Current Topics: Reviews
Membrane States of Saturated Glycerophospholipids: A Thermodynamic Study of Bilayer Phase Transitions
2019 Volume 67 Issue 4 Pages 300-307
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2019 Volume 67 Issue 4 Pages 300-307
Bilayer membranes formed by phospholipids vary in their membrane states by undergoing phase transitions in response to various external environmental factors. Pressure is one of these important environmental factors, but there are very few studies on the effects of pressure on phospholipid bilayer membranes. It is possible to deepen our understanding of the membrane states of phospholipid bilayer membranes by combining information regarding temperature- and/or ligand-responsivity with that regarding pressure-responsivity. In this review, we thermodynamically characterize the bilayer phase transitions of three kinds of saturated glycerophospholipids, each with a different polar head group (phosphatidyl-ethanolamine (PE), -choline (PC) or -glycerol (PG)), and explain their various membrane states depending on temperature and pressure. Both temperature- and pressure-responsivity reveal inherent features of these bilayer membranes: the metastability of the gel phase for PE bilayer membranes, the polymorphism of the gel phases for PC bilayer membranes and morphological changes in bilayer aggregates for PG bilayer membranes.
“Biological membrane” is the generic name of any cell membrane that separates the inside and outside of the cell, as well as the name for inner and outer membranes that constitute small organelles.1–3) A major constituent of biological membranes is phospholipids. When phospholipids are dispersed in water, they spontaneously self-associate and form closed microsomes of a bilayer structure called vesicles or liposomes. Since this bilayer structure is a fundamental structure of biological membranes, phospholipid bilayer membranes are widely used as a model system. The most significant feature of phospholipid bilayer membranes is that they sharply respond to changes in their surrounding environment and transform their bilayer structure accordingly; that is, they undergo phase transitions.4,5) Studies on phospholipid bilayer membranes have used various physicochemical methods to reveal their responsivity to temperature changes and ligand additions, as well as the effect on their aggregate structure.6–8) However, while pressure is an important environmental factor, pressure studies of phospholipid bilayer membranes have been limited to just a few factors,9–11) such as the pressure reversal of anesthesia, the pressure adaptation of deep-sea organisms, and pressure involved in food processing. Studies which analyze the membrane states of phospholipid bilayer membranes using pressure as an experimental variable are very few. It would seem that the application of high pressure is analogous to a depression in temperature, since pressure antagonizes temperature, but this is not the case. Changes in variables such as temperature and concentration include the diffusion process at propagation, followed by a delay in equilibration and a local gradient of the effect. In contrast, pressure generally propagates in a system isotropically and instantaneously following Pascal’s principle, without being subjected to such effects. Accordingly, large mechanical fluctuations caused by the application of pressure produce phenomena characteristic of pressure. In the present review, we focus on the main phospholipids in the biological membranes of living organisms, namely, three glycerophospholipids: phosphatidyl-ethanolamine (PE), -choline (PC) and -glycerol (PG). For these phospholipids, we introduce thermotropic and barotropic changes in the membrane states of bilayers formed by diacylphospholipids with two equivalent saturated fatty acids (myristic acid or palmitic acid), the phospholipids with which are frequently used in biological and model membrane studies.
The primary lipids contained in biological membranes are glycerophospholipids, sphingophospholipids, glycolipids and sterols. Among these, glycerophospholipids represent a large majority of the membrane lipids. The molecular structure of a glycerophospholipid is shown in Fig. 1(a). The basic backbone of this lipid is a tertiary alcohol glycerol, and two fatty acids and a phosphate with a polar head group bind to the glycerol backbone by dehydration condensation. There are many phospholipid species with this combination of two fatty acids and one polar head group binding to the glycerol backbone. The main phospholipids contained in biological membranes of eukaryotes, such as erythrocytes, are PEs and PCs, while those of prokaryotes, such as Escherichia coli, are PEs and PGs.2,3) In particular, bilayer membranes formed by saturated symmetric phospholipids with two equivalent saturated fatty acids of carbon number 14 or 16 (myristic acid or palmitic acid) exhibit a gel-to-liquid crystalline phase transition at around room temperature, and consequently, these have become the preferred phospholipids in physicochemical studies of lipid bilayer membranes.8,12,13) Figure 1(b) depicts the three-dimensional structures of the three kinds of saturated phospholipids in the gel phase described in this review.
(Color figure can be accessed in the online version.)
There are basically three possible phase states for a hydrated phospholipid bilayer membrane: a hydrated crystal phase (Lc), a gel phase (Lβ), and a liquid crystal phase (Lα).4,5,8) The Lc phase normally observed at a low temperature is called the subgel phase. This Lc phase is a membrane state that has a high order of molecular arrangement because the acyl chains of a lipid molecule in the Lc phase take the extended straight form with all zigzag trans conformations. In the Lc phase, the lipid molecules are packed regularly in the bilayer membrane, and the acyl-chain rotation around the long axis of the molecule is significantly restricted. In addition, the number of water molecules hydrated in the vicinity of the polar head group is small. As temperature increases, the Lc phase transforms into the Lβ phase. The acyl chains in the Lβ phase are all trans forms, like those in the Lc phase, but they can rotate around the long axis, and several more water molecules hydrate in the vicinity of the head group. Accompanying the increase in the freedom of this movement, several structures of gel-phase polymorphism, such as the chain tilting,14,15) periodic bilayer undulation,16,17) and bilayer interdigitation,18,19) appear in the gel phase. A further increase in temperature causes a transformation into the Lα phase. This phase transition corresponds to the chain-melting phenomena of lipid molecules in the bilayer membrane. The order of molecular arrangement in the Lα phase is low because the ratio of a gauche conformer in the acyl chains becomes high in this phase, and the number of water molecules hydrated in the vicinity of the head group also increases. Hence, the fluidity of the membrane increases. The membrane states and phase transitions observed for bilayer membranes of dipalmitoyl-PE (DPPE) and dipalmitoyl-PC (DPPC) are illustrated in Fig. 2.
The DPPE bilayer membrane undergoes an Lc/Lα or Lβ/Lα transition, depending on the thermal history of a lipid sample, and the Lβ phase exists as a stable phase only in the high-pressure region. The DPPC bilayer undergoes three phase transitions (sub-, pre- and main) with increasing temperature, and shows the polymorphism of three gel (Lβ′, Pβ′ and LβI) phases.
The thermotropic phase transitions of phospholipid bilayer membranes caused by changing temperature can be measured by a method of high-sensitivity differential scanning calorimetry (DSC).20) For the detection of barotropic phase transitions, we have adopted an optical method, since the light transmittance (i.e., turbidity) of a vesicle suspension distinctly changes at the phase-transition point.21,22) Both experimental methods are advantageous in detecting phase transitions with high accuracy at a low lipid concentration (ca. 1 mM). Moreover, the latter method can be easily applied to measurements under several special conditions such as high pressure. In the lipid concentration range where the water content of a vesicle suspension does not affect the phase-transition temperatures (ca. > 25 wt% of water content),23–25) we have investigated the states of bilayer membranes formed by various kinds of phospholipids by combining the DSC measurements under atmospheric pressure, as well as light-transmittance measurements taken under high pressure, and then characterized them in terms of temperature (T)–pressure (p) phase diagrams and the thermodynamic quantities of phase transitions.26,27)
The DSC thermograms obtained for dimyristoyl-PE (DMPE) and DPPE bilayer membranes are demonstrated in Fig. 3. The three phases explained previously, Lc, Lβ and Lα, were observed for both of these bilayer membranes under atmospheric pressure. The appearance of an Lc or Lβ phase is dependent on the thermal history of the lipid sample: the bilayer membranes undergo the phase transition from the Lc phase to the Lα phase in the first heating scan for thermally pre-treated samples (i.e., thermal annealing), whereas the phase transition from the Lβ phase to the Lα phase occurs in the second heating scan, where the sample is cooled down and reheated immediately after the first scan. This is because it takes a long time to completely form the Lc phase at low temperatures, and consequently the Lβ phase is a metastable phase under atmospheric pressure.28–30) Since the size of an ethanolamine group (–CH2–CH2–N+H3) of the PE molecule is small, there is less steric hindrance among the polar head groups, so the acyl chains are oriented almost perpendicularly to the bilayer surface. Therefore, those phase transitions dependent upon packing states (i.e., between gel phases) are not observed in the PE bilayer membranes. Further, one PE molecule in the bilayer membrane can form hydrogen bonding between a hydrogen atom bound to a nitrogen atom quaternalized in an ethanolamine group and an oxygen atom of the phosphate group in the adjacent PE molecule.29,31) The strong interaction between polar head groups hinders the lateral expansion of the bilayer membrane and creates the dense bilayer packing. This dense packing brings about the easy dehydration of water molecules around the head group, and leads to the formation of a stable Lc phase by a relatively simple thermal annealing procedure.32)
The peak-top temperatures are given in the figure. In the enthalpy diagrams, enthalpy curves of each phase relative to that of the most stable hydrated crystal phase are drawn together with the enthalpy values. (Color figure can be accessed in the online version.)
Figure 4 shows the T–p phase diagrams constructed for DMPE and DPPE bilayer membranes.33,34) By applying pressure, the temperatures of the Lc/Lα and Lβ/Lα transitions increased, and both transition temperatures approached the other since the pressure dependence of the temperature (dT/dp) of the Lβ/Lα transition was larger than that of the Lc/Lα transition. The phase-boundary curves of both transitions do not intersect in the whole pressure range measured in the DMPE bilayer membrane. But since the curve of the Lc/Lα transition greatly shifts to the high-temperature region by a large stabilization of the Lβ phase, accompanied by increasing acyl chain length, the curves of the Lc/Lα and Lβ/Lα transitions intersect at about 22 MPa in the DPPE bilayer membrane, indicating a stability of the gel phase changes under pressure. Namely, the Lβ phase exists as a metastable phase at low pressures, below 22 MPa, while at high pressures above 22 MPa it remains stable. According to this stability variance of the Lβ phase, we can observe one phase transition depending on the thermal history under atmospheric pressure, while we can always detect two consecutive Lc/Lβ and Lβ/Lα transitions under high pressure. There is no polymorphism of the gel phase in the PE bilayer membranes unlike the PC bilayer membranes described in the next section, and the gel phase exists stably only under high pressure. The phase behavior of the PE bilayer membranes can be interpreted by a relatively simple phase diagram defined by the Lβ/Lα and Lc-phase related transitions.
Phase transitions: (□) Lc/Lα or Lc/Lβ transition, (○) Lβ/Lα transition. Solid and dotted lines indicate the phase boundaries between stable phases and those between metastable phases, respectively. Metastable phases are shown in parentheses. (Color figure can be accessed in the online version.)
A PC molecule has a large-sized choline (–CH2–CH2–N+(CH3)3) group as a polar head group, in contrast with a PE molecule. The DSC thermograms obtained for dimyristoyl-PC (DMPC) and DPPC bilayer membranes are drawn in Fig. 5. Four different membrane states, Lc, lamellar gel (Lβ′), ripple gel (Pβ′) and Lα phases, can be observed in the PC bilayer membrane under atmospheric pressure.24,35) Here, a prime symbol “ ′ ” indicates that the hydrophobic chain tilts to the bilayer normal. The three phase transitions occurring in turn, from low temperature, Lc/Lβ′ (or Lc/Pβ′), Lβ′/Pβ′ and Pβ′/Lα transitions, are commonly called the sub, pre and main transitions from the historical background of the PC membrane studies36–38); they correspond to the three endothermic peaks in the thermograms. The formation of the Lc phase in these bilayer membranes requires a sufficiently dynamic annealing treatment that repeats a freeze-thaw cycle before measurement.35,39,40) Thus, no subtransition is observed in the thermogram of the second heating scan immediately after the first heating scan. The pretransition is a transition between gel phases originating from the fluctuation of packing structure in the gel phase, in which a flat bilayer structure changes into a ripple bilayer structure. In the gel phase of these PC bilayer membranes, the stable distance among the acyl chains is maintained by tilting them by about 30 degrees from the bilayer normal, since they cannot be oriented perpendicularly due to the large choline head group; thus, the van der Waals interaction is strengthened.
The peak-top temperatures are given in the Figure. In the enthalpy diagrams, enthalpy curves of each phase relative to that of the most stable hydrated crystal phase are drawn, together with the enthalpy values. (Color figure can be accessed in the online version.)
The T–p phase diagrams constructed for the DMPC and DPPC bilayer membranes are shown in Fig. 6.22,33,41) All three phase-transition temperatures observed under atmospheric pressure were elevated by applying pressure. A noticeable feature of these PC bilayer membranes is that they form one of the non-bilayer membranes, called the interdigitated gel (LβI) phase,42,43) where the acyl chains of the DPPC molecule in a monolayer of one side of a bilayer interpenetrate alternatively into a monolayer of the other side, at high pressures above a certain pressure (ca. 300 MPa for DMPC and ca. 100 MPa for DPPC). The formation of the LβI phase is attributable to the large steric hindrance among adjacent choline groups of the PC molecules. The polymorphism of the gel phases, such as the Lβ′, Pβ′ and LβI phases, is characteristic of bilayers of PCs with a bulky choline head group (cf. Fig. 2(b)). In contrast with other transitions, the temperature of the Lβ′/LβI transition observed at high pressures decreases by applying pressure. Since the dT/dp value of a phase transition can be approximated by Clapeyron’s equation (dT/dp = ΔV/ΔS = TΔV/ΔH), and the ΔH value of the Lβ′/LβI transition is positive, the negative dT/dp value of the transition means that the ΔV value of the transition has a negative value, and is closely related to the difference in compressibility between the Lβ′ and LβI phases.22) The Lc/Lβ′-transition curve intersects the Lβ′/LβI-transition curve under high pressure (ca. 310 MPa for DMPC and ca. 115 MPa for DPPC) because of the negative dT/dp values of the Lβ′/LβI transition. The Lβ′/LβI transition becomes a transition between metastable phases above the pressure of the intersection point.33) In the DMPC bilayer membrane, there exists another intersection point between the sub- (Lc/Pβ′) transition and the pre- (Lβ′/Pβ′) transition curves at ca. 35 MPa, and then the metastable Lβ′ phase under atmospheric pressure converts to a stable one above the pressure of the intersection point.41) On the other hand, since the sub- (Lc/Lβ′) transition curve of the DPPC bilayer membrane is located in a lower-temperature region than the pre- and main (Pβ′/Lα) transitions relative to the entire pressure range, all three gel phases appear as stable phases on the phase diagram.
Phase transitions: (□) Lc/Lβ′, Lc/Pβ′ or Lc/LβI transition, (△) Lβ′/Pβ′, Lβ′/LβI or LβI/Pβ′ transition, (○) Pβ′/Lα transition. Solid and dotted lines indicate the phase boundaries between stable phases and those between metastable phases, respectively. Metastable phases are shown in parentheses. (Color figure can be accessed in the online version.)
PG is an acidic phospholipid with a negative charge in the polar head group. It has a glycerol group (–CH2–CH(OH)–CH2(OH)) that binds to the phosphate group of the head group by dehydration condensation. The head-group interaction of PG is markedly different from that of the neutral phospholipids PE and PC because the repulsive electrostatic interaction among negative charges occurs in the head group. We have so far described the results for the PE and PC bilayer membranes in water. However, the PG bilayer membrane forms vesicles with large membrane curvature in water, and reduces the membrane multiplicity due to the large repulsion among the head group; namely, it forms small unilamellar vesicles. In this case, the cooperativity of phase transitions remarkably diminishes, which makes it difficult to observe the phase transitions experimentally. For comparison of the PG bilayer membrane with the PE and PC bilayer membranes, we here explain the membrane states of dimyristoyl-PG (DMPG) and dipalmitoyl-PG (DPPG) bilayer membranes obtained under the condition that the electrostatic interaction among the head groups is shielded by the addition of a high concentration of a salt, NaCl.
Figure 7 illustrates the DSC thermograms obtained for the DMPG and DPPG bilayer membranes formed in 1.0 mol kg−1 aqueous NaCl solution. Unlike the PE and PC bilayer membranes, the PG bilayer membrane exhibits complicated thermal behavior depending on the methods of annealing treatment.44,45) In the first heating scan, the DSC thermograms of lipid samples obtained by static annealing, which is a cold-storage treatment over a relatively short period, show two (pre- and main) transitions for the DMPG bilayer membrane, and three (sub-, pre- and main) transitions for the DPPG bilayer membrane. In the second heating scan, two (pre- and main) transitions appear in both PG bilayer membranes (data not shown). This thermal behavior is similar to that observed in the PC bilayer membranes. By contrast, the thermograms for the lipid samples obtained by dynamic annealing, which contains a number of freeze-thaw cycles over a long period, show completely different behavior as compared with those obtained by static annealing: an asymmetric endothermic peak and a pair of consecutive small and large endothermic peaks are observed, respectively, for the DMPG and DPPG bilayer membranes in the higher-temperature region than the main-transition temperature. A newly appearing peak, observed in both PG bilayer membranes by the dynamic annealing, is the phase transition from the stable hydrated crystal (Lc2) phase,44) the morphology of which is a rod-like shape, to the Lα phase.46,47) On the other hand, the lowest-temperature peak observed in the DPPG bilayer membrane by the short-period annealing arises from the metastable hydrated crystal (Lc1) phase,44) with a spherical shape.48) The Lc1 phase is not observed in the DMPG bilayer membrane because it promptly transforms into the Lc2 phase. Furthermore, in the two consecutive endothermic peaks observed for the Lc2/Lα transition of the DPPG bilayer membrane, the small peak at a lower temperature corresponds to the transition occurring at the edge, under large packing stress in the rod-like aggregate, while the large peak, at a higher temperature, represents the transition at the body part, with almost no packing stress of the aggregate.45) The DMPG bilayer membrane also shows similar thermal behavior, the transition temperatures of these transitions are close, and the Lc2/Lα transition is observed as an asymmetric peak.46) Thus, variations in the morphology in bilayer aggregates is also caused by changes in the membrane state of the PG bilayer membranes, since the electrostatic interaction among the polar head group significantly affects the surface curvature of the bilayer membrane.
The peak-top temperatures are given in the figure. In the enthalpy diagrams, enthalpy curves of each phase relative to that of the most stable hydrated crystal phase are drawn, together with the enthalpy values. The value of Lc1/Lβ′ transition in parenthesis was obtained under a specific annealing condition.45) (Color figure can be accessed in the online version.)
The T–p phase diagrams for the DMPG and DPPG bilayer membranes obtained by static and dynamic annealing in 1.0 mol kg−1 aqueous NaCl solution are shown in Fig. 8.27,49) By applying pressure to lipid samples prepared by the static annealing, the temperatures of all transitions observed under atmospheric pressure increase, and the DPPG bilayer membrane forms the LβI phase under high pressure.27,49) On the other hand, when pressure was applied to lipid samples of the Lc2 phase prepared by dynamic annealing, only the Lc2/Lα-transition temperature was observed to increase. Since the dT/dp value of the Lc2/Lα-transition curve is smaller than that of the main-transition curve, both curves intersect at about 75 MPa in the DPPG bilayer membrane. In the form of spherical vesicles of the PG bilayer membranes, the binding of Na+ ions to the negatively charged head groups is incomplete, and consequently, the distance between the polar head groups is expanded, and the head-group area becomes larger due to electrostatic repulsion among the head groups. Although this interaction is dependent on the NaCl concentration, the barotropic phase behavior of the PG bilayer membranes at the NaCl concentration presented here resembles that of the PC bilayer membranes,22) with the gel-phase polymorphism as given in Fig. 6. By contrast, the complete dehydration of water molecules around the head group, by dynamic annealing, strengthens the binding of Na+ ions to the head groups and brings about sufficient shielding from the electrostatic effect. In this state, the head-group area contracts and the vesicle curvature is markedly reduced. Thus, the vesicle aggregate transforms into a rod-like one. Since the Lc2/Lα transition is located in the higher-temperature region than the Pβ′/Lα transition under atmospheric pressure, the barotropic phase behavior of the PG bilayer membranes in this case resembles that of the PE bilayer membranes,34) as shown in Fig. 4. Therefore, the complicated behavior of the membrane states induced in the PG bilayer membranes can be understood by the superimposing of those observed for PE and PC bilayer membranes.
Solid lines indicate the phase boundaries between stable phases, and broken lines correspond to the boundaries for the spherical aggregates. Dotted lines in the figure (b) indicate the unobservable Lc2/Pβ′ transition (upper part) and the transition between the metastable Lβ′ and the metastable LβI phases (lower part), respectively. (Color figure can be accessed in the online version.)
Finally, we compared the thermodynamic properties of the phase transitions for these phospholipid bilayer membranes which were determined from both thermal and pressure measurements. Table 1 summarizes the thermodynamic properties of the phase transition for the PE, PC and PG bilayer membranes under atmospheric pressure, according to the chain length of the phospholipids. The phase-transition temperatures and enthalpies are also drawn as enthalpy diagrams in the lower part of Figs. 3, 5 and 7. Each phase-transition temperature of all the phospholipid bilayer membranes increased with an increase in acyl chain length due to the hydrophobic interaction strengthening with the chain length. Among the three phospholipids, the gel/Lα-transition temperature of the bilayer membrane of PE was higher than that of the bilayer membranes of PC and PG at the same hydrophobic chain length, and the Lc/Lα-transition temperature of PE was also higher than that of the bilayer membrane of PG with the same chain length. This fact suggests that the PE molecules form a more tightly packed membrane than the PC and PG molecules through intermolecular hydrogen bonding in the bilayer membrane.34) In addition, each phase transition has a characteristic dT/dp value: 0.13–0.15 K MPa−1 for the pretransition, and 0.21–0.25 K MPa−1 for the gel/Lα transition.22,40) Regarding the Lc-phase related transition, although the dT/dp value ranges widely, from 0.08 K MPa−1 (Lc/Pβ′ transition for the DMPC bilayer membrane) to 0.22 K NPa−1 (Lc/Lα transition for the DMPE bilayer membrane), this difference is attributable to differences in the phase to which the membrane converts, as well as to differences in the stability of the Lc phase. We have utilized these dT/dp values as a method of identification of the various phase transitions.
| Lipid | Transition | T (°C) | dT/dP (K MPa−1) | ΔH (kJ mol−1) | ΔS (J K−1 mol−1) | ΔV (cm3 mol−1) | |
|---|---|---|---|---|---|---|---|
| Myristoyl series | DMPE | Lβ/Lα | 49.8 | 0.251 | 25.1 | 78 | 19.5 |
| Lc/Lα | 56.9 | 0.223 | 80.8 | 245 | 54.5 | ||
| DMPC | Lc/Pβ′ | 17.1 | 0.080 | 25.7 | 89 | 7.1 | |
| Lβ′/Pβ′ | 14.9 | 0.130 | 4.5 | 16 | 2.0 | ||
| Pβ′/Lα | 24.0 | 0.212 | 25.7 | 87 | 18.3 | ||
| DMPGa) | Lβ′/Pβ′ | 19.5 | 0.152 | 3.5 | 12 | 1.8 | |
| Pβ′/Lα | 24.7 | 0.225 | 24.0 | 80 | 18.1 | ||
| Lc2/Lα | 42.5 | 0.153 | 69.0 | 219 | 33.4 | ||
| Palmitoyl series | DPPE | Lβ/Lα | 63.5 | 0.252 | 34.7 | 103 | 26.0 |
| Lc/Lα | 65.3 | 0.210 | 91.2 | 269 | 56.6 | ||
| DPPC | Lc/Lβ′ | 21.5 | 0.180 | 26.3 | 89 | 16.1 | |
| Lβ′/Pβ′ | 34.3 | 0.130 | 4.6 | 15 | 2.0 | ||
| Pβ′/Lα | 42.0 | 0.220 | 36.4 | 116 | 25.4 | ||
| DPPGa) | Lc1/Lβ′ | 32.0 | 0.140 | 28.2 | 92 | 12.9 | |
| Lβ′/Pβ′ | 40.7 | 0.149 | 5.4 | 17 | 2.6 | ||
| Pβ′/Lα | 42.4 | 0.224 | 35.4 | 112 | 25.1 | ||
| Lc2/Lα | 48.0 | 0.170 | 96.9 | 302 | 51.3 | ||
a) The values were determined in 1.0 mol kg−1 aqueous NaCl solution.
The thermodynamic quantities of the phase transitions are also characteristic for each phase transition. The thermodynamic quantities of the gel/Lα transition, i.e., the chain melting transition, have nearly equivalent values to most phospholipids if the acyl chain length is the same, although the values increase in a chain-length dependent manner for each bilayer membrane.22,34,40,45) We say this based on the fact that the melting of acyl chains is almost the same thermodynamically, irrespective of the head-group structures. On the other hand, the thermodynamic quantities of the pretransition are considerably small, and exhibit no acyl-chain length dependence.22,40) Since only the packing state of the bilayer membrane changes, without changing the acyl-chain conformation at the pretransition, it proves that this change is small, energetically. Concerning Lc-phase related transitions, the thermodynamic quantities of the Lc/Lβ transition of the PE bilayer membranes can be obtained by the difference in thermodynamic quantities between the Lc/Lα and Lβ/Lα transitions. These resulting quantities of the Lc/Lβ transition of the PE bilayer membranes is about twice as large as the corresponding quantities of the Lc/Lβ′ or Lc/Pβ′ transition of the PC bilayer membranes. These findings are well correlated to the fact that the PE bilayer membranes form the most stable Lc phase, with a lower energy level, and there exist fewer water molecules in the interlamellar region of the Lc phase.32) In the PG bilayer membranes, judging from the large thermodynamic quantities of the Lc2/Lα transition, the PG molecules assume a highly stable state in the rod-like aggregate.
In this review, we described the phase states of bilayer membranes formed by three types of typical phospholipids: saturated diacyl-PE, -PC and -PG, thermodynamically. Although similar comparative studies on the thermal behavior of these phospholipid bilayer membranes have been reported,50,51) the combination of the pressure study with the thermal study clarifies the difference in bilayer membranes among these phospholipids. The PC bilayer membranes are most frequently used in biological membrane studies; it is understandable from the phase diagrams that the PE bilayer membranes exhibit simpler but fundamental membrane states. One of the characteristics of the PE bilayer membrane is the metastability of the Lβ phase resulting from the highly stable Lc phase, while the PC bilayer membrane is characterized by the polymorphism of the gel phases attributable to the bulky choline head group. For the PG bilayer membranes, the membrane states become further complicated due to the head group with a negative charge, which results in the polymorphism of the Lc phase arising from different aggregate morphology depending on the bilayer packing.
Lipid bilayer membranes that form the basis of biological membranes are not general biomacromolecules, such as nucleic acids and proteins, the constituent units of which are linked by covalent bonds, but rather, self-organized molecular aggregates driven by hydrophobic interaction. Accordingly, the aggregates formed by lipids can produce various membrane states by adapting to their surrounding environments. In particular, the pressure responsivity of lipid bilayer membranes reveals their characteristics inherent to each constituent lipid molecule.
The authors declare no conflict of interest.
