Thermodynamic characterization of the pretrasition of unilamellar dipalmitoyl-phosphatidylcholine vesicles

Protonation of palmitic acid embedded in DPPC lipid bilayers obscures detection of ripple phase by FTIR spectroscopy

The transformation of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers from the gel (Lβ') to the fluid (Lα) phase involves an intermediate ripple (Pβ') phase forming a few degrees below the main transition temperature (Tm). While the exact cause of bilayer rippling is still debated, the presence of amphiphilic molecules, pH, and lipid bilayer architecture are all known to influence (pre)transition behavior. In particular, fatty acid chains interact with hydrophobic lipid tails, while the carboxylic groups simultaneously participate in proton transfer with interfacial water in the polar lipid region which is controlled by the pH of the surrounding aqueous medium. The molecular-level variations in the DPPC ripple phase in the presence of 2% palmitic acid (PA) were studied at pH levels 4.0, 7.3, and 9.1, where PA is fully protonated, partially protonated, or fully deprotonated. Bilayer thermotropic behavior was investigated by differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR) spectroscopy which agreed in their characterization of (pre)transition at pH of 9.1, but not at pH 4.0 and especially not at 7.3. Owing to the different insertion depths of protonated and deprotonated PA, along with the ability of protonated PA to undergo flip-flop in the bilayer, these two forms of PA show a different hydration pattern in the interfacial water layer. Finally, these results demonstrated the hitherto undiscovered potential of FTIR spectroscopy in the detection of the events occurring at the surface of lipid bilayers that obscure the low-cooperativity phase transition explored in this work.

Lamellarity-Driven Differences in Surface Structural Features of DPPS Lipids: Spectroscopic, Calorimetric and Computational Study

Although single-lipid bilayers are usually considered models of eukaryotic plasma membranes, their research drops drastically when it comes to exclusively anionic lipid membranes. Being a major anionic phospholipid in the inner leaflet of eukaryote membranes, phosphatidylserine-constituted lipid membranes were occasionally explored in the form of multilamellar liposomes (MLV), but their inherent instability caused a serious lack of efforts undertaken on large unilamellar liposomes (LUVs) as more realistic model membrane systems. In order to compensate the existing shortcomings, we performed a comprehensive calorimetric, spectroscopic and MD simulation study of time-varying structural features of LUV made from 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS), whereas the corresponding MLV were examined as a reference. A substantial uncertainty of UV/Vis data of LUV from which only T_m was unambiguously determined (53.9 ± 0.8 °C), along with rather high uncertainty on the high-temperature range of DPPS melting profile obtained from DSC (≈50-59 °C), presumably reflect distinguished surface structural features in LUV. The FTIR signatures of glycerol moiety and those originated from carboxyl group serve as a strong support that in LUV, unlike in MLV, highly curved surfaces occur continuously, whereas the details on the attenuation of surface features in MLV were unraveled by molecular dynamics.

Multiscale Dynamics of Lipid Vesicles in Polymeric Microenvironment

Understanding dynamic and complex interaction of biological membranes with extracellular matrices plays a crucial role in controlling a variety of cell behavior and functions, from cell adhesion and growth to signaling and differentiation. Tremendous interest in tissue engineering has made it possible to design polymeric scaffolds mimicking the topology and mechanical properties of the native extracellular microenvironment; however, a fundamental question remains unanswered: that is, how the viscoelastic extracellular environment modifies the hierarchical dynamics of lipid membranes. In this work, we used aqueous solutions of poly(ethylene glycol) (PEG) with different molecular weights to mimic the viscous medium of cells and nearly monodisperse unilamellar DMPC/DMPG liposomes as a membrane model. Using small-angle X-ray scattering (SAXS), dynamic light scattering, temperature-modulated differential scanning calorimetry, bulk rheology, and fluorescence lifetime spectroscopy, we investigated the structural phase map and multiscale dynamics of the liposome–polymer mixtures. The results suggest an unprecedented dynamic coupling between polymer chains and phospholipid bilayers at different length/time scales. The microviscosity of the lipid bilayers is directly influenced by the relaxation of the whole chain, resulting in accelerated dynamics of lipids within the bilayers in the case of short chains compared to the polymer-free liposome case. At the macroscopic level, the gel-to-fluid transition of the bilayers results in a remarkable thermal-stiffening behavior of polymer–liposome solutions that can be modified by the concentration of the liposomes and the polymer chain length.

Characterization of Pegylated Liposomal Mitomycin C Lipid-Based Prodrug (Promitil) by High Sensitivity Differential Scanning Calorimetry and Cryogenic Transmission Electron Microscopy

The effect of a lipidated prodrug of mitomycin C (MLP) on the membrane of a pegylated liposome formulation (PL-MLP), also known as Promitil, was characterized through high-sensitivity differential scanning calorimetry (DSC) and cryo-TEM. The thermodynamic analysis demonstrated that MLP led to the formation of heterogeneous domains in the membrane plane of PL-MLP. MLP concentrated in prodrug-rich domains, arranged in high-ordered crystal-like structures, as suggested by the sharp and high enthalpy endotherm in the first heating scanning. After thiolytic cleavage of mitomycin C from MLP by dithiothreitol (DTT) treatment, the crystal-like prodrug domain disappears and a homogeneous membrane with stronger lipid interactions and higher phase transition temperature compared with the blank (MLP-free) liposomes is observed by DSC. In parallel, the rod-like discoid liposomes and the "kissing liposomes" seen by cryo-TEM in the PL-MLP formulation disappear, and liposome mean size and polydispersity increase after DTT treatment. Both MLP and the residual postcleavage lipophilic moiety of the prodrug increased the rigidity of the liposome membrane as indicated by DSC. These results confirm that MLP is inserted in the PL-MLP liposome membrane via its lipophilic anchor, and its mitomycin C moiety located mainly at the region of the phospholipid glycerol backbone and polar headgroup. We hypothesize that π-π stacking between the planar aromatic rings of the mitomycin C moieties leads to the formation of prodrug-rich domains with highly ordered structure on the PL-MLP liposome membrane. This thermodynamically stable conformation may explain the high stability of the PL-MLP formulation. These results also provide us with an interesting example of the application of high sensitivity DSC in understanding the composition-structure-behavior dynamics of liposomal nanocarriers having a lipid-based drug as pharmaceutical ingredient.

Biological potential of nanomaterials strongly depends on the suspension media: experimental data on the effects of fullerene C₆₀ on membranes

Fullerenes (C60) are some of the most promising carbon nanomaterials to be used for medical applications as drug delivery agents. Computational and experimental studies have proposed their ability to enter cells by penetrating lipid bilayers. The aim of our study was to provide experimental evidence on whether pristine C60 in physiological media could penetrate cell membranes. The effect was tested on phospholipid vesicles (liposomes) composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, and validated on isolated human red blood cells (RBCs). We incubated the liposomes in an aqueous suspension of C60 and dissolved the lipids and C60 together in chloroform and subsequently formatted the liposomes. By differential scanning calorimetry measurements, we assessed the effect of C60 on the phospholipid thermal profile. The latter was not affected after the incubation of liposomes in the C60 suspension; also, a shape transformation of RBCs did not occur. Differently, by dispersing both C60 and the phospholipids in chloroform, we confirmed the possible interaction of C60 with the bilayer. We provide experimental data suggesting that the suspension medium is an important factor in determining the C60-membrane interaction, which is not always included in computational studies. Since the primary particle size is not the only crucial parameter in C60-membrane interactions, it is important to determine the most relevant characteristics of their effects on membranes.

Effect of superparamagnetic iron oxide nanoparticles on fluidity and phase transition of phosphatidylcholine liposomal membranes

Superparamagnetic iron oxide nanoparticles (SPIONs) with multifunctional properties have shown great promise in theranostics. The aim of our work was to compare the effects of SPIONs on the fluidity and phase transition of the liposomal membranes prepared with zwitterionic phosphatidylcholine lipids. In order to study if the surface modification of SPIONs has any influence on these membrane properties, we have used four types of differently functionalized SPIONs, such as: plain SPIONs (primary size was shown to bê11 nm), silica-coated SPIONs, SPIONs coated with silica and functionalized with positively charged amino groups or negatively charged carboxyl groups (the primary size of all the surface-modified SPIONs was ~20 nm). Small unilamellar vesicles prepared with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine lipids and multilamellar vesicles prepared with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine lipids were encapsulated or incubated with the plain and surface-modified SPIONs to determine the fluidity and phase transition temperature of the bilayer lipids, respectively. Fluorescent anisotropy and differential scanning calorimetric measurements of the liposomes that were either encapsulated or incubated with the suspension of SPIONs did not show a significant difference in the lipid ordering and fluidity; though the encapsulated SPIONs showed a slightly increased effect on the fluidity of the model membranes in comparison with the incubated SPIONs. This indicates the low potential of the SPIONs to interact with the nontargeted cell membranes, which is a desirable factor for in vivo applications.

On-demand droplet fusion: a strategy for stimulus-responsive biosensing in solution

A novel strategy is reported for biochemically controlled fusion of oil-in-water (O/W) droplets as an in-solution sensor for biological targets. Inspired by the SNARE complex in cells, the emulsions were stabilized by a combination of phospholipids, phospholipid-poly(ethylene glycol) conjugates, and cholesterol-anchored oligonucleotides. Prior to oligonucleotide binding, the droplets were stable in aqueous media, but hybridization of the oligonucleotides in a zipperlike fashion was shown to initiate droplet fusion. Using image analysis of content mixing of dye-loaded droplets, fusion specificity was studied and optimized as a function of interfacial chemistry. Changing the orientation of the anchored oligonucleotides, using long-chain phospholipids (C18 and C22), and binding a complementary oligonucleotide slowed or even halted fusion completely. Based on these studies, a sensor for the biomarker thrombin was designed using competitive binding of aptamer strands, with droplet fusion increasing as a function of thrombin addition in accordance with a simple binding model, with sensitivity down to 100 nM and with results in as little as 15 min. Future efforts will focus on utilizing this mechanism of content mixing to facilitate highly sensitive detection via modalities such as magnetoresistance or chemiluminescence.

Interlamellar coupling of phospholipid bilayers in liposomes: an emergent property of lipid rearrangement

The thermal phase behaviors of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) large unilamellar vesicles (LUVs) and multilamellar vesicles (MLVs) were compared by fluorescence spectroscopy, using PPDPC (1-palmitoyl-2[10-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine) as a reporter, in parallel with differential scanning calorimetry (DSC). A striking difference is seen between MLVs and LUVs in the lateral organizational dynamics of PPDPC, in particular, below the main phase transition temperature T(m), with efficient clustering of PPDPC into fluid microdomains in the L(beta') and P(beta') (ripple) phases of DPPC MLVs. In the P(beta') phase of MLVs, the probe is likely to become enriched in linear line defects, restricting intermolecular collisions to occur in a quasi one-dimensional system. In contrast, fluorescence and DSC data both suggest that the P(beta') phase is not well-defined in LUVs. Fluorescence anisotropy for 1-palmitoyl-2-[3-(diphenylhexatrienyl)propanoyl]-sn-glycero-3-phosphocholine (DPH-PC) revealed similar acyl chain order for both LUVs and MLVs in the L(beta') and P(beta') phases. However, for MLVs with this probe, T(m) determined from anisotropy was elevated by 0.7 degrees, with higher anisotropy evident in the L(alpha) phase compared to LUVs. These differences in the thermal phase behavior of the two types of liposomes are likely to derive from the augmented acyl chain order due to cooperative coupling of the lamellae of DPPC MLVs, thus manifesting in new, emerging material properties in the latter type of bilayer membrane assembly, as reflected in the organizational dynamics of the pyrene-labeled analogue.

A novel fluorescent probe that senses the physical state of lipid bilayers

Cell membrane lipids and proteins are heterogeneously distributed in the membrane plane. In recent years, much attention has been paid to the heterogeneous distribution of the lipid components, particularly the formation of cholesterol-rich domains that are thought to be important in signaling processes. This has led to renewed interest in the phase diagrams of complex lipid mixtures, such as three-component mixtures containing phospholipids and cholesterol. We report here a novel fluorescent probe (NBD-R595) that is useful for exploring the phase behaviors of one-, two-, and three-component large unilamellar vesicles. In one-component fluid-phase membranes, the probe has the expected spectral characteristic of monomeric 7-nitrobenzo-2-oxa-1,3-diazol, with a fluorescence maximum of 540 nm when excited at 470 nm. But below the gel-to-liquid crystalline phase transition temperature, an additional emission peak appears at approximately 610 nm, because of Förster resonance energy transfer from NBD-R595 monomers to NBD-R595 Jelley aggregates of limited size formed by the association of 7-nitrobenzo-2-oxa-1,3-diazol moieties. This may be the first report of Förster resonance energy transfer from a single fluorophore in two different physical states. In a test of the probe, we found NBD-R595 to be remarkably sensitive to the molar composition of large unilamellar vesicles formed from cholesterol, distearoylphosphatidylcholine, and dioleoylphosphatidylcholine.

Effect of fengycin, a lipopeptide produced by Bacillus subtilis, on model biomembranes

Fengycin is a biologically active lipopeptide produced by several Bacillus subtilis strains. The lipopeptide is known to develop antifungal activity against filamentous fungi and to have hemolytic activity 40-fold lower than that of surfactin, another lipopeptide produced by B. subtilis. The aim of this work is to use complementary biophysical techniques to reveal the mechanism of membrane perturbation by fengycin. These include: 1), the Langmuir trough technique in combination with Brewster angle microscopy to study the lipopeptide penetration into monolayers; 2), ellipsometry to investigate the adsorption of fengycin onto supported lipid bilayers; 3), differential scanning calorimetry to determine the thermotropic properties of lipid bilayers in the presence of fengycin; and 4), cryogenic transmission electron microscopy, which provides information on the structural organization of the lipid/lipopeptide system. From these experiments, the mechanism of fengycin action appears to be based on a two-state transition controlled by the lipopeptide concentration. One state is the monomeric, not deeply anchored and nonperturbing lipopeptide, and the other state is a buried, aggregated form, which is responsible for membrane leakage and bioactivity. The mechanism, thus, appears to be driven mainly by the physicochemical properties of the lipopeptide, i.e., its amphiphilic character and affinity for lipid bilayers.

Effect of the antibiotic azithromycin on thermotropic behavior of DOPC or DPPC bilayers

Azithromycin is a macrolide antibiotic known to bind to lipids and to affect endocytosis probably by interacting with lipid membranes [Tyteca, D., Schanck, A., Dufrene, Y.F., Deleu, M., Courtoy, P.J., Tulkens, P.M., Mingeot-Leclercq, M.P., 2003. The macrolide antibiotic azithromycin interacts with lipids and affects membrane organization and fluidity: studies on Langmuir-Blodgett monolayers, liposomes and J774 macrophages. J. Membr. Biol. 192, 203-215]. In this work, we investigate the effect of azithromycin on lipid model membranes made of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). Thermal transitions of both lipids in contact with azithromycin are studied by (31)P NMR and DSC on multilamellar vesicles. Concerning the DPPC, azithromycin induces a suppression of the pretransition whereas a phase separation between the DOPC and the antibiotic is observed. For both lipids, the enthalpy associated with the phase transition is strongly decreased with azithromycin. Such effects may be due to an increase of the available space between hydrophobic chains after insertion of azithromycin in lipids. The findings provide a molecular insight of the phase merging of DPPC gel in DOPC fluid matrix induced by azithromycin [Berquand, A., Mingeot-Leclercq, M.P., Dufrene, Y.F., 2004. Real-time imaging of drug-membrane interactions by atomic force microscopy. Biochim. Biophys. Acta 1664, 198-205] and could help to a better understanding of azithromycin-cell interaction.

Phase behavior and nanoscale structure of phospholipid membranes incorporated with acylated C14-peptides

The thermotropic phase behavior and lateral structure of dipalmitoylphosphatidylcholine (DPPC) lipid bilayers containing an acylated peptide has been characterized by differential scanning calorimetry (DSC) on vesicles and atomic force microscopy (AFM) on mica-supported bilayers. The acylated peptide, which is a synthetic decapeptide N-terminally linked to a C14 acyl chain (C14-peptide), is incorporated into DPPC bilayers in amounts ranging from 0-20 mol %. The calorimetric scans of the two-component system demonstrate a distinct influence of the C14-peptide on the lipid bilayer thermodynamics. This is manifested as a concentration-dependent downshift of both the main phase transition and the pretransition. In addition, the main phase transition peak is significantly broadened, indicating phase coexistence. In the AFM imaging scans we found that the C14-peptide, when added to supported gel phase DPPC bilayers, inserts preferentially into preexisting defect regions and has a noticeable influence on the organization of the surrounding lipids. The presence of the C14-peptide gives rise to a laterally heterogeneous bilayer structure with coexisting lipid domains characterized by a 10 A height difference. The AFM images also show that the appearance of the ripple phase of the DPPC lipid bilayers is unaffected by the C14-peptide. The experimental results are supported by molecular dynamics simulations, which show that the C14-peptide has a disordering effect on the lipid acyl chains and causes a lateral expansion of the lipid bilayer. These effects are most pronounced for gel-like bilayer structures and support the observed downshift in the phase-transition temperature. Moreover, the molecular dynamics data indicate a tendency of a tryptophan residue in the peptide sequence to position itself in the bilayer headgroup region.

Alpha-synuclein has a high affinity for packing defects in a bilayer membrane: a thermodynamics study

A number of neurodegenerative disorders, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, are characterized by the intracellular deposition of fibrillar aggregates that contain a high proportion of alpha-synuclein (alphaS). The interaction with the membrane-water interface strongly modulates folding and aggregation of the protein. The present study investigates the lipid binding and the coil-helix transition of alphaS, using titration calorimetry, differential scanning calorimetry, and circular dichroism spectroscopy. Titration of the protein with small unilamellar vesicles composed of zwitterionic phospholipids below the chain melting temperature of the lipids yielded exceptionally large exothermic heat values. The sigmoidal titration curves were evaluated in terms of a simple model that assumes saturable binding sites at the vesicle surface. The cumulative heat release and the ellipticity were linearly correlated as a result of simultaneous binding and helix folding. There was no heat release and folding of alphaS in the presence of large unilamellar vesicles, indicating that a small radius of curvature is necessary for the alphaS-membrane interaction. The heat release and the negative heat capacity of the protein-vesicle interaction could not be attributed to the coil-helix transition of the protein alone. We speculate that binding and helix folding of alphaS depends on the presence of defect structures in the membrane-water interface, which in turn results in lipid ordering in the highly curved vesicular membranes. This will be discussed with regard to a possible role of the protein for the stabilization of synaptic vesicle membranes.

Different modulation of phospholipase A2 activity by saturated and monounsaturated N-acylethanolamines

The physiological functions of N-acylethanolamines (NAEs) are poorly understood, although many functions were suggested for these naturally occurring membrane components of plants and animals. The binding with cannabinoid receptors CB1 and CB2 was demonstrated for some NAEs, such as anandamide. However, the chemical nature of these molecules suggests that some of their biological effects on biomembranes could be related, at least partially, to physical interactions with the lipid bilayer. The present work studies the effect of saturated and monounsaturated NAEs on phospholipase A2 (PLA2) activity, which is dependent on lipid bilayer features. The present study, performed by 2-dimethylamino-(6-lauroyl)-naphthalene (Laurdan) fluorescence, demonstrates that the acyl chain length and the presence of a single double bond are crucial for the enzymatic activity modulation by NAEs. In fact, saturated NAEs with 10 carbon atoms don't affect the PLA2 activity, while NAEs with 12 and 16 carbon atoms largely activate the enzyme. On the other hand, an acyl chain length of 18 carbon atoms, with or without the presence of a double bond, only slightly affects the enzymatic activity. A structural model for NAE-lipid interactions is proposed in order to explain the differences in PLA2 activity modulation by these fatty acid derivatives.

Use of laurdan fluorescence intensity and polarization to distinguish between changes in membrane fluidity and phospholipid order

Laurdan is a fluorescent probe that detects changes in membrane phase properties through its sensitivity to the polarity of its environment in the bilayer. Variations in membrane water content cause shifts in the laurdan emission spectrum, which are quantified by calculating the generalized polarization (GP). We tested whether laurdan fluorescence could be used to distinguish differences in phospholipid order from changes in membrane fluidity by examining the temperature dependence of laurdan GP and fluorescence anisotropy in dipalmitoylphosphatidylcholine (DPPC) vesicles. The phase transition from the solid ordered phase to the liquid disordered phase was observed as a decrease in laurdan GP values from 0.7 to -0.14 and a reduction in anisotropy from 0.25 to 0.12. Inclusion of various amounts of cholesterol in the membranes to generate a liquid ordered phase caused an increase in the apparent melting temperature detected by laurdan GP. In contrast, cholesterol decreased the apparent melting temperature estimated from anisotropy measurements. Based on these results, it appeared that laurdan anisotropy detected changes in membrane fluidity while laurdan GP sensed changes in phospholipid order. Thus, the same fluorescent probe can be used to distinguish effects of perturbations on membrane order and fluidity by comparing the results of fluorescence emission and anisotropy measurements.

A model for the lipid pretransition: coupling of ripple formation with the chain-melting transition

Below the thermotropic chain-melting transition, lipid membrane c(P) traces display a transition of low enthalpy called the lipid pretransition. It is linked to the formation of periodic membrane ripples. In the literature, these two transitions are usually regarded as independent events. Here, we present a model that is based on the assumption that both pretransition and main transition are caused by the same physical effect, namely chain melting. The splitting of the melting process into two peaks is found to be a consequence of the coupling of structural changes and chain-melting events. On the basis of this concept, we performed Monte Carlo simulations using two coupled monolayer lattices. In this calculation, ripples are considered to be one-dimensional defects of fluid lipid molecules. Because lipids change their area by approximately 24% upon melting, line defects are the only ones that are topologically possible in a triangular lattice. The formation of a fluid line defect on one monolayer leads to a local bending of the membrane. Geometric constraints result in the formation of periodic patterns of gel and fluid domains. This model, for the first time, is able to predict heat capacity profiles, which are comparable to the experimental c(P) traces that we obtained using calorimetry. The basic assumptions are in agreement with a large number of experimental observations.

Thermodynamics of phospholipase A2-ligand interactions

Future investigations into the role of the structure of phospholipid substrates and the interrelationships between substrate, calcium, and enzyme conformation in the activation process are clearly needed. Enzyme dimerization in the activation of phospholipase A2 has been indicated, and a complex equilibrium between calcium, substrate, and monomer and dimer enzyme apparently exists. The incorporation of proton binding further complicates the scheme, and one is quickly faced with obtaining a large number of equilibrium constants in order to describe the system explicitly. Nevertheless, similarly complex systems have been well characterized using thermodynamic approaches such as those described herein. An excellent example is the complex equilibrium involving the protonation of the histidine residues and the binding of a mononucleotide to ribonuclease A. Achieving a complete thermodynamic description of that system allowed the investigators to make strong mechanistic statements about models for the catalytic mechanism of ribonuclease A. Since phospholipase A2 is available for study at the same level of detail, one can anticipate a similar degree of quantitative detail regarding the important interactions of this enzyme to be forthcoming.

Spontaneous fusion of phosphatidylcholine small unilamellar vesicles in the fluid phase

Using a high-sensitivity differential scanning microcalorimeter capable of performing cooling scans, we have examined the phase behavior of small unilamellar vesicles (SUV) as a function of time of storage above their order-disorder phase transition. Vesicles composed of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were examined. Cooling scans on fresh (5-7-h postsonication) samples revealed broad, relatively simple heat capacity peaks (peak temperatures: 19.9 degrees C for DMPC, 37.8 degrees C for DPPC) free of high-temperature spikes or shoulders. Subsequent heating scans displayed a sharp peak characteristic of previously described fusion products formed below the phase transition. SUV samples stored for 1 or more days above their phase transition displayed a moderately broad, high-temperature shoulder (23.8 degrees C for DMPC and 40.2 degrees C for DPPC) in the cooling profile. For DMPC, the enthalpy associated with this peak increased in a first-order fashion with time. Hydrolysis products were not detected until 12-20 days of storage. Both the rate and extent of shoulder appearance increased with temperature (k = 0.0017 h-1, fraction of total enthalpy = 0.1 at 36 degrees C; k = 0.0037 h-1, fraction = 0.2 at 42 degrees C). Freeze-fracture electron micrographs confirmed that an intermediate-sized vesicle population (diameters 400-500 A) appeared in SUV samples stored above their phase transition. Also, the trapped volume of DMPC SUV increased from 0.26 microL/mumol after 17 h of storage to 0.54 microL/mumol after storage for 16 days at 36 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Action of cobra venom phospholipase A2 on large unilamellar vesicles: comparison with small unilamellar vesicles and multibilayers

Phospholipase A2 (Naja naja naja) catalyzes the hydrolysis of dipalmitoyl phosphatidylcholine in small unilamellar vesicles (SUVs) with a faster initial rate than in large unilamellar vesicles (LUVs) and multilamellar vesicles (MLVs). For the SUVs, the hydrolysis was initially faster for gel phase than liquid crystalline phase phospholipid. For both LUVs and MLVs, hydrolysis was low except in a small temperature range around the thermotropic phase transition of the phospholipid. In this temperature range, the reaction time course of phospholipase action on dipalmitoyl phosphatidylcholine in LUVs and MLVs included a lag period. With SUVs, a lag period also was observed above the phase transition temperature, but it was not observed below it.