1-2-dielaidoylphosphatidylethanolamine and 1-2-dipalmitoyl-3-phosphatidylethanolamine

Additive force fields are designed to account for induced electronic polarization in a mean-field average way, using effective empirical fixed charges. The limitation of this approximation is cause for serious concerns, particularly in the case of lipid membranes, where the molecular environment undergoes dramatic variations over microscopic length scales. A polarizable force field based on the classical Drude oscillator offers a practical and computationally efficient framework for an improved representation of electrostatic interactions in molecular simulations. Building on the first-generation Drude polarizable force field for the dipalmitoylphosphatidylcholine 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) molecule, the present effort was undertaken to improve this initial model and expand the force field to a wider range of phospholipid molecules. New lipids parametrized include dimyristoylphosphatidylcholine (DMPC), dilauroylphosphatidylcholine (DLPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), dipalmitoylphosphatidylethanolamine (DPPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). The iterative optimization protocol employed in this effort led to lipid models that achieve a good balance between reproducing quantum mechanical data on model compound representative of phospholipids and reproducing a range of experimental condensed phase properties of bilayers. A parametrization strategy based on a restrained ensemble-maximum entropy methodology was used to help accurately match the experimental NMR order parameters in the polar headgroup region. All the parameters were developed to be compatible with the remainder of the Drude polarizable force field, which includes water, ions, proteins, DNA, and selected carbohydrates.

Topics: Diffusion; Lipid Bilayers; Molecular Dynamics Simulation; Phosphatidylcholines; Phosphatidylethanolamines; Phospholipids; Quantum Theory; Thermodynamics

We have investigated the effects of the model alpha-helical transmembrane peptide Ac-K(2)L(24)K(2)-amide (L(24)) on the thermotropic phase behavior of aqueous dispersions of 1,2-dielaidoylphosphatidylethanolamine (DEPE) to understand better the interactions between lipid bilayers and the membrane-spanning segments of integral membrane proteins. We studied in particular the effect of L(24) and three derivatives thereof on the liquid-crystalline lamellar (L(alpha))-reversed hexagonal (H(II)) phase transition of DEPE model membranes by differential scanning calorimetry and (31)P nuclear magnetic resonance spectroscopy. We found that the incorporation of L(24) progressively decreases the temperature, enthalpy, and cooperativity of the L(alpha)-H(II) phase transition, as well as induces the formation of an inverted cubic phase, indicating that this transmembrane peptide promotes the formation of inverted nonlamellar phases, despite the fact that the hydrophobic length of this peptide exceeds the hydrophobic thickness of the host lipid bilayer. These characteristic effects are not altered by truncation of the side chains of the terminal lysine residues or by replacing each of the leucine residues at the end of the polyleucine core of L(24) with a tryptophan residue. Thus, the characteristic effects of these transmembrane peptides on DEPE thermotropic phase behavior are independent of their detailed chemical structure. Importantly, significantly shortening the polyleucine core of L(24) results in a smaller decrease in the L(alpha)-H(II) phase transition temperature of the DEPE matrix into which it is incorporated, and reducing the thickness of the host phosphatidylethanolamine bilayer results in a larger reduction in the L(alpha)-H(II) phase transition temperature. These results are not those predicted by hydrophobic mismatch considerations or reported in previous studies of other transmembrane alpha-helical peptides containing a core of an alternating sequence of leucine and alanine residues. We thus conclude that the hydrophobicity and conformational flexibility of transmembrane peptides can affect their propensity to induce the formation of inverted nonlamellar phases by mechanisms not primarily dependent on lipid-peptide hydrophobic mismatch.

Topics: Calorimetry, Differential Scanning; Cell Membrane; Kinetics; Lipid Bilayers; Membrane Proteins; Models, Biological; Models, Chemical; Nuclear Magnetic Resonance, Biomolecular; Peptides; Phosphatidylethanolamines; Phosphorus Isotopes; Protein Structure, Secondary; Temperature; Thermodynamics

By means of x-ray diffraction we show that several sodium salts and the disaccharides sucrose and trehalose strongly accelerate the formation of cubic phases in phosphatidylethanolamine (PE) dispersions upon temperature cycling through the lamellar liquid crystalline-inverted hexagonal (Lalpha-HII) phase transition. Ethylene glycol does not have such an effect. The degree of acceleration increases with the solute concentration. Such an acceleration has been observed for dielaidoyl PE (DEPE), dihexadecyl PE, and dipalmitoyl PE. It was investigated in detail for DEPE dispersions. For DEPE (10 wt% of lipid) aqueous dispersions at 1 M solute concentration, 10-50 temperature cycles typically result in complete conversion of the Lalpha phase into cubic phase. Most efficient is temperature cycling executed by laser flash T-jumps. In that case the conversion completes within 10-15 cycles. However, the cubic phases produced by laser T-jumps are less ordered in comparison to the rather regular cubic structures produced by linear, uniform temperature cycling at 10 degrees C/min. Temperature cycles at scan rates of 1-3 degrees C/min also induce the rapid formation of cubic phases. All solutes used induce the formation of Im3m (Q229) cubic phase in 10 wt% DEPE dispersions. The initial Im3m phases appearing during the first temperature cycles have larger lattice parameters that relax to smaller values with continuation of the cycling after the disappearance of the Lalpha phase. A cooperative Im3m --> Pn3m transition takes place at approximately 85 degrees C and transforms the Im3m phase into a mixture of coexisting Pn3m (Q224) and Im3m phases. The Im3m/Pn3m lattice parameter ratio is 1. 28, as could be expected from a representation of the Im3m and Pn3m phases with the primitive and diamond infinite periodic minimal surfaces, respectively. At higher DEPE contents ( approximately 30 wt%), cubic phase formation is hindered after 20-30 temperature cycles. The conversion does not go through, but reaches a stage with coexisting Ia3d (Q230) and Lalpha phases. Upon heating, the Ia3d phase cooperatively transforms into a mixture of, presumably, Im3m and Pn3m phases at about the temperature of the Lalpha-HII transition. This transformation is readily reversible with the temperature. The lattice parameters of the DEPE cubic phases are temperature-insensitive in the Lalpha temperature range and decrease with the temperature in the range of the HII phase.

Topics: Crystallization; Kinetics; Liposomes; Phosphates; Phosphatidylethanolamines; Solutions; Structure-Activity Relationship; Thermodynamics; Thiocyanates; X-Ray Diffraction

The damaging effects of synchrotron-derived x rays on aqueous phospholipid dispersions have been evaluated. The effect of degree of lipid hydration, phospholipid chemical structure, mesophase identity, aqueous medium composition, and incident flux on the severity and progress of damage was quantified using time-resolved x-ray diffraction and chromatographic analysis of damage products. Electron spin resonance measurements of spin-trapped intermediates generated during irradiation suggest a free radical-mediated process. Surprisingly, radiation damage effects revealed by x-ray diffraction were imperceptible when the lamellar phases were prepared under water-stressed conditions, despite the fact that x-ray-induced chemical breakdown of the lipid occurred regardless of hydration level. Of the fully hydrated lipid systems studied, saturated diacyl-phosphatidylcholines were most sensitive to radiation damage compared to the ester- and ether-linked phosphatidylethanolamines and the ether-linked phosphatidylcholines. The inclusion of buffers or inorganic salts in the dispersing medium had only a minor effect in reducing damage development. A small inverse dose-rate effect was found when the x-ray beam intensity was changed 15-fold. These results contribute to our understanding of the mechanism of radiation damage, to our appreciation of the importance of monitoring both structure and composition when evaluating biomaterials radiation sensitivity, and to the development of strategies for eliminating or reducing the severity of damage due to an increasingly important source of x rays, synchrotron radiation. Because damage is shown to be free radical mediated, these results have an important bearing on age-related accumulation of free radicals in cells and how these might compromise membrane integrity, culminating in cell death.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Chromatography, Thin Layer; Dimyristoylphosphatidylcholine; Dose-Response Relationship, Drug; Electron Spin Resonance Spectroscopy; Free Radicals; Liposomes; Phosphatidylethanolamines; Spin Labels; Structure-Activity Relationship; Synchrotrons; X-Ray Diffraction; X-Rays

CTP:phosphocholine cytidylyltransferase present in rat liver cytosol was activated almost 30-fold when assayed in the presence of liposomes containing 60 mole % dioleoyl phosphatidylethanolamine (DOPE). During the assay, some of the DOPE was degraded to lysoPE and oleic acid. Whereas cytidylyltransferase activity was not affected when assayed in the presence of liposomes containing lysoPE, liposomes containing oleic acid activated the enzyme. Activation by oleic acid could be eliminated by the addition of fatty acid-free bovine serum albumin (BSA) to the assay. When cytidylyltransferase activity was measured in the presence of both BSA and liposomes containing DOPE, enzyme activity was increased almost 20-fold, as compared with assays performed in the absence of added lipid. The 1.5-fold difference in cytidylyltransferase activity observed when cytosol was assayed with DOPE containing liposomes in the absence or presence of BSA (30-fold stimulation vs 20-fold stimulation) cannot be explained by the loss of activation attributable to oleic acid alone. Activation of the enzyme in the presence of liposomes containing DOPE and oleic acid is several-fold greater than the sum of the activations caused by the individual compounds. These data suggest that PE and oleic acid act synergistically in activating the cytidylyltransferase.

Topics: Animals; Choline-Phosphate Cytidylyltransferase; Cytosol; Drug Synergism; Enzyme Activation; Lipids; Liver; Nucleotidyltransferases; Oleic Acid; Oleic Acids; Phosphatidylcholines; Phosphatidylethanolamines; Rats; Tissue Preservation

Dimethylsuberimidate was reacted with aqueous dispersions of dipalmitoylphosphatidylethanolamine, dimyristoylphosphatidylethanolamine, dilauroylphosphatidylethanolamine, and dielaidoylphosphatidylethanolamine at pH 10 and at pH 8. The amount of amidine dimer formation was about four times greater above the gel-to-fluid phase transition of each lipid than below the transition. The transition temperature of each phosphatidylethanolamine, measured by steady-state fluorescence anisotropy of cis-parinaric acid, was lower at pH 10 than at pH 8 or in water. The ability of dimethylsuberimidate to discriminate between phosphatidylethanolamines in the fluid and gel phases should allow use of this reagent to identify phosphatidylethanolamine species within the gel or fluid lipid phase.

Topics: Cross-Linking Reagents; Dimethyl Suberimidate; Fatty Acids, Unsaturated; Fluorescence Polarization; Fluorescent Dyes; Hydrogen-Ion Concentration; Imidoesters; Macromolecular Substances; Membrane Fluidity; Membrane Lipids; Phosphatidylethanolamines; Temperature; Water

Fourier transform infrared (FT-IR) spectroscopy and differential scanning calorimetry (DSC) have been used to elucidate the phase behavior of two binary lipid mixtures, acyl chain perdeuterated 1,2-dipalmitoylphosphatidylethanolamine (DPPE-d62)/1,2-dielaidoylphosphatidylcholine (DEPC) and acyl chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC-d62)/1,2-dimyristoylphosphatidylethanolamine (DMPE). The former shows gel state immiscibility over most of the composition range. The FT-IR data indicate that one of the solid phases is essentially pure DEPC, while the other solid phase contains both lipids. The DPPC-d62/DMPE pair are miscible over the entire composition range. The use of deuterated lipids as one component in the mixture permits the melting characteristics of each component to be separately determined in the FT-IR experiment. The FT-IR data are used to assign the endotherms observed in the DSC to particular molecular components. For the DPPE-d62/DEPC system, two endotherms are observed at compositions between 10 and 67 mol% DPPE-d62. The lower transition is assigned to the DEPC component, while the higher event contains contributions to the enthalpy from both lipids in the mixture. The midpoint of the DEPC melting occurs substantially below that for DPPE-d62. For the miscible pair, each of the lipids melt over approximately the same temperature range. The complementary and consistent nature of the information available from FT-IR and from DSC is demonstrated from the current work.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Calcium-Transporting ATPases; Calorimetry, Differential Scanning; Fourier Analysis; Phosphatidylethanolamines; Phospholipids