Temperature dependence of the ripple structure in dimyristoylphosphatidylcholine studied by synchrotron X-ray small-angle diffraction

Membrane States of Saturated Glycerophospholipids: A Thermodynamic Study of Bilayer Phase Transitions

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.

Interactions of a cationic surfactant--(benzyloxymethyl) dodecyldimethylammonium chloride with model biomembrane systems

Phospholipids are the main components of biological membranes. The aim of the present study was to determine the influence of a cationic surfactant on phospholipid structure and dynamics. Fourier transform infrared (FTIR) and dielectric relaxation (DRS) spectroscopies as well as small-angle X-ray scattering (SAXS) with synchrotron radiation have been used to analyse the structure of fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) in the presence of a quaternary ammonium surfactant: (benzyloxymethyl) dodecyldimethylammoniumchloride (BzMDDAC). The presence of the surfactant caused changes in the temperature of the DMPC phase transition, as revealed using FTIR and DRS measurements. This change results from the disappearance of the multilamellar phase of DMPC and the formation of the unilamellar (most likely bicellar) phase, as indicated by the SAXS results.

Competitive molecular interaction among paeonol-loaded liposomes: differential scanning calorimetry and synchrotron X-ray diffraction studies

Thermotropic phase behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes containing 5 mol% cholesterol, or 5 mol% stigmasterol, or 5 mol% paeonol have been investigated by differential scanning calorimetry (DSC) and synchrotron X-ray diffraction (XRD) techniques, to investigate the competitive molecular interaction among paeonol-loaded liposomes. The results show that both sterol and paeonol can incorporate into hydrophobic region and interact with acyl chains of DPPC. Both 5 mol% sterols and 5 mol% paeonol can promote the formation of rippled gel phase of DPPC liposomes at room temperature. 5 mol% paeonol can induce the occurrence of phase separation in DPPC liposomes, but 5 mol% cholesterol or 5 mol% stigmasterol cannot induce this phenomenon. Both the repeat distance and the correlation length of paeonol-poor domain are larger than those of coexisted paeonol-rich domain. Both calorimetric data and SAXS patterns show that sterols have more favorable, stabilizing interactions with DPPC than paeonol, implying that high concentrations of sterols will have a negative effect on the loading of paeonol. In addition, calorimetric data show that cholesterol have a little more favorable, stabilizing interactions with DPPC than stigmasterol. The results of this study will play an important role in optimizing the formulation of paeonol-loaded liposomes.

Structural calorimetry of main transition of supported DMPC bilayers by temperature-controlled AFM

Atomic force microscopy at high temperature resolution (DeltaT < or approximately 0.1 K) provided a quantitative structural calorimetry of the transition from the fluid (Lalpha)- to the gel (Pbeta')-phase of supported dimyristoylphosphatidylcholine bilayers. Besides a determination of the main transition temperature (T0) and the van't Hoff transition enthalpy (DeltaHvH), the structural analysis in the nm-scale at T close to T0 of the ripple phase allowed an experimental estimation of the area of cooperative units from small lipid domains. Thereby, the corresponding transition enthalpy (DeltaH) of single molecules could be determined. The lipid organization and the corresponding parameters T0 and DeltaHvH (DeltaH) were modulated by heptanol or external Ca2+ and compared with physiological findings. The size of the cooperative unit was not significantly affected by the presence of 1 mM heptanol. The observed linear relationship of DeltaHvH and T0 was discussed in terms of a change in heat capacity.

Characterization of swollen lamellar phase of dimyristoylphosphatidylcholine-gramicidin A mixed membranes by DSC, SAXS, and densimetry

For the fully hydrated multilamellar stack of dimyristoylphosphatidylcholine (DMPC) fluid membranes containing hydrophobic peptide gramicidin A (GrA), the membrane thickness and the bilayer-bilayer separation (i.e., water layer thickness) were determined by measuring small-angle X-ray scattering and the density of aqueous suspensions of DMPC-GrA mixtures. When the molar ratio of GrA to DMPC was 0.04, the membrane thickness decreased by 2-3 A by the incorporation of GrA molecules into DMPC bilayers, whereas the water layer thickness increased by 3-4 A. As the cause of the increment of water layer thickness, two possibilities were considered; (1) attractive van der Waals force acting between the bilayer membranes weakened by the decrease of membrane thickness, and (2) repulsive undulation force enhanced by the incorporation of GrA which may stabilize the gauche conformers of the lipid acyl chains.

Phase stability of phosphatidylcholines in dimethylsulfoxide solutions

The temperature dependence of the phase stability of dispersions of dimyristoyl, dipalmitoyl, and distearoyl derivatives of phosphatidylcholines in excess aqueous dimethylsulfoxide has been examined by differential scanning calorimetry and synchrotron x-ray diffraction methods. There was a close correlation between the enthalpic transitions and the structural changes associated with the pre- and main transitions of the phospholipids in the range of concentrations up to mole fractions of dimethylsulfoxide in water of 0.1333. The temperature of the pre- and main transitions of the three phospholipids were found to increase linearly with increasing mole fraction of dimethylsulfoxide. The difference in phase stability between the lamellar gel and ripple phases induced by increasing dimethylsulfoxide concentration resulted in disappearance of the ripple phase and direct transition between lamellar gel and lamellar liquid-crystal phases. The effect of changing the properties of the solvent by the addition of dimethylsulfoxide on the dimensions of dipalmitoylphosphatidylcholine and solvent layers of the bilayer repeat structure has been determined from electron density distribution calculations. The lamellar repeat spacing recorded at 25 degrees C decreased from 6.36 nm in aqueous dispersion to 6.04 nm in a dispersion containing a mole fraction of 0.1105 dimethylsulfoxide. The results indicate that dipole interactions between solvent and phospholipid and dielectric properties of the solvent are important factors in the determination of the structure of saturated phosphatidylcholines.

Real-time X-ray diffraction study at different scan rates of phase transitions for dipalmitoylphosphatidylcholine in KSCN

Multibilayer arrays of dipalmitoylphosphatidylcholine (DPPC) in 1 M KSCN were characterized using real-time X-ray diffraction and differential scanning calorimetry. A phase transition sequence was observed as a function of increasing temperature which involved changes from the interdigitated subgel (Lc(inter)) to interdigitated gel (L beta(inter)) to disordered (L alpha) bilayer states. The phase transition mechanisms were unambiguously determined by comparison of results from fast and slow scans. The Lc(inter)-->L beta(inter) phase transition was shown to involve a continuous change in acyl chain spacing between the rectangular subgel acyl chain unit cell into an hexagonal gel acyl chain unit cell. The mechanism is similar to that for subgel to gel state transitions involving non-interdigitated DPPC bilayers.

Temperature change of the ripple structure in fully hydrated dimyristoylphosphatidylcholine/cholesterol multibilayers

The ripple structure was studied as a function of temperature in fully hydrated dimyristoylphosphatidylcholine (DMPC)/cholesterol multibilayers using synchrotron x-ray small-angle diffraction and freeze-fracture electron microscopy. In the presence of cholesterol, the ripple structure appears below the pretransition temperature of pure DMPC multibilayers. In this temperature range the ripple periodicity is relatively large (25-30 nm) and rapidly decreases with increasing temperature. In this region, defined as region I, we observed coexistence of the P beta' phase and the L beta' phase. The large ripple periodicity is caused by the formation of the P beta' phase region in which cholesterol is concentrated and the L beta' phase region from which cholesterol is excluded. An increase in ripple periodicity also takes place in the narrow temperature range just below the main transition temperature. We define this temperature region as region III, where the ripple periodicity increases dramatically toward the main transition temperature. In region II, between regions I and III, the ripple periodicity decreases gradually with temperature. This behavior is quite similar to that of pure DMPC. Temperature-versus-ripple periodicity curves are parallel among pure DMPC and DMPCs with various cholesterol contents. We explain this behavior in terms of a model proposed by other workers.

A new liquid crystalline phase in phosphatidylcholine bilayers as studied by X-ray diffraction

Model membranes of diacylphosphatidylcholines (CnPC), with saturated linear acyl chains of n > 12 carbons, show a single sharp phase transition (known as the main transition) between the gel phase P beta' and the liquid crystalline phase L alpha with differential scanning calorimetry. However, C12PC (dilauroylphosphatidylcholine) shows, as well as the sharp transition at -2 degrees C, a broad peak at 5 degrees C, originally observed by S. Mabrey and J.M. Sturtevant. The broad peak is not artificial: between the two peaks a new phase Lx was predicted for (C12PC) bilayers on the basis of calorimetry (Finegold, Shaw and Singer, Chem. Phys. Lipids 53 (1990), 177-184). The existence of Lx has now been confirmed by synchrotron X-ray diffraction on samples identical to those of the previous work, of similar preparation and at corresponding scan rates. With temperature, both small-angle (long lamellar) and wide-angle (hydrocarbon chain) spacings show abrupt discontinuities, and separate broader changes, at temperatures corresponding to the calorimetric sharp and broad peaks, respectively. All the X-ray diffraction profiles and spacing results are consistent with the following phase scheme with increasing temperature: gel ripple phase P beta'-->new, less ordered liquid crystalline phase Lx-->most disordered liquid crystalline phase L alpha. The phase Lx possibly exists in other CnPCs, and its examination may provide details of the main transition. Because Lx exists at a higher temperature than the main transition from P beta', it promises to be of biological relevance.

Condition for the appearance of the metastable P beta' phase in fully hydrated phosphatidylcholines as studied by small-angle x-ray diffraction

In the ripple phase of fully hydrated multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC), two kinds of small-angle x-ray diffraction profiles are observed on cooling through the main transition. One is a seemingly normal profile similar to that observed on heating and the other is the superposition of the diffraction profiles for the primary (normal) and the secondary ripple structures. We found that the profile obtained depended on the cooling rate. Increasing the cooling rate from 0.1 degrees C/min to 1 degrees C/min caused the peaks originating from the secondary ripple structure to diminish. After a cooling scan at 43 degrees C/min, the profile became similar to that of the normal ripple structure, although a trace of the secondary ripple structure remains. The results are interpreted in terms of the rise and fall of three-dimensional correlated domains composed of both primary and secondary ripple structures. At slow cooling rates, correlated domains of both kinds of ripple structures develop. As the cooling rate is increased, the domain of the primary ripple structure remains correlated, while that of the secondary ripple structure becomes less correlated. In addition, the multipeak profile appears even at rapid cooling rates, if the final low temperature lies just below the Tm for the main transition. This results suggests that formation of the correlated domains of the secondary ripple structure requires a certain time interval during which the DPPC vesicles experience the temperature just below the main transition. The secondary ripple structure takes place in phosphatidylcholines having more than 15 carbons in each hydrocarbon chain upon cooling through the main transition.

Kinetics of the barotropic ripple (P beta')/lamellar liquid crystal (L alpha) phase transition in fully hydrated dimyristoylphosphatidylcholine (DMPC) monitored by time-resolved x-ray diffraction

We present here the first study of the use of a pressure-jump to induce the ripple (P beta')/lamellar liquid crystal (L alpha) phase transition in fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The transition was monitored by using time-resolved x-ray diffraction (TRXRD). Applying a pressure-jump from atmospheric to 11.3 MPa (1640 psig, 111.6 atm) in 2.5 s induces the L alpha to P beta' phase transition which takes place in two stages. The lamellar repeat spacing initially increases from a value of 66.0 +/- 0.1 A (n = 4) to a maximum value of 70.3 +/- 0.8 A (n = 4) after 10 s and after a further 100-150 s decreases slightly to 68.5 +/- 0.3 A (n = 4). The reverse transition takes place following a pressure jump in 5.5 s from 11.3 MPa to atmospheric pressure. Again, the transition occurs in two stages with the repeat spacing steadily decreasing from an initial value of 68.5 +/- 0.3 A (n = 3) to a minimum value of 66.6 +/- 0.3 A (n = 3) after 50 s and then increasing by approximately 0.5 A over a period of 100 s. The transition temperature increases linearly with pressure up to 14.1 MPa in accordance with the Clapeyron relation, giving a dT/dP value of 0.285 degrees C/MPa (28.5 degrees C/kbar) and an associated volume change of 40 microliters/g. A dynamic compressibility of 0.13 +/- 0.01 A/MPa has been determined for the L alpha phase. This value is compared with the equilibrium compressibilities of bilayer and nonbilayer phases reported in the literature. The results suggest testable mechanisms for the pressure-induced transition involving changes in periodicity, phase hydration, chain order, and orientation. A more complete understanding of the transition mechanism will require improvement in detector spatial resolution and sensitivity, and data on the pressure sensitivity of phase hydration.