Kaori Tada

Thermotropic and Barotropic Phase Behavior of Phosphatidylcholine Bilayers

Bilayers formed by phospholipids are frequently used as model biological membranes in various life science studies. A characteristic feature of phospholipid bilayers is to undergo a structural change called a phase transition in response to environmental changes of their surroundings. In this review, we focus our attention on phase transitions of some major phospholipids contained in biological membranes, phosphatidylcholines (PCs), depending on temperature and pressure. Bilayers of dipalmitoylphosphatidylcholine (DPPC), which is the most representative lipid in model membrane studies, will first be explained. Then, the bilayer phase behavior of various kinds of PCs with different molecular structures is revealed from the temperature–pressure phase diagrams, and the difference in phase stability among these PC bilayers is discussed in connection with the molecular structure of the PC molecules. Furthermore, the solvent effect on the phase behavior is also described briefly.

Barotropic and thermotropic bilayer phase behavior of positional isomers of unsaturated mixed-chain phosphatidylcholines

The bilayer phase transitions of six kinds of mixed-chain phosphatidylcholines (PCs) with an unsaturated acyl chain in the sn-1 or sn-2 position, 1-oleoyl-2-stearoyl- (OSPC), 1-stearoyl-2-oleoyl- (SOPC), 1-oleoyl-2-palmitoyl- (OPPC), 1-palmitoyl-2-oleoyl- (POPC), 1-oleoyl-2-myristoyl- (OMPC) and 1-myristoyl-2-oleoyl-sn-glycero-3-phosphocholine (MOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light transmittance measurements. Bilayer membranes of SOPC, POPC and MOPC with an unsaturated acyl chain in the sn-2 position exhibited only one phase transition, which was identified as the main transition between the lamellar gel (L(beta)) and liquid crystalline (L(alpha)) phases. On the other hand, the bilayer membranes of OSPC, OPPC and OMPC with an unsaturated acyl chain in the sn-1 position exhibited not only the main transition but also a transition from the lamellar crystal (L(c)) to the L(beta) (or L(alpha)) phase. The stability of their gel phases was markedly affected by pressure and chain length of the saturated acyl chain in the sn-2 position. Considering the effective chain lengths of unsaturated mixed-chain PCs, the difference in the effective chain length between the sn-1 and sn-2 acyl chains was proven to be closely related to the temperature difference of the main transition. That is, a mismatch of the effective chain length promotes a temperature difference of the main transition between the positional isomers. Anomalously small volume changes of the L(c)/L(alpha) transition for the OPPC and OMPC bilayers were found despite their large enthalpy changes. This behavior is attributable to the existence of a cis double bond and to significant inequivalence between the sn-1 and sn-2 acyl chains, which brings about a small volume change for chain melting due to loose chain packing, corresponding to a large partial molar volume, even in the L(c) phase. Further, the bilayer behavior of unsaturated mixed-chain PCs containing an unsaturated acyl chain in the sn-1 or sn-2 position was well explained by the chemical-potential diagram of a lipid in each phase.

Pressure effect on the bilayer phase transition of asymmetric lipids with an unsaturated acyl chain

The bilayer phase transitions of mixed‐chain lipids with monounsaturated acyl chain in the sn‐2 position, 1‐myristoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (MOPC), 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (POPC), and 1‐stearoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (SOPC), and with a polyunsaturated acyl chain in the sn‐2 position, 1‐stearoyl‐2‐linoleoyl‐sn‐glycero‐3‐phosphocholine (SLPC), 1‐stearoyl‐2‐arachidonoyl‐sn‐glycero‐3‐phosphocholine (SAPC), and 1‐stearoyl‐2‐docosahexaenoyl‐sn‐glycero‐3‐phosphocholine (SDPC), were observed by differential scanning calorimetry (DSC) under ambient pressure and by light‐transmittance measurements under high pressure. The DSC thermogram for each lipid bilayer showed only one transition between the lamellar gel and liquid crystalline phases. The introduction of one or two cis double bonds into the sn‐2 acyl chain caused the significant depression of the main‐transition temperature and an obvious decrease of enthalpy and volume changes associated with the transition. These features are attributable to loose packing of saturated and unsaturated acyl chains in the bilayer gel phase. The existence of four or six double bonds in the sn‐2 chain produced no further decrease in the transition temperature, and in fact six double bonds caused a slight increase in the transition temperature. Thermodynamic properties associated with the bilayer phase transition were discussed.

Sudachitin, a polymethoxyflavone from Citrus sudachi, suppresses lipopolysaccharide-induced inflammatory responses in mouse macrophage-like RAW264 cells.

Although some polymethoxyflavones possess several important biological properties, including neuroprotective, anticancer, and anti-inflammatory ones, sudachitin, a polymethoxyflavone from Citrus sudachi, has been little studied. In this study, we found that sudachitin inhibited nitric oxide production by suppressing the expression of inducible nitric oxide synthase in lipopolysaccharide-stimulated macrophages, indicating that sudachitin has an anti-inflammatory effect.

High-pressure study on bilayer phase behavior of oleoylmyristoyl- and myristoyloleoyl-phosphatidylcholines

We investigated the thermotropic and barotropic bilayer phase behavior of 1-myristoyl-2-oleoyl-sn-glycero-3-phosphocholine (MOPC) and 1-oleoyl-2-myristoyl-sn-glycero-3-phosphocholine (OMPC) by means of the differential scanning calorimetry (DSC) and high-pressure light-transmittance technique. Water could be used as a solvent for measurements at high pressures because of the elevation of the transition temperatures above 0 degrees C by pressurization, whereas aqueous 50 wt.% ethylene glycol solution was used mainly for those at low pressures. Only one phase transition was observed in the DSC thermogram of the MOPC bilayer membrane as an endothermic peak, and also observed at high pressures as an abrupt change of the light-transmittance. The transition was assigned as a main transition between the lamellar gel (L(beta)) and liquid-crystalline (L(alpha)) phases on the basis of the values of enthalpy change (DeltaH) and slope of the transition temperature with respect to pressure (dT/dP). The DSC thermogram of the OMPC bilayer membrane similarly showed a single endothermic peak but two kinds of phase transitions were observed at different temperatures in the light-transmittance profile at high pressures. The extrapolation of the lower-temperature transition in the high-pressure range to an ambient pressure coincided with the transition observed in the DSC thermogram. This transition was identified as a transition between the lamellar crystal (L(c)) and L(alpha) (or L(beta)) phases from the DeltaH and dT/dP values. The higher-temperature transition, appearing only at high pressures, was identified as the L(beta)/L(alpha) transition considering the topological resemblance of its temperature-pressure phase diagram as that of the dioleoylphosphatidylcholine bilayer membrane. The phase diagram of the OMPC bilayer membrane demonstrated that the L(beta) phase cannot exist at pressures below ca. 190 MPa while it can exist stably in a finite temperature range at pressures above the pressure.

Thermodynamic characterization of bilayer-nonbilayer phase transitions of phospholipid membranes

We determined thermodynamic properties of phase transitions between bilayer and nonbilayer for phosphatidylcholines with saturated hydrophobic chains (C18:0-PC, O-C18:0-PC) and phosphatidylethanolamines with unsaturated ones (C18:1(cis)-PE, C18:1(trans)-PE) by means of calorimetry under ambient pressure and optical measurements under high pressure. The thermodynamic quantities of the transitions between bilayer and nonbilayer were much smaller than those of the transition between bilayers (gel-liquid crystal or hydrated crystal-liquid crystal transition) for the corresponding phospholipids. Although the nonbilayer formations correspond to a dynamic transformation between lamellar structure and nonlamellar structure, we can say that the order of the lipid molecule in both structures may not appreciably change judging from the smaller thermodynamic quantities. A notable feature of the bilayer-nonbilayer transitions is the large pressure dependence of the transition temperature as compared with that of the bilayer-bilayer transitions. Comparing the enthalpy and volume changes of the bilayer-nonbilayer transitions with those of the bilayer-bilayer transitions, we concluded that the former transitions can be regarded as the volume-driven transitions for the reconstruction of molecular packing.

Effect of pressure on the bilayer phase transitions of asymmetric lipids with an unsaturated acyl chain in sn-1 position

The bilayer phase transitions of a series of mixed-chain phosphatidylcholines (PCs) with an unsaturated chain in the sn-1 position and saturated chain with different lengths in the sn-2 position, 1-oleoyl-2-myristoyl-PC (OMPC), 1-oleoyl-2-palmitoyl-PC (OPPC) and 1-oleoyl-2-stearoyl-PC (OSPC), were observed by differential scanning calorimetry (DSC) under ambient pressure and light transmittance measurements under high pressure. All the PC bilayer membranes exhibited not only the main transition from the lamellar gel (Lβ) phase to the liquid crystalline (Lα) phase but also the transition from the lamellar crystal (Lc) to the Lβ (or Lα) phase. The thermodynamic quantities of the transition between the Lc and Lα phases for bilayers of unsaturated mixed-chain PCs were compared with those of the corresponding mixed-chain PCs with saturated acyl chains such as 1-stearoyl-2-myristoyl-PC (SMPC), 1-stearoyl-2-palmitoyl-PC (SPPC) and 1,2-distearoyl-PC (DSPC). It turned out that the partial molar quantities of the unsaturated mixed-chain PC bilayers in the Lc phase are larger than those of the saturated mixed-chain PCs bilayers. The increase in the partial molar quantities in the Lc phase by the introduction of a cis double bond into the sn-1 acyl chain is attributable to the loose packing of the saturated and unsaturated acyl chains in the Lc phase.

Packing state in bilayer membranes of diacylphosphatidylcholines with varying acyl chain lengths under high pressures

The bilayer packing states of a series of diacylphosphatidylcholines (CnPC) containing linear saturated acyl chains were examined by a high-pressure fluorescence method. We revealed from the second derivatives of Prodan fluorescence spectra for all bilayer membranes that the Prodan molecules can be distributed into multiple sites in these bilayer membranes and move around the head-group region, depending on the phase state. The hydrophobicity of the PC molecules markedly affected the distribution quantities of the Prodan molecules between the gel and liquid crystalline phases. The distribution of the Prodan molecules into the gel phase decreased with the increasing acyl chain length while that into the liquid crystalline phase conversely increased. The present study suggests that Prodan can sensitively recognize the packing states and strengths in the bilayer membranes and becomes a good packing indicator.