Unbinding transition in lipid multibilayers induced by copper(II) ions

We describe our observations on an unbinding transition in a multilamellar dispersion of phosphatidylcholine (PC) vesicles induced by copper(II) ions. The small-angle X-ray measurements clearly show that the increasing amount of CuCl2 in the millimolar concentration range continuously increases the amount of the unbound bilayers in the gel phase. Moreover, this phenomenon becomes more pronounced when the samples are heated above the so-called pretransition temperature between the gel and the ripple gel phase. The proposed reason for the latter is the increased repulsive electrostatic interaction due to the appearance of the surface modulation in the ripple gel phase. The observed effects reveal a new aspect of the unbinding phenomena since only the transition induced by the steric repulsion due to the layer fluctuations has been considered so far. Here, we show that the unbinding can also be triggered by the change in the electrostatic interactions. These findings are connected to the physical basis of the crucial role of copper(II) ions in biological processes such as neurodegenerative diseases and cell evolution.     

The biologically important surfactin lipopeptide induces nanoripples in supported lipid bilayers

Under specific conditions, lipid membranes form ripple phases with intriguing nanoscale undulations. Here, we show using in situ atomic force microscopy (AFM) that the biologically important surfactin lipopeptide induces nanoripples of 30 nm periodicity in dipalmitoyl phosphatidylcholine (DPPC) bilayers at 25 degrees (i.e. well below the pretransition temperature of DPPC). Whereas most undulations formed the classical straight orientation with characteristic angle changes of 120 degrees , some of them also displayed unusual circular orientations. Strikingly, ripple structures were formed at 15% surfactin but were rarely or never observed at 5 and 30% surfactin, emphasizing the important role played by the surfactin concentration. Theoretical simulations corroborated the AFM data by revealing the formation of stable surfactin/lipid assemblies with positive curvature.     

The effect of some phenyltin compounds on the thermotropic phase behaviour and the structure of model membranes

The effects of diphenyltin dichloride (DPhT), triphenyltin chloride (TPhT) and tetraphenyltin (TTPhT) on the thermotropic phase behaviour of phosphatidylcholine bilayers were studied. All the phenyltin compounds investigated affected phase transitions differently. TTPhT broadened the main phase transition but it left the transition temperatures and enthalpy unchanged. TPhT reduced the transition temperatures and the enthalpies while DPhT showed a dual effect on the pretransition and the main transition. At low concentrations DPhT reduced the temperatures of the transitions slightly and at higher concentrations it increased them. Based on differential scanning calorimetry (DSC) and also ¹H NMR and ³¹P NMR measurements, it is suggested that DPhT induces interdigitated gel phase formation and TPhT induces hexagonal phase formation. TTPhT seems to affect the structure only a little. The toxic activity of DPhT and TPhT seems to be connected with their ability to induce changes in the membrane structure. Copyright © 2000 John Wiley & Sons, Ltd.     

Differential scanning calorimetry and Fourier transform infrared spectroscopic studies of phospholipid organization and lipid-peptide interactions in nanoporous substrate-supported lipid model membranes

High-sensitivity differential scanning calorimetry was utilized to examine whether lipids capable of forming an inverted nonlamellar hexagonal II (HII) phase can be deposited into nanoporous substrate-supported arrays. Particularly, we compare the thermotropic phase properties of nanoconfined unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine lipid bilayers with unsupported dispersions to assess nanoconfinement effects, focusing on the lamellar fluid (Lalpha) to HII phase transition. Experimental results provide direct and clear evidence for the formation of an HII phase upon both heating and cooling. However, a small shift in the Lalpha/HII phase transition temperature, as well as an increase in the magnitude of the associated temperature hysteresis, was observed in the nanoporous substrate-supported system. Additionally, nanoconfinement effects on the interaction and location of the antimicrobial peptide gramicidin S (GS) with nanoporous substrate-supported cardiolipin bilayers were examined by Fourier transform infrared spectroscopy as a function of temperature and phospholipid phase state. Upon heating, GS molecules began to insert into nanoconfined, substrate-supported cardiolipin bilayers at lower temperatures relative to the gel/liquid-crystalline phase transition temperature than into unsupported bilayers. The reduction in the polarity and hydrogen-bonding potential environment of GS in the Lalpha state suggests that GS is located at the polar/apolar interfacial region in both supported and unsupported cardiolipin bilayers and that the capacity of GS to interact with nanoporous substrate-supported cardiolipin bilayers was not significantly hindered by nanoconfinement. These studies further demonstrate the usefulness of supported lipid bilayers inside nanoporous substrates.     

磷脂DHPC和短杆菌肽D相互作用的DSC研究

Changes of the behavior of the thermotropic phase transitions of the phospholipid DHPC by the action of miniprotein gramicidin D (G.D) have been studied by differential scanning calorimetry. The experimental results demonstrated that G.D could not remove the pretransition of DHPC, but the pretransition peak and the main transition peak of DHPC were incorporated into a broad peak. The pretransition temperature and the main transition temperature of DHPC decreased to different ex-tent. The sums of the pretransition and main transition enthalpies of DHPC showed a lineally decreasing relationship with increasing the mole fraction x_g of gramicidin D.     

Density, DSC, X-ray and NMR measurements through the gel and lamellar phase transitions of 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC): observation of slow relaxation processes and mechanisms of phase transitions

Dialkyl lecithin dispersions in water exhibit two phase transitions upon cooling from the lamellar phase (L(α)). At the main transition (T(M)) the L(α) phase changes to a ripple (gel) phase (P(β')) which then transforms to a second gel phase (L(β')) at the "pretransition" (T(P)). We have made accurate density measurements through the various phases for two lecithins having unequal chains: 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC). The measurements were carried out over five heat/cool cycles from 5 to 55 °C, followed by cooling back to 5 °C. The samples were then held at 50 °C for 24 hours, followed by a further three cool/heat cycles. For SMPC we observe an increase in density of the gel phases over the first 5 cycles, followed by much smaller changes after incubation at 50 °C. The lamellar phase also shows an increase in density, albeit much smaller. This parallels the behaviour of 1,2-di-palmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-di-myristoyl-sn-glycero-3-phosphatidylcholine (DMPC) reported earlier (Jones et al., Liquid Crystals 32, 1465 (2005)). For MSPC we observe a decrease in density within the gel phases while T(P) almost disappears after the first cycle. The lamellar phase shows little evidence of any change with each cycle. Within the lamellar phases there is a marked reduction in density on approaching T(M), which is attributed to the formation of transitory gel phase domains. Additional measurements by DSC and X-ray diffraction show that the changes in densities are not accompanied by large changes in transition enthalpies or phase structures. NMR data indicate that the pretransitional event within the L(α) phase is accompanied by ordering of the alkyl chains. The results indicate that the exact nature of the lipid alkyl chains could play a key role in the formation of gel phase patches within membrane bilayers. Their detailed chemical structures merit more attention than by simply assuming a uniform "bending energy" to describe the behaviour.     

Low-frequency dielectric study of the pretransition region in DPL-water model membrane

A low-frequency dielectric investigation of the pretransition region in dipalmitoylphosphatidylcholine (DPL)-water bilayers is reported. Results show a marked increase of the permittivity in the pretransition region. A possible interpretation in terms of capillary waves is suggested.     

Light scattering study of the isotropic-to-blue phase transition in cholesteryl oleyl carbonate (COC) and its mixtures with a nematogen

Light scattering experiments have been performed on cholesteryl oleyl carbonate (COC) and its mixtures with a nematogen in order to investigate pretransition phenomena accompanying the transition from the isotropic to the blue phase. Estimates of the critical temperature for the flat-spiral (m = 2) mode have been obtained in each case. The critical temperatures approach the transition temperature, T_c, in the mixtures, probably due to an impurity-induced blurring of the phase transition. Measurements of the optical rotatory power have also been carried out on pure COC as well as its mixtures with the nematogen and possible origins for the pretransition phenomena observed are discussed.     

Light scattering study of the isotropic-to-blue phase transition in cholesteryl oleyl carbonate (COC) and its mixtures with a nematogen

Abstract

Light scattering experiments have been performed on cholesteryl oleyl carbonate (COC) and its mixtures with a nematogen in order to investigate pretransition phenomena accompanying the transition from the isotropic to the blue phase. Estimates of the critical temperature for the flat-spiral (m = 2) mode have been obtained in each case. The critical temperatures approach the transition temperature, T _c, in the mixtures, probably due to an impurity-induced blurring of the phase transition. Measurements of the optical rotatory power have also been carried out on pure COC as well as its mixtures with the nematogen and possible origins for the pretransition phenomena observed are discussed.     

Molecular characterization of gel and liquid-crystalline structures of fully hydrated POPC and POPE bilayers

Molecular dynamics simulations were used for a comprehensive study of the structural properties of monounsaturated POPC and POPE bilayers in the gel and liquid-crystalline state at a number of temperatures, ranging from 250 to 330 K. Though the chemical structures of POPC and POPE are largely similar (choline versus ethanolamine headgroup), their transformation processes from a gel to a liquid-crystalline state are contrasting. In the similarities, the lipid tails for both systems are tilted below the phase transition and become more random above the phase transition temperature. The average area per lipid and bilayer thickness were found less sensitive to phase transition changes as the unsaturated tails are able to buffer reordering of the bilayer structure, as observed from hysteresis loops in annealing simulations. For POPC, changes in the structural properties such as the lipid tail order parameter, hydrocarbon trans-gauche isomerization, lipid tail tilt-angle, and level of interdigitation identified a phase transition at about 270 K. For POPE, three temperature ranges were identified, in which the lower one (270-280 K) was associated with a pre-transition state and the higher (290-300 K) with the post-transition state. In the pre-transition state, there was a significant increase in the number of gauche arrangements formed along the lipid tails. Near the main transition (280-290 K), there was a lowering of the lipid order parameters and a disappearance of the tilted lipid arrangement. In the post-transition state, the carbon atoms along the lipid tails became less hindered as their density profiles showed uniform distributions. This study also demonstrates that atomistic simulations of current lipid force fields are capable of capturing the phase transition behavior of lipid bilayers, providing a rich set of molecular and structural information at and near the main transition state.     

Effect of leucinyl-phenylalanyl-valine on DMPC liposome membrane

Leucinyl-phenylalanyl-valine (LFV) is a hydrophobic tripeptide with a flat egg shaped structure with the long axis dimension of about 12 A. The effect of LFV on dimyristoylphosphatidylcholine (DMPC) liposome membrane has been studied by differential scanning calorimetry (DSC) and fluorescence spectroscopy. Calorimetric studies shows that incorporation of LFV completely abolishes the pretransition temperature with broadening of main transition temperature. Four conceptually different fluorescence probes, 1-naphthol (1-ROH) an excited state proton transfer probe, 8-anilino-1-naphthalenesulphonate (ANS) a solvent polarity probe, 1-6-diphenylhexatriene (DPH) an anisotropy probe and pyrene an excimer-forming probe have been used for fluorescence spectroscopic studies. For 1-ROH, ANS and DPH, a decreased partitioning with increasing mol.% of LFV was observed. Increasing LFV mol.% caused a decrease in the neutral form emission of 1-ROH, and a decrease in fluorescence intensity with red shift in ANS. The excimer formation ability of pyrene also decreased. The phase transition behavior of DMPC membrane in the presence of LFV was similar to the known effect of cholesterol on lipid bilayers. These results suggest that LFV cause an increased compactness of membrane.     

Mixing behavior of 10-(perfluorohexyl)-decanol and DPPC

Fluorocarbon alcohol such as 10-(perfluorohexyl)-decanol are of interest for novel pulmonary drug delivery approaches. The purpose of this study was to investigate the mixing behavior of 10-(perfluorohexyl)-decanol with dipalmitoylphosphatidylcholine (DPPC), the major component of lung surfactant as an aid in assessing usefulness for this and other biomedical applications. The impact of 10-(perfluorohexyl)-decanol on the phase transitions of DPPC bilayers fully hydrated with a 0.15 M sodium chloride solution were studied using differential scanning calorimetry (DSC). No peak corresponding to excess alcohol was observed. The fluorinated alcohol caused DPPC peak broadening, especially below X(DPPC) < 0.95, and elimination of the pretransition of DPPC at X(DPPC) approximately 0.91. The onset of the main phase transition remains constant down to X(DPPC) approximately 0.91, suggesting limited miscibility in the gel phase. Hydration of the 10-(perfluorohexyl)-decanol-DPPC mixtures with calcium chloride (2 mM) in place of sodium chloride did not alter the macroscopic phase behavior. In addition to the thermal properties, the miscibility of 10-(perfluorohexyl)-decanol in DPPC in monolayers at the air water interface was investigated on water, sodium chloride (0.15 M), calcium chloride (2 mM) or hydrochloric acid (pH 1.9) subphases. The concentration dependence of the onset pressure of the liquid-expanded to liquid condensed phase transition of DPPC showed a slight change with increasing mole fraction on all four subphases. The surface area-mole fraction diagrams of 10-(perfluorohexyl)-decanol and DPPC on water, sodium chloride and calcium chloride showed near ideal behavior with slight negative deviations at higher surface pressure. A more significant negative deviation was observed for the hydrochloric acid subphase. Overall, both the DSC and the monolayer studies suggest that 10-(perfluorohexyl)-decanol and DPPC are partially miscible in biological mono- and bilayers. The macroscopic phase behavior 10-(perfluorohexyl)-decanol-DPPC system is significantly different from the analogous hydrocarbon system, which is attributed to a less favorable packing of the partially fluorinated hydrophobic tails in the mono- and bilayer.     

Audio to microwave frequency dielectric study of the pretransition region in DPL-water systems

Abstract

The phospholipid–water mixtures are used as model systems to investigate the structure and function of biological membranes. The dielectric behaviour of DPPC–water and DPPE–water systems was explored at audio (≥ 2kHz) and microwave (10 GHz) frequencies versus temperature. Particular care was devoted to the pretransition below the main gel-liquid-crystalline phase transition. A possible interpretation of the pretransition mechanism is given in terms of capillary waves.     

Mixing of perfluorooctanesulfonic acid (PFOS) potassium salt with dipalmitoyl phosphatidylcholine (DPPC)

Perfluorooctane-1-sulfonic acid (PFOS) is emerging as an important persistent environmental pollutant. To gain insight into the interaction of PFOS with biological systems, the mixing behavior of dipalmitoylphosphatidylcholine (DPPC) with PFOS was studied using differential scanning calorimetry (DSC) and fluorescence anisotropy measurements. In the DSC experiments the onset temperature of the DPPC pretransition (Tp) decreased with increasing PFOS concentration, disappearing at XDPPC < or = 0.97. The main DPPC phase transition temperature showed a depression and peak broadening with increasing mole fraction of PFOS in both the DSC and the fluorescence anisotropy studies. From the melting point depression in the fluorescence anisotropy studies, which was observed at a concentration as low as 10 mg/L, an apparent partition coefficient of K = 5.7 x 10(4) (mole fraction basis) was calculated. These results suggest that PFOS has a high tendency to partition into lipid bilayers. These direct PFOS-DPPC interactions are one possible mechanism by which PFOS may contribute to adverse effects, for example neonatal mortality, in laboratory studies and possibly in humans.     

High pressure effect on phase transition behavior of lipid bilayers

Phase behavior of lipid bilayers at high pressure is critical to biological processes. Using coarse grained molecular dynamic simulations, we report critical characteristics of dipalmitoylphosphatidylcholine bilayers with applied high pressure, and also show their phase transition by cooling bilayer patches. Our results indicate that the phase transition temperature of dipalmitoylphosphatidylcholine bilayers obviously shifts with pressure increasing in the rate of 37 °C kbar(-1), which are in agreement with experimental data. Moreover, the main phase transition is revealed to be strongly dependent on lipid area. A critical lipid area of ~0.57 nm(2) is found on the main phase transition boundary. Similar structures of acyl chains lead to the same sensitivity of phase transition temperature of different lipids to the pressure. Based on the lateral density and pressure profiles, we also discuss the different effects on bilayer structure induced by high temperature and high pressure, e.g., increasing temperature induces higher degree of interdigitation of lipid tails and thinner bilayers, and increasing pressure maintains the degree of interdigitation and bilayer thickness.     

High pressure optical studies of the chirality and phase behaviour of liquid crystalline blue phases

The blue phase (BP) polymorphism of chiral S -(+)-4-(2-methylbutyl)phenyl-4-decyloxy benzoate (CE6) and S -(+)-4-(2-methylbutyl)phenyl-4-dodecyloxy benzoate (CE7) was studied at elevated transition pressures up to 280MPa using optical activity measurements. The increased density causes an increase in phase chirality which in the case of CE6 manifests itself in the disappearance of BP II (BP I-BP II-BP III triple point) and in the case of CE7 in the appearance of BP II. At atmospheric pressure, CE6 exhibits BP I, BP II and BP III, while CE7 only possesses BP I. This pressure behaviour of CE6 and CE7 is contrary to that of cholesteryl nonanoate, which has been investigated previously. This result is particularly interesting when discussed in connection with the effect of elongation of the terminal n alkylcarboxy chain of the cholesteryl n -alkanoates and of the terminal n -alkyloxy chain in the homologous series to which CE6 and CE7 belong. In the first case the phase chirality increases; in the second case it decreases. Since CE6 shows an obvious pretransitional optical activity in its isotropic liquid phase, pretransition and BP phase behaviour could be compared with each other at elevated transition pressures and temperatures. For those transition pressures or temperatures where anomalies in the BP phase behaviour occur, anomalies in the pretransition behaviour are also observed.     

Dilatometric and calorimetric studies of the effect of staphylococcus aureus delta-lysin on the phospholipid phase transition

The interaction of the protein -lysin from Staphylococcus aureus with model membrane of dipalmitol-sn-glycero-3-phosphocholine has been studied by the method of temperature scanning densitometry and high precision differential scanning calorimetry. The studies were performed in the range of low relative protein concentrations (between 10^–4 and 3×10^–2 moles -lysin per mole phospholipid) in order to observe those effects which may be relevant to the high efficiency of membranolysis. -lysin was found to have strong effects on the main transition as well as on the pretransition, i.e., significant reduction of the transition volume and transition enthalpy and complete abolition of the pretransition. The temperature of the main transition was nearly unaffected in this concentration range. These effects can be interpreted as long-range structural changes in the lipid bilayers caused by the protein and are discussed in terms of a cooperative cluster model. The results are in many respects qualitatively similar to those observed earlier with the bee venom protein melittin.Presented at the sixth Italian meeting on Calorimetry and Thermal Analysis (AICAT) held in Naples, December 4–7, 1984     

Structure and phase behavior of self-assembled DPPC-DNA-metal cation complexes

Multilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) in solution with DNA and bivalent metal cations (Ca2+, Mn2+, Mg2+) self-assemble into a ternary DPPC-DNA-Me2+ complex. The supramolecular structure of the complex consists of an ordered multilamellar assembly where hydrated DNA helices are sandwiched between the lipid bilayers and the metal cations bind the phosphate groups of DNA to the lipid polar heads. In the range of explored incubation times, the complex coexists with the uncomplexed DPPC over the whole temperature range investigated (20-55 degrees C). Accordingly, two distinct coexisting lamellar phases are observed, one corresponding to the ternary complex and the other to the uncomplexed lipid. The structure and thermotropic phase behavior of both of these have been investigated by means of synchrotron X-ray diffraction, and the relevant structural data are deduced from experimental electron density profiles. While the uncomplexed lipid exhibits the same phase behavior as pure DPPC, that is, L beta'-P beta'-L alpha, the thermotropic behavior of the bound lipid in the complex is partially altered. This is manifested as an increase in the main transition temperature and the disappearance of the ripple phase leading to the single -phase transition. The role of the different metal cations in promoting and stabilizing the DNA condensation into the ternary complex is also discussed.     

Pressure-induced re-entrant cholesteric phase behaviour of non-polar liquid crystals

In a previous paper we reported the existence of a pressure-induced re-entrant cholesteric phase in mixtures of non-polar liquid crystals. Now the influence of the mixing ratio on this behaviour has been studied up to 3000 bar and 190°C and the phase boundaries based on light reflection measurements have been confirmed by transmission and texture observations in a diamond anvil cell. Additional thermodynamic investigations show that when the cholesteric/smectic A phase transition line possesses a maximum temperature the pretransition enthalpy and volume disappear.     

Subgel studies of dimyristoylphosphatidylcholine bilayers

It is known that when bilayers of some saturated phosphatidylcholines are stored for 3 or more days at approximately 0 degrees C, a lamellar subgel (Lc) phase is detected at temperatures below the pretransition by differential scanning calorimetry (DSC). However, the subgel (Lc) phase and the corresponding subtransition (Lc--> Lbeta') for dimyristoylphosphatidylcholine (DMPC) has not been clearly characterized. In this study, using the temperature jump protocol first developed by Tristram-Nagle et al. for the dipalmitoylphosphatidylcholine (DPPC) system, new and accurate data characterizing the subgel formation and subtransition of DMPC were obtained through DSC and fluorescence spectroscopy with 1,6-diphenyl-1,3,5-hexatriene (DPH). It was discovered that the formation of the DMPC subgel phase requires incubation at temperatures of -5 degrees C or lower for 2 h or more. Kinetics of the subgel formation indicate that it is a very complex process and demonstrates that the planar gel phase is merely metastable below the subtransition, and not the thermodynamically stable phase. The subgel growth of DMPC is proven to be the dehydration of the headgroup region, and the subtransition is a process in which poorly hydrated DMPC becomes hydrated.