Lamellar Gel Phase Research Articles

Glass Transition of a Synthetic Phospholipid in the Lamellar Phase

A synthetic phospholipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), was studied by differential scanning calorimetry at different hydration levels. A glass transition was observed in the lamellar gel (Lβ) phase but was not detected in a more ordered lamellar crystalline phase. The glassy state of the Lβ phase of POPE has properties that are typical of other glass-forming materials, i.e., nonexponential relaxation behavior below the glass transition temperature, a broad distribution of relaxation times, and plasticization by water. The significance of the glass transition in biological membranes is discussed.

Thermotropic and piezotropic phase behaviour of phospholipids in propanediols and water

The investigation of the solvation process of phospholipids includes the investigation of the thermotropic and piezotropic phase behaviour in presence of non-aqueous polar solvents. 1,2-propanediol and 1,3-propanediol were chosen to investigate the influence of the solvents on the parameters of the main phase transition. The non-polar parts of the propanediols slightly reduce the main phase transition temperature in contrast to water. On the other hand, the pressure induced increase of the transition temperature hardly differs between propanediols and water. Furthermore, a coexistence of a lamellar gel phase and an isotropic liquid phase was found for a number of phospholipid/1,2-propanediol dispersions.

Structural, dynamic and mechanical properties of POPC at low cholesterol concentration studied in pressure/temperature space.

We have studied the structural, dynamic and mechanical properties of 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphatidylcholine (POPC)/cholesterol binary mixtures by small-angle X-ray scattering. Our investigations were concentrated on the biologically most relevant pressure-temperature-cholesterol regime, i.e. the liquid crystalline phase and its phase boundary to the lamellar gel phase within a cholesterol concentration up to 25 mol%. From the dependence of the transition pressure we derived a value of 19 kJ/mol for the transition enthalpy Delta H(m) of POPC in excess water. With increasing cholesterol concentration, Delta H(m) drops to about 7 kJ/mol at 20 mol% cholesterol. Time-resolved pressure-scan (p-scan) and temperature-jump (T-jump) experiments reveal that at low cholesterol content (<5-8 mol%) the fluidity and also the bilayer compressibility increase remarkably. In contrast, at concentrations between 5 and 25 mol% cholesterol the bilayer becomes again more rigid and the lipid bilayer spacing increases about 2 A. Theses changes are attributed to the onset of phase separation between liquid disordered and liquid ordered phases. The fluid-fluid miscibility gap for this mono-unsaturated lecithin species is strongly enlarged compared with saturated lecithins.

Regulation of lipid composition in biological membranes—biophysical studies of lipids and lipid synthesizing enzymes

The study of the role played by membrane lipids, functional lipidomics, has become increasingly important in membrane biology. The physico-chemical properties of the lipids in biological membranes are subject to some fundamental requirements. In general, the acyl chains shall be in a liquid-like state to keep the membrane proteins active, and the lipids must form a bilayer structure in order for the membrane to be an insulating barrier. However, a potential ability of the lipids to form nonbilayer structures seems to be a prequisite for several membrane-associated cell processes. Therefore, organisms exposed to changes in the environmental conditions, such as temperature and uptake of fatty acids, adjust their membrane lipid composition. Examples of prokaryotic organisms that have been studied in this respect are the cell wall-less bacterium Acholeplasma laidlawii, the Gram-negative bacterium Escherichia coli, and the Gram-positive bacteria Bacillus megaterium and Clostridium butyricum, and among eukaroytic organisms are found fungi, higher plants, and poikilothermic animals. By synthesizing a proper combination of acyl chain and polar head group structures, the organisms modify the phase transition temperatures of the membrane lipids so that they are maintained in a lamellar liquid crystalline phase, and the formation of a lamellar gel phase as well as reserved nonlamellar phases is avoided. It has been shown that A. laidlawii and E. coli maintain a balance between lamellar-formed and nonlamellar-forming lipids. A growing body of evidence shows that nonlamellar-forming membrane lipids play essential roles in many aspects of membrance functioning. Short-lived nonbilayer structures are probably formed in the processes of fusion and fission of lipid bilayers, and long-lived bilayer structures with a small radius of curvature occur in several types of biological membranes (e.g. smooth endoplasmic reticulum, inner mitochondrial membrane, and prolamellar bodies). The activity of membrane-associated proteins can be modulated by adding detergents or nonlamellar-forming lipids to bilayers. Some examples are the regeneration of denatured bacteriorphodopsin, and the activities of protein kinase C, some phospholipases, and some key lipid synthases involved in the lipid metabolism of eukaryotic cells, A. laidlawii, and E. coli. The physico-chemical properties of the lipid matrix can be a direct feed-back signal on the activity of the lipid synthases. Finally, nonlamellar-forming lipids are essential for a proper cell division, and an efficient translocation of proteins across the plasma membrane, of E. coli cells.

Crystallization behavior of DSPE in dimethyl sulfoxide by time-resolved infrared spectroscopy and differential scanning calorimetry

Phase behavior of distearoylphosphatidylethanolamine (DSPE) dispersed in excess dimethyl sulfoxide has been examined by differential scanning calorimetry (DSC) and time-resolved Fourier-transform infrared spectroscopy (FT-IR). Transformation from lamellar-crystalline to liquid-crystalline phase was found to take place at temperature near 94°C upon initial heating and transformation from liquid-crystalline to lamellar-gel phase took place near 81°C during cooling scans. The kinetics of the transformation from liquid crystalline phase to lamellar-gel phase were analyzed by the well-known Avrami equation. The apparent Avrami exponents were found to be about 3 by DSC and time-resolved FT-IR. The effective dimensionality of the growth pattern can then be set as 2.

Effect of urea, dimethylurea, and tetramethylurea on the phase behavior of dioleoylphosphatidylethanolamine

The phase behavior of dioleoylphosphatidylethanolamine in aqueous solutions of urea, N, N′-dimethylurea (DMU), and N, N, N′, N′-tetramethylurea (TMU) has been characterized by synchrotron X-ray diffraction and differential scanning calorimetry. All three solutes stabilize the lamellar liquid-crystalline phase at the expense of lamellar-gel phase and inverted hexagonal phase of the phospholipid when present in concentrations up to 3 M. X-ray diffraction data demonstrated that the repeat spacing of DOPE increased with increasing urea concentration, but decreased as the DMU and TMU concentrations increased. The repeat spacing of DOPE in the liquid-crystal phase dispersed in the three solutes is d(urea)> d(DMU)> d(TMU). The molecular mechanisms underlying these observations are discussed in terms of either membrane Hofmeister effect, where urea acts as a water structure breaker, or a direct insertion effect of the amphiphilic DMU and TMU molecules into the lipid head groups in the interfacial region of the phospholipid bilayer.

Phase separations of α-tocopherol in aqueous dispersions of distearoylphosphatidylethanolamine

The effect of α-tocopherol on the structure and thermotropic phase behaviour of distearoylphosphatidylethanolamine was examined by using synchrotron X-ray diffraction methods. There was evidence that α-tocopherol does not distribute randomly in the dispersed phospholipid but instead phospholipid phases enriched in α-tocopherol are formed. Heating codispersions from lamellar gel phase induced formation of hexagonal-II phase at temperatures below the main transition of the pure phospholipid and which were enriched in α-tocopherol. Codispersions containing 5 or 10 mol% α-tocopherol were induced to form a cubic phase at temperatures above the lamellar to hexagonal-II phase transition. Such phases were not observed in codispersions containing 2.5 or 20 mol% α-tocopherol in which only lamellar and hexagonal-II phases were formed. The space group of the cubic phases were tentatively assigned as Pn3 m. Equilibration of codispersions at 4 °C results in the formation of lamellar crystalline phases enriched in α-tocopherol and phase separated domains of pure phospholipid. Two lamellar crystalline phases were characterized on the basis of their particular wide-angle X-ray scattering patterns. The lamellar crystalline phases were also distinguished from other lamellar phases of the pure phospholipid by the lamellar repeat. Partitioning of α-tocopherol into phosphatidylethanolamine domains in membranes may introduce instability into the structure.

Skin Barrier Structure and Function: The Single Gel Phase Model

A new model for the structure and function of the mammalian skin barrier is postulated. It is proposed that the skin barrier, i.e., the intercellular lipid within the stratum corneum, exists as a single and coherent lamellar gel phase. This membrane structure is stabilized by the very particular lipid composition and lipid chain length distributions of the stratum corneum intercellular space and has virtually no phase boundaries. The intact, i.e., unperturbed, single and coherent lamellar gel phase is proposed to be mainly located at the lower half of stratum corneum. Further up, crystalline segregation and phase separation may occur as a result of the desquamation process. The single gel phase model differs significantly from earlier models in that it predicts that no phase separation, neither between liquid crystalline and gel phases nor between different crystalline phases with hexagonal and orthorhombic chain packing, respectively, is present in the unperturbed barrier structure. The new skin barrier model may explain: (i) the measured water permeability of stratum corneum; (ii) the particular lipid composition of the stratum corneum intercellular space; (iii) the absence of swelling of the stratum corneum intercellular lipid matrix upon hydration; and (iv) the simultaneous presence of hexagonal and orthorhombic hydrocarbon chain packing of the stratum corneum intercellular lipid matrix at physiologic temperatures. Further, the new model is consistent with skin barrier formation according to the membrane folding model of Norlén (2001). This new theoretical model could fully account for the extraordinary barrier capacity of mammalian skin and is hereafter referred to as the single gel phase model.

Characterization of the Kinetic Phase Transition of Phospholipids Using Avrami and Tobin Models

AbstractMechanism of the lamellar crystalline phase formation of distearoyl‐phosphatidylethanolamine (DSPE) dispersed in excess glycerol has been examined by differential scanning calorimetry. It was found that transformation of liquid‐crystal phase to a crystalline phase must be mediated by a lamellar‐gel phase. Further examination of the kinetic phase behavior using Avrami and Tobin models suggested a single dimensional growing pattern and a three‐step mechanism of the crystallization, consisting of nucleation, normal growth, and restricted growth.

Partition coefficients of charged and uncharged local anesthetics into dipalmitoylphosphatidylcholine bilayer membrane: estimation from pH dependence on the depression of phase transition temperatures

Effects of the local anesthetics, dibucaine, bupivacaine and lidocaine on the phase transition temperatures of dipalmitoylphosphatidylcholine (DPPC) bilayer membrane were studied by the optical method. We focus our attention on pH dependence of the depression of main transition and pretransition temperatures. The temperatures of both transitions of DPPC bilayer membrane were depressed by the addition of anesthetics; the higher the value of pH, the larger the depression of main transition temperature and/or pretransition temperature by anesthetics. By extending the colligative thermodynamic framework to the depression of main transition temperature by an anesthetic, we can estimate the differential partition coefficient, which is defined by the difference in partition coefficients of an anesthetic into the ripple gel and liquid crystal phases. The difference in partition coefficient between the lamellar and ripple gel phases can also be estimated from the depression of pretransition temperature. Since the differential partition coefficients include both contributions of the charged and uncharged anesthetics, we could estimate the partition coefficients of the charged and uncharged anesthetic into the membranes from the pH dependence of differential partition coefficients. The liquid crystalline membrane of DPPC bilayer was more receptive to the uncharged local anesthetics than the charged species. The partition coefficients of the charged and uncharged anesthetics into the liquid crystalline phase of DPPC bilayer membrane were 3540 and 249 000 (for dibucaine), 1120 and 83 900 (for bupivacaine), 256 and 11 700 (for lidocaine), respectively. The transfer free energy of uncharged anesthetics from the aqueous phase to the liquid crystalline membrane was well correlated to the local anesthetic potency.

Packing characteristics of a model system mimicking cytoplasmic bacterial membranes

The phase diagram of fully hydrated mixtures of dipalmitoylphosphatidylethanolamine and -phosphatidylglycerol was constructed and the coexistence lines of the solidus and liquidus curve calculated based on regular solution theory using two nonideality parameters for each of the phase to account for nonideal and nonsymmetric mixing. Both lipids show nonideal miscibility in the liquid-crystalline phase, while a region of immiscibility exists in the lamellar-gel phase between the mole fraction x DPPE=0.05–0.4. Two lines of three-phase coexistence around 35 and 40 °C reflects the presence of lipid domains predominantly composed of phosphatidylglycerol as well as of the mixed lipid system. This is reflected in the positive nonideality parameters of the gel phase obtained from the simulation of the phase diagram. Moreover, segregation of pure phosphatidylethanolamine domains was detected in mixtures x DPPE>0.9, which formed multilamellar liposomes, while unilamellarity was observed for the mixed lipid systems owing to the presence of the negatively charged phosphatidylglycerol. The packing constraints of these phospholipids, major components of cytoplasmic bacterial membranes, may be of importance in the interaction with various solutes like antimicrobial peptides, and were explained based on the nature of the headgroups and the molecular geometry of the phospholipids.

Nanoparticle Arrays Formed by Spatial Compartmentalization in a Complex Fluid

A mesoscopically ordered lamellar gel phase of a polymer-grafted, lipid-based complex fluid is used as a scaffolding to spatially organize inorganic nanoparticles. The complex fluid provides both a highly anisotropic environment and a segregated aqueous and organic domains in which inorganic nanoparticles can be selectively placed by tailoring their size and surface characteristics. Three types of silver nanoparticlesunderivatized, surfactant-stabilized, and dodecanthiol-derivatizedwere evaluated. Comparison of the surface plasmon resonance of the various silver particles dispersed in conventional solvents to those contained within the complex fluid was used to determine the region of spatial localization in the lamellar gel phase. Silver particles rendered hydrophobic by capping with an alkane thiol insert into the hydrocarbon bilayer region. Surfactant-stabilized and underivatized silver nanoparticles reside in the aqueous channels, with the latter particles preferentially interacting with the grafted PEG chains/charged membrane interface region. Interparticle interaction between encapsulated hydrophilic silver particles can be enhanced by increasing the number of PEG repeat units (i.e., the length of the lipid-appended polymer). Examination of the X-ray diffraction profiles indicates that the gel-phase structure of the complex fluid is preserved upon introduction of all three types of nanoparticles. Guinier analysis of the low-q SAXS data for the intercalated silver yields particle sizes that are in good agreement with those determined by TEM prior to introduction, indicating that they remain as nonaggregated, discrete nanoparticles. These results not only demonstrate the use of complex fluids as a matrix in which to produce periodic arrays of encapsulated nanoparticle guests, but also suggest the possibility of employing them to modulate interactions between guests and, hence, their optical and electronic properties.

Cholesterol favors phase separation of sphingomyelin

The phase behavior of mixed lipid dispersions representing the inner leaflet of the cell membrane has been characterized by X-ray diffraction. Aqueous dispersions of phosphatidylethanolamine:phosphatidylserine (4:1 mole/mole) have a heterogeneous structure comprising an inverted hexagonal phase H II and a lamellar phase. Both phases coexist in the temperature range 20–45°C. The fluid-to-gel mid-transition temperature of the lamellar phase assigned to phosphatidylserine is decreased from 27 to 24°C in the presence of calcium. Addition of sphingomyelin to phosphatidylethanolamine/phosphatidylserine prevents phase separation of the hexagonal H II phase of phosphatidylethanolamine but the ternary mixture phase separates into two lamellar phases of periodcity 6.2 and 5.6 nm, respectively. The 6.2-nm periodicity is assigned to the gel phase enriched in sphingomyelin of molecular species comprising predominantly long saturated hydrocarbon chains because it undergoes a gel-to-fluid phase transition above 40°C. The coexisting fluid phase we assign to phosphatidylethanolamine and phosphatidylserine and low melting point molecular species of sphingomyelin which suppresses the tendency of phosphatidylethanolamine to phase-separate into hexagonal H II structure. There is evidence for considerable hysteresis in the separation of lamellar fluid and gel phases during cooling. The addition of cholesterol prevents phase separation of the gel phase of high melting point sphingomyelin in mixtures with phosphatidylserine and phosphatidylethanolamine. In the quaternary mixture the lamellar fluid phase, however, is phase separated into two lamellar phases of periodicities of 6.3 and 5.6 nm (20°C), respectively. The lamellar phase of periodicity 5.6 nm is assigned to a phase enriched in aminoglycerophospholipids and the periodicity 6.3 nm to a liquid-ordered phase formed from cholesterol and high melting point molecular species of sphingomyelin characterized previously by ESR. Substituting 7-dehydrocholesterol for cholesterol did not result in evidence for lamellar phase separation in the mixture within the temperature range 20–40°C. The specificity of cholesterol in creation of liquid-ordered lamellar phase is inferred.

The distribution of α-tocopherol in mixed aqueous dispersions of phosphatidylcholine and phosphatidylethanolamine

The effect of α-tocopherol on the structure and phase behaviour of mixed aqueous dispersions of phosphatidylcholine and phosphatidylethanolamine has been examined by synchrotron X-ray diffraction. Equimolar mixtures of dioleoylphosphatidylethanolamine:dioleoylphosphatidylcholine and dimyristoylphosphatidylcholine:dioleoylphosphatidylethanolamine did not show evidence of phase separation of an inverted hexagonal structure typical of α-tocopherol and phosphatidylethanolamine from lamellar phase. Mixed dispersions of dioleoyl derivatives of phosphatidylethanolamine:phosphatidylcholine (3:1) form a typical miscible gel phase at low temperatures but which phase separates into lamellar liquid–crystal and inverted hexagonal phases at temperatures greater than 65°C. The presence of 1, 2 or 5 mol% α-tocopherol caused a decrease in the temperature at which the inverted hexagonal phase appears. Phase separation of non-lamellar phase from lamellar gel phase can be detected in the presence of 7.5 and 10 mol% α-tocopherol, indicating a limited capacity of the phosphatidylcholine to incorporate α-tocopherol into the lamellar domain. A partial phase diagram of the ternary mixture has been constructed from the X-ray scattering data. It was concluded that there is no preferential interaction of α-tocopherol with phosphatidylethanolamine in mixed aqueous dispersions containing phosphatidylcholines.

Microemulsion polymerization of styrene stabilized by sodium dodecyl sulfate and diethylene glycol monoalkyl ether

The effects of the cosurfactants diethylene glycol monoalkyl ether [CiH2i+1O(CH2CH2O)jOH (CiEj; i=4, 6 and j=1, 2)] on the formation of an oil-in-water styrene (ST) microemulsion and the subsequent free radical polymerization were studied. For comparison, the data for the CiH2i+1OH (CiOH; i=4, 6) systems obtained from the literature were also included in this work. Sodium dodecyl sulfate was used as the surfactant. The pseudo three-component phase diagram (macroemulsion, microemulsion and lamellar gel phases) was constructed for each cosurfactant. The primary parameters selected for the polymerization study are the concentrations of cosurfactant and styrene. The number of latex particles nucleated is much smaller than that of the microemulsion droplets initially present in the reaction system. Limited flocculation of the latex particles occurs to some extent during polymerization. Among the cosurfactants investigated, the C4OH-containing polymerization system is the least stable. By contrast, the diethylene glycol monoalkyl ether group of CiEj tends to enhance the latex stability. CiEj is more effective in stabilizing the ST microemulsion and the subsequent polymerization in comparison with the CiOH counterpart.

Synchrotron X-ray investigations into the lamellar gel phase formed in pharmaceutical creams prepared with cetrimide and fatty alcohols

Semisolid liquid paraffin-in-water emulsions (aqueous creams) prepared from cetrimide/fatty alcohol mixed emulsifiers, and ternary systems formed by dispersing the mixed emulsifier in controlled percentages of water were examined as they aged using a combination of low and high angle X-ray diffraction measurements (Daresbury Laboratory Synchrotron Radiation Source). The results were correlated with the rheological properties measured in earlier studies. The cationic emulsifying wax showed phenomenal swelling in water. The reflection that incorporates interlamellar water increased continuously from 74 Å at 28% water to over 500 Å at 93% water. The trend was not influenced by the method of incorporation of the components and swollen lamellar phase was also identified in the corresponding emulsion. The swelling, which was due to electrostatic repulsion, was suppressed by salt and was reduced when the surfactant counterion was changed from Br − to Cl −. Changes in rheological properties on storage and in the presence of salt were correlated with changes in water layer thickness. High angle diffraction confirmed that the hydrocarbon bilayers were in the hexagonal α-crystalline mode of packing. Ternary systems and creams prepared from pure alcohols, although initially semisolid, were rheologically unstable and broke down. Low angle X-ray study into the kinetics of structure breakdown showed that the swollen lamellar gel phase formed initially swells even further on storage before separating.

Ripple Phases Induced by α-Tocopherol in Saturated Diacylphosphatidylcholines

The effect of α-tocopherol on the structure and phase behavior of dilauroyl-, dimyristoyl-, dipalmitoyl-, and distearoyl-phosphatidylcholines was examined using X-ray diffraction and freeze-fracture electron microscopic methods. A ripple phase was observed in all of the mixtures at temperatures well below the pretransition temperature of the corresponding pure phospholipid. Freeze-fracture studies indicated that with proportion of α-tocopherol less than 5 mol% a ripple phase with large periodicity (50–150 nm) predominated and with about 10 mol% α-tocopherol a ripple phase of periodicity about 16 nm was formed. With more than 10 mol% α-tocopherol planar bilayers tended to be formed. Partial phase diagrams of mixed aqueous dispersions of saturated phosphatidylcholines and α-tocopherol over temperature ranges about the gel to liquid-crystal phase boundary have been constructed. α-Tocopherol-enriched domains form ripple phases that coexist with regions of lamellar gel phase of the pure phospholipid in mixtures containing less than 10 mol% α-tocopherol. The presence of increasing amounts of α-tocopherol in the phospholipid causes an increase in the proportion of ripple phase at the expense of pure phospholipid bilayer indicating that the α-tocopherol-enriched domains might possess a defined stoichiometry of the two constituents.

Magnetic Field-Induced Ordering of a Polymer-Grafted Biomembrane−Mimetic Hydrogel

A biomembrane−mimetic complex fluid that spontaneously orients in the presence of a magnetic field to yield a highly ordered lamellar structure is described. Macroscopically oriented lamellae were produced by exploiting the inverted thermoreversible phase transition of the material, that is, by aligning the sample below the phase transition temperature (<16 °C) (i.e., in the fluid, hexagonal micellar phase) and warming to produce the lamellar gel phase in a 7.05 T magnetic field. The in situ field-induced alignment was studied by deuterium NMR. The lamellar domains were found to preferentially orient perpendicular to the applied field (negative order). Characterization of the magnetic field-induced anisotropy by polarized optical microscopy and small-angle X-ray scattering/diffraction (SAXS) indicates that it persists even upon field termination. The directional alignment was flipped by 90°, with the lamellar domains oriented parallel to the field (positive order), simply by modifying the composition through the addition of a lanthanide ion (Eu3+). The system offers the opportunity to spatially organize both membrane and aqueous soluble proteins in an anisotropic matrix, thereby facilitating structure and dynamic studies using a range of techniques, including magnetic resonance (both NMR as well as EPR), optical spectroscopy, and small-angle neutron and X-ray scattering.

Cardiolipin, α- d-glucopyranosyl, and l-lysylcardiolipin from Gram-positive bacteria: FAB MS, monofilm and X-ray powder diffraction studies

Cardiolipin preparations from Streptococcus B, Listeria welshimeri, Staphylococcus aureus, and a glucosyl and lysyl derivative of cardiolipin were analysed for fatty acid composition and fatty acid combinations. Three different fatty acid patterns are described and up to 17 molecular species were identified in Streptococcus B lipids by high resolution FAB MS. The physicochemical properties of these lipids were characterised in the sodium salt form by monofilm experiments and X-ray powder diffraction. All lipids formed stable monofilms. The minimal space requirement of unsubstituted cardiolipin was dictated by the fatty acid pattern. Substitution with l-lysine led to a decrease of the molecular area, substitution with d-glucopyranosyl to an increase. On self assembly at 100% relative humidity, all preparations adopted lamellar structures. They showed a high degree of order, in spite of the heterogeneous fatty acid compositions and numerous fatty acid combinations. The repeat distances in lamellar fluid phase varied between 4.99 and 5.52 nm, the bilayer thickness between 3.70 and 4.46 nm. Surprising were the low values of sorbed water per molecule of the glucosyl and lysyl derivatives which were 58 and 60%, compared with those of the respective cardiolipin. When Na + was replaced as counterion by Ba 2+, the bilayer structure was retained, but the lipids were in the lamellar gel phase and the fatty acids were tilted between 32 and 53° away from the bilayer normal. Wide angle X-ray diffraction studies and electron density profiles are also reported. Particular properties of glucosyl cardiolipin are discussed.

Phase Behavior of Phospholipid Bilayer Membranes under High Pressure.

High-pressure studies on the bilayer phase transitions of phosphatidylcholines containing linear saturated acyl chains are reviewed. Temperature-pressure phase diagrams of lipid bilayer membranes are shown; the pressure-induced, interdigitated gel phase is found in addition to liquid crystal, ripple gel and lamellar gel phases. The minimum pressure for the interdigitation of lipid bilayers decreases with an increase in acyl chain length in a non-linear manner. Thermodynamic quantities for phase transitions are also discussed.