1-2-oleoylphosphatidylcholine and 1-2-linoleoylphosphatidylcholine

The formation and closure of aqueous pores in lipid bilayers is a key step in various biophysical processes. Large pores are well described by classical nucleation theory, but the free-energy landscape of small, biologically relevant pores has remained largely unexplored. The existence of small and metastable "prepores" was hypothesized decades ago from electroporation experiments, but resolving metastable prepores from theoretical models remained challenging. Using two complementary methods-atomistic simulations and self-consistent field theory of a minimal lipid model-we determine the parameters for which metastable prepores occur in lipid membranes. Both methods consistently suggest that pore metastability depends on the relative volume ratio between the lipid head group and lipid tails: lipids with a larger head-group volume fraction (or shorter saturated tails) form metastable prepores, whereas lipids with a smaller head-group volume fraction (or longer unsaturated tails) form unstable prepores.

Topics: Cell Membrane Permeability; Computer Simulation; Dimyristoylphosphatidylcholine; Hydrophobic and Hydrophilic Interactions; Lipid Bilayers; Membrane Lipids; Models, Biological; Models, Chemical; Models, Molecular; Phosphatidylcholines; Porosity; Stress, Physiological

We address the critically important ionization properties of histidine side chains of membrane proteins, when exposed directly to lipid acyl chains within lipid bilayer membranes. The problem is important for addressing general principles that may underlie membrane protein function. To this end, we have employed a favorable host peptide framework provided by GWALP23 (acetyl-GGALW(5)LALALALALALALW(19)LAGA-amide). We inserted His residues into position 12 or 14 of GWALP23 (replacing either Leu(12) or Leu(14)) and incorporated specific [(2)H]Ala labels within the helical core sequence. Solid-state (2)H NMR spectra report the folding and orientation of the core sequence, revealing marked differences in the histidine-containing transmembrane helix behavior between acidic and neutral pH conditions. At neutral pH, the GWALP23-H12 and GWALP23-H14 helices exhibit well defined tilted transmembrane orientations in dioleoylphosphatidylcholine (DOPC)and dilauroylphosphatidylcholine (DLPC) bilayer membranes. Under acidic conditions, when His(12) is protonated and charged, the GWALP23-H12 helix exhibits a major population that moves to the DOPC bilayer surface and a minor population that occupies multiple transmembrane states. The response to protonation of His(14) is an increase in helix tilt, but GWALP23-H14 remains in a transmembrane orientation. The results suggest pKa values of less than 3 for His(12) and about 3-5 for His(14) in DOPC membranes. In the thinner DLPC bilayers, with increased water access, the helices are less responsive to changes in pH. The combined results enable us to compare the ionization properties of lipid-exposed His, Lys, and Arg side chains in lipid bilayer membranes.

Topics: Histidine; Lipid Bilayers; Peptides; Phosphatidylcholines; Protein Structure, Secondary

In this work, we apply the recently developed constant pH molecular dynamics technique to study protonation equilibria of titratable side chains in the context of simple transmembrane (TM) helices and explore the effect of pH on their configurations in membrane bilayers. We observe that, despite a significant shift toward neutral states, considerable population of different side chains stay in the charged state that give rise to pKa values around 9.6 for Asp and Glu and 4.5 to 6 for His and Lys side chains, respectively. These charged states are highly stabilized by favorable interactions between head groups, water molecules, and the charged side chains that are facilitated by substantial changes in the configuration of the peptides. The pH dependent configurations and the measured pKa values are in good agreement with relatively recent solid state NMR measurements. Our results presented here demonstrate that all-atom constant pH molecular dynamics can be applied to membrane proteins and peptides to obtain reliable pKa values and pH dependent behavior for these systems.

Topics: Hydrogen-Ion Concentration; Lipid Bilayers; Molecular Dynamics Simulation; Peptides; Phosphatidylcholines; Protein Structure, Secondary; Protons; Water

In contrast to expectations that unsaturated fatty acids contribute to oxidative stress by providing a source of lipid peroxides, we demonstrated the protective effect of double bonds in lipids on oxidative damage to membrane proteins. Photodynamic inactivation of gramicidin channels was decreased in unsaturated lipid compared to saturated lipid bilayers. By estimating photosensitizer (boronated chlorine e6 amide) binding to the membrane with the current relaxation technique, the decrease in gramicidin photoinactivation was attributed to singlet oxygen scavenging by double bonds in lipids rather than to the reduction in photosensitizer binding. Gramicidin protection by unsaturated lipids was also observed upon induction of oxidative stress with tert-butyl hydroperoxide.

Topics: Adsorption; Fatty Acids, Unsaturated; Gramicidin; Lipid Bilayers; Membrane Potentials; Oxidants; Oxidation-Reduction; Oxidative Stress; Phosphatidylcholines; Singlet Oxygen; tert-Butylhydroperoxide

A significant modification to the additive all-atom CHARMM lipid force field (FF) is developed and applied to phospholipid bilayers with both choline and ethanolamine containing head groups and with both saturated and unsaturated aliphatic chains. Motivated by the current CHARMM lipid FF (C27 and C27r) systematically yielding values of the surface area per lipid that are smaller than experimental estimates and gel-like structures of bilayers well above the gel transition temperature, selected torsional, Lennard-Jones and partial atomic charge parameters were modified by targeting both quantum mechanical (QM) and experimental data. QM calculations ranging from high-level ab initio calculations on small molecules to semiempirical QM studies on a 1,2-dipalmitoyl-sn-phosphatidylcholine (DPPC) bilayer in combination with experimental thermodynamic data were used as target data for parameter optimization. These changes were tested with simulations of pure bilayers at high hydration of the following six lipids: DPPC, 1,2-dimyristoyl-sn-phosphatidylcholine (DMPC), 1,2-dilauroyl-sn-phosphatidylcholine (DLPC), 1-palmitoyl-2-oleoyl-sn-phosphatidylcholine (POPC), 1,2-dioleoyl-sn-phosphatidylcholine (DOPC), and 1-palmitoyl-2-oleoyl-sn-phosphatidylethanolamine (POPE); simulations of a low hydration DOPC bilayer were also performed. Agreement with experimental surface area is on average within 2%, and the density profiles agree well with neutron and X-ray diffraction experiments. NMR deuterium order parameters (S(CD)) are well predicted with the new FF, including proper splitting of the S(CD) for the aliphatic carbon adjacent to the carbonyl for DPPC, POPE, and POPC bilayers. The area compressibility modulus and frequency dependence of (13)C NMR relaxation rates of DPPC and the water distribution of low hydration DOPC bilayers also agree well with experiment. Accordingly, the presented lipid FF, referred to as C36, allows for molecular dynamics simulations to be run in the tensionless ensemble (NPT), and is anticipated to be of utility for simulations of pure lipid systems as well as heterogeneous systems including membrane proteins.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Dimyristoylphosphatidylcholine; Lipid Bilayers; Lipids; Molecular Dynamics Simulation; Phosphatidylcholines; Phosphatidylethanolamines; Quantum Theory; Thermodynamics; X-Ray Diffraction

We report a new type of gel-liquid phase segregation in giant unilamellar vesicles (GUVs) of mixed lipids. Coexisting patch- and stripe-shaped gel domains in GUV bilayers composed of DOPC/DPPC or DLPC/DPPC are observed by confocal fluorescence microscopy. The lipids in stripe domains are shown to be tilted according to the DiIC18 fluorescence intensity dependence on the excitation polarization. The patch domains are found to be mainly composed of DPPC-d62 according to the coherent anti-Stokes Raman scattering (CARS) images of DOPC/DPPC-d62 bilayers. When cooling GUVs from above the miscibility temperature, the patch domains start to appear between the chain melting and the pretransition temperature of DPPC. In GUVs containing a high molar percentage of DPPC, the stripe domains form below the pretransition temperature. Our observations suggest that the patch and stripe domains are in the Pbeta' and Lbeta' gel phases, respectively. According to the thermoelastic properties of GUVs described by Needham and Evans [(1988) Biochemistry 27, 8261-8269], the Pbeta' and Lbeta' phases are formed at relatively low and high membrane tensions, respectively. GUVs with high DPPC percentage have high membrane surface tension and thus mainly exhibit Lbeta' domains, while GUVs with low DPPC percentage have low membrane surface tension and form Pbeta' domains accordingly. Adding negatively charged lipid to the lipid mixtures or applying an osmotic pressure to GUVs using sucrose solutions releases the surface tension and leads to the disappearance of the Lbeta' gel phase. The relationship between the observed domains in free-standing GUV bilayers and those in supported bilayers is discussed.

Topics: Androstanes; Hot Temperature; Lipid Bilayers; Liposomes; Microscopy, Confocal; Microscopy, Fluorescence; Phosphatidylcholines; Sucrose; Surface Properties

The nanomechanical response of supported lipid bilayers has been studied by force spectroscopy with atomic force microscopy. We have experimentally proved that the amount of ions present in the measuring system has a strong effect on the force needed to puncture a 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer with an atomic force microscope tip, thus highlighting the role that monovalent cations (so far underestimated, e.g., Na(+)) play upon membrane stability. The increase in the yield threshold force has been related to the increase in lateral interactions (higher phospholipid-phospholipid interaction, decrease in area per lipid) promoted by ions bound into the membrane. The same tendency has also been observed for other phosphatidylcholine bilayers, namely, 2-dilauroyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, and 1,2-dioleoyl-sn-3-phosphocholine, and also for phosphatidylethanolamine bilayers such as 1-palmitoyl-2-oleoyl-sn-3-phosphoethanolamine. Finally, this effect has been also tested on a natural lipid bilayer (Escherichia coli lipid extract), showing the same overall tendency. The kinetics of the process has also been studied, together with the role of water upon membrane stability and its effect on membrane nanomechanics. Finally, the effect of the chemical structure of the phospholipid molecule on the nanomechanical response of the membrane has also been discussed.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Biophysical Phenomena; Biophysics; Dimyristoylphosphatidylcholine; Dose-Response Relationship, Drug; Escherichia coli; Ethanolamines; Ions; Kinetics; Lipid Bilayers; Lipids; Microscopy, Atomic Force; Nanotechnology; Phosphatidylcholines; Phosphatidylethanolamines; Phospholipids; Phosphorylcholine; Silicon Compounds; Sodium; Sodium Chloride; Spectrophotometry

Small-angle neutron scattering on extruded unilamellar vesicles in water was used to study bilayer thickness when cholesterol (CHOL) was added to dilauroylphosphatidylcholine (DLPC) and dioleoylphosphatidylcholine (DOPC) bilayers in molar fraction 0.44. Using the H2O/2H2O contrast variation and the small-angle form of Kratky-Porod approximation, the bilayer gyration radius at infinite contrast R(g,infinity) and the bilayer thickness parameter d(g,infinity) = 12(0.5)R(g,infinity) were obtained at 25 degrees C. Addition of CHOL to DLPC increased the d(g,infinity) from 4.058 +/- 0.028 nm to 4.62 +/- 0.114 nm, while in case of DOPC the d(g,infinity) values were the same in the absence (4.618 +/- 0.148 nm) and in the presence (4.577 +/- 0.144 nm) of CHOL within experimental errors. The role of CHOL-induced changes of bilayer thickness in the protein insertion, orientation and function in membranes is discussed.

Topics: Cholesterol; Lipid Bilayers; Liposomes; Macromolecular Substances; Membrane Fluidity; Molecular Conformation; Neutron Diffraction; Phase Transition; Phosphatidylcholines

In organic solvents gramicidin A (gA) occurs as a mixture of slowly interconverting double-stranded dimers. Membrane-spanning gA channels, in contrast, are almost exclusively single-stranded beta(6,3)-helical dimers. Based on spectroscopic evidence, it has previously been concluded that the conformational preference of gA in phospholipid bilayers varies as a function of the degree of unsaturation of the acyl chains. Double-stranded pi pi(5,6)-helical dimers predominate (over single-stranded beta(6,3)-helical dimers) in lipid bilayer membranes with polyunsaturated acyl chains. We therefore examined the characteristics of channels formed by gA in 1-palmitoyl-2-oleoylphosphatidylcholine/n-decane, 1,2-dioleoylphosphatidylcholine/n-decane, and 1,2-dilinoleoylphosphatidylcholine/n-decane bilayers. We did not observe long-lived channels that could be conducting double-stranded pi pi(5,6)-helical dimers in any of these different membrane environments. We conclude that the single-stranded beta(6,3)-helical dimer is the only conducting species in these bilayers. Somewhat surprisingly, the average channel duration and channel-forming potency of gA are increased in dilinoleoylphosphatidylcholine/n-decane bilayers compared to 1-palmitoyl-2-oleoylphosphatidylcholine/n-decane and dioleoylphosphatidylcholine/n-decane bilayers. To test for specific interactions between the aromatic side chains of gA and the acyl chains of the bilayer, we examined the properties of channels formed by gramicidin analogues in which the four tryptophan residues were replaced with naphthylalanine (gN), tyrosine (gT), and phenylalanine (gM). The results show that all of these analogue channels experience the same relative stabilization when going from dioleoylphosphatidylcholine to dilinoleoylphosphatidylcholine bilayers.

Topics: Alkanes; Amino Acid Sequence; Fatty Acids, Unsaturated; Gramicidin; Ion Channels; Lipid Bilayers; Models, Biological; Models, Structural; Molecular Sequence Data; Phosphatidylcholines; Protein Structure, Secondary; Structure-Activity Relationship

Multilamellar lipid vesicles (MLV) composed of egg yolk lecithin (EYL) suppressed the response of bovine peripheral blood lymphocytes (BPBL) to phytohemagglutinin (PHA). EYL contains 18:1 and 18:2 as the major unsaturated phospholipids. Dioleoyllecithin (DOL; cis 9) MLV did not suppress BPBL blastogenesis. Dilinoeyllecithin (DLL; cis, cis 9, 12) MLV suppressed BPBL blastogenesis. The suppressive effect could be reversed by increasing the MLV DML concentration. The addition of alpha-tocopherol (alpha-T) at 10 mole% into MLV containing DLL reversed blastogenic suppression of BPBL. MLV composed of mixed saturated phospholipids (dimyristoyllecithin and dipalmitoyllecithin) and alpha-T enhanced the BPBL blastogenic response to PHA. BPBL incubated with varying PHA concentrations (11.82-375 mg/ml) and a constant concentration (2 mumoles/ml) of MLV composed of EYL remained suppressed either when PHA and MLV were added simultaneously or when MLV were incubated for 1 hr prior to the addition of PHA. This suggests that alpha-T may act as an immunomodulator in the blastogenic response to PHA. Results suggest that alpha-T reversion of EYL suppression of BPBL blastogenesis may be due to interactions of alpha-T with unsaturated acyl chains in EYL phospholipids.

Topics: Animals; Cattle; Dimyristoylphosphatidylcholine; Dose-Response Relationship, Drug; Liposomes; Lymphocyte Activation; Lymphocytes; Phosphatidylcholines; Phospholipids; Phytohemagglutinins; Pulmonary Surfactants; Vitamin E

Topics: Chemical Phenomena; Chemistry; Circular Dichroism; Hydrogen-Ion Concentration; Liposomes; Microscopy, Electron; Models, Molecular; Models, Structural; Molecular Conformation; Peptides; Phosphatidylcholines; Phosphatidylserines; Polylysine; Pulmonary Surfactants; Spectrophotometry; Temperature