ESR investigation on the phase transitions of DPPC vesicles in presence of high concentrations of Li+, Na+, K+ and Cs+

The partition of the spin label TEMPO in the hydrophobic region of di-palmitoyl-phosphatidylcholine unilamellar vesicles has been used to investigate the influence of high concentration (up 3M) of Li⁺, Na⁺, K⁺, and Cs⁺ on the phase transitions at 20–60°C. All of the above salts increase the permeation of TEMPO. The efficiency of monovalent cations in inducing the partition of the spin label in the hydrophobic environment of the bilayer increases in the order: Cs⁺+++. The disappearing of the pretransition and the downward shift of the main phase transition temperature from 37°C to 33.5°C is related to the increased permeation of TEMPO into the bilayer. The presence of salts in the bulk solution disturbs the hydration of the zwitterionic polar head of the DPPC molecules and changes the electrical interaction between the polar groups of the bilayer. This reduces the packing density of the lipid molecules and promotes the permeation of TEMPO.     

Formation of ring calcium oxalate patterns induced by domains in DPPC Langmuir-Blodgett films

The ring patterns of calcium oxalate crystals were induced by domains in Langmuir-Blodgett （LB） films of dipalmitoylpho- sphatidylcboline （DPPC）. The result was explained by the defects at the ring boundaries of liquid condensed （LC） and liquid expanded （LE） phases of LB film. These boundaries could provide less free energy and much more nucleating sites for COM crystals.     

Effects of nonionic sugar surfactants on the phase transition of DPPC membranes

The influence of newly synthesized N-alkanoyl-N-methyllactitolamines (decanoyl [C₁₀MELA], lauoroyl [C₁₂MELA] and miristoyl [C₁₄MELA]) on the thermotropic phase transition of phosphatidylcholine bilayer was compared with common sugar-based surfactants N-dodecyl-β-d-glucopyranoside [C12G1] and decanoyl-N-methyl glucamide [MEGA-10]. The results indicate that C_nMELA are very active at the membrane surface and disturb the phospholipid bilayer structure less than commercially used MEGA-10 and C12G1.     

The perturbing effect of cholesterol on the interaction between labdanes and DPPC bilayers

Differential scanning calorimetry (DSC) was used to study the effect of cholesterol on the perturbation of DPPC bilayers induced by eight bioactive structurally related labdanes isolated from the resin ‘ladano’ of Cistus creticus subsp. creticus (Cistaceae) or semisynthesized from the mother compounds. Labdanes themselves induced profound modifications in DPPC bilayer organization and thermotropic properties that were altered when cholesterol was incorporated in equimolar amounts to the labdanes. The present work shows that, up to 10 mol% of the equimolar mixture of cholesterol and the labdanes, the modifications evoked on DPPC bilayer organization are in accordance to these induced by the labdanes themselves. When the concentration exceeded 20 mol%, cholesterol influence dominated while the effect of the labdanes was suppressed and their interaction with the bilayer was probably prevented. The degree by which cholesterol modulated the labdane interaction with the bilayer depended on their structural characteristics that determine their localization in the bilayer interior. Polar groups that force the labdanes to localize themselves at the interfacial region broadened the concentration range by which labdanes interacted with the DPPC bilayer even in the presence of high concentration of cholesterol where cholesterol-rich domains are preferentially formed. On the other hand, labdanes possessing functional groups that promote their deeper penetration in the bilayer interior compete with cholesterol in a high extent for the same localization sites resulting in their possible elimination from the bilayer when the concentration of cholesterol present exceeds the 20 mol%.     

Sum frequency generation spectroscopic study of the condensation effect of cholesterol on a lipid monolayer

Sum frequency generation (SFG) spectra and surface pressure–molecular area (π–A) isotherms have been obtained for mixed cholesterol–DPPC monolayers with cholesterol mole fractions, x(chol.), from 0 to 1.0, at the air–water interface, under same conditions, at 22 °C. Analysis of the spectra indicated that incorporation of cholesterol into the monolayers at 3 mN m⁻¹ greatly increases the conformational and orientational order of the alkyl chains of DPPC, maximizing these properties at x(chol.)=0.4. Analysis also indicated that order in the mixed monolayers at 15 and 35 mN m⁻¹ is not affected by incorporation of cholesterol. The π–A isotherms measured at 3 mN m⁻¹ for the mixed monolayer with x(chol.)=0.4 have the largest negative deviation of the molecular area relative to those of ideal mixtures (the so-called “condensation effect” of cholesterol), indicating the most thermodynamically stable state. Comparison of results from SFG spectra and π–A isotherms explicitly proved that the condensation effect can be interpreted in terms of conformational and orientational ordering of the alkyl chains of DPPC.     

Langmuir monolayer properties of the fluorinated-hydrogenated hybrid amphiphiles with dipalmitoylphosphatidylcholine (DPPC)

Surface pressure–area (π–A), surface potential–area (ΔV–A), and dipole moment–area (μ–A) isotherms were obtained for the Langmuir monolayer of two fluorinated-hydrogenated hybrid amphiphiles (sodium phenyl 1-[(4-perfluorohexyl)-phenyl]-1-hexylphosphate (F6PH5PPhNa) and (sodium phenyl 1-[(4-perfluorooctyl)-phenyl]-1-hexylphosphate (F8PH5PPhNa)), DPPC and their two-component systems at the air/water interface. Monolayers spread on 0.02 M Tris buffer solution (pH 7.4) with 0.13 M NaCl at 298.2 K were investigated by the Wilhelmy method, ionizing electrode method and fluorescence microscopy. Moreover, the miscibility of two components was examined by plotting the variation of the molecular area and the surface potential as a function of the molar fraction for the fluorinated-hydrogenated hybrid amphiphiles on the basis of the additivity rule. The miscibility of the monlayers was also examined by construction of two-dimensional phase diagrams. Furthermore, assuming the regular surface mixture, the Joos equation for analysis of the collapse pressure of two-component monolayers allowed calculation of the interaction parameter (ξ) and the interaction energy (−Δ) between the fluorinated-hydrogenated hybrid amphiphiles and DPPC. The observations by a fluorescence microscopy also supported our interpretation as for the miscibility in the monolayer state. Comparing the monolayer behavior between the two binary systems, no remarkable difference was found among various aspects. Among the two combinations, the mole fraction dependence in monlayer properties was commonly classified into two ranges: 0 ≤ X ≤ 0.3 and 0.3 < X ≤ 1. Dependence of the chain length of fluorinated part was reflected for the molecular packing and surface potential.     

Molecular dynamics simulation of hydrated DPPC monolayers using charge equilibration force fields

We present results of molecular dynamics simulations of a model DPPC-water monolayer using charge equilibration (CHEQ) force fields, which explicitly account for electronic polarization in a classical treatment of intermolecular interactions. The surface pressure, determined as the difference between the monolayer and pure water surface tensions at 323 K, is predicted to be 22.92 ±1.29 dyne/cm, just slightly below the broad range of experimental values reported for this system. The surface tension for the DPPC-water monolayer is predicted to be 42.35 ±1.16 dyne/cm, in close agreement with the experimentally determined value of 40.9 dyne/cm. This surface tension is also consistent with the value obtained from DPPC monolayer simulations using state-of-the-art nonpolarizable force fields. The current results of simulations predict a monolayer-water potential difference relative to the pure water-air interface of 0.64 ±0.02 Volts, an improved prediction compared to the fixed-charge CHARMM27 force field, yet still overestimating the experimental range of 0.3 to 0.45 Volts. As the charge equilibration model is a purely charge-based model for polarization, the current results suggest that explicitly modeled polarization effects can offer improvements in describing interfacial electrostatics in such systems.     

Interactions of cationic surfactantswith DPPC

The effects of different cationic surfactants (n-undecylammonum chloride, UDACl and dodecyldimethyl (dodecyloxymethyl) ammonium chloride, DDMDDACl) on fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles have been studied. In the studied systems the molar ratio (x) of DPPC/surfactant ranged between x=0.0164–0.82 and from x=0.0352–1.76 for DPPC/DDMDDACl and DPPC/UDACl, respectively. For both systems, the enthalpy associated with the phase transition significantly decreases even at the lowest surfactant concentration. Also the main phase transition temperature is shifted towards lower temperatures. The structural parameters of the phases have been characterised by small angle X-ray scattering (SAXS). The SAXS results have proved that UDACl at x=0.0352 molar ratio significantly influences the DPPC lamellar structure, while its total disappearance was observed for x=0.176. The presence of DDMDDACl causes a total disappearance of the DPPC lamellar structure already at the lowest molar ratio (x=0.0352). Each surfactant in the system with DDPPC leads to a mixed micellar phase formation.     

缺陷LB膜诱导特异形貌草酸钙堆积图形

A special pattern of calcium oxalate monohydrate (COM) crystals was induced by DPPC Langmuir-Blodgett films with defects. The pattern was that a large hexagonal COM crystal located in the middle of a ring-shaped pattern of needle-liked crystals. The number of such special patterns with a diameter between 10 μm and 40 μm accounted for about 5% of the whole crystal patterns. It is due to the “islands” composed of aggregated film-forming molecules in the middle of the liquid-condensed domains. The effects of the concentration of potassium oxalate and the growth time on the crystal patterns were studied. When the concentration of potassium oxalate(c_K2C2O4) was 0.6 mmol·L^-1, the needle-liked COM crystals on the rings grew towards inside part of the patterns as the induced-time increased from 1 h to 4 h. After 2 h, the solid circle patterns were formed. On the other hand, the size of the hexagonal COM crystal in the middle of the pattern increased. The size of the COM crystals on the rings were about 3 μm × 2 μm, and that of three-dimensional COM crystals in the middle of the patterns were about 16 μm × 7 μm × 6 μm after 4 h. However, when c_K2C2O4 was increased to 5.0 mmol·L^-1, there was only a large COM crystal grown in the middle of the circular COM crystallites no matter 1 h or 4 h of crystal growth, and the ratio of these special COM patterns increased. It can be explained that the interaction between oxalate and DPPC molecules is enhanced as the increase of c_K2C2O4, so more and more DPPC molecules fled away from the substrate surface, resulting in decrease of the nucleating sites in the circle.     

Solubilization of Homologous ω-Phenylalkanols into Gel and Liquid-Crystalline Liposome Membranes of Dipalmitoylphosphatidylcholine

The gel-to-liquid-crystalline phase transition temperature T _m of dipalmitoylphosphatidylcholine (DPPC) liposome membrane was measured in the presence of homologous -phenylalkanols (phenol to 8-phenyl-1-octanol). The decrease in T _m induced by the alkanols allowed us, by applying the van't Hoff equation for freezing-point depression, to estimate two partition coefficients of each alkanol: gel membrane/bulk water K _x ^g and liquid-crystalline membrane/bulk water K _x ¹ . Shorter alkyl chain alkanols were solubilized only in the liquid-crystalline membrane, i.e., K _x ^g =0, whereas longer-chain alkanols were solubilized not only in the liquid-crystalline membrane but also in the gel membrane. The former result suggests that the fraction of liquid-crystalline phase in the liposome membrane is 0.83 at T _m. From the latter result, the values of the free energy changes of transfer of the alkanol molecules from bulk water to liposome membrane were estimated to be – 3.46 kJ-mol^–1 (liquid-crystalline membrane) and – 3.85 kJ-mol^–1 (gel membrane) per CH₂ group in the alkanol molecules.