Effect of chain length and asymmetry on material properties of bilayer membranes

Dissipative particle dynamics is used to extract the material parameters (bending and area stretch moduli) of a bilayer membrane patch. Some experiments indicate that the area stretch modulus of lipid vesicles varies little as the chain length of the lipids composing the bilayer increases. Here we show that making the interactions between the hydrophilic head groups of the model amphiphiles proportional to the hydrophobic tail length reproduces the above result for the area stretch modulus. We also show that the area stretch modulus of bilayers composed of amphiphiles with the same number of tail beads but with asymmetric chains is less than that of bilayers with symmetric chains. The effects on the bilayer density and lateral stress profiles of changes to the amphiphile architecture are also presented.     

Hydrostatic pressure reveals bilayer phase behavior of dioctadecyldimethylammonium bromide and chloride

Bilayer phase transitions of dioctadecyldimethylammonium bromide (2C(18)Br) and chloride (2C(18)Cl) were observed by differential scanning calorimetry and high-pressure light-transmittance measurements. The 2C(18)Br bilayer membrane showed different kinds of transitions depending on preparation methods of samples under atmospheric pressure. Under certain conditions, the 2C(18)Br bilayer underwent three kinds of transitions, the metastable transition from the metastable lamellar crystal (L(c(2))) phase to the metastable lamellar gel (L(β)) phase at 35.4 °C, the metastable main transition from the metastable L(β) phase to the metastable liquid crystalline (L(α)) phase at 44.5 °C, and the stable transition from the stable lamellar crystal (L(c(1))) phase to the stable L(α) phase at 52.8 °C. On the contrary, the 2C(18)Cl bilayer underwent two kinds of transitions, the stable transition from the stable L(c) phase to the stable L(β) phase at 19.7 °C and the stable main transition from the stable L(β) phase to the stable L(α) phase at 39.9 °C. The temperatures of the phase transitions of the 2C(18)Br and 2C(18)Cl bilayers were almost linearly elevated by applying pressure. It was found from the temperature (T)-pressure (p) phase diagram of the 2C(18)Br bilayer that the T-p curves for the main transition and the L(c(1))/L(α) transition intersect at ca. 130 MPa because of the larger slope of the former transition curve. On the other hand, the T-p phase diagram of the 2C(18)Cl bilayer took a simple shape. The thermodynamic properties for the main transition of the 2C(18)Br and 2C(18)Cl bilayers were comparable to each other, whereas those for the L(c(1))/L(α) transition of the 2C(18)Br bilayer showed considerably high values, signifying that the L(c(1)) phase of the 2C(18)Br bilayer is extremely stable. These differences observed in both bilayers are attributable to the difference in interaction between a surfactant and its counterion.     

Thermotropic behavior of asymmetric chain length catanionic surfactants: the influence of the polar head group

Catanionic surfactants formed by the pairing of two ionic amphiphilic chains of opposite charge are now recognized as an important class of amphiphiles. Many aspects of their phase behavior have yet to be explored. In this work, two homologous series of catanionic surfactants were synthesized, based on the cationic headgroups trimethylammonium and pyridinium. Within each series, the headgroup and chain length of the cationic counterpart remains constant while for the anionic counterpart the headgroup is varied, while its alkyl chain length is also kept constant. Thus, one can directly monitor the influence of headgroup chemistry on the thermal behavior of these compounds. Differential scanning calorimetry (DSC) and polarizing light microscopy show that these compounds bear a rich and often complex thermotropic behavior, with the headgroup chemistry in some instances having a rather dramatic influence on phase behavior. Several liquid crystalline phases appear between the solid crystalline phase and the isotropic liquid phase. A qualitative correlation between the observed thermotropic behavior and the chemical nature of headgroup is presented.     

Determination of lipid phase transition temperatures in hybrid bilayer membranes

The main gel-to-liquid-crystal (LC) phase transition temperature, T(m), of the lipid monolayer in hybrid bilayer membranes (HBMs) was investigated using vibrational sum frequency spectroscopy (VSFS). In the gel phase, the acyl chains of the lipid molecules assume an ordered, all-trans configuration, whereas in the LC phase, the acyl chains exhibit a significant number of disordered gauche conformers. VSFS has unique sensitivity to the order/disorder transitions in the acyl chains and was used to determine T(m) for a series of saturated phosphatidylcholine lipids on octadecanethiolate self-assembled monolayers (SAMs). The values obtained for T(m) for all lipids studied are significantly higher than for the corresponding lipids in vesicles in solution. Additionally, the transition widths are broader for the lipids in HBMs. The underlying SAM clearly influences the phase behavior of the overlying lipid monolayer.     

Effects of poly(ethylene glycol) (PEG) chain length of PEG-lipid on the permeability of liposomal bilayer membranes

The effects of poly(ethylene glycol) (PEG) chain length of PEG-lipid on the membrane characteristics of liposomes were investigated by differential scanning calorimetry (DSC), freeze-fracture electron microscopy (FFEM), fluorescence polarization measurement and permeability measurement using carboxyfluorescein (CF). PEG-liposomes were prepared from mixtures of dipalmitoyl phosphatidylcholine (DPPC) and distearoyl phosphatidylethanolamines with covalently attached PEG molecular weights of 1000, 2000, 3000 and 5000 (DSPE-PEG). DSC and FFEM results showed that the addition of DSPE-PEG to DPPC in the preparation of liposomes caused the lateral phase separation both in the gel and liquid-crystalline states. The fluidity in the hydrocarbon region of liposomal bilayer membranes was not significantly changed by the addition of DSPE-PEG, while that in the interfacial region was markedly increased. From these results, it was anticipated that the CF leakage from PEG-liposomes is accelerated compared with DPPC liposomes. However, CF leakage from liposomes containing DSPE-PEG with a 0.060 mol fraction was depressed compared with regular liposomes, and the leakage decreased with increasing PEG chain length. Furthermore, the CF leakage from liposomes containing DSPE-PEG with a 0.145 mol fraction was slightly increased compared with that of liposomes containing DSPE-PEG with a 0.060 mol fraction. It is suggested that the solute permeability from the PEG-liposomes was affected by not only properties of the liposomal bilayer membranes such as phase transition temperature, phase separation and membrane fluidity, but also the PEG chain of the liposomal surface.     

Combined effect of chain length and phase state on adhesion/friction behavior of self-assembled monolayers

The combined effects of phase state and chain length on the adhesion and friction behavior of self-assembled monolayers (SAMs) are demonstrated using atomic force microscopy (AFM). The phase state of n-alkyltrichlorosilane monolayers of varying chain length (C-8, C-12, and C-18) was controlled by adjusting the preparation temperature. The adhesion forces and friction coefficients were observed to increase dramatically around the phase-transition temperature of each monolayer. The phase state effect was more prominent for the longer chain SAMs, which is attributed to the larger deformation volume associated with disordered monolayers. The adhesion/friction diagram for chain length effect with a wide range of phase states is successfully presented. This study reveals that the chain length effect on adhesion/friction can be correctly evaluated by examining the phase-state dependence of adhesion/friction behind the chain length effect.     

The effect of chain length of bonded organic phases in reversed phase high performance liquid chromatography

Summary Bonded hydrocarbon phases have been prepared by the reaction of organotrichlorosilanes with silica (Lichrosorb Si-100, 10 m, Merck) using conditions for maximum coverage. Alkyl phases, CH₃ (CH₂)_n-With n=7, 10, 12, 14, 17 and 20 and arylakyl phases, Ph (CH₂)_n—with n=0,2,4,6 were investigated. In reverse phase chromatography using water/methanol phases a linear relationship was between the capacity ratios k and the amount of coverage, independent of chain length. The selectivity, however, depends on the chain length of the bonded phase and molecular structure of the solute, whereas for halogenated benzene derivatives the selectivity is constant, and changing the mobile phase composition shows only a minor effect. The selectivity of 4,4-dibromodiphenyl in relation to the benzene derivatives is strongly influenced by the chain length and solvent composition. The utilization of chemically bonded organic long chain phases with maximum coverage makes it possible to reduce the water content in the water/methanol phase increasing the efficiency and loading capacity. The extent of maximum coverage was slightly dependent on the chain length and showed only a 10% decrease from n-octyl-to heneicosyl phase. Treatment of the bonded organic phase with TMCS was not effective with long chain materials, but further reaction occurred with bonded phenyl phase, which could be shown by IR-analysis.     

Amphiphile bilayer films from DPPC: bilayer lipid membranes and Newton black films

Amphiphile bilayer films are obtained from 1,2 dipalmitoyl-glycero-3-phosphocholine (DPPC): bilayer lipid membranes (BLM) and Newton black films (NBF), through thinning of the respective thin liquid films, thus allowing for a very precise determination of the moment of their formation. Stability (or rupture) and formation of BLM and NBF are considered from a unified point of view with the microscopic theory of Kashchiev–Exerowa [J. Colloid Interface Sci., 77 (1980) 501–511], based on the formation of nanoscopic holes in them. BLM and NBF are obtained and studied with the microinterferometric method of Scheludko–Exerowa in its contemporary version. The equivalent thickness of both BLM (in benzene solution between two water phases with 0.1 M NaCl) and NBF in aqueous DPPC solution (in the presence of 0.1 M NaCl) is determined as being h_w = 7.0 nm for BLM and h_w = 7.8 nm for NBF. By means of the dependences: BLM lifetime versus DPPC concentration and probability for BLM formation versus DPPC concentration, it is established that there exist metastable BLM and stable NBF. The good fit between the experimental results of τ(C) dependence and theory in the case of BLM allow to determine the three constants: pre-exponential factor A = 1.5 × 10⁻³ s, related to the process kinetics; constant B = 20.2 ± 0.2, related to the specific hole energy γ = 1.7 × 10⁻¹¹ J/m and the equilibrium concentration C_e = 6 × 10⁻⁴ ± 7.2 × 10⁻⁶ m/l. The specific hole linear energy γ = 1.7 × 10⁻¹¹ J/m determined as well as the binding energy Q between first neighbor molecules in the bilayers Q = 1.48 × 10⁻¹⁹ J (36 kT) are lower than the ones determined for DPPC foam bilayer in gel state γ = 9.1 × 10⁻¹¹ J/m and Q = 55 kT. This means that interaction is weaker in the case of BLM. The critical concentration C_c at which bilayer formation starts is: for BLM C_c = 30 μg/ml and for NBF C_c = 70 μg/ml. This concentration characterizes quantitatively the formation of the amphiphile bilayer and is a very useful parameter that can be used for various purposes.     

Mercury extraction by ionic liquids: temperature and alkyl chain length effect

The preliminary results described here show the complete transfer of Hg(II) ions, in the absence of a chelating agent in 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids; the lag time required to gain the quantitative metal ion partition turned out to be strongly dependent both on alkyl chain length on the imidazolium ring and on the working temperature.     

Structure, dynamics, and phase transitions of tethered membranes: a Monte Carlo simulation study

A coarse-grained model of a self-avoiding tethered membrane with hexagonal coordination, embedded in three-dimensional space, is studied by means of extensive Monte Carlo computer simulations. The simulations are performed at various temperatures for membranes with linear size 5< or =L< or =50. We find that the membrane undergoes several folding transitions from a high-temperature flat phase to multiple-folded structure as the temperature is steadily decreased. Using a suitable order parameter and finite size scaling analysis, these phase transitions are shown to be of first order. The equilibrium shape of the membranes is analyzed by calculating the eigenvalues lambda(max) (2)> or =lambda(med) (2)> or =lambda(min) (2) of the inertia tensor. We present a systematic finite size scaling analysis of the radius of gyration and the eigenvalues of the inertia tensor at different phases of the observed folding transitions. In the high-temperature flat phase, the radius of gyration R(g) grows with the linear size of the membrane L as R(g) proportional to L(nu), where the exponent nu is approximately equal to 1.0. The eigenvalues of the inertia tensor scale as lambda(max) proportional to lambda(med) proportional to L(nu) and lambda(min) proportional to L(nu(min) ), whereby the roughness exponent nu(min) is approximately equal to 0.7. We also find that the time tau(R) of a self-avoiding membrane to diffuse a distance R(g) scales as tau(R) proportional to L(2nu+2), which is in good agreement with the theoretical predictions.     

Thermotropic and lyotropic phase behaviour of monoalkyl glycosides

We have studied the thermotropic and lyotropic phase behaviour of seven monoalkyl glycosides using polarizing microscopy, differential scanning calorimetry and X-ray diffraction. The phase behaviour of these non-ionic surfactants is compared with that of two previously studied monoalkyl glycosides \[1]. We have found that small modifications in chemical structure can lead to large changes in the phase behaviour. The results reveal the effect of changes in alkyl glycoside chemical structure on the phase behaviour, in particular, on the melting point, the clearing point, the solubility in water, and the extent of the lamellar and curved phases. Seven out of the nine surfactants were found to exhibit thermotropic mesomorphism. Furthermore, in the presence of water, three formed lamellar (L ), type I cubic (Q ) and type I hexagonal (H ) phases, two formed lamellar and cubic phases, and two I I formed only lamellar phases. The cubic phase in each case was indexed, either from powderlike or monodomain samples, as spacegroup I a3 d . The lowest order low angle X-ray spacings were found to lie on single, continuous lines with varying water content across the L , Q and H phases, implying epitaxial relationships between these phases at the phase boundaries. I a a I     

Effect of chirality and length on the penetrability of single-walled carbon nanotubes into lipid bilayer cell membranes

The ability of carbon nanotubes to enter the cell membrane acting as drug-delivery vehicles has yielded a plethora of experimental investigations, mostly with inconclusive results because of the wide spectra of carbon nanotube structures. Because of the virtual impossibility of synthesizing CNTs with distinct chirality, we report a parametric study on the use of molecular dynamics to provide better insight into the effect of the carbon nanotube chirality and the aspect ratio on the interaction with a lipid bilayer membrane. The simulation results indicated that a single-walled carbon nanotube utilizes different time-evolving mechanisms to facilitate their internalization within the membrane. These mechanisms comprise both penetration and endocytosis. It was observed that carbon nanotubes with higher aspect ratios penetrate the membrane faster whereas shorter nanotubes undergo significant rotation during the final stages of endocytosis. Furthermore, nanotubes with lower chiral indices developed significant adhesion with the membrane. This adhesion is hypothesized to consume some of the carbon nanotube energy, thus resulting in longer times for the nanotube to translocate through the membrane.     

Ceramide N-acyl chain length: a determinant of bidimensional transitions, condensed domain morphology, and interfacial thickness

Several lipids of biological interest are able to form monomolecular surfaces with a rich variety of thickness and lateral topography that can be precisely controlled by defined variations of the film composition. Ceramide is one of the simplest sphingolipids, consisting of a sphingosine base N-linked to a fatty acid, and is a membrane mediator for cell-signaling events. In this work, films of ceramides N-acylated with the saturated fatty acids C10, C12, C14, and C16 were studied at the air-aqueous interface. The dipole moment contribution (from surface potential measurements) and the surface topography and thickness (as revealed by Brewster angle microscopy) were measured simultaneously with the surface pressure at different molecular areas. Several surface features were observed depending on the asymmetry between the sphingosine and the N-linked acyl chains. At 21 °C, the C16:0 and C14:0 ceramides showed condensed isotherms and the film topography revealed solid film patches (17.3-15.7 ? thick) that coalesced into a homogeneous surface by further compression. On the other hand, in the more asymmetric C12:0 and C10:0 ceramides, liquid expanded states and liquid expanded-condensed transitions occurred. In the phase coexistence region, the condensed state of these compounds formed flowerlike domains (11.1-13.3 ? thick). C12:0 ceramide domains were larger and more densely branched than those of C10:0 ceramide. Both the film thickness and the surface dipole moment of the condensed state increased with ceramide N-acyl chain length. Bending of the sphingosine chain over the N-linked acyl chain in the more asymmetric ceramides can account for the variation of the surface electrostatics, topography, and thickness of the films with the acyl chain mismatch.     

DSC and Raman studies of the side chain length effect of ubiquinones on the thermotropic phase behavior of liposomes

Differential scanning calorimetry and Raman spectroscopy have been used to examine the effects of ubiquinones (UQn) on the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) in multilamellar vesicles, for UQ/DPPC molar ratios ranging from 0.01 to 0.1. The influence of the side chain length has been investigated by comparing the effect of a series of UQ with 2 (UQ2), 4 (UQ4), 6 (UQ6) and 10 isoprene units (UQ10).In the presence of increasing amount of UQ2 or UQ4, concomitant shift of the gel to liquid crystalline phase transition towards lower temperatures and vanishing of the pretransition are observed. Short-chain ubiquinones are thus inserted parallel to phospholipid chains, their benzoquinone ring being close to the DPPC polar headgroups. In addition, broadening and skewing of the main transition peak support the fact that UQ2 and UQ4 are laterally self-organized in highly concentrated regions located at the boundary of lipid domains. The lipid thermotropic behavior is not affected by the presence of other analogues of the series, UQ6 and UQ10. They remain homogeneously dispersed within the midplane of the phospholipid bilayer. Such a chain length dependence on the location and the organization of ubiquinones analogues may be correlated with their biological activities in biomembranes.     

Low-pH-induced transformation of bilayer membrane into bicontinuous cubic phase in dioleoylphosphatidylserine/monoolein membranes

Cubic biomembranes, nonbilayer membranes with connections in three-dimensional space that have a cubic symmetry, have been observed in various cells. Interconversion between the bilayer liquid-crystalline (L(alpha)) phase and cubic phases attracted much attention in terms of both biological and physicochemical aspects. Herein we report the pH effect on the phase and structure of dioleoylphosphatidylserine (DOPS)/monoolein (MO) membranes under a physiological ion concentration condition, which was revealed by small-angle X-ray scattering (SAXS) measurement. At neutral pH, DOPS/MO membranes containing high concentrations of DOPS were in the L(alpha) phase. First, the pH effect on the phase and structure of the multilamellar vesicles (MLVs) of the DOPS/MO membranes preformed at neutral pH was investigated by adding various low-pH buffers into the MLV suspension. For 20%-DOPS/80%-MO MLVs, at and below pH 2.9, a transition from the L(alpha) to cubic (Q(224)) phase occurred within 1 h. This phase transition was reversible; a subsequent increase in pH to a neutral one in the membrane suspension transformed the cubic phase into the original L(alpha) phase. Second, we found that a decrease in pH transformed large unilamellar vesicles of DOPS/MO membranes into the cubic phase under similar conditions. We have proposed the mechanism of the low-pH-induced phase transition and also made a quantitative analysis on the critical pH of the phase transition. This finding is the first demonstration that a change in pH can induce a reversible phase transition between the L(alpha) and cubic phases of lipid membranes within 1 h.     

Thermotropic liquid-crystalline polymers. XXVII. Reentrant nematic phase of side chain liquid-crystalline polymers and their optical properties

The phase diagrams of side chain liquid-crystalline acrylic copolymers with cyanobiphenyl mesogenic groups are described. These copolymers are shown to form a reentrant nematic phase. The main regularities of the reentrant behaviour of polymer systems are studied. Certain peculiarities of the electric field induced orientation phenomena are discussed.     

Gauging the effect of impurities on lipid bilayer phase transition temperature

We report on the gel-to-fluid phase transition behavior of unilamellar vesicles formed with 1,2-dimyristoyl-sn-phosphatidylcholine (14:0 DMPC). We have interrogated the gel-to-fluid transition temperature of these bilayer structures using the chromophore perylene incorporated in their nonpolar region. We observe a discontinuous change in the reorientation time of perylene sequestered within the bilayer at the known melting transition temperature of 14:0 DMPC, 24 degrees C. The perylene reorientation data reveal a local viscosity of 14.5 +/- 2.5 cP in the gel phase, and 8.5 +/- 1.5 cP in the fluid phase. We have also incorporated small amounts of 1,2-dimyristoleoyl-sn-glycero-3-phosphocholine (14:1 DMPC) into these unilamellar vesicles and find that the melting transition temperature for these bilayers varies in a regular manner with the amount of 14:1 DMPC present. These data demonstrate that very little "contaminant" is required to cause a substantial change in the gel-to-fluid transition temperature, even though these contaminants do not alter the viscosity of the bilayer sensed by perylene, either above or below the melting transition.     

Novel liquid crystalline structures of a chiral side chain polymer and its phase transitions

Two novel smectic bilayer structures have been identified in an enantiomerically enriched chiral side chain polymer containing the highly dipolar nitrile group at stereocentres. The structures were characterized by electron diffraction, electron microscopy, and X-ray diffraction. In both phases each smectic layer has a bilayer structure with backbones and spacers confined in a thin disordered region between two sublayers of mesogenic segments. One of the structures which we denote as CrE* has the unusual feature of having its side chains arranged parallel to the layer normal in spite of its enantiomeric bias and twisted nature. In the second structure side chains are tilted by 34.8° with respect to the layer normal and we denote this phase as CrH*_c In both structures each sublayer contains three different orientations of orthorhombic (CrE) or monoclinic (CrH*_c) lattices which are related to one another by rotations of ± 60° about the c-axis. In both the CrH*_c and the CrE* phases, lattices in each sublayer are regularly rotated about the c-axis by 5.9° relative to those in the adjacent sublayer. The observation of a chiral CrH phase is uncommon and in this specific case the structure is unique since the rotation between adjacent layers occurs about the sidechain axis (c-axis) (CrH*_c) and not about the layer normal (c-axis) (CrH*_c). We believe the system undergoes a change in molecular organization from CrH*_c to CrE* as a result of a chemical reaction which joins a fraction of the stereocentres through covalent bonds. With increasing temperature the CrE* structure was found to transform to a special orthorhombic untwisted smectic phase in which a = 31/2b, denoted here as CrE_h. The structure then transforms to a hexatic S_B phase and finally to a S_A phase at yet higher temperatures.     

Aggregation properties of diacyl lysine surfactant compounds: hydrophobic chain length and counterion effect

In this paper, we report on the study of aqueous solution and aggregation properties of diacyl Lysine surfactant salts with several surfactant counterions at a fixed hydrophobic chain length. They present a critical micellar concentration nearly independent of the counterion. The area per surfactant molecule is around 1.3 nm (2) also independent of the counterion. We have also studied the dry state crystallization of these surfactant salts. We show that small counterion systems tend to form bicontinuous cubic structures and that the increase in counterion size tends to form lamellar structures. We have compared this behavior with the dry state crystallization of the diacyl Lysine surfactants as a function of hydrophobic chain length. For long hydrophobic chains, the crystal structure is lamellar, while for intermediate, length is cubic. Among the structures studied, the one with the shortest chain length crystallizes in a hexagonal inverse phase.