Hydration pressure and phase transitions of phospholipids. II. Thermotropic approach

It is widely known that dehydration increases the main phase transition temperature of phospholipids. A mathematical analysis now shows that hydration pressure can be calculated by the dehydration-induced shift of the phase transition temperature. The hydration-dependent piezotropic and thermotropic phase transitions were determined by using calorimetry and FT-IR spectroscopy, and the application of our approach gives hydration pressure parameters that agree very well with the values obtained with the osmotic stress method.     

Hydration pressure and phase transitions of phospholipids: I. Piezotropic approach

Dehydration reduces the main phase transition pressure of phospholipids. An analysis based on the Gibbs-Duhem equation shows how the shift of the transition pressure is correlated to the hydration pressure.By using Fourier transform infrared (FT-IR) spectroscopy we determined the hydration-dependent phase transition pressure. The application of our new approach gives hydration pressure values which agree with the values obtained with the osmotic stress method.     

Hydration pressure and phase transitions of phospholipids: II. Thermotropic approach

It is widely known that dehydration increases the main phase transition temperature of phospholipids. A mathematical analysis now shows that hydration pressure can be calculated by the dehydration-induced shift of the phase transition temperature.The hydration-dependent piezotropic and thermotropic phase transitions were determined by using calorimetry and FT-IR spectroscopy, and the application of our approach gives hydration pressure parameters that agree very well with the values obtained with the osmotic stress method.     

Lyotropic phase transitions in phospholipids as evidenced by small-angle synchrotron X-ray scattering.

Hydration is an important factor in regulating the phase behaviour of lipids and besides affects their interactions with other compounds relevant for biological membranes. We present a reliable and fast method to detect and characterise hydration-induced phase transitions in phospholipids by means of small-angle synchrotron X-ray scattering. Films consisting of aggregations of representatives of the two important lipid classes lecithins (DPPC a, POPC and OPPC,a for abbreviations, see below) and cephalins (DPPE and DOPE) were investigated at room temperature in dependence on relative humidity. Qualitative changes in the sets of the diffraction patterns obtained in dynamic hydration/dehydration scans were taken as markers indicating the existence of lyotropic phase transitions. The efficiency of this methodology is demonstrated by illustrating the course of hydration-driven phase transitions between lamellar as well as nonlamellar phases. In detail, this was realised for chain melting in the mixed-chain lipids, POPC and OPPC, and for a novel nonlamellar-phase transition for DOPE between a disordered inverted ribbon phase designated as Palpha and the canonical H(II), phase, respectively.     

Dynamic approach to thermotropic and inotropic phase transitions

Thermotropic and inotropic phase transitions have been analysed with a dynamic theory on a self-organization. An equation of motion of a molecular assembly with strong interactions may be approximately described as: dQ/dt' congruent to -K1Q-K3Q3, where Q is a displacement from the equilibrium point Q0(identical to 0) in a vibrational state, K1 is a transition parameter. When the parameter K1 concerned with an internal driving force (partial system) changes from positive to negative through the potential bifurcation, the system transfers to a new stable state breaking down the symmetry. Such a sign change of K1 serves as a trigger to a phase transition. Using Weiss' approximation, we have evaluated the change of K1 by a function of temperature, kappa (T-TC), and have obtained the critical temperature TC of thermotropic phase transition. We have furthermore treated inotropic phase transition caused by the binding of divalent cations like Ca2+ using the function kappa (T-beta TC), where beta is a shift parameter of the critical temperature.     

Phase transitions of polymerizable phospholipids

Polymerizable lipids have received considerable attention in the last ten years as polymerization of lipids in vesicle systems is a possibility to increase the stability of lipid bilayers. Lipids with various polymerizable groups have been synthesized in the last years. This paper is focussed on those lipids which are closely related to natural phospholipids, i.e. molecules which have two hydrophobic chains and a head group containing a phosphate moiety. The phase behaviour of polymerizable phospholipids as lipid monomers and in the polymerized state is reviewed and discussed.     

Bilayer phase transitions of N-methylated dioleoylphosphatidylethanolamines under high pressure

The bilayer phase transitions of four kinds of unsaturated phospholipids with different-sized polar head groups, dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidyl-N-methylethanolamine (DOMePE), dioleoylphosphatidyl-N,N-dimethylethanolamine (DOMe2PE) and dioleoylphosphatidylcholine (DOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light-transmittance. DSC thermogram and light-transmittance curve for each phospholipid vesicle solution exhibited only one phase transition under ambient pressure, respectively. The light-transmittance of DOPC solution at pressure higher than 234 MPa abruptly increased stepwise at two temperatures, which corresponds to the appearance of stable subgel and lamellar gel phases under high pressure in addition to the liquid crystal phase. The constructed temperature (T)-pressure (p) phase diagrams were compared among these phospholipids. The phase-transition temperatures of the phospholipids decreased stepwise by N-methylation of the head group. The slops of the T-p phase boundary (dT/dp) of DOPE, DOMePE and DOMe2PE bilayers (0.127-0.145 K MPa-1) were found to be close to that of the transition from the lamellar crystal (or subgel; Lc) phase to the liquid crystal (Lalpha) phase for DOPC bilayer (0.131 K MPa-1). On the other hand, the dT/dp value of the main transition from the lamellar gel (Lbeta) phase to the Lalpha phase for DOPC bilayer (0.233 K MPa-1) was significantly different from that of the Lc/Lalpha transition, hence both curves intersected with each other at 234 MPa. The thermodynamic quantities associated with the phase transition of DOPE, DOMePE and DOMe2PE bilayers had also similar values to those for the Lc/Lalpha transition of DOPC bilayer. Taking into account of the values of transition temperature, dT/dp and thermodynamic quantities compared with the corresponding results of saturated phospholipids, we identified the phase transitions observed in the DOPE, DOMePE and DOMe2PE bilayers as the transition from the Lc phase to the Lalpha phase although they have been regarded as the main transition in the previous studies. The Lbeta phase is probably unstable for DOPE, DOMePE and DOMe2PE bilayers at all pressures, it exists as a metastable phase at pressures below 234 MPa while as a stable phase at pressures above 234 MPa in DOPC bilayer. The difference in phase stability among the phospholipid bilayers is originated from that in hydration structure of the polar head groups.     

The pressure dependence of the lipid phase transitions. Effect of the pressure on the pre- and subtransition

The pressure dependence of the pre- and subtransitions is explained by a statistical physical model. Using this theoretical model the shift of the transition temperatures can be shown to be in agreement with the experimental results. Both the hysteresis effect which appears at standard pressure and the absence of the hysteresis at high pressures are explained.     

Quantum-chemical and empirical calculations on phospholipids. III. Hydration of the dimethylphosphate anion

The binding of water to the dimethylphosphate anion (DMP^?) was calculated using the PCILO method. We found binding energies of 25.95 kcal·mol^?1 in the O1-P-O3 plane bridging the anionic oxygen atoms and 19.3 kcal·mol^?1 for the one-site association of a water molecule to an anionic oxygen atom of DMP^?. In this range one water molecule added to DMP^? in the O1 … O3 bridged configuration has a significantly higher binding energy to DMP^? than water molecules added to other binding sites. The total binding energy of 5 water molecules to DMP^? is 92 kcal·mol^?1, a quantity which is about 10% less than the sum of the binding energies of the corresponding monohydrates.     

Thermotropic and barotropic phase transitions in bilayer membranes of ether-linked phospholipids with varying alkyl chain lengths

The bilayer phase transitions of a series of ether-linked phospholipids, 1,2-dialkylphosphatidylcholines containing linear saturated alkyl chain (C_n = 12, 14, 16 and 18), were observed by differential scanning calorimetry (DSC) under ambient pressure and light-transmittance measurements under high pressure. The thermodynamic quantities of the pre- and main-transitions for the ether-linked PC bilayer membranes were calculated and compared with those of a series of ester-linked PCs, 1,2-diacylphosphatidylcholines. The thermodynamic quantities of the main transition for the ether-linked PC bilayers showed distinct dependence on alkyl-chain length and were slightly different from those of the ester-linked PC bilayers. From the comparison of thermodynamic quantities for the main transition between both PC bilayers, we revealed that the attractive interaction in the gel phase for the ether-linked PC bilayers is weaker than that for the ester-linked PC bilayers. Regarding the pretransition, although changes in enthalpy and entropy for both PC bilayers were comparable to each other, the volume changes of the ether-linked PC bilayers roughly doubled those of the ester-linked PC bilayers. The larger volume change results from the smallest partial molar volume of the ether-linked PC molecule in the interdigitated gel phase. Further, we constructed the temperature-pressure phase diagrams for the ether-linked PC bilayers by using the phase-transition data. The region of the interdigitated gel phase in the phase diagrams was extended by applying pressure and by increasing the alkyl-chain length of the molecule. Comparing the phase diagrams with those for the ester-linked PC bilayers, it was proved that the phase behavior of the ester-linked PC bilayers under high temperature and pressure is almost equivalent to that of the ether-linked PC bilayers in the vicinity of ambient pressure.     

Thermotropic and barotropic phase transitions in bilayer membranes of ether-linked phospholipids with varying alkyl chain lengths

The bilayer phase transitions of a series of ether-linked phospholipids, 1,2-dialkylphosphatidylcholines containing linear saturated alkyl chain (C(n)=12, 14, 16 and 18), were observed by differential scanning calorimetry (DSC) under ambient pressure and light-transmittance measurements under high pressure. The thermodynamic quantities of the pre- and main-transitions for the ether-linked PC bilayer membranes were calculated and compared with those of a series of ester-linked PCs, 1,2-diacylphosphatidylcholines. The thermodynamic quantities of the main transition for the ether-linked PC bilayers showed distinct dependence on alkyl-chain length and were slightly different from those of the ester-linked PC bilayers. From the comparison of thermodynamic quantities for the main transition between both PC bilayers, we revealed that the attractive interaction in the gel phase for the ether-linked PC bilayers is weaker than that for the ester-linked PC bilayers. Regarding the pretransition, although changes in enthalpy and entropy for both PC bilayers were comparable to each other, the volume changes of the ether-linked PC bilayers roughly doubled those of the ester-linked PC bilayers. The larger volume change results from the smallest partial molar volume of the ether-linked PC molecule in the interdigitated gel phase. Further, we constructed the temperature-pressure phase diagrams for the ether-linked PC bilayers by using the phase-transition data. The region of the interdigitated gel phase in the phase diagrams was extended by applying pressure and by increasing the alkyl-chain length of the molecule. Comparing the phase diagrams with those for the ester-linked PC bilayers, it was proved that the phase behavior of the ester-linked PC bilayers under high temperature and pressure is almost equivalent to that of the ether-linked PC bilayers in the vicinity of ambient pressure.     

Phase transitions of phospholipids in monolayers and surface viscosity

The surface pressures and the surface viscosities of lecithin, cephalin and its analogs were measured at the air—water and the oil—water interfaces. It was found that the surface viscosity of the phospholipids used in this study was very high, and was comparable to those of some polymer films at the oil—water interface as well as at the air—water interface under the conditions where the monolayers were condensed. The plateaus indicating the phase transitions in monolayers were clearly observed on the pressure-area curves at the oil—water interface in all of the specimens studied. It was found that the phase transitions exactly corresponded to the abrupt increases in surface viscosity. From the results thus obtained, an intermolecular ionic linkage between neighboring molecules in the monolayers is discussed.