Lamellar Crystalline Phase Research Articles

The thermotropic phase behavior of cationic lipids: calorimetric, infrared spectroscopic and X-ray diffraction studies of lipid bilayer membranes composed of 1,2-di-O-myristoyl-3-N,N,N-trimethylaminopropane (DM-TAP)

The thermotropic phase behavior of lipid bilayer model membranes composed of the cationic lipid 1,2-di-O-myristoyl-3-N,N,N-trimethylaminopropane (DM-TAP) was examined by differential scanning calorimetry, infrared spectroscopy and X-ray diffraction. Aqueous dispersions of this lipid exhibit a highly energetic endothermic transition at 38.4°C upon heating and two exothermic transitions between 20 and 30°C upon cooling. These transitions are accompanied by enthalpy changes that are considerably greater than normally observed with typical gel/liquid–crystalline phase transitions and have been assigned to interconversions between lamellar crystalline and lamellar liquid–crystalline forms of this lipid. Both infrared spectroscopy and X-ray diffraction indicate that the lamellar crystalline phase is a highly ordered, substantially dehydrated structure in which the hydrocarbon chains are essentially immobilized in a distorted orthorhombic subcell. Upon heating to temperatures near 38.4°C, this structure converts to a liquid–crystalline phase in which there is excessive swelling of the aqueous interlamellar spaces owing to charge repulsion between, and undulations of, the positively charged lipid surfaces. The polar/apolar interfaces of liquid–crystalline DM-TAP bilayers are not as well hydrated as those formed by other classes of phospho- and glycolipids. Such differences are attributed to the relatively small size of the polar headgroup and its limited capacity for interaction with moieties in the bilayer polar/apolar interface.

Kinetic phase behavior of distearoylphosphatidylethanolamine dispersed in glycerol

Phase behavior of distearoylphosphatidylethanolamine dispersed in excess glycerol has been examined by differential scanning calorimetry. Transformation from lamellar-gel to lamellar crystalline phase was found to take place at temperatures near 74.9°C upon cooling and near 76.3°C during heating scans. The transition can also be observed under isothermal conditions at temperature in this range. The kinetics of the transformation from lamellar-gel to lamellar-crystal phase was analyzed by the well-known Avrami equation. The apparent Avrami exponents were found to be approximately 1.6. The effective dimensionality of the growth pattern can then be set as 1, after taking into account the contribution of nucleation at the examination temperatures. The activation energy of the phase transition was estimated as approximately 255 kJ mol −1. The data are discussed in terms of development of successful cryoprotective strategies using glycerol.

Calorimetric and Spectroscopic Studies of the Thermotropic Phase Behavior of Lipid Bilayer Model Membranes Composed of a Homologous Series of Linear Saturated Phosphatidylserines

The thermotropic phase behavior of lipid bilayer model membranes composed of the even-numbered, n-saturated 1,2-diacyl phosphatidylserines was studied by differential scanning calorimetry and by Fourier-transform infrared and 31P-nuclear magnetic resonance spectroscopy. At pH 7.0, 0.1 M NaCl and in the absence of divalent cations, aqueous dispersions of these lipids, which have not been incubated at low temperature, exhibit a single calorimetrically detectable phase transition that is fully reversible, highly cooperative, and relatively energetic, and the transition temperatures and enthalpies increase progressively with increases in hydrocarbon chain length. Our spectroscopic observations confirm that this thermal event is a lamellar gel (L β )-to-lamellar liquid crystalline (L α ) phase transition. However, after low temperature incubation, the L β /L α phase transition of dilauroyl phosphatidylserine is replaced by a higher temperature, more enthalpic, and less cooperative phase transition, and an additional lower temperature, less enthalpic, and less cooperative phase transition appears in the longer chain phosphatidylserines. Our spectroscopic results indicate that this change in thermotropic phase behavior when incubated at low temperatures results from the conversion of the L β phase to a highly ordered lamellar crystalline (L c) phase. Upon heating, the L c phase of dilauroyl phosphatidylserine converts directly to the L α phase at a temperature slightly higher than that of its original L β /L α phase transition. Calorimetrically, this process is manifested by a less cooperative but considerably more energetic, higher-temperature phase transition, which replaces the weaker L β /L α phase transition alluded to above. However, with the longer chain compounds, the L c phase first converts to the L β phase at temperatures some 10–25°C below that at which the L β phase converts to the L α phase. Our results also suggest that shorter chain homologues form L c phases that are structurally related to, but more ordered than, those formed by the longer chain homologues, but that these L c phases are less ordered than those formed by other phospholipids. These studies also suggest that polar/apolar interfaces of the phosphatidylserine bilayers are more hydrated than those of other glycerolipid bilayers, possibly because of interactions between the polar headgroup and carbonyl groups of the fatty acyl chains.

Differential Scanning Calorimetric and Fourier Transform Infrared Spectroscopic Studies of the Effects of Cholesterol on the Thermotropic Phase Behavior and Organization of a Homologous Series of Linear Saturated Phosphatidylserine Bilayer Membranes

We have examined the effects of cholesterol on the thermotropic phase behavior and organization of aqueous dispersions of a homologous series of linear disaturated phosphatidylserines by high-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy. We find that the incorporation of increasing quantities of cholesterol progressively reduces the temperature, enthalpy, and cooperativity of the gel-to-liquid-crystalline phase transition of the host phosphatidylserine bilayer, such that a cooperative chain-melting phase transition is completely or almost completely abolished at 50 mol % cholesterol, in contrast to the results of previous studies. We are also unable to detect the presence of a separate anhydrous cholesterol or cholesterol monohydrate phase in our binary mixtures, again in contrast to previous reports. We further show that the magnitude of the reduction in the phase transition temperature induced by cholesterol addition is independent of the hydrocarbon chain length of the phosphatidylserine studied. This result contrasts with our previous results with phosphatidylcholine bilayers, where we found that cholesterol increases or decreases the phase transition temperature in a chain length-dependent manner (1993. Biochemistry, 32:516–522), but is in agreement with our previous results for phosphatidylethanolamine bilayers, where no hydrocarbon chain length-dependent effects were observed (1999. Biochim. Biophys. Acta, 1416:119–234). However, the reduction in the phase transition temperature by cholesterol is of greater magnitude in phosphatidylethanolamine as compared to phosphatidylserine bilayers. We also show that the addition of cholesterol facilitates the formation of the lamellar crystalline phase in phosphatidylserine bilayers, as it does in phosphatidylethanolamine bilayers, whereas the formation of such phases in phosphatidylcholine bilayers is inhibited by the presence of cholesterol. We ascribe the limited miscibility of cholesterol in phosphatidylserine bilayers reported previously to a fractional crystallization of the cholesterol and phospholipid phases during the removal of organic solvent from the binary mixture before the hydration of the sample. In general, the results of our studies to date indicate that the magnitude of the effect of cholesterol on the thermotropic phase behavior of the host phospholipid bilayer, and its miscibility in phospholipid dispersions generally, depend on the strength of the attractive interactions between the polar headgroups and the hydrocarbon chains of the phospholipid molecule, and not on the charge of the polar headgroups per se.

Macroscopic Helical and Cylindrical Morphologies from Achiral 1,3-Diynes

Diacetylenes containing an anhydride function (1) or two s-triazine groups (2) precipitate from organic solvents in large helical or cylindrical forms, respectively, which have been studied by optical and scanning electron microscopy. X-ray diffraction studies on helices of 10,12-tricosadynoic anhydride, {n-C10H21C⋮CC⋮C(CH2)8CO}2O (1), show a lamellar crystalline phase with layer spacing of 42.5 ± 0.5 A and support a noninterdigitated bilayer structure. The crystal structure of a related amphiphilic diacetylene, n-C10H21C⋮CC⋮C(CH2)8CH2OSO2C6H4Me-4 (3), is also reported and discussed: triclinic, P1, a = 5.9730(10) A, b = 8.0690(10) A, c = 29.524(3) A; α = 95.020(10)°, β = 93.260(10)°, γ = 97.480(10)°. The nonamphiphilic, α,ω-difunctionalized diacetylene, 6,6‘-(deca-4,6-diyne-1,10-diyl)di-2,4-diamino-1,3,5-triazine, 2-{4,6-(NH2)2C3N3}(CH2)3C⋮CC⋮C(CH2)3-{C3N3(NH2)2-4,6}-2 (2), forms solid cylindrical particles of mean length ∼0.5 mm, which may derive from helix formation. The origin of macroscopic chiralit...

Bolaamphiphilic Phosphocholines: Structure and Phase Behavior in Aqueous Media

The phase behavior of 1,1‘-di-O-eicosamethylene-2,2‘-di-O-decylbis(glycero-3-phosphocholine) (1) and 1,1‘-di-O-hexadecamethylene-2,2‘-di-O-octylbis(glycero-3-phosphocholine) (2) was studied at the air−water interface and in aqueous dispersions using monolayer film balance, small-angle X-ray scattering, Raman spectroscopy, and cryo-TEM techniques. The limiting molecular areas (20 °C, pH 6.8) were observed to be 172 Å2 for 2 and 105 Å2 for 1, indicating a U-shaped configuration of the long polymethylene chains at the air−water interface. The larger molecular area of 2 relative to 1 at the air−water interface suggests that the alkyl chains of 2 are less ordered than those of 1 at similar pressures. The extrapolated d spacings for 1 from SAXS experiments were 32 Å in the liquid crystalline lamellar phase and 40 Å in the gel phase where the hydrocarbon chains are in their all trans configuration. The extrapolated d spacings for 2 were 27 and 33 Å in the liquid crystalline and gel phases, respectively. DSC analysis revealed phase transition temperatures of 1 and 2 in excess water at 15.3 and −23.5 °C, respectively, with observed transition enthalpies of 9.1 and 2.9 kJ/mol, respectively. These values are in good agreement with Raman spectroscopy experiments that detect alkyl melting transitions near these temperatures. Addition of 30 mol % cholesterol to both bolaamphiphile samples lowered the observed transition temperatures (1, 9.5 °C; 2, −25.1 °C) and enthalpies (1, 5.7 kJ/mol; 2, 2.5 kJ/mol), suggesting that cholesterol reduces the transition cooperativity. Extruded samples of pure 1 in buffer (30 mg/mL in 20 mM Tris, pH 8) produced vesicle structures that were variable in size (0.15−1.5 μm), with multilamellar and nonspherical morphologies apparent. Incorporation of cholesterol in the extruded samples (7:3 1/Chol), however, gave stable spherical unilamellar vesicles with a mean diameter of 857 ± 237 Å. Similar results were observed for dispersions of 2 and 7:3 2/Chol. These results suggest that stable, unilamellar monolayer membrane vesicles can be formed from bolaamphiphiles with relatively short (C16 and C20) membrane-spanning alkyl chains.

Molecular Dynamics Study of the Poly(oxyethylene) Surfactant C12E2 and Water

Constant pressure and temperature (NPT) molecular dynamics (MD) simulations have been carried out to investigate the properties of a binary mixture of the poly(oxyethylene) surfactant C12E2 and water in its liquid crystalline lamellar phase (Lα). The calculated interlamellar spacing and the area per surfactant were found to be in reasonable agreement with X-ray diffraction results. The water molecules were observed to form hydrogen bonded bridged structures linking the oxygen atoms of the same surfactant chain. This interaction leads to a strong preference for the C−C bonds in the headgroup to attain a gauche conformation.

Microphase separation in poly(isoprene-b-ethylene oxide) diblock copolymer melts. I. Phase state and kinetics of the order-to-order transitions

The phase state and the kinetics of the order-to-order transitions have been studied in a series of poly(isoprene-b-ethylene oxide) (PI-PEO) diblock copolymers with a PI volume fraction in the range 0.25<fPI<0.92, using small angle x-ray scattering (SAXS), and rheology. The mean-field theory (MFT) structure factor is used to describe the SAXS profiles in the disordered phase and to extract the temperature dependence of the interaction parameter χ(T). In general, an agreement is found with the phase diagram proposed by an extended MFT, except at fPI=0.61 where the following sequence of phases was found: Lc→Hex→Gyroid→Dis (Lc is the crystalline lamellar phase, Hex signifies hexagonally packed cylinders, Gyroid is the bicontinuous cubic network with the Ia3̄d symmetry, and Dis is the disordered phase). We found that crystallization disrupts the amorphous ordered morphologies and imposes a layered structure (Lc). The study of the kinetics of the Hex to Lc and the Hex to Gyroid transitions is facilitated by the different viscoelastic contrast and the distinctly different scattering patterns of the three phases involved (Lc, Hex, Gyroid). Our studies show that it is possible to undercool and overheat ordered phases just as we can undercool the disordered phase. The transformation from the Hex to the Lc phase proceeds via a heterogeneous nucleation and growth process and results in the formation of a spherulitic superstructure composed from stacks of lamellar crystals. The transformation of the Hex to the Gyroid phase involves two steps. The first step—which is too fast to be picked up by rheology—involves fluctuations of the hexagonal phase. The second “slow” step involves a nucleation and growth process of elongated objects. The transformation proceeds nearly epitaxially and has an activation energy of 47 kcal/mol which is typical for a collective process.

Calorimetric and spectroscopic studies of the effects of cholesterol on the thermotropic phase behavior and organization of a homologous series of linear saturated phosphatidylethanolamine bilayers

Aqueous dispersions of cholesterol-containing phosphatidylethanolamine (PE) bilayers were examined by a combination of high-sensitivity differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) and 31P-nuclear magnetic resonance spectroscopy. Regardless of hydrocarbon chain length, the incorporation of low levels of cholesterol into these bilayers causes progressive reductions in the temperature, enthalpy and overall cooperativity of the lipid hydrocarbon chain-melting phase transition. Moreover, at low cholesterol levels, the heating and cooling thermograms observed for the cholesterol/PE binary mixtures are similar, indicating comparable levels of lateral miscibility of cholesterol with PE bilayers in the gel and liquid-crystalline states. However, at higher levels of cholesterol incorporation, marked differences between the heating and cooling thermograms are noted. Upon heating, complex multicomponent thermograms are observed in PE bilayers containing large amounts of cholesterol, and the temperature and overall enthalpy values increase discontinuously from the pattern of monotonic decrease observed at lower cholesterol levels. Moreover, these discontinuities begin to emerge at progressively lower cholesterol concentrations as PE hydrocarbon chain length increases. Upon cooling, a simpler pattern of thermotropic behavior is observed, and the measured temperature and enthalpy values continue to decrease monotonically with increases in cholesterol content. These results suggest that at higher concentrations cholesterol exhibits a decreased degree of lateral miscibility in the gel or crystalline as compared to the liquid-crystalline states of PE bilayers, particularly in the case of the longer-chain PEs. Our FTIR and 31P-nuclear magnetic resonance spectroscopic studies also show that the thermotropic events observed with mixtures of low cholesterol content are analogous to the gel/liquid-crystalline phase transitions exhibited by the pure PEs. However, lamellar crystalline phases readily form when mixtures of high cholesterol content are cooled to low temperatures. Moreover, these crystalline phases are spectroscopically indistinguishable from those formed by the pure PEs, indicating that cholesterol is excluded from such phases. Upon subsequent heating, the melting of these crystalline phases gives rise to the complex thermograms detected by DSC and to the discontinuities in the phase transition temperature and enthalpy noted above. This pattern of behavior differs markedly from that observed with the corresponding phosphatidylcholines (PCs), where comparable degrees of cholesterol miscibility are observed in the gel and liquid-crystalline states even at high cholesterol concentrations, and where cholesterol inhibits rather than facilitates the formation of lamellar crystalline phases. We also find that the presence of cholesterol does not result in the hydrophobic mismatch-dependent shifts in the phase transition temperature in PE bilayers previously observed in PC bilayers of varying thickness. We attribute these differences in the effects of cholesterol on phospholipid thermotropic phase behavior to stronger electrostatic and hydrogen bonding interactions at the surfaces of PE and compared to PC bilayers.

Investigation of Lipid Organization in Biological Membranes by Two-Dimensional Nuclear Overhauser Enhancement Spectroscopy

The cross-peaks between lipid resonances in two-dimensional nuclear Overhauser enhancement spectroscopy 1H NMR spectra, recorded with magic-angle spinning, contain valuable information about the structure and dynamics of the lipid bilayer. We have attempted a quantitative analysis of magnetization exchange between lipid resonances in the biologically relevant liquid crystalline lamellar phase. Spectra of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) water dispersions were recorded at mixing times from 0.005 to 1 s, and all diagonal and cross-peak volumes were determined by integration. The full relaxation-rate matrix was computed for the 10 lipid resonances by a matrix equation algorithm. Results of this mathematically rigorous approach were compared with simplified approaches to calculate cross-relaxation rates. In a second series of experiments, the DMPC was mixed with increasing amounts of perdeuterated DMPC-d67 to determine the percentage of intra- vs intermolecular magnetization transfer. With the exception of transfer between nearest neighbor protons by chemical bond, cross-relaxation in DMPC bilayers is exclusively intermolecular. There is no evidence for the transfer of magnetization over several bonds within lipid molecules by spin diffusion. Lipids exist in multiple conformations with rapid transitions between them, and, therefore, cross-relaxation rates do not represent stable molecular arrangements with fixed distances. Instead, the cross-relaxation rates reflect the probability of close approach between protons of neighboring lipid molecules. We established experimentally that cross-relaxation rates are proportional to the statistical average of lateral lipid−lipid contacts in binary mixtures. The location of lipid segments along the bilayer normal is best described by a distribution function. Segments which are located at the same depth in the bilayer approach each other with higher probability and have higher rates of magnetization transfer, compared with groups which are, on average, more distant and approach each other less frequently. The per-proton intermolecular cross-relaxation rates vary over the bilayer by only 1 order of magnitude, indicating surprisingly high probabilities of close approach even between the most distant groups, like the choline methyl groups and the terminal methyl groups of hydrocarbon chains. The results reflect the high degree of lipid motional disorder and the substantial variations in location of neighboring lipid molecules.

The structure and organization of phospholipid bilayers as revealed by infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy is a powerful yet relatively inexpensive spectroscopic technique for studying the structure and organization of phospholipid bilayers. This technique yields information about all regions of the phospholipid molecule simultaneously without the necessity of introducing perturbing extrinsic probes. In this review we summarize some recent FTIR spectroscopic studies of the structure and organization of phospholipids in their lamellar liquid-crystalline, gel, and crystalline phases and show that interconversions between these phases can be readily monitored by this technique. We also demonstrate that this technique can yield valuable information about nonlamellar phospholipid phases as well. Finally, we point out that in principle FTIR spectroscopy is capable of yielding fairly high resolution structural and conformational information about phospholipid molecules in multi-molecular aggregates and give several examples of how this potential is beginning to be realized.

Characterization of the Nanostructures in Liquid Crystalline Mesophases Present in the Ternary System Brij-35/Dibutyl Ether/H2O by Small- and Wide-Angle X-ray Scattering

We investigated the liquid crystalline phase structures and their temperature-dependent dynamics in the ternary system composed of Brij-35 [poly(oxyethylene(∼23))lauryl ether, C12OE∼23], dibutyl ether (DBE) and 10 mM Na/K buffer. A sequence of schematic ternary phase diagrams at 10, 20, 30, 40, and 60 °C, derived from temperature-scanning small- and wide-angle X-ray scattering (SWAX), has been established. In the binary Brij-35/H2O system, the amphiphilic component Brij-35, being totally miscible with water, self-assembles as a function of its concentration into a normal micellar solution L1, a normal packed micellar cubic I1-phase region, and an extended region of various lamellar structures. The cubic I1 phases show diffraction patterns with Im3m (unit cell parameter aIm3m = 85−94 Å) and Ia3d (aIa3d = 162−167 Å) symmetry. Increasing temperature destabilizes the cubic phases between 15 and 55 °C, depending on the weight ratio of the used components. The lamellar phases show unit cell dimensions of a = 179−181 Å. A phase transition from Lc (crystalline lamellar phase), Lβ (gel lamellar phase), to a isotropic micellar phase, here denoted as L2, occurs at temperatures from 15 to 35 °C, as a function of the water content. The DBE uptake of these structures is limited to 20−50%(w/w), which generates biphasic regions at higher DBE amounts, where a DBE excess phase containing approximately 2% (w/w) H2O and 0.02% (w/w) Brij-35 is formed. Increasing amounts of DBE leadwith exception of the Lc phaseto enhanced phase temperature stabilities in the water-lean region and induce normal hexagonal H1 (a = 68−135 Å) and various cubic structures. We found the cubic space groups Im3m and Ia3d with unit cell dimensions aIm3m = 83−112 Å and aIa3d = 162−195 Å in the cubic I1 region and Pn3m (aPn3m = 164 Å) and Ia3d (aIa3d = 187−214 Å) as well as Fd3m (aFd3m = 218−275 Å) in the cubic region with low water contents.

Barotropic phase transitions of dioleoylphosphatidylcholine and stearoyl-oleoylphosphatidylcholine bilayer membranes

In order to understand the effect of cis unsaturation on the thermotropic and barotropic phase behavior of phospholipid bilayer membranes, the phase transitions of 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC), 1-stearoyl-2-oleoyl- sn-glycero-3-phosphocholine (SOPC), 1,2-distearoyl- sn-glycero-3-phosphocholine (DSPC) bilayer membranes were observed by high-pressure optical method. With respect to DOPC bilayer membrane, the so-called main transition between the liquid crystalline (L α) and the lamellar gel (L β) phases was observed in water at above 0°C under high pressure, in addition to the transition between the L α and the lamellar crystalline (L C) phases in 50% aqueous ethylene glycol. The pressure of main transition increased linearly with an increase in temperature. Extrapolation of temperature ( T)-pressure ( P) phase boundary to ambient pressure suggests the temperature of the main transition to be −40.3°C, which has never been found by the DSC method. On the other hand, the temperature of L C/L α phase transition in 50% aqueous ethylene glycol was found to be −12.0°C at ambient pressure. The main transition temperatures for DSPC, SOPC and DOPC are 55.6, 6.7 and −40.3°C, respectively, at ambient pressure. The substitution of cis unsaturated chain for saturated chains of DSPC brings about the depression of the main transition temperature by about 48 (±1)°C for each chain. The volume changes (Δ V) associated with the transitions were calculated from the transition enthalpy (Δ H) and the slope of T-P diagram (d T/d P) by means of the Clapeyron-Clausius equation. The value of Δ V for the main transition of SOPC bilayer membranes was reduced to half the volume change for DSPC bilayers, which means the introduction of the cis double bond in the acyl chain of lipids brings about the reduction of Δ V because of the disordered packing of unsaturated chains in the gel phase of lipid bilayer membranes.

Liquid Crystalline Phases in the Structuring of Food Products

It has been explored whether liquid crystalline phases of surfactants and water would offer opportunities for the development of low-calorie food products. Commercial monoglycerides and esters of monoglycerides are used as nonionic and anionic surfactants, respectively. Such mixtures allow the incorporation of large amounts of water in a liquid crystalline lamellar phase. On cooling, this phase transforms into a so-called α-gel state, with a remarkable ointment-like consistency. In turn, the α-gel may transform into a crystalline coagel state with a more fat-like consistency, which also is able to incorporate large amounts of water. The characteristics of these gel phases and their transformation have been investigated. The transformation appears to be a rather slow process which can be influenced by thermal, mechanical or chemical treatments. Shear and temperature increase, in general, enhance the transition rate. For proper gel formation the amounts of nonionic and anionic surfactants and of salt and pH have to be matched carefully. Addition of compounds such as other surfactants or biopolymers, in general, retards the transition process. Electrostatic interactions between added proteins and the bilayer in the lamellar phase strongly stabilize the α-gel state. Coagel and α-gel phases can be used as main components in low-calorie food products. Both phases show a different functionality, in particular with respect to firmness and foaming characteristics. Applications are discussed.

NMR Study of Collective Motions and Bending Rigidity in Multilamellar System of Lipid and Surfactant Bilayers

Abstract The frequency dependence of the longitudinal proton spin relaxation time T1 was measured by field-cycling and standard NMR techniques at different temperatures in the liquid crystalline lamellar phases of bilayer systems, composed of lipids, nonionic surfactants, and lipid-surfactant mixtures. We show by our data analysis, comparing various motional models such as layer undulations (LUs) and relaxation by translational diffusion mediated reorientations (TR), that collective layer undulations with their typical T1~ν behaviour determine the low frequency T1 dispersion in both unoriented and glass plate-oriented bilayer systems. The angular dependence of the T1 dispersion for the oriented bilayer system supports these findings and provides a more critical analysis of the two dimensional self-diffusion than in unoriented samples. The evaluated fitting parameters of the LU model allows, together with the measured second moment of the proton NMR signal for the lipid, calculation of the bending rigidity ϰc for these bilayers at different levels of hydration. The obtained values of ϰc turn out to be too large compared with the literature. However, using recent LU models (B. Halle) which include the obvious couplings between neighbouring bilayers at low Larmor frequencies, the corrected ϰc of the fully hydrated membrane systems are comparable to those obtained from the standard videooptical experiments. Therefore proton spin relaxation measure-ments at low Larmor frequencies with the field-cycling technique are a suitable means to determine the bending rigidity ϰc of model membrane systems at low hydrations and of systems containing surfactants.

Phase Behavior and Phase Structure for Catanionic Surfactant Mixtures: Dodecyltrimethylammonium Chloride−Sodium Nonanoate−Water System

The phase behavior and phase structure of catanionic surfactant mixtures of DoTAC (dodecyltrimethylammonium chloride) and SN (sodium nonanoate) with water are studied by combined 2H NMR, SAXS, and microscopy techniques at 40 °C. The system forms a large isotropic micellar solution phase with excess water. As the concentration of total surfactant is increased, the solution phase coexists with different liquid crystalline phasesa lamellar phase at equimolar ratio of the two surfactants and hexagonal phases with excess DoTAC and excess SN. The lamellar and hexagonal liquid crystalline phases formed by the binary DoTAC system extensively swell with water on adding the anionic surfactant, and the swelling is more dramatic for the lamellar phase which extends to an equimolar ratio of the two surfactants. The mesophase of the short alkyl chain is incapable of solubilizing any substantial amounts of the long chain DoTAC molecules. SAXS data shows a decrease in bilayer thickness and an unchanged average area per p...

Liquid crystalline phase metastability of phosphatidylglycerols

Aqueous dispersions of dilauroylphosphatidylglycerol (DLPG) and dimyristoylphosphatidylglycerol (DMPG) form stable subgel and liquid crystalline lamellar phases, Lc and Lα, and also metastable gel phases, Lβ′ and Pβ′, at physiologically relevant conditions (neutral pH, <0.2 M salt). The metastable gel phase Lβ′ relaxes to the stable subgel phase upon low-temperature storage, with at least one metastable intermediate in the case of DMPG. The gel–liquid crystalline phase transition takes place at temperature with 15–30°C lower as compared to that of the subgel–liquid crystalline phase transition in these phosphatidylglycerols. Therefore, the lamellar liquid crystalline phase Lα is expected to exhibit metastability in the temperature range between the gel–Lα and subgel–Lα phase transitions, in which the subgel phase is the equilibrium one. Here we demonstrate by using differential scanning calorimetry that crystallisation from the liquid crystalline phase in this temperature interval takes place in DMPG and DLPG. The kinetics of the observed Lα-to-Lc phase relaxation is sensitive to the thermal prehistory of the lipid dispersion. This is possibly related to the different morphologies of the lipid aggregates in the subgel and liquid crystalline phases (Epand et al., 1992) and the slow interconversion between them.

Physical−Chemical Properties of Some Branched Alkyl Glucosides

Four different alkylglucosides have been examined with respect to their physicochemical properties, viz. 2-ethylhexyl glucoside, isooctyl glucoside, 2-propylheptyl glucoside, and isodecyl glucoside. The substances have been examined by means of microscopy, crossed polarizers, 1H-NMR self-diffusion, 2H-NMR quadrupolar splittings, and small angle X-ray scattering. Properties such as critical micelle concentrations, solubilities, Krafft points, aggregation structures, and liquid crystallinity have been determined. For all substances, the only phases appearing in the binary (surfactant/water) system are the micellar and lamellar phases. For the two substances with eight carbons in the hydrophobic tail, there is a broad micellar region, a minor two-phase region, and a lamellar crystalline phase. The isodecyl glucoside, with 10 carbons in the hydrophobic tail, shows a liquid−liquid coexistence region from low surfactant concentrations up to ca. 18 wt % surfactant. In the case of the 2-propylheptyl glucoside, th...

NMR investigations of non-lamellar phase promoters in the lamellar phase state

A delicate balance of attractive and repulsive forces between lipid molecules determines the average area per lipid in the liquid crystalline lamellar phase. Formation of inverted non-lamellar phases is linked to an imbalance in lateral tension between lipid headgroups and chains favoring formation of curved lipid monolayers, such as in inverse hexagonal (H II) and cubic phases. If assembled in bilayers, non-lamellar forming lipids are under a curvature related lateral stress. This tension imbalance influences molecular cross-sectional area in the lamellar phase which is reflected in order parameter changes of a perdeuterated lipid chain. 2H NMR order parameters are sensitive to variations as small as 0.2 Å 2 in area per molecule. Curvature stress caused by a reduction of repulsive forces between headgroups, e.g. replacement of PC with PE raises chain order and reduces area per lipid by a few square angstrom. Stress caused by increased repulsive tension between lipid hydrocarbon chains, e.g. an increase in chain length, chain unsaturation or a rise in temperature, lowers chain order and increases area per molecule. Addition of alkanes, alcohols, cholesterol and other substances also changes lipid order, but interpretation of these changes in terms of tension in headgroup and chain regions does not follow simple patterns. In addition to altering tension, these additives may stabilize non-lamellar phases by lowering interstitial energies.

Effect of unsaturation on the chain order of phosphatidylcholines in a dioleoylphosphatidylethanolamine matrix

The properties of phosphatidylcholines (PCs) having a perdeuterated stearic acid, 18:0d35, in the sn-1 position and the fatty acid 18:0, 18:1 omega 9, 18:2 omega 6, 18:3 omega 3, 20:4 omega 6, 20:5 omega 3, or 22:6 omega 3 at the sn-2 position were investigated in a matrix of dioleoylphosphatidylethanolamine (DOPE) by 2H and 31P NMR spectroscopy. At a mole ratio of DOPE/PC = 5:1, the lipids form liquid crystalline lamellar phases below 40 degrees C and coexisting lamellar, inverse hexagonal (Hll), and cubic phases at higher temperatures. The sn-1 chain of the PCs in a DOPE matrix is appreciably more ordered than in pure PCs, corresponding to an increase in the hydrophobic bilayer thickness of approximately 1 A. Distearoylphosphatidylcholine in the DOPE matrix has a higher sn-1 chain order than the unsaturated PCs. We observed distinct differences in the lipid order of upper and lower sections of the hydrocarbon chains caused by changes of temperature, unsaturation, headgroups, and ethanol. Unsaturation lowers chain order, mostly in the lower third of the hydrocarbon chains. By contrast, the increase in chain order caused by the DOPE matrix and the decrease in order with increasing temperature have a constant magnitude for the upper two-thirds of the chain and are smaller for the lower third. Addition of 2 M ethanol reduced order parameters, in effect reversing the increase in chain order caused by the DOPE matrix.