved by conformational changes of lipid chains toward a more compact state. This process is impossible for a membrane in the most compact, gel phase.

DISCUSSION AND CONCLUSIONS

We show that the geometry of the pores formed in the lipid bilayer in the gel phase must be different from the cylinder shape formed in the lipid bilayer in a liquid crystalline state, confirming experimental studies. Furthermore, we characterize the occurrence of the 'buffer' zone surrounding pores in the liquid crystalline phase as a mechanism of separation of neighbouring pores.

Role of amphipathicity and hydrophobicity in the balance between hemolysis and peptide-membrane interactions of three related antimicrobial peptides

Cationic antimicrobial peptides (CAMPs) represent important self defense molecules in many organisms, including humans. These peptides have a broad spectrum of activities, killing or neutralizing many Gram-negative and Gram-positive bacteria. The emergence of multidrug resistant microbes has stimulated research on the development of alternative antibiotics. In the search for new antibiotics, cationic antimicrobial peptides (CAMPs) offer a viable alternative to conventional antibiotics, as they physically disrupt the bacterial membranes, leading to lysis of microbial membranes and eventually cell death. In particular, the group of linear α-helical cationic peptides has attracted increasing interest from clinical as well as basic research during the last decade. In this work, we studied the biophysical and microbiological characteristics of three new designed CAMPs. We modified a previously studied CAMP sequence, in order to increase or diminish the hydrophobic face, changing the position of two lysines or replacing three leucines, respectively. These mutations modified the hydrophobic moment of the resulting peptides and allowed us to study the importance of this parameter in the membrane interactions of the peptides. The structural properties of the peptides were also correlated with their membrane-disruptive abilities, antimicrobial activities and hemolysis of human red blood cells.

Singlet oxygen effects on lipid membranes: implications for the mechanism of action of broad-spectrum viral fusion inhibitors

It was reported recently that a new aryl methyldiene rhodanine derivative, LJ001, and oxazolidine-2,4-dithione, JL103, act on the viral membrane, inhibiting its fusion with a target cell membrane. The aim of the present study was to investigate the interactions of these two active compounds and an inactive analogue used as a negative control, LJ025, with biological membrane models, in order to clarify the mechanism of action at the molecular level of these new broad-spectrum enveloped virus entry inhibitors. Fluorescence spectroscopy was used to quantify the partition and determine the location of the molecules on membranes. The ability of the compounds to produce reactive oxygen molecules in the membrane was tested using 9,10-dimethylanthracene, which reacts selectively with singlet oxygen (1O2). Changes in the lipid packing and fluidity of membranes were assessed by fluorescence anisotropy and generalized polarization measurements. Finally, the ability to inhibit membrane fusion was evaluated using FRET. Our results indicate that 1O2 production by LJ001 and JL103 is able to induce several changes on membrane properties, specially related to a decrease in its fluidity, concomitant with an increase in the order of the polar headgroup region, resulting in an inhibition of the membrane fusion necessary for cell infection.

Improvement of HIV fusion inhibitor C34 efficacy by membrane anchoring and enhanced exposure

OBJECTIVES

The aim of the present work was to evaluate the interaction of two new HIV fusion inhibitors {HIVP3 [C34-polyethylene glycol (PEG)₄-cholesterol] and HIVP4 [(C34-PEG₄)₂-cholesterol]} with membrane model systems and human blood cells in order to clarify where and how the fusion inhibitors locate, allowing us to understand their mechanism of action at the molecular level, and which strategies may be followed to increase efficacy.

METHODS

Lipid vesicles with defined compositions were used for peptide partition and localization studies, based on the intrinsic fluorescence of HIVP3 and HIVP4. Lipid monolayers were employed in surface pressure studies. Finally, human erythrocytes and peripheral blood mononuclear cells (PBMCs) isolated from blood samples were used in dipole potential assays.

RESULTS

Membrane partition, dipole potential and surface pressure assays indicate that the new fusion inhibitors interact preferentially with cholesterol-rich liquid-ordered membranes, mimicking biological membrane microdomains known as lipid rafts. HIVP3 and HIVP4 are able to interact with human erythrocytes and PBMCs to a similar degree as a previously described simpler drug with monomeric C34 and lacking the PEG spacer, C34-cholesterol. However, the pocket-binding domain (PBD) of both HIVP3 and HIVP4 is more exposed to the aqueous environment than in C34-cholesterol.

CONCLUSIONS

The present data allow us to conclude that more efficient blocking of HIV entry results from the synergism between the membranotropic behaviour and the enhanced exposure of the PBD.

Biological activity of antibacterial peptides matches synergism between electrostatic and non electrostatic forces

Substitution of Ala 108 and Ala 111 in the 107-115 human lysozyme (hLz) fragment results in a 20-fold increased anti-staphylococcal activity while its hemolytic activity becomes significant (30%) at very high concentrations. This analog displays an additional positive charge near the N-terminus (108) and an extra Trp residue at the center of the molecule (111), indicating that this particular amino acid sequence improves its interaction with the bacterial plasma membrane. In order to understand the role of this arrangement in the membrane interaction, studies with model lipid membranes were carried out. The interactions of peptides, 107-115 hLz and the novel analog ([K(108)W(111)]107-115 hLz) with liposomes and lipid monolayers were evaluated by monitoring the changes in the fluorescence of the Trp residues and the variation of the monolayers surface pressure, respectively. Results obtained with both techniques revealed a significant affinity increase of [K(108)W(111)]107-115 hLz for lipids, especially when the membranes containing negatively charged lipids, such as phosphatidylglycerol. However, there is also a significant interaction with zwitterionic lipids, suggesting that other forces in addition to electrostatic interactions are involved in the binding. The analysis of adsorption isotherms and the insertion kinetics suggest that relaxation processes of the membrane structure are involved in the insertion process of novel peptide [K(108)W(111)]107-115 hLz but not in 107-115 hLz, probably by imposing a reorganization of water at the interphases. In this regard, the enhanced activity of peptide [K(108)W(111)]107-115 hLz may be explained by a synergistic effect between the increased electrostatic forces as well as the increased hydrophobic interactions.

Conjugation of cholesterol to HIV-1 fusion inhibitor C34 increases peptide-membrane interactions potentiating its action

Recently, the covalent binding of a cholesterol moiety to a classical HIV-1 fusion inhibitor peptide, C34, was shown to potentiate its antiviral activity. Our purpose was to evaluate the interaction of cholesterol-conjugated and native C34 with membrane model systems and human blood cells to understand the effects of this derivatization. Lipid vesicles and monolayers with defined compositions were used as model membranes. C34-cholesterol partitions more to fluid phase membranes that mimic biological membranes. Importantly, there is a preference of the conjugate for liquid ordered membranes, rich in cholesterol and/or sphingomyelin, as observed both from partition and surface pressure studies. In human erythrocytes and peripheral blood mononuclear cells (PBMC), C34-cholesterol significantly decreases the membrane dipole potential. In PBMC, the conjugate was 14- and 115-fold more membranotropic than T-1249 and enfuvirtide, respectively. C34 or cholesterol alone did not show significant membrane activity. The enhanced interaction of C34-cholesterol with biological membranes correlates with its higher antiviral potency. Higher partitions for lipid-raft like compositions direct the drug to the receptor-rich domains where membrane fusion is likely to occur. This intermediary membrane binding step may facilitate the drug delivery to gp41 in its pre-fusion state.

Regional cooperativity in the phase transitions of dipalmitoylphosphatidylcholine bilayers: the lipid tail triggers the isothermal crystallization process

We have a long-standing interest to explore the answer of the question: Which part of the amphiphilic molecule triggers the phase transition of the self-assembled aggregates consisting of these amphiphiles? This is an important issue regarding the phase transition kinetics of amphiphiles. To this end, we studied the phase transition behaviors of dipalmitoylphosphatidylcholine (DPPC) by differential scanning calorimetry, synchrotron X-ray scattering, Fourier transform infrared spectroscopy, and image analysis. We found that different parts (head, interface, and tail) of DPPC molecules all exhibit nonsynchronous changes during the sub-, pre-, and main transitions. Particular efforts have been devoted to studying the isothermal subgel (L(c')) formation process. It was found that only the lipid interface and tail regions change, and only when the rearrangement of the lipid hydrocarbon chain packing reaches a certain extent can the interfacial C═O groups be induced to undergo vibrational environment changes. The result means that the hydrocarbon tail is the part that triggers the gel (L(β')) to L(c') phase transition. The present work deepens our understanding on the phase transition mechanisms of DPPC and may shed light on those of other phospholipids and other types of amphiphiles.

Surface-induced phase separation of a sphingomyelin/cholesterol/ganglioside GM1-planar bilayer on mica surfaces and microdomain molecular conformation that accelerates Abeta oligomerization

Ganglioside GM1 mediates the amyloid beta (Abeta) aggregation that is the hallmark of Alzheimer's disease (AD). To investigate how ganglioside-containing lipid bilayers interact with Abeta, we examined the interaction between Abeta40 and supported planar lipid bilayers (SPBs) on mica and SiO(2) substrates by using atomic force microscopy, fluorescence microscopy, and molecular dynamics computer simulations. These SPBs contained several compositions of sphingomyelin, cholesterol, and GM1 and were treated at physiological salt concentrations. Surprisingly high-speed Abeta aggregation of fibril formations occurred at all GM1 concentrations examined on the mica surface, but on the SiO(2) surface, only globular agglomerates formed and they formed slowly. At a GM1 concentration of 20mol%, unique triangular regions formed on the mica surface and the rapidly formed Abeta aggregations were observed only outside these regions. We have found that some unique surface-induced phase separations are induced by the GM1 clustering effects and the strong interactions between the GM1 head group and the water layer adsorbed in the ditrigonal cavities on the mica surface. The speed of Abeta40 aggregation and the shape of the agglomerates depend on the molecular conformation of GM1, which varies depending on the substrate materials. We identified the conformation that significantly accelerates Abeta40 aggregation, and we think that the detailed knowledge about the GM1 molecular conformation obtained in this work will be useful to those investigating Abeta-GM1 interactions.

Sifuvirtide screens rigid membrane surfaces. establishment of a correlation between efficacy and membrane domain selectivity among HIV fusion inhibitor peptides

Sifuvirtide, a 36 amino acid negatively charged peptide, is a novel and promising HIV fusion inhibitor, presently in clinical trials. Because of the aromatic amino acid residues of the peptide, its behavior in aqueous solution and the interaction with lipid-membrane model systems (large unilammelar vesicles) were studied by using mainly fluorescence spectroscopy techniques (both steady-state and time-resolved). No significant aggregation of the peptide was observed with aqueous solution. Various biological and nonbiological lipid-membrane compositions were analyzed, and atomic force microscopy was used to visualize phase separation in several of those mixtures. Results showed no significant interaction of the peptide, neither with zwitterionic fluid lipid membranes (liquid-disordered phase), nor with cholesterol-rich membranes (liquid-ordered phase). However, significant partitioning was observed with the positively charged lipid models (K(p) = (2.2 +/- 0.3) x 10(3)), serving as a positive control. Fluorescence quenching using Förster resonance acrylamide and lipophilic probes was carried out to study the location of the peptide in the membrane models. In the gel-phase DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) membrane model, an adsorption of the peptide at the surface of these membranes was observed and confirmed by using Förster resonance energy-transfer experiments. These results indicate a targeting of the peptide to gel-phase domains relatively to liquid-disordered or liquid-ordered phase domains. This larger affinity and selectivity toward the more rigid areas of the membranes, where most of the receptors are found, or to viral membrane, may help explain the improved clinical efficiency of sifuvirtide, by providing a local increased concentration of the peptide at the fusion site.

Miscibility and interaction between 1-alkanol and short-chain phosphocholine in the adsorbed film and micelles

The miscibility and interaction of 1-hexanol (C6OH) and 1-heptanol (C7OH) with 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) in the adsorbed films and micelles were investigated by measuring the surface tension of aqueous C6OH-DHPC and aqueous C7OH-DHPC solutions. The surface density, the mean molecular area, the composition of the adsorbed film, and the excess Gibbs energy of adsorption g(H,E), were estimated. Further, the critical micelle concentration of the mixtures was determined from the surface tension versus molality curves; the micellar composition was calculated. The miscibility of the 1-alkanols and DHPC molecules in the adsorbed film and micelles was examined using the phase diagram of adsorption (PDA) and that of micellization (PDM). The PDA and the composition dependence of g(H,E) indicated the non-ideal mixing of the 1-alkanols and DHPC molecules due to the attractive interaction between the molecules in the adsorbed film, while the PDM indicated that the 1-alkanol molecules were not incorporated in the micelles within DHPC rich region. The dependence of the mean molecular area of the mixtures on the surface composition suggested that the packing property of the adsorbed film depends on the chain length of 1-alkanol: C6OH expands the DHPC adsorbed film more than C7OH.

Osmotic stress induces a phase transition from interdigitated gel phase to bilayer gel phase in multilamellar vesicles of dihexadecylphosphatidylcholine

We have investigated the effects of poly(ethylene glycol) (PEG) on the structure and phase behavior of multilamellar vesicles of dihexadecylphosphatidylcholine (DHPC-MLVs) using an X-ray diffraction method. At low concentrations of PEG-6K (MW = 7500), DHPC-MLVs were in an interdigitated gel (L(beta)I) phase, a gel phase with interdigitated hydrocarbon chains. At around 24% (w/v) PEG 6K, a phase transition from the L(beta)I phase to a bilayer gel phase occurred in the DHPC-MLVs, and above this concentration, they were in a bilayer gel phase. On the other hand, ethylene glycol (EG), the monomer of PEG, did not induce this phase transition in the DHPC-MLVs. A mechanism of this phase transition is proposed and discussed; a decrease in the repulsive interaction between the head groups of the phospholipids in the bilayer gel phase with an increase in PEG concentration, which is due to a decrease in the cross-sectional area of the head group region by osmotic stress, may be the main reason for this phase transition.

Intermembrane distance in multilamellar vesicles of phosphatidylcholine depends on the interaction free energy between solvents and the hydrophilic segments of the membrane surface

To investigate the interaction of the surface of biomembranes with solvents systematically, we have studied the structure and phase behavior of multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) in dimethylformamide (DMF)-water mixture by X-ray diffraction and differential scanning calorimetry. The solubility of phosphorylcholine, which is the same molecular structure as the head-group of phosphatidylcholine (PC), decreased with an increase in DMF concentration. This result indicates that DMF is a poor solvent for the hydrophilic segments of the surface of the PC membrane, and interaction free energy of the hydrophilic segments of the membrane surface with solvents increases with an increase in DMF concentration. X-ray diffraction data indicated that DPPC-MLVs were in the bilayer gel phase from 0 to 80% (v/v) DMF, and that the spacing (lamellar repeat period) and intermembrane distance of DPPC-MLV decreased with an increase in DMF concentration. Main transition temperature and pre-transition temperature of DPPC-MLV increased with an increase in DMF concentration, and above 50% (v/v) DMF there was no pre-transition. In the interaction of POPC-MLV with DMF, X-ray diffraction data indicated that POPC-MLVs were in L(alpha) phase (liquid-crystalline phase) from 0 to 80% (v/v) DMF, and that the spacing and intermembrane distance of POPC-MLV decreased with an increase in DMF concentration. These results are discussed by the change of the interaction free energy between the hydrophilic segments of the membrane surface and solvents. As DMF concentration increases, this interaction free energy may increase, resulting in the decrease of the intermembrane distance of PC-MLVs.

Dipalmitoylphosphatidylcholine (DPPC) and DPPC/Cholesterol Liposomes as Predictors of the Cytotoxicity ofBis‐GMA Related Compounds

In light of recent development, dental materials such as 2, 2-bis [4-2(-hydroxy-3-methacryloyloxypropoxy)phenyl] propane, ( bis-GMA); 2, 2-bis [4-(1-hydroxymethyl-2-methacryloxy)phenyl] propane, (iso-bis-GMA); and triethyleneglycol dimethacrylate, (TEGDMA) were investigated to determine whether their phase transition properties (phase transition temperature, temperature width, cooperativity) could be induced in samples of DPPC or DPPC/cholesterol (CHOL) liposomes using differential scanning calorimetry (DSC). The changes in phase transition properties of DPPC liposomes caused by addition of TEGDMA were greater than those caused by addition of bis-GMA or iso-bis-GMA, but the extent of changes in the properties of DPPC/CHOL (10:1 or 4:1) liposomes declined in the order of bis-GMA > iso-bis-GMA > TEGDMA. The degree of alteration was related to the cytotoxicity of these compounds. DPPC/CHOL liposomes were found to be better predictors of cytotoxicity than DPPC liposomes. Whether the computational approach to studying the molecular mechanism of alteration is applicable using descriptors such as reactivity of energy of the highest occupied molecular orbital (HOMO) and/or lowest unoccupied molecular orbital (LUMO) was investigated, and the data suggested that these descriptors are useful for studying the interactive roles of dental materials.

Low concentration of DMSO stabilizes the bilayer gel phase rather than the interdigitated gel phase in dihexadecylphosphatidylcholine membrane

We have investigated effects of dimethylsulfoxide (DMSO) on the phase stability of multilamellar vesicles of the ether-linked 1,2-dihexadecyl-sn-glycero-3-phosphatidylcholine (DHPC-MLV), which is known to be in the interdigitated gel (LbetaI) phase in excess water at 20 degrees C. The results of X-ray diffraction experiments indicate that the DHPC membrane was in the Lbeta, phase at X> or =0.12 (X=mole fraction of DMSO in DMSO/water mixture). The result of differential scanning calorimetry indicate that the gel to liquid-crystalline phase transition temperature increased, but the LbetaI to Pbeta, phase transition temperature decreased with an increase in DMSO concentration. These results show that DMSO stabilizes the bilayer gel phase rather than the LbetaI phase at its low concentration. The solubility of phosphorylcholine, which is the same structure as the headgroup of DHPC, decreased with an increase in DMSO concentration, indicating that the interaction free energy of the hydrophilic segments of the membrane with solvents increases with an increase in DMSO concentration. On the basis of the thermodynamic analysis, the mechanism of the stabilization of the bilayer gel phase of DHPC-MLV by DMSO is discussed. The decrease in the repulsive interaction between the headgroups of the phospholipid induced by the low concentrations of DMSO in water plays an important role in this stabilization.

The structure and function of gramicidin A embedded in interdigitated bilayer

The effects of phase transition from normal to interdigitated lipid bilayer on the function and structure of membrane proteins were studied using linear gramicidin (gramicidin A) as a model. Interdigitated bilayer structure of dipalmitoylphosphatidylglycerol (DPPG) liposomes that was induced by atropine could not be changed notably by intercalating of gramicidin. The K+ transportation of gramicidin in both normal and interdigitated bilayer was assayed by measuring the membrane potential. Results showed that gramicidin in interdigitated bilayer exhibited lower transport capability. Intrinsic fluorescence spectrum of gramicidin in interdigitated bilayer blue-shifted 2.8 nm from the spectrum in normal bilayer, which means that interdigitation provides a more hydrophobic environment for gramicidin. Circular dichroism measurement results indicated that the conformation of gramicidin in interdigitated bilayer is not the typical beta6.3 helix as in the normal bilayer. The results suggested that the interdigitated lipid bilayer might largely affect the structure and function of membrane proteins.

Low pH induces an interdigitated gel to bilayer gel phase transition in dihexadecylphosphatidylcholine membrane

We have investigated the influence of pH on the structures and phase behaviors of multilamellar vesicles of the ether-linked dihexadecylphosphatidylcholine (DHPC-MLV). This phospholipid is known to be in the interdigitated gel (L(beta)I) phase in excess water at 20 degrees C at neutral pH. The results of X-ray diffraction experiments indicate that a phase transition from L(beta)I phase to the bilayer gel phase occurred in DHPC-MLV in 0.5 M KCl around pH 3.9 with a decrease in pH, and that at low pH values, less than pH 2.2, DHPC-MLVs were in L(beta') phase. The results of fluorescence and light scattering method indicate that the gel to liquid-crystalline phase transition temperature (T(m)) of DHPC-MLV increased with a decrease in pH. On the basis of a thermodynamic analysis, we conclude that the main mechanism of the low-pH induced L(beta)I to bilayer gel phase transition in DHPC-MLV and the increase in its T(m) is connected with the decrease in the repulsive interaction between the headgroups of these phospholipids. As pH decreases, the phosphate groups of the headgroups begin to be protonated, and as a result, the apparent positive surface charges appear. However, surface dipoles decrease and the interaction free energy of the hydrophilic segments with water increases. The latter effect dominates the pure electrostatic repulsion between the charged headgroups, and thereby, the total repulsive interaction in the interface decreases.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

The effect of ethanol on the structure of phosphatidylserine bilayers

Thermotrophic and structural effects of ethanol on phosphatidylserine (PS) membranes were investigated by differential scanning calorimetry (DSC) and X-ray diffraction. It was found that up to 15% (v/v) added ethanol, there is little change in the melting temperature of the phospholipid and no change in the interbilayer (d) spacing in the gel phase, indicating that there is no interdigitation of the hydrocarbon chains. Above the melting temperature of the phospholipid, a large decrease of the d spacing, due primarily to a decrease in the thickness of the bilayer, was found. Ethanol molecules located in the headgroup region apparently expand the area available to the headgroups with concomitant coiling of the acyl chains, resulting in marked thinning of the lipid layer.

Steady-state fluorescence quenching for detecting acyl chain interdigitation in phosphatidylcholine vesicles

In the present study we have demonstrated the detection of the transition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar vesicles from the noninterdigitated gel to the fully interdigitated gel phase in the presence of ethanol or ethylene glycol (EG) using the method of fluorescence quenching. This method is based on the change of accessibility of 2-(12-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3- phosphatidyl-choline (NBD-PC), a membrane-buried fluorophore, to iodide, a quencher in the aqueous solution, during the phase transition. It is found that accessible fluorophore appears to increase at ethanol and EG concentrations known for inducing DPPC interdigitation. This increase in accessibility is either due to the relocation of the fluorescent moiety closer to the lipid-water interface or an increase in the ability of the quencher to penetrate into the loosely packed headgroup region of the interdigitated domain or both. Our results suggest the coexistence of interdigitated and noninterdigitated phases in the phospholipid vesicles and the method of fluorescence quenching might be useful in quantitating the percentage of phospholipids which are interdigitated.

The growth of bilayer defects and the induction of interdigitated domains in the lipid-loss process of supported phospholipid bilayers

The lipid-loss process has been studied with in situ atomic force microscopy (AFM) at six different temperatures for supported dipalmitoylphosphatidylcholine (DPPC) bilayers. A typical structural characteristic is the creation and the growth of bilayer defects as lipid molecules are lost from the bilayer. The rate of the lipid loss has an Arrhenius behavior, with an activation energy of 37 kT, where kT is the thermal energy at room temperature. For the lipid-loss processes at temperatures above 45 degrees C, interdigitated membrane domains are induced and are mostly in contact with some bilayer defects. These domains disappear at the increase of the area of bilayer defects. Possible mechanisms of these phenomena are discussed.

Organic solvents induce interdigitated gel structures in multilamellar vesicles of dipalmitoylphosphatidylcholine

We have investigated the effects of several water-soluble organic solvents which also have a high solubility for alkanes, on the structure and phase behavior of multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC-MLV). Of these solvents, we selected five organic solvents; such as, acetonitrile, acetone, propionaldehyde, ethanol, and tetrahydrofuran. The main transition temperature of DPPC-MLV decreased with an increase in acetonitrile concentration from 0% to 6.0%(v/v) acetonitrile, and increased above 6.0%(v/v). X-ray diffraction data indicated that a phase transition from L beta', to L beta I phase (interdigitated gel phase) in DPPC-MLV, occurred at 5.0%(v/v) and DPPC-MLV were completely in the L beta I phase above 6.0%(v/v) acetonitrile at 20 degrees C. Results of the excimer method (Yamazaki, M. et al. (1992) Biochim. Biophys. Acta 1106, 94-98) supported the above results; the ratio of excimer to monomer fluorescence intensity (E/M) of pyrene-PC in DPPC-MLV rapidly decreased at 5.1%(v/v) and E/M became very low above 6.0%(v/v) acetonitrile. By the excimer method, we have found that other organic solvents; such as, acetone, propionaldehyde, and tetrahydrofuran, induced a phase transition from L beta', to L beta I phase in DPPC-MLV. Threshold concentrations of acetone, ethanol, propionaldehyde, and tetrahydrofuran for this phase transition at 20 degrees C were 9.4%(v/v), 5.5%(w/v), 3.5%(w/v), and 3.7%(w/v), respectively. Substitution of H2O by D2O (deuterium oxide) increased the threshold concentrations of all the organic solvents. A mechanism of these phase transitions and the effect of the substitution of H2O by D2O is proposed and discussed; a concept of the chi parameter, which is an interaction energy parameter between the surface segments of DPPC-MLV and solvents, may explain these phenomena reasonably.

Ethanol-enhanced permeation of phosphatidylcholine/ phosphatidylethanolamine mixed liposomal membranes due to ethanol-induced lateral phase separation

Effects of ethanol on permeability of large unilamellar vesicles (ca. 160 nm in diameter), composed of dipalmitoyl phosphatidylcholine/dilauroyl phosphatidylethanolamine (DLPE) mixture, were studied by monitoring leakage of the fluorescent dye, calcein, entrapped in the inner aqueous phase of the vesicles. In the presence of 2.1 M ethanol, permeabilities of membranes in various phases were G (bilayer gel) phase > L (bilayer liquid-crystalline) phase with a high mole fraction of DLPE and (I (ethanol-induced interdigitated gel phase) + G) phase > (I + L) at 20 mol % DLPE. Arrhenius plots of the leakage rate constants demonstrated that the permeability was greater with 2.1 M ethanol than without ethanol, especially in the temperature above 33 degrees C, suggesting that the presence of ethanol can induce lateral phase separation of liposomal membranes and cause them to have a high permeability even if they are stable and have low permeability in its absence.

Chronic ethanol consumption alters effects of ethanol in vitro on brain membrane structure of high alcohol sensitivity and low alcohol sensitivity rats

In this study, we examined if differences in initial membrane sensitivity to ethanol were associated with development of membrane tolerance to ethanol. High Alcohol Sensitivity (HAS) and Low Alcohol Sensitivity (LAS) rats were administered a 15% ethanol solution in water as the sole source of fluid for 30 days. The amount of ethanol consumed per day did not significantly differ between the HAS and LAS rats. Development of membrane tolerance to in vitro effects of ethanol has been previously reported for bulk membrane fluidity and protein-lipid interaction. Our data expands the understanding of "membrane tolerance" phenomenon to protein distribution and bilayer interdigitation. We also introduce genotype-dependent and genotype-independent properties of the membrane tolerance to ethanol. ethanol treatment produced genotype-dependent and genotype-independent membrane tolerance to ethanol. The in vitro effects of ethanol on synaptic plasma membrane (SPM) protein distribution and lipid bilayer interdigitation were abolished or decreased in the SPM of chronic ethanol-treated HAS rats, as compared with the SPM of HAS control rats (genotype-dependent tolerance). Protein distribution and bilayer interdigitation were not affected by ethanol in vitro in either chronic ethanol-treated or control LAS rats. Genotype-independent tolerance to ethanol in vitro was observed for SPM annular and bulk bilayer fluidity in chronic ethanol-treated HAS and LAS rats. It is concluded that initial sensitivity to ethanol contributes to the development of membrane tolerance to ethanol in HAS and LAS rats.

Increased permeability of phase-separated liposomal membranes with mixtures of ethanol-induced interdigitated and non-interdigitated structures

It has been suggested by many workers using model membranes that the interdigitated structure formation, in which the acyl chains fully interpenetrate the hydrocarbon chains of the opposing monolayer, plays an important role in regulating many functions of biomembranes. In the present study the control of permeability was focused on as one of the biomembrane functions, and the effects of ethanol on the permeability of large unilamellar vesicles made by the extrusion technique (LUVET) (average diameter: about 250 nm), composed of dipalmitoyl or egg yolk phosphatidylcholines, were studied by monitoring the leakage of fluorescent dye, calcein, entrapped in the inner aqueous phase of the LUVET. The permeability was estimated from the apparent rate constant of calcein leakage at 25 degrees C. Large permeabilities were observed in the region of 0.6 M to 1.3 M ethanol, with a concentration dependence. In this range of ethanol concentrations the normal bilayer and interdigitated structure coexist and the membrane is in a phase-separated state. The large permeability is due to the instability of the boundary regions, the interdigitated membrane being characterized by a thinner structure and more rigid hydrocarbon regions in the layer than its non-interdigitated counter part. These results suggest the possibility of biomembrane-permeability regulation by interdigitated membrane formation.

Ethanol-induced aggregation and fusion of small phosphatidylcholine liposome: participation of interdigitated membrane formation in their processes

The mechanism for the ethanol-induced aggregation/fusion of uniform-sized small liposomes comprised of dipalmitoyl (DPPC) or egg yolk (eggPC) phosphatidylcholines was studied by measuring the average size using a photon correlation spectroscopy, by observing directly the states in the liposomal solutions using freeze-fracture electron microscopy and by attempting resonance energy transfer using flurophore-labeled phospholipids. Abrupt increases in the apparent size of DPPC liposomes were observed in the presence of above 44 mg/ml ethanol, where microscopically plateau membranes form interdigitated structure, in which the acyl chains fully interpenetrate the hydrocarbon chains of the apposing monolayer. On the contrary, in the eggPC liposome, where the membranes cannot form interdigitated structures even in the presence of high concentration of ethanol, such intense aggregation and fusion were not observed, suggesting their intimate relation to the interdigitated structure formation. The formation of interdigitated structures in the adhering region leads to an increase in the interfacial area and an exposure of hydrophobic acyl chain terminal on the surface area, and enhances hydrophobic interactions between two interdigitated bilayers. Thus, the resultant interdigitated structure makes the aggregated state stable and partially initiates the bilayer mixing between the two apposed membranes, leading to fusion.