A theoretical description of phase diagrams for nonideal lipid mixtures

Thermally-assisted ultrasonic separation of giant vesicles

We report on a newly-developed membrane stiffness-based separation of vesicles using a thermally-assisted acoustophoretic approach. By tuning the temperature, we achieved the separation of vesicles of the same size, shape, and charge but with different stiffness values. It was observed that at a specific transition point, the acoustic contrast factor of vesicles changed sign from positive to negative. This change was mainly due to the change in the acoustic compressibility of the vesicles, which is inversely proportional to stiffness. The acoustic contrast temperature, corresponding to the temperature at which the acoustic contrast factor switches sign, was determined to be unique to the composition of the vesicles. This unique temperature signature allowed us to develop a separation method of vesicles with distinct membrane stiffness with target outlet purities exceeding 95%. Our studies suggest that this method may be applied for the separation of cells affected by diseases that affect the cellular stiffness.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Are lipid phase transitions responsible for chilling damage in human platelets?

In previous studies we have proposed that the well-known chilling-induced activation of human blood platelets can be ascribed at least in part to a thermotropic phase transition in membrane lipids. The evidence that this is the case is reviewed and amplified in this review, followed by an examination of the available physical data concerning phase transitions in lipid mixtures that mimic the mixture found in platelet membranes. Assuming complete mixing at all temperatures and equal contributions of the members of the mixture to the phase transition, the lipid mixture found in platelets should give values for Tm ranging from about 1 degrees C to about 16 degrees C, depending on the isomers present in the mixture. (The former value is not in agreement with the observed Tm, but the latter is in excellent agreement.) However, examination of the phase diagram for a binary pair of lipids found in platelet membranes shows that ideal mixing almost certainly does not occur; instead of a linear phase diagram, a convex one was obtained. This shape for the phase diagram, which would displace Tm to an unexpectedly elevated temperature, is in agreement with previously published phase diagrams for mixtures of this type. The prediction, based on thermodynamic properties of lipids found in the platelets, is that Tm will be displaced upward in more complex mixtures of the composition found in platelets, a prediction that requires experimental testing.

New approaches to the simulation of heat-capacity curves and phase diagrams of pseudobinary phospholipid mixtures

A simulation program using least-squares minimization was developed to calculate and fit heat capacity (cp) curves to experimental thermograms of dilute aqueous dispersions of phospholipid mixtures determined by high-sensitivity differential scanning calorimetry. We analyzed cp curves and phase diagrams of the pseudobinary aqueous lipid systems 1,2-dimyristoyl-sn-glycero-3-phosphatidylglycerol/ 1,2-dipalmitoyl-sn-glycero-3phosphatidylcholine (DMPG/DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphatidic acid/1, 2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DMPA/DPPC) at pH 7. The simulation of the cp curves is based on regular solution theory using two nonideality parameters rho g and rho l for symmetric nonideal mixing in the gel and the liquid-crystalline phases. The broadening of the cp curves owing to limited cooperativity is incorporated into the simulation by convolution of the cp curves calculated for infinite cooperativity with a broadening function derived from a simple two-state transition model with the cooperative unit size n = delta HVH/delta Hcal as an adjustable parameter. The nonideality parameters and the cooperative unit size turn out to be functions of composition. In a second step, phase diagrams were calculated and fitted to the experimental data by use of regular solution theory with four different model assumptions. The best fits were obtained with a four-parameter model based on nonsymmetric, nonideal mixing in both phases. The simulations of the phase diagrams show that the absolute values of the nonideality parameters can be changed in a certain range without large effects on the shape of the phase diagram as long as the difference of the nonideality parameters for rho g for the gel and rho l for the liquid-crystalline phase remains constant. The miscibility in DMPG/DPPC and DMPA/DPPC mixtures differs remarkably because, for DMPG/DPPC, delta rho = rho l -rho g is negative, whereas for DMPA/DPPC this difference is positive. For DMPA/DPPC, this difference is interpreted as being caused by a negative rho g value, indicating complex formation of unlike molecules in the gel phase.

Association probabilities between the single-chain amphiphiles into a binary mixture in plan monolayers (II)

A test system of homologous binary mixtures of single-chain amphiphiles was considered. The dependence of the association probabilities on the hydrophobic chain length, and on the dipole moment of the shorter chain component when the longer chain component has a low dipole moment, was studied. The free-end effect of each hydrophobic chain on the association probability was also analysed. Generally, both the shorter chain end effect and the longer chain end effect on the association process are independent of their own initial chain length, but are dependent on the chain length of the other component and dipole moment. The changes in every association probability produced by the addition of one methylene group to the shorter chain and to the longer chain are unequal and opposite. For this reason the efficiency ratios were defined and analysed. The thermotropic behaviour of lipid mixtures can be related to association processes.

Nonideal mixing of phosphatidylserine and phosphatidylcholine in the fluid lamellar phase

The mixing of phosphatidylserine (PS) and phosphatidylcholine (PC) in fluid bilayer model membranes was studied by measuring binding of aqueous Ca2+ ions. The measured [Ca2+]aq was used to derive the activity coefficient for PS, gamma PS, in the lipid mixture. For (16:0, 18:1) PS in binary mixtures with either (16:0, 18:1)PC, (14:1, 14:1)PC, or (18:1, 18:1)PC, gamma PS > 1; i.e., mixing is nonideal, with PS and PC clustered rather than randomly distributed, despite the electrostatic repulsion between PS headgroups. To understand better this mixing behavior, Monte Carlo simulations of the PS/PC distributions were performed, using Kawasaki relaxation. The excess energy was divided into an electrostatic term Uel and one adjustable term including all other nonideal energy contributions, delta Em. Uel was calculated using a discrete charge theory. Kirkwood's coupling parameter method was used to calculate the excess free energy of mixing, delta GEmix, hence In gamma PS,calc. The values of In gamma PS,calc were equalized by adjusting delta Em in order to find the simulated PS/PC distribution that corresponded to the experimental results. We were thus able to compare the smeared charge calculation of [Ca2+]surf with a calculation ("masked evaluation method") that recognized clustering of the negatively charged PS: clustering was found to have a modest effect on [Ca2+]surf, relative to the smeared charge model. Even though both PS and PC tend to cluster, the long-range nature of the electrostatic repulsion reduces the extent of PS clustering at low PS mole fraction compared to PC clustering at an equivalent low PC mole fraction.

Association probabilities between the single chain amphiphiles into a binary mixture in planar monolayers

In this work, we have calculated the probabilities of selfassociation of single chain amphiphile molecules for 36 binary mixtures of molecules having an even number of carbon atoms (from 8 to 24 carbon atoms). The association probabilities depend on the dipole moment of head groups, the length of chains and the mole fraction of the mixture of both types of amphiphile. These calculations are useful for the quantitative and qualitative explanation of the cluster formation on the surface of the mono- or bilayers.

Differential scanning calorimetry and 2H NMR studies of the phase behavior of gramicidin-phosphatidylcholine mixtures

The extents of two-phase coexistence in the phase diagrams of mixtures of gramicidin with 1,2-bis(perdeuteriopalmitoyl)-sn-glycero-3-phosphocholine (DPPC-d62) and with 1,2-bis(perdeuteriomyristoyl)-sn-glycero-3-phosphocholine (DMPC-d54) mixtures have been explored with differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (2H NMR). For both systems, increased gramicidin content causes a decrease in transition enthalpy and a broadening of the peak in excess heat capacity at the transition. In DMPC-d54-based mixtures, the broadening is roughly symmetric about the pure lipid transition temperature. Addition of gramicidin to DPPC-d62 extends the excess heat capacity peak on the low-temperature side, resulting in a slightly asymmetric scan. Deuterium NMR spectra showing a superposition of gel and liquid-crystalline components, observed for both mixtures, indicate the presence of two-phase coexistence. For the DPPC-d62-based mixtures, two-phase coexistence is restricted to an approximately 2 degrees C temperature range below the pure transition temperature. For DMPC-d54-based mixtures, the region of two-phase coexistence is even narrower. For both mixtures, beyond a gramicidin mole fraction of 2%, distinct gel and liquid-crystal contributions to the spectra cannot be distinguished. Along with the broad featureless nature of the DSC scan in this region, this is taken to indicate that the transition has been replaced by a continuous phase change. These results are consistent with the existence of a closed two-phase region having a critical concentration of gramicidin below 2 mol%.

Simultaneous modeling of phase and calorimetric behavior in an amphiphilic peptide/phospholipid model membrane

Differential scanning calorimetry (DSC) experiments have been performed on the amphiphilic peptide/1,2-bis(perdeuteriopalmitoyl)-sn-glycero-3-phosphocholine system for which partial phase diagrams have been measured by deuterium nuclear magnetic resonance. The solute concentration dependence of the transition enthalpy in such systems is often interpreted in terms of an annulus of lipid withdrawn, by the solvent, from participation in the transition while the bulk lipid melts with its fully enthalpy. This idea is equivalent to postulating ideal mixing between the lipid and the peptide/lipid complex, and there is little justification for such an assumption. Adaptation of regular solution theory to this system demonstrates that the peptide concentration dependence of the transition enthalpies can be incorporated into a thermodynamic model which reproduces the observed phase behavior fairly well without postulating that a complexing annulus of lipid around the peptide be withdrawn from participating in the chain-melting transition. The model parameters determined by simultaneous fitting of the phase behavior and transition enthalpies are used to simulate the DSC scan shapes. The asymmetry of the calorimetric scans for chi 2 less than or equal to 0.02 is reproduced by the model, but a broad component observed for higher concentration is not. In light of the results presented here, previous analyses of the calorimetric behavior of two-component systems in terms of symmetric transitions which do not account for the possible extent of a region of two-phase equilibrium must be questioned.

Landau theory of two-component phospholipid bilayers. I. Phosphatidylcholine/phosphatidylethanolamine mixtures

Priest's phenomenological model (Mol. Cryst. Liq. Cryst. 60 (1980) 167.) on one- and two-component PC bilayers is extended here. We constructed a new excess free energy term in the state function to describe the thermodynamic properties of the two-component phospholipid bilayers where the chain lengths and the polar heads of the components can be different simultaneously. By means of this generalized state function, we can calculate the phase diagrams of DPPC/DPPE, DMPC/DMPE, DMPC/DPPE, DPPC/DMPE and DSPC/DMPE mixtures. We obtained complete miscibility both in the liquid crystalline and in the gel phase if the chain lengths of the components were the same. If the chain length of the PE component was longer than that of the PC component, we obtained a peritectic system. A eutectic system was obtained in the reverse case. The results of the model were compared with the experimental data available. Applying the quasichemical approximation, we determined the molecular meaning of the phenomenological model parameters. Namely, sigma and gamma are proportional to the sublimation heat of the CH2 group in the long-chain alkanes and to the hydrogen-bonding energy between the polar heads of the ethanolamines; otherwise the model resulted in--1.94 kcal/mol per CH2 for the sublimation heat and --1.4 kcal/mol for the hydrogen-bond energy.

Effect of pH, mono- and divalent cations on the mixing of phosphatidylglycerol with phosphatidylcholine. A monolayer (pi, delta V) and fluorescence study

The mixing of various molecular species of phosphatidylglycerol and phosphatidylcholine differing in their acyl chain lengths has been studied both in monolayers (pi, delta V), and in water dispersions (fluorescence polarization) with varying pH and ionic strength of the aqueous phase and in the presence of the divalent cations Mg2+ and Ca2+. In dilauroylphosphatidylglycerol/dipalmitoylphosphatidylcholine mixtures, both in monolayers and in water dispersions, no phase separation was detected at pH 2.9 where phosphatidylglycerol was protonated. With dipalmitoylphosphatidylglycerol/dipalmitoylphosphatidylcholine mixtures, in monolayers and at the same pH, no phase separation was detected for surface pressures below pi = 40 mN.m-1. In monolayers, and under ionic conditions such that phosphatidylglycerol was ionized (pH 5.6, 10 mM NaCl) miscibility was observed with dilauroylphosphatidylglycerol and dipalmitoylphosphatidylcholine and also with dipalmitoylphosphatidylglycerol and dilauroylphosphatidylcholine. Varying the ionic strength did not alter the miscibility of these lipids. The divalent cations Mg2+ and Ca2+ did not modify that of dilauroylphosphatidylglycerol with dilauroylphosphatidylcholine or with dipalmitoylphosphatidylcholine. Both in monolayers and in water dispersions, dipalmitoylphosphatidylglycerol and dilauroylphosphatidylcholine appeared to be at least partly miscible, in the presence of magnesium. Only in the presence of calcium and at high surface pressure might the monolayer data account for phase separation between these two lipids. The data presented demonstrate the existence of strong cohesive forces between phosphatidylcholine and phosphatidylglycerol with a marked influence of the former on the physical state of the latter. From an analysis of the delta V data, it is suggested that intrafacial hydrogen bonds may play a significant role in stabilizing phosphatidylcholine/phosphatidylglycerol mixtures.