Time-resolved x-ray diffraction and calorimetric studies at low scan rates: I. Fully hydrated dipalmitoylphosphatidylcholine (DPPC) and DPPC/water/ethanol phases

Detection of the metastable rippled gel phase in hydrated phosphatidylcholine by fluorescence spectroscopy

Steady-state and time-resolved emission spectroscopy of 1-anilinonaphthalene-8-sulfonic acid (ANS) have been used for characterization of the metastable rippled gel phase, Pbeta'(mst), formed in fully-hydrated dipalmitoylphosphatidylcholine (DPPC) upon cooling from the liquid crystalline phase Lalpha [Tenchov et al., Biophys. J. 56 (1989) 757]. The Pbeta'(mst) phase of DPPC clearly differs from the stable Pbeta' phase by increased (approximately 27%) ANS emission intensity, by enhanced (approximately 23%) average radiative rate constant, and by reduced (approximately 18%) non-radiative quenching rate constant. The fluorescence intensity peak at the Pbeta'-->Lalpha transition temperature is replaced by a large, reversible stepwise intensity drop at the Pbeta'(mst)-->Lalpha transition. No such effects have been found for dimiristoylphosphatidylcholine (DMPC) dispersions confirming previous results that DMPC does not form a Pbeta'(mst) phase. Since ANS is known to predominantly reside in the interfacial region, the observed effects indicate differences between the stable and metastable rippled phases in the organization and dynamics of their lipid/water interfaces. The data demonstrate that the metastable rippled phase manifests its appearance also through interactions with small molecules (ANS size approximately 8 A).

Effects of alcohols on lipid bilayers with and without cholesterol: the dipalmitoylphosphatidylcholine system

Differential scanning calorimetry is a useful method to study the thermotropic phase transitions of a phospholipid bilayer. In the present study DSC is used to determine the effects of methanol and ethanol on DPPC and DPPC/2 mol% cholesterol bilayers. The biphasic effect of the main transition and the presence of an extra peak on the DSC cooling scans were observed above certain alcohol concentrations. In the presence of 2% cholesterol, the concentration at which the biphasic effect occurs is increased by both short-chain alcohols. 1,6-Diphenyl-1,3,5-hexatriene (DPH) is used as a fluorescent probe to directly determine the onset of interdigitation in these systems as reflected by a drop in the DPH fluorescence intensity.

Polymorphism in myristoylpalmitoylphosphatidylcholine

This study focuses on the mixed-chain lipid myristoylpalmitoylphosphatidylcholine (MPPC) near full hydration. The lipid, synthesized according to the procedure of (Mason et al., 1981a, has a low degree of acyl chain migration. When MPPC is temperature-jumped (T-jumped) from the L alpha phase (T = 38 degrees C) to T = 20 degrees C or below, a subgel phase forms; this formation takes less than 1 h at a temperature below T = 12 degrees C. The subgel remains stable up to T = 29 degrees C. When MPPC is T-jumped from the L alpha phase to T = 24 degrees C or above, a ripple phase forms with coexisting ripple wavelengths of 240 A and 130 A. In contrast, when MPPC is melted from the subgel phase, the ripple phase is characterized by bilayers having a single ripple wavelength of 130 A. In agreement with earlier studies (Stumpel et al., 1983; Serrallach et al., 1984. Structure and thermotropic properties of mixed-chain phosphatidylcholine bilayer membranes. Biochemistry 23:713-720.), no stable gel phase was observed. Instead, an ill-defined low-angle X-ray pattern is initially observed, which gradually transforms into the subgel phase below 20 degrees C, or into the ripple phase above 24 degrees C. In the wide-angle X-ray diffraction, a single peak is observed, similar to the ripple phase wide-angle pattern, that either persists above 24 degrees C or transforms into a multi-peaked subgel wide-angle pattern below 20 degrees C. The absence of a gel phase can be understood phenomenologically as the relative dominance of the subgel phase in mixed-chain PCs compared to same-chain PCs. The subgel structure and molecular interactions responsible for this comparative behavior are interesting open issues.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

The effect of ethanol on the structure of phosphatidylserine bilayers

Thermotrophic and structural effects of ethanol on phosphatidylserine (PS) membranes were investigated by differential scanning calorimetry (DSC) and X-ray diffraction. It was found that up to 15% (v/v) added ethanol, there is little change in the melting temperature of the phospholipid and no change in the interbilayer (d) spacing in the gel phase, indicating that there is no interdigitation of the hydrocarbon chains. Above the melting temperature of the phospholipid, a large decrease of the d spacing, due primarily to a decrease in the thickness of the bilayer, was found. Ethanol molecules located in the headgroup region apparently expand the area available to the headgroups with concomitant coiling of the acyl chains, resulting in marked thinning of the lipid layer.

Metastable rippled gel phase in saturated phosphatidylcholines: calorimetric and densitometric characterization

A long-living metastable rippled phase P beta' mst has been earlier reported to form in aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) upon cooling from the lamellar liquid crystalline phase L alpha (Tenchov et al. (1989) Biophys. J. 56, 757-768). Here we demonstrate that similar metastable phases form also in distearoylphosphatidylcholine (DSPC) and dihexadecylphosphatidylcholine (DHPC) but not in dimyristoylphosphatidylcholine (DMPC). The thermodynamic parameters of P beta' mst in DPPC, DSPC and DHPC have been characterized in detail by means of differential scanning calorimetry and scanning densitometry. It is shown that the P beta' mst phase in these three lipids has higher specific heat capacity by 0.1-0.4 kcal K-1 mol-1 and higher specific volume by 1-2 microliters/g than the equilibrium P beta' phase formed upon heating from the L beta' phase. The P beta' mst-->L alpha transition in these three lipids takes place at 0.06-0.14 degree C lower temperature than the P beta'-->L alpha transition. Its enthalpy is lower by 5-11% and the apparent maximum specific heat Cp max is lower by 11-14%. The P beta' mst phase is a long-living phase-it does not relax into the equilibrium P beta' for at least several hours. The replacement of the ester glycerol-hydrocarbon chain linkages in DPPC with ether bonds in DHPC does not influence the formation of P beta' mst.

Structure and phase behaviour of dimyristoylphosphatidic acid/poly(L-lysine) systems

Differential scanning calorimetry (DSC) and X-ray diffraction studies on (DMPA)/poly(L-lysine) systems are reported. DSC studies revealed that addition of poly(L-lysine) to DMPA bilayers raises the gel to liquid-crystalline phase transition of the systems, and that this effect depends on the molecular weight of the poly(L-lysine). Small-angle X-ray diffraction measurements showed that, in the liquid-crystalline phase, the lamellar spacing of a DMPA/short-poly(L-lysine) (approximately 4000 mol. wt.) system is shorter than that of a DMPA/long-poly(L-lysine) (approximately 22000 mol. wt.). In this connection wide-angle X-ray diffraction measurements indicate that the long-poly(L-lysine) adopts a beta-sheet conformation on the DMPA bilayers in both the gel and the liquid-crystalline phases, but the short-poly(L-lysine) adopts this conformation only on gel phase DMPA bilayers. We found that the spacings of the hydrocarbon chain packing in a DMPA bilayer in the gel phase increases with temperature, while the spacing between neighbouring polypeptide chains in long-poly(L-lysine) in the beta-sheet conformation remains almost constant. These observations indicate that the positively charged lysine residues are structurally independent of the negatively charged head groups of the phospholipid. On the basis of the present results we propose a model to explain the elementary behaviour of extrinsic membrane proteins in biomembranes.

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.

Structural and thermotropic properties of calcium-dimyristoylphosphatidic acid complexes at acidic and neutral pH conditions

Two kinds of calcium-dimyristoylphosphatidic acid (DMPA) complexes at acidic and neutral pH conditions were prepared in the following ways. The complex at pH 4 was obtained by adding Ca2+ to DMPA dispersion in pure water. On the other hand, the complex at pH 7.4 was obtained by adding Ca2+ to DMPA dispersion in the presence of NaOH. The stoichiometries of Ca2+ ion to DMPA molecule are 0.5-0.67 and approximately 1 for the complexes at pH 4 and 7.4, respectively. Static x-ray diffraction shows that the hydrocarbon chains of the Ca(2+)-DMPA complex at pH 4 at 20 degrees C are more tightly packed than those of the complex at pH 7.4 at 20 degrees C. Furthermore, the complex at pH 4 at 20 degrees C gives rise to several reflections that might be related to the ordered arrangement of the Ca2+ ions. These results indicate that the structure of the complex at pH 4 is crystalline-like. In the differential scanning calorimetry (DSC) thermogram, the complex at pH 7.4 undergoes no phase transition in a temperature range between 30 and 80 degrees C. On the other hand, in the DSC thermogram for the complex at pH 4, a peak appears at 65.8 degrees C in the first heating scan. In the successive second heating scan, a transition peak appears at 63.5 degrees C. In connection with the DSC results, the structural changes associated with these phase transitions were studied with temperature-scan x-ray diffraction. In the first heating scan, although a peak appears at 65.80C in the DSC thermogram, the hydrocarbon chain packing gradually converts from an orthorhombic lattice to a hexagonal lattice near 52 degree C, and successively the chain melting phase transition occurs near 670C. In the second heating scan, the hydrocarbon chains are packed in a hexagonal lattice over the whole temperature range and the chain melting phase transition occurs near 63.5 degree C. Therefore,the Ca2+-DMPA complex at pH 4 has a metastable state. The metastable state transforms to a stable state by maintaining the complex at pH 4 for about 90 h at 200C.

Kinetics of lipid phase changes

In the past two years, the kinetics of transitions involving the assorted lamellar and inverted hexagonal and cubic phases in bulk hydrated lipid systems have been established using a variety of physical techniques. In several cases, the kinetic data have lead to a transition mechanisms being deciphered.

Rapid reversible formation of a metastable subgel phase in saturated diacylphosphatidylcholines

Formation of well ordered lamellar subgel (SGII) phase in aqueous dispersions of L-dipalmitoylphosphatidylcholine upon cooling from the lamellar gel phase, without low-temperature equilibration, is observed in real time using synchrotron x-ray diffraction. It has the same lamellar repeat period as the gel phase from which it was formed but differs in its wide-angle diffraction pattern. The SGII phase forms at about 7 degrees C upon cooling at 2 degrees C/min. In temperature jump experiments at 1 degree C/s from 50 to -5 degrees C, the relaxation time of the lamellar gel-SGII transition is found to be approximately 15 s. The conversion between the lamellar gel and SGII phase is cooperative and rapidly reversible. Upon heating, it coincides in temperature with an endothermic event with a calorimetric enthalpy of 0.35 kcal/mol, the so-called sub-subtransition. Similar sub-subtransitions are also observed calorimetrically at temperatures approximately 10 degrees C below the subtransition, without low-temperature storage, in aqueous dispersions of L-dimyristoylphosphatidylcholine and L-distearoylphosphatidylcholine, but not in racemic DL-dipalmitoylphosphatidylcholine. The formation of the equilibrium lamellar crystalline Lc phase appears to take place only from within the SGII phase.

Interdigitated structure of phospholipid-alcohol systems studied by x-ray diffraction

In the interdigitated structure of phosphatidylcholine/alcohol systems, the one-dimensional electron density profile in the direction normal to the membrane surface is generated from the x-ray diffraction pattern. The membrane thickness for these systems is expressed by the sum of the hydrocarbon chain lengths of phosphatidylcholine and alcohol molecules. For this study, various sets of phosphatidylcholines and 1-alcohols were used; a phosphatidylcholine has a carbon number from 14 to 18 in a hydrocarbon chain, and an alcohol has a carbon number from 1 (methanol) to 4 (1-butanol). Based upon the results, we propose a model for the interdigitated structure in which 1) two alcohol molecules occupy a volume whose surface is surrounded interstitially by the headgroups of phosphatidylcholine molecules, and 2) the methyl ends of both hydrocarbon chains in alcohol and phosphatidylcholine molecules face each other at the bottom of the volume.

Characterization of complexes formed in fully hydrated dispersions of dipalmitoyl derivatives of phosphatidylcholine and diacylglycerol

The phase diagram of fully hydrated binary mixtures of dipalmitoylphosphatidylcholine (DPPC) with 1,2-dipalmitoylglycerol (DPG) published recently by López-García et al. identifies regions where stoichiometric complexes of 1:1 and 1:2 DPPC:DPG, respectively, are formed. In this study, the structural parameters of the 1:1 complex in the presence of pure DPPC was characterized by synchrotron low angle and static x-ray diffraction methods. Structural changes upon transitions through phase boundaries were correlated with enthalpy changes observed by differential scanning calorimetry in mixtures of DPPC with 5, 7.5, 10, and 20 mol% DPG dispersed in excess water. Phase separation of a complex in gel phase could be detected by calorimetry in the mixture containing 5 mol% DPG but was not detectable by synchrotron low angle x-ray diffraction. Static x-ray measurements show evidence of phase separation, particularly in the reflections indexing chain packing. In the mixture containing 7.5 mol% DPG, two distinct lamellar repeat spacings could be seen in the temperature range from 25 to 34 degrees C. The lamellar spacing of about 6.6 nm was assigned to pure gel phase DPPC because the change in the spacing corresponds with thermal transition of the pure phospholipid, and a longer repeat spacing of about 7.2 nm was assigned to domains of the 1:1 complex of DPPC-DPG. In the temperature range from 34 to 420C, i.e., in the region of coexistence of the ripple phase of DPPC and the gel phase of the complex, a single, rather broad lamellar reflection appears because of superposition of two reflections of DPPC and the complex; the lamellar spacing of DPPC in the ripple phase is similar to that of the gel phase of complex. In the coexistence region of the liquid-crystalline phase of DPPC and the gel phase of complex (-42-480C), the lamellar reflections of the both phases are present. The fluidus boundary lies between the coexistence region and the fluid region.In the fluid region (-48-550C), the gel state of complex persists up to the fluidus boundary, whereupon the liquid-crystalline state of complex replaces the gel state of the complex. This indicates that the complex is also immiscible with DPPC even above the fluidus boundary at least in the temperature range close to the phase boundary. For mixtures comprising 10 and 20 mol%DPG in DPPC, complex formation is clearly detectable in both the gel region and the coexistence region by x-ray diffraction.Synchrotron x-ray measurements indicate phase separation between pure DPPC and liquid-crystalline complex just above thefluidus boundary. Static, wide angle x-ray measurements also suggest phase separations of the 1:1 complex not only from the gel phase but also the liquid-crystalline phase of pure DPPC. Two distinct diffraction peaks were detected for the mixture of DPPC with 5, 10, and 20 mol% DPG. One is due to the chain spacing of the complex, and the other is due to that of the pure DPPC. In the coexistence region of the liquid-crystalline phase of DPPC and the gel phase of complex, two kinds of diffraction peaks of the hydrocarbon chain of the gel phase complex and the broad scattering profile for the chain melting of DPPC were observed in the wide angle region. Electron density reconstructed from the lamellar reflections indicates that the thicknesses of both the bilayer and the water layer of the gel phase complex are greater than those of the respective thicknesses of gel phase DPPC.

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.

Synchrotron X-ray studies of lipids and membranes: a critique

This review gives a description of techniques, suitable for the study of lipid dispersions and unorientated membranes, that are available at synchrotron facilities to determine either the kinetics of transitional phenomena in the time after a temperature or pressure jump is initiated, or the phases present while a sample undergoes a phase transition. Included in this description is information about synchrotron X-ray sources, sample holders and temperature controllers, detection systems, as well as data reduction. Examples involving lipid dispersions are provided to illustrate the application of these methods using synchrotron radiation.

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.

Effects of fibrinogens on phase transitions in lipid model membrane systems

An abnormal fibrinogen that caused aggregation of red blood cells (RBC) in a patient with gangrene was examined by real-time X-ray diffraction to determine its effects on dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylethanolamine (POPE) phase transitions. Similar studies were done with normal fibrinogen and results were compared. Both types of fibrinogen slightly increased the L alpha-->HII phase transition temperature and the HII phase parameters for POPE, while neither fibrinogen significantly affected the order-disordered acyl chain transitions in the lipid bilayer phase. However, fibrinogen differentially influenced the bilayer unit cell parameter of the gel and disordered bilayer and the gel state ripple phase. These results can be interpreted as indicating that fibrinogen has little effect on the balance of gel and disordered acyl chains in the lipid bilayer, but may influence membrane functions dependent on non-bilayer phases.

Thermal transitions in dimyristoylphosphatidylcholine foam bilayers

Thermal transitions in the system dimyristoylphosphatidylcholine/water/ethanol/sodium chloride were studied in the temperature range 10-31 degrees C. The water-ethanol dispersions were investigated by differential scanning calorimetry and the foam bilayers by the microinterferometric method for investigation of thin liquid films. Calorimetry showed that an increase in ethanol content (up to 47.5 vol.%-the concentration used in the experiments with foam bilayers) did not significantly influence the temperature of the main phase transition and led to the disappearance of the pretransition. The microinterferometric study of the foam bilayer thickness showed that there were two thermal transitions-at 13 and 23 degrees C. An Arrhenius type dependence was obtained for the critical concentration of dimyristoylphosphatidyl-choline (DMPC) in the solution, which was necessary for the formation of the foam bilayer. A steep change in the slope of the linearized Arrhenius dependence was found at 23 degrees C. Values of the binding energy of a DMPC molecule in the foam bilayers were calculated using the hole-nucleation theory of stability and permeability of bilayers. It was proved that the phase transition at 23 degrees C was due to melting of the hydrocarbon tails of phospholipid molecules. The low-temperature phase transition was assumed to be due to a change in the tilt of the hydrocarbon tails. These experiments demonstrate for the first time the occurrence of phase transitions in foam bilayers.

The phase behavior of mixed aqueous dispersions of dipalmitoyl derivatives of phosphatidylcholine and diacylglycerol

The phases and transition sequences for aqueous dispersions of mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycerol (1,2-DPG) have been studied by differential scanning calorimetry, dynamic x-ray diffraction, freeze-fracture electron microscopy, 31P-nuclear magnetic resonance spectroscopy, and Fourier-transform infrared spectroscopy. The results have been used to construct a dynamic phase diagram of the binary mixture as a function of temperature over the range 20 degrees-90 degrees C. It is concluded that DPPC and 1,2-DPG form two complexes in the gel phase, the first one with a DPPC/1,2-DPG molar ratio of 55:45 and the second one at a molar ratio of approximately 1:2, defining three different regions in the phase diagram. Two eutectic points are postulated to occur: one at a very low 1,2-DPG concentration and the other at a 1,2-DPG concentration slightly higher than 66 mol%. At temperatures higher than the transition temperature, lamellar phases were predominant at low 1,2-DPG concentrations, but nonlamellar phases were found to be predominant at high proportions of 1,2-DPG. A very important aspect of these DPPC/1,2-DPG mixtures was that, in the gel phase, they showed a ripple structure, as seen by freeze-fracture electron microscopy and consistent with the high lamellar repeat spacings seen by x-ray diffraction. Ripple phase characteristics were also found in the fluid lamellar phases occurring at concentrations up to 35.6 mol% of 1,2-DPG. Evidence was obtained by Fourier transform infrared spectroscopy of the dehydration of the lipid-water interface induced by the presence of 1,2-DPG. The biological significance of the presence of diacylglycerol in membrane lipid domains is discussed.

Kinetics of subgel formation in DPPC: X-ray diffraction proves nucleation-growth hypothesis

Wide-angle and low-angle X-ray diffraction data were obtained during the time course of the gel to subgel phase transformation in fully hydrated DPPC. When the system was kept close to equilibrium by following a T-jump protocol, the X-ray data unequivocally demonstrate the coexistence of growing subgel and shrinking gel domains. When the system was supercooled and held further from equilibrium as in previous studies, the kinetic behavior was more complicated. These data prove that the basic mechanism for the gel to subgel phase transformation is one of nucleation of subgel domains followed by growth of the domains.

Slow relaxation process in the main transition of phosphatidylcholines studied with heat capacity spectroscopy. I. Multilamellar vesicles

Extremely slow relaxation processes have been examined near the main transition of multilamellar vesicle samples of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) with a heat capacity spectroscopic technique. The dynamic heat capacity showed a significant frequency dependence in the studied frequency region of 0.5 mHz to 50 mHz. The relaxation observed here has been analyzed with the Cole-Cole equation. The mean relaxation times were 120 s in DMPC, and 260 s in DPPC. The relaxation showed a polydispersive character. The parameter beta was around 0.5 in both DMPC and DPPC.

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.

Phases and phase transitions of the hydrated phosphatidylethanolamines

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Here, a review of the LIPIDAT data subset referring to hydrated phosphatidylethanolamines (PE) is presented together with an analysis of these data. The PE subset represents 14% of all LIPIDAT records. It includes data collected over a 38-year period and consists of 1511 records obtained from 203 articles in 35 different journals. An analysis of the data in the subset has allowed us to identify trends in synthetic PE phase behavior reflecting changes in lipid chain length, chain unsaturation (number, isomeric type and position of double bonds), chain asymmetry and branching, type of chain-glycerol linkage (ether vs. ester) and headgroup modification. Also included is a summary of the data concerning the effect of pH, stereochemical purity, and different additives such as salts, saccharides, alcohols, amino adds and alkanes on PE phase behavior. Information on the phase behavior of biologically derived PE is also presented. This review includes 236 references.

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.

Electrostatic interaction of poly(L-lysine) with dipalmitoylphosphatidic acid studied by X-ray diffraction

Structure of dipalmitoylphosphatidic acid (DPPA) bilayers in the presence of poly(L-lysine) is proposed from the results of X-ray diffraction obtained by a storage phosphor detector with a high resolution called an imaging plate. The small-angle X-ray diffraction pattern exhibits that DPPA/poly(L-lysine) complex forms a highly ordered multilamellar structure. The electron density profile of the DPPA/poly(L-lysine) complex draws that only one poly(L-lysine) layer is intercalated between the neighboring DPPA bilayers. The wide-angle X-ray diffraction pattern suggests that the presence of poly(L-lysine) hardly affects the nature of hydrocarbon chain packing in the DPPA bilayers. The X-ray reflection from the DPPA/poly(L-lysine) complex indicates that the poly(L-lysine) molecules adopt a beta-sheet conformation on the surface of the DPPA bilayers. The both surface areas occupied by a headgroup of the DPPA and by a lysine residue in poly(L-lysine) are estimated from the observed spacings. The number ratio of lysine residues to DPPA headgroups per unit area is greater than unity. Therefore, one DPPA headgroup interacts with more than one lysine residue electrostatically, i.e., the electric charge distributions in both the surface of a DPPA bilayer and the poly(L-lysine) beta-sheet are incommensurate.

Relaxation dynamics of the gel to liquid-crystalline transition of phosphatidylcholine bilayers. Effects of chainlength and vesicle size

The relaxation kinetics of the gel to liquid-crystalline transition of five phosphatidylcholine (DC14PC to DC18PC) bilayer dispersions have been investigated using volume perturbation calorimetry, a steady-state technique which subjects a sample to sinusoidal changes in volume. Temperature and pressure responses to the volume perturbation are measured to monitor the relaxation to a new equilibrium position. The amplitude demodulation and phase shift of these observables are analyzed with respect to the perturbation frequency to yield relaxation times and amplitudes. In the limit of low perturbation frequency, the temperature and pressure responses are proportional to the equilibrium excess heat capacity and bulk modulus, respectively. At all temperatures, the thermal response data are consistent with a single primary relaxation process of the lipid. The less accurate bulk modulus data exhibit two relaxation times, but it is not clear whether they reflect lipid processes or are characteristic of the instrument. The observed thermal relaxation behavior of all multilamellar vesicles are quantitatively similar. The relaxation times vary from approximately 50 ms to 4 s, with a pronounced maximum at a temperature just greater than Tm, the temperature of the excess heat capacity maximum. Large unilamellar vesicles also exhibit a single relaxation process, but without a pronounced maximum in the relaxation time. Their relaxation time is approximately 80 ms over most of the transition range.

Effects of small organic molecules on phospholipid phase transitions

Small organic molecules are known to exhibit a wide spectrum of physiological or pharmacological effects and many of them are thought to be membrane associated. Therefore a great number of studies is devoted to the interaction between these molecules and phospholipid model membranes. Results obtained for molecular species of varying hydrophobic/hydrophilic balances will be described. It will be shown that, in general, these different molecules induce similar effects on phospholipid phase transitions, although they are located differently in the membrane. Detailed studies of these interactions will help to understand these processes on a molecular level.

Phospholipid phase transitions: kinetics and structural mechanisms

A brief review is given on the principles and methods used to investigate structural phase transitions in phospholipid supramolecular structures. The conceptual differences of approaches close to and far from equilibrium are addressed, and the consequences in terms of the limits of interpretation for different types of methods, in particular referring to jump-relaxation and steady-state techniques, are surveyed. With the emphasis on connecting dynamic and structural information, the results obtained so far from different techniques are reviewed, and the open questions addressed. The more recent advances by millisecond time-resolved X-ray diffraction with synchrotron radiation and their main results obtained for transitions triggered by IR-laser temperature jumps are summarized. As a major novel aspect in the field, the necessity of considering martensitic, diffusionless transformation mechanisms and the occurrence of intermediate structures is highlighted.

On the reversibility of the phase transitions in lipid-water systems

Heating and cooling phase sequences observed in phospholipid and glycolipid dispersions in excess water have been summarized. Differences between heating and cooling sequences and also with respect to a reference phase sequence "subgel-gel-lamellar liquid crystalline-cubic-inverted hexagonal" have been pointed out. Together with kinetic data obtained by alternating current (AC) calorimetry, these data have been used for a discussion on the reversibility of the lipid phase transitions. Several typical symptoms of irreversible behavior are (i) undercooling of stable phases; (ii) formation of phases which are metastable over the whole range of their existence; (iii) slow formation of the nascent phase requiring isothermal annealing out of the transition region; (iv) different nonconvergent transition pathways in heating and cooling. These phenomena are related to the appearance of slow rearrangement modes during the phase transitions with characteristic times longer than experimental time scales. Similarly slow relaxations supporting the existence of long-lived non-equilibrium lipid states in the biomembranes may have also certain physiological significance.

Ethanol induces interdigitated gel phase (L beta I) between lamellar gel phase (L beta') and ripple phase (P beta') in phosphatidylcholine membranes: a scanning density meter study

Effects of ethanol on dipalmitoylphosphatidylcholine (DPPC) and distearoylphosphatidylcholine (DSPC) dispersions were investigated with an automated scanning density meter and a differential scanning calorimeter (DSC). The temperature-dependent profile of specific volume measured by the density meter clearly exhibited phase transitions of the DPPC and the DSPC dispersions as drastic changes in the thermal expansion coefficients. On increasing the ethanol concentration in the DPPC dispersions, the pretransition temperature was reduced faster than the main transition temperature was. An interdigitated gel phase (L beta I) appeared as a region of lower specific volume at the pretransition temperature when the ethanol concentration reached 40 mg/ml. The L beta I phase spread both its ends in an ethanol-dependent fashion, and the high-temperature end merged to the main transition at 50 mg/ml of ethanol. The temperature-ethanol phase diagram has been determined for DPPC. The transitions L beta' to L beta I and from L beta I to P beta' were also observed on the thermograms of DSC measurements. In the DSPC dispersions, the L beta I phase was induced between the L beta' and the P beta' phases by a lower ethanol concentration (about 20 mg/ml).

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

The ripple structure of 1,2-dimyristoyl-L-phosphatidylcholine (DMPC) multibilayer containing excess water (60 wt%) was studied by synchrotron X-ray small-angle diffraction. The (0,1) spacing which corresponds to the ripple repeat distance depends on temperature: At 13 degrees C the (0,1) spacing is 14.15 nm, the spacing decreases at higher temperatures and reaches 12.1 nm at 23.5 degrees C, just below the main transition temperature. The spacing is in good agreement between heating process and cooling process except for the supercooling region. The result suggests that the rearrangement of the ripple structure takes place during temperature change successively. The Landau-de Gennes free energy equation explains well the temperature dependence of the ripple repeat distance.

Kinetics of the premelting (L beta'-P beta') and main transition (P beta'-L alpha) in hydrated dipalmitoylphosphatidylcholine. A time-resolved x-ray diffraction study using microwave-induced temperature-jumps

The dynamics and mechanism of the premelting (L beta'-P beta') and main transitions (P beta'-L alpha) in fully hydrated dipalmitoylphosphatidylcholine were examined by low-angle time-resolved x-ray diffraction (TRXRD) using microwave radiation to effect uniform, internal sample heating. Equilibrium and dynamic aspects of the transitions were investigated. The dynamic studies involved applying a temperature jump of sufficient amplitude to effect the two transitions sequentially. Our findings are as follows. (a) Microwave radiation has proven useful as a means for implementing rapid and uniform internal heating in temperature-jump studies of lipid-phase transition kinetics. (b) Heating rate can be controlled by adjusting microwave power setting. (c) The thermal expansion coefficient of the three lyotropic phases follows the sequence L beta' not equal to O greater than P beta' much greater than L alpha. (d) Regardless of temperature-jump amplitude and sample heating rate the P beta' phase was always in evidence as an intermediate between the L beta' and L alpha phases. (e) The degree of development of the P beta' phase was inversely proportional to temperature-jump amplitude and heating rate. (f) The shortest transit time recorded for the combined L beta'-P beta', and P beta'-L alpha transitions was less than 1 s. (g) Upon cooling from the L alpha phase the onset of the chain disorder/order transition was apparent as a dramatic change of slope in the scattering angle vs. time plot which is interpreted as arising from sample heating by the latent heat of the transition. (h) Based on the shape of the low-angle diffraction pattern of the P beta' phase the P beta'-L alpha transition appears to be reversible with no evidence of metastability as was observed in the slow scan TRXRD measurements of Tenchov et al. (1989. Biophys. J. 56:757-768).