Interaction of N-myristoyldimyristoylphosphatidylethanolamine with dimyristoylphosphatidylcholine investigated by differential scanning calorimetry: binary phase diagram

Interaction of N-acyltaurines with phosphatidylcholines

N-acyltaurines (NATs) are biologically active amphiphilic lipids. They come under the group of compounds known as N-acyl amino acids. NATs were first detected in the brain and other tissues in mice lacking the enzyme fatty acid amide hydrolase FAAH (-/-). N-arachidonoyltaurine (20:4 NAT) acts as an excellent ligand for the subset of transient receptor potential (TRP) channels, especially vanilloid type channels TRPV1 and TRPV4. Also, hydrophobic and hydrophilic regions of NATs enable them to interact with membrane lipids. Here, we have investigated the interaction of NATs, N-myristoyltaurine (NMT), and N-palmitoyltaurine (NPT) with their corresponding diacyl phosphatidylcholines (PCs), dimyristoylphosphatidylcholine (DMPC), and dipalmitoylphosphatidylchoine (DPPC). The miscibility and phase behavior of the hydrated binary mixtures have been investigated by differential scanning calorimetry (DSC). Studies on the interaction of NMT/NPT with DMPC/DPPC revealed that the two amphiphiles mix well up to 50 mol% of NAT and phase separation is observed at higher contents of the NAT. The phase transition of the equimolar mixtures of NAT:PC (50:50) studied by fluorescence, also supported the DSC results. PXRD and FTIR analysis show that the NAT:PC equimolar mixture (50:50) forms different supramolecular structures when compared to that of individual NATs and PCs. From transmission electron microscopic studies it is observed that the equimolar mixtures of NMT and NPT with their corresponding diacylphosphatidylcholines (50:50, mol/mol) forms unilamellar vesicles whose diameter range between 30 and 50 nm.

Analytical approaches to study domain formation in biomimetic membranes

Novel characterization methods open new horizons in the study of membrane mixtures.

Synthesis, characterization and thermotropic phase behavior of a homologous series of N-acyl-L-alaninols and interaction of N-myristoyl L-alaninol with dimyristoylphosphatidylcholine

N-acylethanolamines (NAEs) and their precursors, N-acylphosphatidylethanolamines are present in the cell membranes of a variety of species and exhibit interesting biological properties. N-acyl-L-alaninols (NAAOHs) are chiral homologues of NAEs and reduced forms of N-acyl-L-alanines (NAAs) and were reported to induce apoptosis in human lymphocytes. In the present study, we have synthesized and characterized a homologous series of N-acyl-L-alaninols (n=9-20). In DSC studies in the dry as well as hydrated states NAAOHs with different chain lengths showed single sharp transitions similar to N-acyl-L-alanines. Transition enthalpies (ΔHt) and entropies (ΔSt) of NAAOHs are linearly dependent on the acyl chain length in both dry and hydrated states. Powder X-ray diffraction studies showed that the d-spacings of NAAOHs exhibit linear dependence on the chain length and the incremental increase in the d values suggest that they may be packed in a tilted bilayer pattern. Studies on the interaction of N-myristoyl L-alaninol (NMAOH) with DMPC revealed that the two amphiphiles mix well up to 45 mol% of NMAOH, whereas phase separation is observed at higher contents of the alaninol. Transmission electron microscopic studies show that the NMAOH:DMPC (45:55, mol/mol) mixture forms unilamellar vesicles of about 120-150 nm in diameter.

Model cell membranes: discerning lipid and protein contributions in shaping the cell

The high complexity of biological membranes has motivated the development and application of a wide range of model membrane systems to study biochemical and biophysical aspects of membranes in situ under well defined conditions. The aim is to provide fundamental understanding of processes controlled by membrane structure, permeability and curvature as well as membrane proteins by using a wide range of biochemical, biophysical and microscopic techniques. This review gives an overview of some currently used model biomembrane systems. We will also discuss some key membrane protein properties that are relevant for protein-membrane interactions in terms of protein structure and how it is affected by membrane composition, phase behavior and curvature.

Miscibility and phase behavior of N-acylethanolamine/diacylphosphatidylethanolamine binary mixtures of matched acyl chainlengths (N=14, 16)

The miscibility and phase behavior of hydrated binary mixtures of two N-acylethanolamines (NAEs), N-myristoylethanolamine (NMEA), and N-palmitoylethanolamine (NPEA), with the corresponding diacyl phosphatidylethanolamines (PEs), dimyristoylphosphatidylethanolamine (DMPE), and dipalmitoylphosphatidylethanolamine (DPPE), respectively, have been investigated by differential scanning calorimetry (DSC), spin-label electron spin resonance (ESR), and (31)P-NMR spectroscopy. Temperature-composition phase diagrams for both NMEA/DMPE and NPEA/DPPE binary systems were established from high sensitivity DSC. The structures of the phases involved were determined by (31)P-NMR spectroscopy. For both systems, complete miscibility in the fluid and gel phases is indicated by DSC and ESR, up to 35 mol % of NMEA in DMPE and 40 mol % of NPEA in DPPE. At higher contents of the NAEs, extensive solid-fluid phase separation and solid-solid immiscibility occur depending on the temperature. Characterization of the structures of the mixtures formed with (31)P-NMR spectroscopy shows that up to 75 mol % of NAE, both DMPE and DPPE form lamellar structures in the gel phase as well as up to at least 65 degrees C in the fluid phase. ESR spectra of phosphatidylcholine spin labeled at the C-5 position in the sn-2 acyl chain present at a probe concentration of 1 mol % exhibit strong spin-spin broadening in the low-temperature region for both systems, suggesting that the acyl chains pack very tightly and exclude the spin label. However, spectra recorded in the fluid phase do not exhibit any spin-spin broadening and indicate complete miscibility of the two components. The miscibility of NAE and diacyl PE of matched chainlengths is significantly less than that found earlier for NPEA and dipalmitoylphosphatidylcholine, an observation that is consistent with the notion that the NAEs are most likely stored as their precursor lipids (N-acyl PEs) and are generated only when the system is subjected to membrane stress.

Miscibility and phase behaviour of binary mixtures of N-palmitoylethanolamine and dipalmitoylphosphatidylcholine

The content of N-acylethanolamines (NAEs) increases dramatically in cell membranes when the parent organism is subjected to injury or stress. This increase has been attributed to stress-combating mechanisms of the organism. In this study, a binary phase diagram of hydrated mixtures of N-palmitoylethanolamine (NP-E)--an endogenous ligand for the peripheral cannabinoid receptor (CB-2)--with dipalmitoylphosphatidylcholine (DPPC) is established by high-sensitivity differential scanning calorimetry (DSC). The structures of the phases involved were determined by using 31P-NMR spectroscopy and low-angle X-ray scattering. DSC studies show that NP-E and DPPC mix well in the composition range DPPC/NP-E=100:0 to 40:60 (mol/mol). At higher contents of NP-E, phase separation is indicated by the presence of additional transitions in the thermograms. Characterization of the structures formed by the mixtures with 31P-NMR shows that, up to 80 mol% NP-E, DPPC remains in the lamellar phase. The low-angle X-ray diffraction data are also consistent with a lamellar gel-phase structure for DPPC/NP-E mixtures up to 60 mol% NP-E. Above 70 mol% NP-E, NP-E phase separates in the gel-phase region, while complete miscibility is observed in the fluid phase. These results provide a structural basis for understanding the membrane interactions of NAEs, which is necessary for understanding the mechanism of their putative stress-combating role in the parent organisms.

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.

N-Myristoylated Phosphatidylethanolamine: Interfacial Behavior and Interaction with Cholesterol

The interfacial packing behavior of N-myristoyldimyristoylphosphatidylethanolamine (N-14:0 DMPE) and its interaction with cholesterol were characterized and compared to the behavior of dimyristoylphosphatidylethanolamine (DMPE) using an automated Langmuir type film balance. Surface pressure and surface potential were monitored as a function of lipid cross-sectional molecular area. N-14:0 DMPE exhibited two-dimensional (2D) phase transitions of a liquid-expanded to condensed nature at many temperatures in the 15-30 °C range, but isotherms showed only condensed behavior at 15 °C. The sharp decline in the surface compressional moduli upon entering the 2D-transition region is consistent with differences in the partial molar areas of coexisting liquid-expanded (chain-disordered) and condensed (chain-ordered) phases. Including Ca(2+) in the subphase beneath the negatively charged N-14:0 DMPE caused a downward shift in the 2D-transition onset pressure even in the presence of 100 mM NaCl. The average dipole moments perpendicular to the lipid-water interface for N-14:0 DMPE's liquid-expanded and condensed phases were higher than those of DMPE. At surface pressures sufficiently low (<10 mN/m) to produce liquid-expanded phase behavior in pure N-14:0 DMPE, mixing with cholesterol resulted in a classic "condensing effect". Maximal area condensation was observed near equimolar N-14:0 DMPE/cholesterol. Insights into mixing behavior at high surface pressures that mimic the lipid cross-sectional areas of biomembranes were provided by analyzing the surface compressional moduli as a function of cholesterol mole fraction. Complex mixing patterns were observed that deviated significantly from theoretical ideal mixing behavior suggesting the presence of lipid "complexes" and/or a liquid-ordered phase at high sterol mole fractions (>0.35) and low to intermediate surface pressures (<20 mN/m) as well as the possible coexistence of relatively immiscible solid phases at higher surface pressures (e.g., 35 mN/m).