Binary phase diagram of hydrated dimyristoylglycerol-dimyristoylphosphatidylcholine mixtures

Large Decrease in the Melting Point of Benzoquinones via High-n Eutectic Mixing Predicted by a Regular Solution Model

Decreasing the melting point (T_m) of a mixture is of interest in cryopreservatives, molten salts, and battery electrolytes. One general strategy to decrease T_m, exemplified by deep eutectic solvents, is to mix components with favorable (negative) enthalpic interactions. We demonstrate a complementary strategy to decrease T_m by mixing many components with neutral or slightly positive enthalpic interactions, using the number of components (n) to increase the entropy of mixing and decrease T_m. In theory, under certain conditions this approach could achieve an arbitrarily low T_m. Furthermore, if the components are small redox-active molecules, such as the benzoquinones studied here, this approach could lead to high energy density flow battery electrolytes. Finding the eutectic composition of a high-n mixture can be challenging due to the large compositional space yet is essential for ensuring the existence of a purely liquid phase. We reformulate and apply fundamental thermodynamic equations to describe high-n eutectic mixtures of small redox-active molecules (benzoquinones and hydroquinones). We illustrate a novel application of this theory by tuning the entropy of melting, rather than the enthalpy, in systems highly relevant to energy storage. We demonstrate with differential scanning calorimetry measurements that 1,4-benzoquinone derivatives exhibit eutectic mixing that decreases their T_m, despite slightly positive enthalpies of mixing (0-5 kJ/mol). By rigorously investigating all 21 binary mixtures of a set of seven 1,4-benzoquinone derivatives with alkyl substituents (T_m's between 44 and 120 °C), we find that the eutectic melting point of a mixture of all seven achieves a large decrease in T_m to -6 °C. We further determine that the regular solution model shows improvement over an ideal solution model in predicting the eutectic properties for this newly investigated type of mixture composed of many small redox-active organic molecules.

Simultaneous action of microbial phospholipase C and lipase on model bacterial membranes - Modeling the processes crucial for bioaugmentation

Bioaugmentation is a promising method of the remediation of soils polluted by persistent organic pollutants (POP). Unfortunately, it happens frequently that the microorganisms inoculated into the soil die out due to the presence of enzymes secreted by autochthonous microorganisms. Especially destructive are here phospholipases C (PLC) and lipases which destruct the microorganism's cellular membrane. The composition of bacterial membranes differs between species, so it is highly possible that depending on the membrane constitution some bacteria are more resistant to PLCs and lipases than other. To shed light on these problems we applied phospholipid Langmuir monolayers as model microbial membranes and studied their interactions with α-toxin (model bacterial PLC) and the lipase isolated from soil fungus Candida rugosa. Membrane phospholipids differing in their headgroup (phosphatidylcholines, phosphatidylethanolamines, phosphatidylglycerols and cardiolipins) and in their tail structure were applied. The monolayers were characterized by the Langmuir technique, visualized by Brewster angle microscopy, and the packing mode of the phospholipid molecules was verified by the application of the diffraction of synchrotron radiation. We also studied the mutual miscibility of diacylglycerols and the native phospholipids as their interaction is crucial for the understanding of the PLC and lipase activity. It turned out that all the investigated phospholipid classes can be hydrolyzed by PLC; however, they differ profoundly in the hydrolysis degree. Depending on the effects of the initial PLC action and the mutual organization of the diacylglycerol and phospholipid molecules the lipase can ruin the model membranes or can be completely neutral to them.

The Effects of Alkyl Chain Combinations on the Structural and Mechanical Properties of Biomimetic Ion Pair Amphiphile Bilayers

Ion pair amphiphile (IPA), a lipid-like complex composed of a pair of cationic and anionic surfactants, has great potentials in various pharmaceutical applications. In this work, we utilized molecular dynamics (MD) simulation to systematically examine the structural and mechanical properties of the biomimetic bilayers consist of alkyltrimethyl-ammonium-alkylsulfate (C_mTMA⁺-C_nS⁻) IPAs with various alkyl chain combinations. Our simulations show an intrinsic one-atom offset for the C_mTMA⁺ and C_nS⁻ alignment, leading to the asymmetric index definition of ΔC = m − (n + 1). Larger |ΔC| gives rise to higher conformational fluctuations of the alkyl chains with the reduced packing order and mechanical strength. In contrast, increasing the IPA chain length enhances the van der Waals interactions within the bilayer and thus improves the bilayer packing order and mechanical properties. Further elongating the C_mTMA⁺-C_nS⁻ alkyl chains to m and n ≥ 12 causes the liquid disorder to gel phase transition of the bilayer at 298 K, with the threshold membrane properties of 0.45 nm² molecular area, deuterium order parameter value of 0.31, and effective bending rigidity of 20 k_BT, etc. The combined results provide molecular insights into the design of biomimetic IPA bilayers with wide structural and mechanical characteristics for various applications.

Diacylglycerol levels modulate the cellular distribution of the nicotinic acetylcholine receptor

Diacylglycerol (DAG), a second messenger involved in different cell signaling cascades, activates protein kinase C (PKC) and D (PKD), among other kinases. The present work analyzes the effects resulting from the alteration of DAG levels on neuronal and muscle nicotinic acetylcholine receptor (AChR) distribution. We employ CHO-K1/A5 cells, expressing adult muscle-type AChR in a stable manner, and hippocampal neurons, which endogenously express various subtypes of neuronal AChR. CHO-K1/A5 cells treated with dioctanoylglycerol (DOG) for different periods showed augmented AChR cell surface levels at short incubation times (30min-4h) whereas at longer times (18h) the AChR was shifted to intracellular compartments. Similarly, in cultured hippocampal neurons surface AChR levels increased as a result of DOG incubation for 4h. Inhibition of endogenous DAG catabolism produced changes in AChR distribution similar to those induced by DOG treatment. Specific enzyme inhibitors and Western blot assays revealed that DAGs exert their effect on AChR distribution through the modulation of the activity of classical PKC (cPKC), novel PKC (nPKC) and PKD activity.

Alteration of lipid membrane structure and dynamics by diacylglycerols with unsaturated chains

Diacylglycerols (DAGs) with unsaturated acyl chains play many important roles in biomembranes, such as a second messenger and activator for protein kinase C. In this study, three DAGs of distinctly different chain unsaturations (i.e. di16:0DAG (DPG), 16:0-18:1DAG (POG), and di18:1DAG (DOG)) are studied using atomistic MD simulation to compare their roles in the structure and dynamics of 16:0-18:1phosphatidylcholine (POPC) membranes. All three DAGs are able to produce the so-called 'condensing effect' in POPC membranes: decreasing area-per-lipid, and increasing acyl chain order and bilayer thickness. Our visual and quantitative analyses clearly show that DAG with unsaturated chains induce larger spacing between POPC headgroups, compared with DAG with saturated chains; this particular effect has long been hypothesized to be crucial for activating enzymes and receptors in cell membranes. DAGs with unsaturated chains are also located closer to the bilayer/aqueous interface than DPG and are more effective in slowing down lateral diffusion of molecules. We show that DAG molecules seek the "umbrella coverage" from neighboring phospholipid headgroups - similar to cholesterol. Unlike cholesterol, DAGs also hide their chains from water by laterally inserting their chains into the surrounding. Thus, acyl chains of DAG are more spread and disordered than those of PC due to the insertion. By calculating the potential of mean force (PMF) for POPC in POPC/DAG bilayers, we found that all three DAGs can significantly increase the free energy barrier for POPC to flip-flop, but only DAGs with unsaturated chains can additionally increase the free energy of POPC desorption.

Water-binding phospholipid nanodomains and phase-separated diacylglycerol nanodomains regulate enzyme reactions in lipid monolayers

Phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) nanodomains covered with bound water as well as diacylglycerol 1-palmitoyl-2-oleoyl-sn-glycerol (POG) nanodomains separated from a lipid membrane were studied, using monolayer surfaces of POPC hydrolyzed by phospholipase C (PLC). The investigation was based on the analysis of compression isotherms and on atomic force microscope (AFM) observations of Langmuir-Blodgett (LB) films and Langmuir-Schaefer (LS) films. The results included reaction rate constants obtained by kinetic analysis of phosphocholine at surface pressures from 0.1 to 31 mN/m and determined by a luminol-enhanced chemiluminescence method. Monolayer elastic modulus values and fluorescence microscopic images confirmed that hydrolysis by PLC progressed in the intermediate monolayer between a liquid-expanded (L1) film and a liquid-condensed (L2) film at 2-17 mN/m. Furthermore, the intermediate film was confirmed to consist of L1 film and the POPC nanodomains in the L2 state are covered with bound water, conclusions based on the following AFM results: (1) nanodomains in POPC LS films were catalyzed by PLC, (2) POG nanodomains extended out from LB films of mixed POPC/POG 9/1 (mol/mol) monolayers, and (3) POPC LS films were covered with bound water, as indicated by cross-sectional analysis. At the optimal surface pressure of 10 mN/m, when POPC nanodomains (L2), with internal diameters of ∼75 nm, were hydrolyzed by PLC, they shrank down into pockets of the same size as those that appeared with POG. The resulting pocket sizes on LS films were in agreement with POG nanodomain sizes on LB films. This study demonstrated that PLC reacted with POPC nanodomains (L2) dispersed in L1/L2 mixed phase monolayers selectively and that POG nanodomains were phase-separated from the monolayer as hydrolysis proceeded.

Protein-lipid interactions role of membrane plasticity and lipid specificity on peripheral protein interactions

Lipid mixtures are inherently nonrandom as each lipid species differs slightly in its chemical structure. A protein associates not with a lipid but with a membrane comprised of lipids where the chemical activities of each lipid is determined by the composition of the mixture. There can be selectivity in this association because a protein can enhance the underlying tendency of lipids to be heterogeneously distributed. This is dependent on the protein having a preferential association of sufficient magnitude with some of the lipids within the membrane. To measure and model protein-lipid interactions, an understanding of the underlying lipid behavior is necessary to interpret their association constants. Methods to measure protein-lipid interactions are discussed within the context of using these techniques in modeling and a general framework is presented for the use of a signal arising from these interactions. The use of binding partition functions is presented as this allows the modeling of cooperative or independent (noncooperative) interactions of protein with lipids and of proteins with additional ligands as well as lipids. A model is also provided using the binding partition function formalism where protein dimerization, and by extension, oligomerization is enhanced at the membrane compared to in solution.

The intermediate state of DMPG is stabilized by enhanced positive spontaneous curvature

1,2-Dimyristoyl-sn-glycero-3-phospho-rac-glycerol (DMPG) at low salt concentrations has a complex endotherm with at least four components and extending over the span of 20 degrees. During this ongoing melting, the solution becomes viscous and scatters light poorly. This multipeak endotherm was suggested to result from the effects of curvature on the relative free energies of gel and fluid DMPG bilayers, further relating to the formation of an intermediate sponge phase between the lamellar gel and fluid phases. Although later studies appear to exclude a connected bilayer network, the relation of the endotherm peaks to curvature remains an appealing hypothesis. This was tested by including in the system both water-soluble small molecules (dimethyl sulfoxide, ethanol, and urea) as well as amphiphiles (myristoyl-lyso-PG, cholesterol, cholesterol-3-sulfate, and dimyristoylglycerol) known to alter the spontaneous curvature of bilayers. All compounds increasing the monolayer positive spontaneous curvature (ethanol, urea, myristoyl-lyso-PG, cholesterol-3-sulfate) increased the temperature span of the intermediate state and elevated the temperature of its dissolution, while all compounds increasing the negative spontaneous curvature (dimethyl sulfoxide, cholesterol, dimyristoylglycerol) had the opposite effect, implying that the intermediate state contains a structure with positive curvature. The results support the view that the intermediate state consists of vesicles with a large number of holes. The viscosity increase could be related to vesicle expansion needed to accommodate the numerous holes.

Phase studies of model biomembranes: complex behavior of DSPC/DOPC/cholesterol

We have undertaken a series of experiments to examine the behavior of individual components of cell membranes. Here we report an initial stage of these experiments, in which the properties of a chemically simple lipid mixture are carefully mapped onto a phase diagram. Four different experimental methods were used to establish the phase behavior of the 3-component mixture DSPC/DOPC/chol: (1) confocal fluorescence microscopy observation of giant unilamellar vesicles, GUVs; (2) FRET from perylene to C20:0-DiI; (3) fluorescence of dilute dyes C18:2-DiO and C20:0-DiI; and (4) wide angle X-ray diffraction. This particular 3-component mixture was chosen, in part, for a high level of immiscibility of the components in order to facilitate solving the phase behavior at all compositions. At 23 degrees C, a large fraction of the possible compositions for this mixture give rise to a solid phase. A region of 3-phase coexistence of {Lalpha+Lbeta+Lo} was detected and defined based on a combination of fluorescence microscopy of GUVs, FRET, and dilute C20:0-DiI fluorescence. At very low cholesterol concentrations, the solid phase is the tilted-chain phase Lbeta'. Most of the phase boundaries have been determined to be within a few percent of the composition. Measurements of the perturbations of the boundaries of this accurate phase diagram could serve as a means to understand the behaviors of a range of added lipids and proteins.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

Determination of partition coefficient of spin probe between different lipid membrane phases

Model lipid membranes made from binary mixtures of dimyristoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DMPC/DPPC) and dimyristoylphosphatidylcholine/cholesterol (DMPC/Chol) exhibit coexistence of diverse lipid phases at appropriate temperature and composition. Since lipids in different phases show different structural and motional properties, it is expected that the corresponding spin probe electron paramagnetic resonance (EPR) spectra will be superposition of several spectral components. From comparison of proportions of spectral components of the EPR spectrum with the fractions of the corresponding lipid phases obtained from known phase diagrams the partition coefficient of spin probe methyl ester of 5-doxyl palmitate between different lipid phases was determined. The results indicate that the used spin probe partitions approximately equally between different phases.

Diacylglycerol-rich domain formation in giant stearoyl-oleoyl phosphatidylcholine vesicles driven by phospholipase C activity

We have studied the effect of phospholipase C from Bacillus cereus and Clostridium perfringens (alpha-toxin) on giant stearoyl-oleoyl phosphatidylcholine (SOPC) vesicles. Enzyme activity leads to a binary mixture of SOPC and the diacylglycerol SOG, which phase separates into a SOPC-rich bilayer phase and a SOG-rich isotropic bulk-like domain embedded within the membrane, as seen directly by phase contrast microscopy. After prolonged enzymatic attack, all bilayer membranes are transformed into an isotropic pure SOG phase as characterized by fluorescence microscopy, differential scanning calorimetry, fluorescence anisotropy measurements, and small angle x-ray scattering. These domains may have biological relevance, serving as storage compartments for hydrophobic molecules and/or catalyzing cellular signaling events at their boundaries. Furthermore, in the early stages of asymmetric enzymatic attack to the external monolayer of giant vesicles, we observe a transient coupling of the second-messenger diacylglycerol to membrane spontaneous curvature, which decreases due to enzyme activity, before domain formation and final vesicle collapse occurs.

Streptomyces chromofuscus phospholipase D interaction with lipidic activators at the air–water interface

The phospholipase D from Streptomyces chromofuscus (PLDSc) is a soluble enzyme that interacts with membranes to catalyse phosphatidylcholine (PC) transformation. In this work, we focused on the interaction between PLDSc and two lipid activators: a neutral lipid, diacylglycerol (DAG), and an anionic one, phosphatidic acid (PA). DAG is a naturally occurring alcohol, so it is a potent nucleophile for the transphosphatidylation reaction catalysed by PLD. Concerning PA, it is a widely described activator of PLDSc-catalysed hydrolysis of PC. The monolayer technique allowed us to define PLDSc interaction with DAG and PA. In the case of DAG, the results suggest an insertion of PLDSc within the acyl chains of the lipid with an exclusion pressure of approximately 45 mN/m. PLDSc-DAG interaction seemed to occur preferentially with the lipid in the liquid-expanded (LE) phase. PLDSc interaction with PA was found to be more effective at high surface pressures. The overall results obtained with PA show a preferential interaction of the protein with condensed PA domains. No exclusion pressure could be found for PLDSc-PA interaction indicating only superficial interaction with the polar head of this lipid. Brewster angle microscopy (BAM) images were acquired in order to confirm these results and to visualise the patterns induced by PLDSc adsorption.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

Characterization of the main phase transition in 1,2-dipalmitoyl-phosphatidylcholine luvs by 1H NMR

The main phase transition (Tm) of 100 nm large unilamellar vesicles (LUVs) of 1,2-dipalmitoylphosphatidylcholine (DPPC) was investigated using 1H NMR (proton magnetic resonance) in deuterium oxide, and both DSC (differential scanning calorimetry) and IR (infrared) spectroscopy in water and deuterium oxide. The ability of 1H NMR to determine Tm was demonstrated and the values obtained were in general agreement with those observed with DSC and IR. However, the temperature range of the transition observed by NMR was significantly broader than that observed with either DSC or IR. The effect of deuterium oxide on Tm was studied by comparing results obtained in water and deuterium oxide with DSC and IR. The results showed no significant difference in Tm or temperature range of transition determined in these solvents.

The effects of acyl chain length and saturation of diacylglycerols and phosphatidylcholines on membrane monolayer curvature

The second messenger, diacylglycerol (DAG), introduces negative curvature in phospholipid monolayers and strongly induces the lamellar (L(alpha)) to reverse hexagonal (H(II)) phase transition. The chain lengths and degree of unsaturation of symmetric DAGs influence this effect. Within dioleoylphosphatidylcholine (DOPC) monolayers, the apparent spontaneous radius of curvature (R(0)) of the short, saturated dicaprylglycerol (C10-DCG) itself was determined to be -13.3 A, compared with an R(0) value of -10.1 A for the long, di-monounsaturated dioleoylglycerol (C18-DOG). Such increased length and unsaturation of the DAG acyl chains produces this small change. Di-saturated phosphatidylcholines (PCs) with equal length chains (from C10-C18) with 25 mol % DOG do not form the H(II) phase, even under the unstressed conditions of excess water and alkane. Di-unsaturated PCs with equal chain length (from C14-C18) with 25 mol % DOG do form the H(II) phase. Asymmetric chained PCs (position 1 saturated with varying lengths, position 2 differentially unsaturated with varying lengths) all form the H(II) phase in the presence of 25 mol % DOG. As a general rule for PCs, their unsaturation is critical for the induction of the H(II) phase by DOG. The degree of curvature stress induced by the second messenger DOG in membranes, and any protein that might be affected by it, would appear to depend on chain unsaturation of neighboring PCs.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Effects of phospholipid headgroup and phase on the activity of diacylglycerol kinase of Escherichia coli

Diacylglycerol kinase (DGK) of Escherichia coli has been reconstituted into a variety of phospholipid bilayers and its activity determined as a function of lipid headgroup structure and phase preference. The anionic phospholipids dioleoylphosphatidic acid, dioleoylphosphatidylserine, and cardiolipin were all found to support activities lower than that supported by dioleoylphosphatidylcholine. In mixtures of dioleoylphosphatidylcholine and 20 mol % anionic phospholipids, the presence of anionic phospholipids all resulted in lower activities than in dioleoylphosphatidylcholine, except for dioleoylphosphatidylglycerol whose presence had little effect on activity. In some cases, the low activity in the presence of anionic phospholipid followed from a decrease in v(max); in some cases, it followed from an increase in the K(m) for diacylglycerol, and in the case of dioleoylphosphatidic acid, it followed from both. Activities in mixtures containing 80 mol % dioleoylphosphatidylethanolamine were lower than in dioleoylphosphatidylcholine at temperatures where both lipids adopted a bilayer phase; at higher temperatures where dioleoylphosphatidylethanolamine preferred a hexagonal H(II) phase, the differences in activity were greater. These experiments suggest that the presence of lipids preferring a hexagonal H(II) phase leads to low activities. Activities of DGK are low in a gel phase lipid.

Activation of protein kinase C alpha by lipid mixtures containing different proportions of diacylglycerols

Lipid activation of protein kinase C alpha (PKC alpha) was studied using a model mixture containing POPC/POPS (molar ratio 4:1) and different proportions of either DPG or POG. The lipid mixtures containing DPG were physically characterized by using different physical techniques, and a phase diagram was constructed by keeping a constant POPC/POPS molar ratio of 4:1 and changing the concentration of 1,2-DPG. The phase diagram displayed three regions delimited by two compounds: compound 1 (CO(1)) with 35 mol % of 1,2-DPG and compound 2 (CO(2)) with 65 mol % of 1,2-DPG. PKC alpha activity was assayed at increasing concentrations of 1,2-DPG, maximum activity being reached at 30 mol % 1,2-DPG, which decreased at higher concentrations. Maximum activity occurred, then, at concentrations of 1,2-DPG which corresponded to the transition from region 1 to region 2 of the phase diagram. It was interesting that this protein was maximally bound to the membrane at all DPG concentrations. Similar results were observed when the enzyme was activated by POG, when a maximum was reached at about 10 mol %. This remained practically constant up to 50 mol %, about which it decreased, the binding level remaining maximal and constant at all POG concentrations. The fact that in the assay conditions used maximal binding was already reached even in the absence of diacylglycerol was attributed to the interaction of the C2 domain with the POPS present in the membrane through the Ca(2+) ions also present. To confirm this, the isolated C2 domain was used, and it was also found to be maximally bound at all DPG concentrations and even in its absence. Since the intriguing interaction patterns observed seemed to be due then to the C1 domain, the PKC alpha mutant D246/248N was used. This mutant has a decreased Ca(2+)-binding capacity through the C2 domain and was not activated nor bound to membranes by increasing concentrations of DPG. However, POG was able to activate the mutant, which showed a similar dependence on POG concentration with respect to activity and binding to membranes. These data underline the importance of unsaturation in one of the fatty acyl chains of the diacylglycerol.

Effects of bilayer thickness on the activity of diacylglycerol kinase of Escherichia coli

We have developed a procedure for the reconstitution of Escherichia coli diacylglycerol kinase (DGK) into phospholipid bilayers containing diacylglycerol substrate. When DGK is reconstituted into a series of phosphatidylcholines containing monounsaturated fatty acyl chains, activity against dihexanoylglycerol (DHG) as a substrate was found to be markedly dependent on the fatty acyl chain length with the highest activity in dioleoylphosphatidylcholine [di(C18:1)PC] and a lower activity in bilayers with shorter or longer fatty acyl chains. Low activities in the short chain phospholipid dimyristoleoylphosphatidylcholine [di(C14:1)PC] followed from an increase in the K(m) value for DHG and ATP, with no effect on v(max). In contrast, in the long chain lipid dierucoylphosphatidylcholine [di(C24:1)PC], the low activity followed from a decrease in v(max) with no effect on K(m). In mixtures of two phosphatidylcholines with different chain lengths, the activity corresponded to that expected for the average chain length of the mixture. Cholesterol increased the activity in di(C14:1)PC but slightly decreased it in di(C18:1)PC or di(C24:1)PC, effects that could follow from changes in bilayer thickness caused by cholesterol.

Interaction of ceramides with phosphatidylcholine, sphingomyelin and sphingomyelin/cholesterol bilayers

Ceramides (Cers) may exert their biological activity through changes in membrane structure and organization. To understand this mechanism, the effect of Cer on the biophysical properties of phosphatidylcholine, sphingomyelin (SM) and SM/cholesterol bilayers was determined using fluorescence probe techniques. The Cers were bovine brain Cer and synthetic Cers that contained a single acyl chain species. The phospholipids were 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glyero-3-phosphocholine (DPPC) and bovine brain, egg yolk and bovine erythrocyte SM. The addition of Cer to POPC and DPPC bilayers that were in the liquid-crystalline phase resulted in a linear increase in acyl chain order and decrease in membrane polarity. The addition of Cer to DPPC and SM bilayers also resulted in a linear increase in the gel to liquid-crystalline phase transition temperature (T(M)). The magnitude of the change was dependent upon Cer lipid composition and was much higher in SM bilayers than DPPC bilayers. The addition of 33 mol% cholesterol essentially eliminated the thermal transition of SM and SM/Cer bilayers. However, there is still a linear increase in acyl chain order induced by the addition of Cer. The results are interpreted as the formation of DPPC/Cer and SM/Cer lipid complexes. SM/Cer lipid complexes have higher T(M)s than the corresponding SM because the addition of Cer reduces the repulsion between the bulky headgroup and allows closer packing of the acyl chains. The biophysical properties of a SM/Cer-rich bilayer are dependent upon the amount of cholesterol present. In a cholesterol-poor membrane, a sphingomyelinase could catalyze the isothermal conversion of a liquid-crystalline SM bilayer to a gel phase SM/Cer complex at physiological temperature.

Mixed membranes of sphingolipids and glycerolipids as studied by spin-label ESR spectroscopy. A search for domain formation

The temperature dependences of the ESR spectra from different positional isomers of sphingomyelin and of phosphatidylcholine spin-labeled in their acyl chain have been compared in mixed membranes composed of sphingolipids and glycerolipids. The purpose of the study was to identify the possible formation of sphingolipid-rich in-plane membrane domains. The principal mixtures that were studied contained sphingomyelin and the corresponding glycerolipid phosphatidylcholine, both from egg yolk. Other sphingolipids that were investigated were brain cerebrosides and brain gangliosides, in addition to sphingomyelins from brain and milk. The outer hyperfine splittings in the ESR spectra of sphingomyelin and of phosphatidylcholine spin-labeled on C-5 of the acyl chain were consistent with mixing of the sphingolipid and glycerolipid components, in fluid-phase membranes. In the gel phase of egg sphingomyelin and its mixtures with phosphatidylcholine, the outer hyperfine splittings of sphingomyelin spin-labeled at C-14 of the acyl chain of sphingomyelin are smaller than those of the corresponding sn-2 chain spin-labeled phosphatidylcholine. This is in contrast to the situation with sphingomyelin and phosphatidylcholine spin-labeled at C-5, for which the outer hyperfine splitting is always greater for the spin-labeled sphingomyelin. The behavior of the C-14 spin-labels is attributed to a different geometry of the acyl chain attachments of the sphingolipids and glycerolipids that is consistent with their respective crystal structures. The two-component ESR spectra of sphingomyelin and phosphatidylcholine spin-labeled at C-14 of the acyl chain directly demonstrate a broad two-phase region with coexisting gel and fluid domains in sphingolipid mixtures with phosphatidylcholine. Domain formation in membranes composed of sphingolipids and glycerolipids alone is related primarily to the higher chain-melting transition temperature of the sphingolipid component.

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.

Ceramides modulate protein kinase C activity and perturb the structure of Phosphatidylcholine/Phosphatidylserine bilayers

We studied the effects of natural ceramide and a series of ceramide analogs with different acyl chain lengths on the activity of rat brain protein kinase C (PKC) and on the structure of bovine liver phosphatidylcholine (BLPC)/dipalmitoylphosphatidylcholine (DPPC)/dipalmitoylphosphatidylserine (DPPS) (3:1:1 molar ratio) bilayers using (2)H-NMR and specific enzymatic assays in the absence or presence of 7.5 mol % diolein (DO). Only a slight activation of PKC was observed upon addition of the short-chain ceramide analogs (C(2)-, C(6)-, or C(8)-ceramide); natural ceramide or C(16)-ceramide had no effect. In the presence of 7.5 mol % DO, natural ceramide and C(16)-ceramide analog slightly attenuated DO-enhanced PKC activity. (2)H-NMR results demonstrated that natural ceramide and C(16)-ceramide induced lateral phase separation of gel-like and liquid crystalline domains in the bilayers; however, this type of membrane perturbation has no direct effect on PKC activity. The addition of both short-chain ceramide analogs and DO had a synergistic effect in activating PKC, with maximum activity observed with 20 mol % C(6)-ceramide and 15 mol % DO. Further increases in C(6)-ceramide and/or DO concentrations led to decreased PKC activity. A detailed (2)H-NMR investigation of the combined effects of C(6)-ceramide and DO on lipid bilayer structure showed a synergistic effect of these two reagents to increase membrane tendency to adopt nonbilayer structures, resulting in the actual presence of such structures in samples exceeding 20 mol % ceramide and 15 mol % DO. Thus, the increased tendency to form nonbilayer lipid phases correlates with increased PKC activity, whereas the actual presence of such phases reduced the activity of the enzyme. Moreover, the results show that short-chain ceramide analogs, widely used to study cellular effects of ceramide, have biological effects that are not exhibited by natural ceramide.

Influence of the physical state of the membrane on the enzymatic activity and energy of activation of protein kinase C alpha

The activation of protein kinase C alpha was studied by using a lipid system consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) (molar ratio 4:1) and different proportions of 1-palmitoyl-2-oleoyl-sn-glycerol (POG). The phase behavior of the lipidic system was characterized by using differential scanning calorimetry and 31P NMR, and a phase diagram was elaborated. The results suggested the formation of two diacylglycerol/phospholipid complexes, one at 15 mol % of POG and the second at 30 mol % of POG. These two complexes would define the three regions of the phase diagram: in the first region (concentrations of POG lower than 15 mol %) there is gel-gel immiscibility at temperatures below that of the phase transition between C1 and pure phospholipid, and a fluid lamellar phase above of the phase transition. In the second region (between 15 and 30 mol % of POG), gel-gel immiscibility between C1 and C2 with fluid-fluid immiscibility was observed, while inverted hexagonal HII and isotropic phases were detected by 31P NMR. In the third region (concentrations of POG higher than 30 mol %), gel-gel immiscibility seemed to occur between C2 and pure POG along with fluid-fluid immiscibility, while an isotropic phase was detected by 31P NMR. When PKC alpha activity was measured, as a function of POG concentration, maximum activity was found at POG concentrations as low as 5-10 mol %; the activity slightly decreased as POG concentration was increased to 45 mol % at 32 degrees C (above Tc) whereas activity did not change with increasing concentrations of POG at 5 degrees C (below Tc). When the activity was studied as a function of temperature, at different POG concentrations, and depicted as Arrhenius plots, it was found that the activity increased with increasing temperatures, showing a discontinuity at a temperature very close to the phase transition of the system and a lower activation energy at the upper slope of the graph, indicating that the physical state of the membrane affected the interaction of PKC alpha with the membrane.

Simultaneous X-ray diffraction and calorimetric study of metastable- to-stable solid phase transformation of 1,2-dipalmitoyl-sn-glycerol

The thermal behaviour and structural changes associated with the phase transformation of 1,2-dipalmitoyl-sn-glycerol (DPG) were studied by means of simultaneous X-ray diffraction and differential scanning calorimetry. Metastable DPG solid phases are crystallized from the melted sample by thermal quenching. The metastable phase (alpha-phase) formed initially is converted into a stable phase (beta' phase) at approximately 50 degrees C on heating. It was found that the behaviour of the alpha- to beta'-phase transformation depends on the thermal history. DPG solid samples incubated at approximately 3 degrees C for more than 10 h after cooling transformed directly into the beta'-phase with heat release. On the other hand, in the solid samples without incubation, the alpha-phase once melted and then the crystallization of the beta'-phase occurred successively from the melted state.

Modulation of polymorphic properties of dielaidoylphosphatidylethanolamine by the antineoplastic ether lipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine

The capacity of the antineoplastic ether lipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine (ET-18-OCH3) to modulate the polymorphic properties of dielaidoylphosphatidylethanolamine has been studied using biophysical techniques. Differential scanning calorimetry showed that ET-18-OCH3 depresses the onset of the Lbeta to Lalpha phase transition, decreasing also DeltaH of the transition. At the same time, the onset of the transition from Lalpha to inverted hexagonal HII phase was gradually increased as the ether lipid concentration was increased, totally disappearing at concentrations higher than 5 mol%. Small-angle X-ray diffraction and 31P-NMR confirmed that ET-18-OCH3 induced that the appearance of the inverted hexagonal HII phase was shifted towards higher temperatures completely disappearing at concentrations higher than 5 mol%. These results were used to elaborate a partial phase diagram and they were discussed as a function of the molecular action of ET-18-OCH3.

Effects of diacylglycerol on the structure and phase behaviour of non-bilayer forming phospholipid

The phase behaviour of mixed aqueous dispersions of the monomethyl derivative of dioleoylphosphatidylethanolamine and dipalmitoylglycerol has been characterised by X-ray diffraction, differential scanning calorimetry and freeze-fracture electron microscopy for mixtures containing dipalmitoylglycerol in the concentration range 0-20 mol%. Dispersions prepared at temperatures where the phospholipid exhibits a liquid-crystalline lamellar phase show that dipalmitoylglycerol is completely phase separated into aggregates of stable crystal phase (beta'-phase). Heating mixed dispersions results in transformation of lamellar into hexagonal-II structure commencing at approximately 45 degrees C. This temperature coincides with a disappearance of beta'-phase of DPG which becomes incorporated into hexagonal-II phase. The pure phospholipid is transformed upon cooling from hexagonal-II into characteristic cubic phases; the formation of cubic phase is prevented by the presence of dipalmitoylglycerol and mixed dispersions initially form a lamellar liquid-crystalline phase in which the lipids are phase separated. The X-ray and thermal data suggest that relatively small domains of metastable crystal phase (alpha-phase) of DPG form initially on cooling and these subsequently coalesce and transform to beta'-phase.

A comparative study of the activation of protein kinase C alpha by different diacylglycerol isomers

The lipid activation of protein kinase C alpha (PKC alpha) has been studied by comparing the activation capacity of different 1, 2-diacylglycerols and 1,3-diacylglycerols incorporated into mixed micelles or vesicles. Unsaturated 1,2-diacylglycerols were, in general, more potent activators than saturated ones when 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS)/Triton X-100 mixed micelles and pure POPS vesicles were used. In contrast, these differences were not observed when 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/POPS (4:1, molar ratio) vesicles were used. Diacylglycerols bearing short fatty acyl chains showed a very high activation capacity, however, the capacity was less in mixed micelles. Furthermore, 1, 2-diacylglycerols had a considerably higher activating capacity than 1,3-diacylglycerols in POPS/Triton X-100 mixed micelles and in POPC/POPS vesicles. However, the differences between the two types of diacylglycerols were smaller when pure POPS vesicles were used. Differential scanning calorimetry (DSC) showed that POPC/POPS membrane samples containing diacylglycerols had endothermic transitions in the presence of 200 microM Ca2+ and 5 mM Mg2+. Transitions were not detected when using pure POPS vesicles due to the formation of dehydrated phases as demonstrated by FTIR (Fourier-transform infrared) spectroscopy. PKC alpha binding studies, performed by differential centrifugation in the presence of 200 microM Ca2+ and 5 mM Mg2+, showed that 1,2-sn-dioleoylglycerol (1, 2-DOG) was more effective than 1,3-dioleoylglycerol (1,3-DOG) in promoting binding to POPC/POPS vesicles. However, when pure POPS vesicles were used, PKC alpha was able to bind to membranes containing either 1,2-DOG or 1,3-DOG to the same extent.

Correlation between protein kinase C alpha activity and membrane phase behavior

Lipid activation of protein kinase C alpha (PKC alpha) was studied by using a model mixture containing 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1, 2-dimyristoyl-sn-glycero-3-phosphoserine (DMPS), and 1, 2-dimyristoyl-sn-glycerol (1,2-DMG). This lipid mixture was physically characterized by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and 31P-nuclear magnetic resonance (31P-NMR). Based on these techniques, a phase diagram was constructed by keeping a constant DMPC/DMPS molar ratio of 4:1 and changing the concentration of 1,2-DMG. This phase diagram displayed three regions and two compounds: compound 1 (C1), with 45 mol% 1,2-DMG, and compound 2 (C2), with 60 mol% 1,2-DMG. When the phase diagram was elaborated in the presence of Ca2+ and Mg2+, at concentrations similar to those used in the PKC alpha activity assay, the boundaries between the regions changed slightly and C1 had 35 mol% 1,2-DMG. The activity of PKC alpha was studied at several temperatures and at different concentrations of 1,2-DMG, with a maximum of activity reached at 30 mol% 1,2-DMG and lower values at higher concentrations. In the presence of Ca2+ and Mg2+, maximum PKC alpha activity occurred at concentrations of 1,2-DMG that were close to the boundary in the phase diagram between region 1, where compound C1 and the pure phospholipid coexisted in the gel phase, and region 2, where compounds C1 and C2 coexisted. These results suggest that the membrane structure corresponding to a mixture of 1,2-DMG/phospholipid complex and free phospholipid is better able to support the activity of PKC alpha than the 1,2-DMG/phospholipid complex alone.

Ceramides in phospholipid membranes: effects on bilayer stability and transition to nonlamellar phases

The effects of ceramides of natural origin on the gel-fluid and lamellar-inverted hexagonal phase transitions of phospholipids (mainly dielaidoylphosphatidylethanolamine) have been studied by differential scanning calorimetry, with additional support from infrared and 31P nuclear magnetic resonance (NMR) spectroscopy. In the lamellar phase, ceramides do not mix ideally with phospholipids, giving rise to the coexistence of domains that undergo the gel-fluid transition at different temperatures. The combination of differential scanning calorimetry and infrared spectroscopy, together with the use of deuterated lipids, allows the demonstration of independent melting temperatures for phospholipid and ceramide in the mixtures. In the lamellar-hexagonal phase transitions, ceramides (up to 15 mol %) decrease the transition temperature, without significantly modifying the transition enthalpy, thus facilitating the inverted hexagonal phase formation. 31P-NMR indicates the coexistence, within a certain range of temperatures, of lamellar and hexagonal phases, or hexagonal phase precursors. Ceramides from egg or from bovine brain are very similar in their effects on the lamellar-hexagonal transition. They are also comparable to diacylglycerides in this respect, although ceramides are less potent. These results are relevant in the interpretation of certain forms of interfacial enzyme activation and in the regulation and dynamics of the bilayer structure of cell membranes.

Structure and functional properties of diacylglycerols in membranes

1. 1,2-Diacyl-sn-glycerols (DAG) are minor components of cell membranes (about 1 mole% of the lipids) and yet they are potent regulators of both the physical properties of the lipid bilayer and the catalytic behaviour of several membrane-related enzymes. 2. In the pure state DAG's present a considerable polymorphism, with several crystalline phases in addition to the neat fluid phase. The most stable crystalline phase is the so-called beta' phase, a monoclinic crystalline form with orthorhombic perpendicular subcell chain packing, in which both acyl chains lie parallel to each other in a hairpinlike configuration about the sn-1 and sn-2 glycerol carbon atoms. The molecules are organized in a bilayer, with the glycerol backbone roughly parallel to the plane of the bilayer, and the acyl chains tilted at approximately 60 degrees with respect to that plane. Acyl chain unsaturation, and particularly a single cis unsaturation, impairs chain packing in mixed-chain DAG's, and this results in an increased number of metastable crystalline phases. 3. DAG's mix with phospholipids in fluid bilayers when their melting temperature is below or close enough to the melting temperature of the bilayer system. When incorporated in phospholipid bilayers, the conformation of DAG is such that the glycerol backbone is nearly perpendicular to the bilayer, with the sn-1 chain extending from the glycerol Cl carbon into the hydrophobic matrix of the bilayer and the sn-2 chain first extending parallel to the bilayer surface, then making a 90 degrees bend at the position of the sn-1 carbonyl to become parallel to the sn-1 chain. DAG's are located in phospholipid bilayers about two CH2 units deeper than the adjacent phospholipids. DAG's mix nonideally with phospholipids, giving rise to in-plane separations of DAG-rich and -poor domains, even in the fluid state. DAG molecules also increase the separation between phospholipid headgroups, and decrease the hydration of the bilayer surface. Also, because the transversal section of the DAG headgroup is small when compared to that of the acyl chains, DAG favours the (negative) curvature of the lipid monolayers, and DAG-phospholipid mixtures tend to convert into inverted nonlamellar hexagonal or cubic phases. 4. A number of membrane enzyme activities are modulated (activated) by DAG, most notably protein kinase C, phospholipases and other enzymes of lipid metabolism. Protein kinase C activation (and perhaps that of other enzymes as well) occurs as the combined result of a number of DAG-induced modifications of lipid bilayers that include: changes in lipid headgroup conformation, interspacing and hydration, changes in the bilayer propensity to form inverted nonlamellar phases, and lateral phase separations of DAG-rich and -poor domains. Among the DAG-activated enzymes, phospholipases C show the peculiarity of yielding the activator DAG as their reaction product, and this allows the self-induced transition from a low- to a high-activity status. 5. DAG's induce or enhance membrane fusion in a number of ways, mainly through partial dehydration of the bilayer surface, increase in lipid monolayer curvature and perhaps lateral phase separation. DAG-increased fusion rates have been demonstrated in several instances of cation-induced fusion of model membranes, as well as in Ca(2+)-induced fusion of chromaffin granules with plasma membrane vesicles. Also phospholipase C has been shown to induce vesicle aggregation and fusion through the catalytic generation of DAG in the bilayers. A rather general property of DAG is that it promotes vesicular or interparticle aggregation. 6. In the living cell, DAG is often generated through phospholipid degradation in response to an extracellular agonist binding a specific receptor in the cell surface. DAG is said to act as an intracellular second messenger. (ABSTRACT TRUNCATED)

The phase behavior of aqueous dispersions of unsaturated mixtures of diacylglycerols and phospholipids

The phase behavior of mixtures of 1-palmitoyl-2-oleoyl-sn-glycerol (1,2-POG) with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) was studied by using DSC, small-angle X-ray diffraction and 31P-NMR. The results have been used to construct phase diagrams for both type of mixtures, in the 0-45 degreesC range. It is concluded that 1, 2-POG form complexes in the gel phases with both POPC and POPS. In the case of POPC, two complexes are postulated, the first one at a 1, 2-POG/POPC molar ratio of 40:60, and the second one at 70:30, defining three different regions in the phase diagram. Two eutectic points are proposed to occur: one at a very low 1,2-POG concentration and the other at a 1,2-POG concentration slightly lower than 70%. In the case of the 1,2-POG/POPS mixtures, the pattern was similar, but the first complex was seen to happen at a higher concentration, about 50 mol% of 1,2-POG, whereas the second was found at 80 mol% of 1,2-POG. This indicated a bigger presence of 1,2-POG in the complexes with POPS than with POPC. In the first region of the phase diagram, i.e. at concentrations of 1,2-POG lower than that required for the formation of the first complex, and at temperatures above the phase transition, lamellar phases were seen in all the cases. In region 2 of the phase diagram, i.e. at concentrations where the first and the second complexes coexist, a mixture of lamellar and non-lamellar phases was observed. Finally, at high concentrations of 1,2-POG, non-lamellar phases were detected as predominant, these phases being of an isotropic nature, according to 31P-NMR. An important conclusion of this study is that, using unsaturated lipids, similar to those found in biological membranes, it has been shown that diacylglycerols are found separated in domains, and that this process starts at very low concentrations of diacylglycerols. The formation of separated domains enriched in diacylglycerol is biologically relevant as it will allow them to have important effects on the membrane structure besides the fact that their concentration in the biomembrane is relatively low.

Cholesterol-induced variations in the volume and enthalpy fluctuations of lipid bilayers

The sound velocity and density of suspensions of large unilamellar liposomes from dimyristoylphosphatidylcholine with admixed cholesterol have been measured as a function of temperature around the chain melting temperature of the phospholipid. The cholesterol-to-phospholipid molar ratio xc has been varied over a wide range (0 </= xc </= 0.5). The temperature dependence of the sound velocity number, of the apparent specific partial volume of the phospholipid, and of the apparent specific adiabatic compressibility have been derived from the measured data. These data are particularly discussed with respect to the volume fluctuations within the samples. A theoretical relation between the compressibility and the excess heat capacity of the bilayer system has been derived. Comparison of the compressibilities (and sound velocity numbers) with heat capacity traces display the close correlation between these quantities for bilayer systems. This correlation appears to be very useful as it allows some of the mechanical properties of membrane systems to be calculated from the specific heat capacity data and vice versa.

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.

Effects of dipalmitoylglycerol and fatty acids on membrane structure and protein kinase C activity

The individual and combined effects of the saturated diacylglycerol (DAG) dipalmitin (DP) and saturated or polyunsaturated unesterified fatty acids (PUFAs) on both the structure of phosphatidylcholine/phosphatidylserine (PC/PS; 4:1 mol/mol) bilayers and on protein kinase C (PKC) activity were studied using 2H nuclear magnetic resonance (NMR) and enzyme activity assays. In the absence of DP, PUFAs only slightly activated PKC whereas palmitic acid had no effect. In the absence of fatty acids, DP induced lateral phase separation of the bilayer into liquid-crystalline and gel phases. Under these conditions virtually all DP was sequestered into the gel phase and no activation of PKC was observed. The addition of polyunsaturated arachidonic or docosahexaenoic acids to the DP-containing bilayers significantly increased the relative amounts of DP and other lipid components in the liquid-crystalline phase, correlating with a dramatic increase in PKC activity. Furthermore, the effect was greater with PS, resulting in an enrichment of PS in the liquid-crystalline domains. In the presence of DP, palmitic acid did not decrease the amount of gel phase lipid and had no effect on PKC activity. The results explain the observed lack of PKC-activating capacity of long-chain saturated DAGs as due to the sequestration of DAG into gel domains wherein it is complexed with phospholipids and thus not available for the required interaction with the enzyme.

Spin-label electron spin resonance studies on the interactions of lysine peptides with phospholipid membranes

The interactions of lysine oligopeptides with dimyristoyl phosphatidylglycerol (DMPG) bilayer membranes were studied using spin-labeled lipids and electron spin resonance spectroscopy. Tetralysine and pentalysine were chosen as models for the basic amino acid clusters found in a variety of cytoplasmic membrane-associating proteins, and polylysine was chosen as representative of highly basic peripherally bound proteins. A greater motional restriction of the lipid chains was found with increasing length of the peptide, while the saturation ratio of lipids per peptide was lower for the shorter peptides. In DMPG and dimyristoylphosphatidylserine host membranes, the perturbation of the lipid chain mobility by polylysine was greater for negatively charged spin-labeled lipids than for zwitterionic lipids, but for the shorter lysine peptides these differences were smaller. In mixed bilayers composed of DMPG and dimyristoylphosphatidylcholine, little difference was found in selectivity between spin-labeled phospholipid species on binding pentalysine. Surface binding of the basic lysine peptides strongly reduced the interfacial pK of spin-labeled fatty acid incorporated into the DMPG bilayers, to a greater extent for polylysine than for tetralysine or pentalysine at saturation. The results are consistent with a predominantly electrostatic interaction with the shorter lysine peptides, but with a closer surface association with the longer polylysine peptide.

Evidence for superlattice arrangements in fluid phosphatidylcholine/phosphatidylethanolamine bilayers

Recently, evidence for cholesterol and phosphatidylcholine (PC) molecules to adapt superlattice arrangements in fluid lipid bilayers has been presented. Whether superlattice arrangements exist in other biologically relevant lipid membranes, such as phosphatidylethanolamine (PE)/PC, is still speculative. In this study, we have examined the physical properties of fluid 1-palmitoyl-2-oleoyl-PC (POPC) and 1-palmitoyl-2-oleoyl-PE (POPE) binary mixtures as a function of the POPE mole fraction (X(PE)) using fluorescence and Fourier transform infrared spectroscopy. At 30 degrees C, i.e., above the Tm of POPE and POPC, deviations, or dips, as well as local data scattering in the excimer-to-monomer fluorescence intensity ratio of intramolecular excimer forming dipyrenylphosphatidylcholine probe in POPE/POPC mixtures were detected at X(PE) approximately 0.04, 0.11, 0.16, 0.26, 0.33, 0.51, 0.66, 0.75, 0.82, 0.91, and 0.94. The above critical values of X(PE) coincide (within +/-0.03) with the critical mole fractions X(HX,PE) or X(R,PE) predicted by a headgroup superlattice model, which assumes that the lipid headgroups form hexagonal or rectangular superlattice, respectively, in the bilayer. Other spectroscopic data, generalized polarization of Laurdan and infrared carbonyl and phosphate stretching frequency, were also collected. Similar agreements between some of the observed critical values of X(PE) from these data and the X(HX,PE) or X(R,PE) values were also found. However, all techniques yielded critical values of X(PE) (e.g., 0.42 and 0.58) that cannot be explained by the present headgroup superlattice model. The effective cross-sectional area of the PE headgroup is smaller than that of the acyl chains. Hence, the relief of "packing frustration" of PE in the presence of PC (larger headgroup than PE) may be one of the major mechanisms in driving the PE and PC components to superlattice-like lateral distributions in the bilayer. We propose that headgroup superlattices may play a significant role in the regulation of membrane lipid compositions in cells.