Binary lipid mixtures

This review emphasizes an intriguing and potentially useful aspect of the polymerization of lipid assemblies, i.e. polymerization and domain formation within an ensemble of molecules that is usually composed of more than one amphiphile. General aspects of domain formation in binary lipid mixtures and the polymerization of lipid bilayers are discussed in Sects. 1.1 and 1.2, respectively. More detailed reviews of these topics are available as noted. The mutual interactions of lipid domains and lipid polymerization are described in the subsequent sections. Given the proper circumstances the polymerization of lipid monolayers or bilayers can lock in the phase separation of lipids, i.e. pre-existing lipid domains within the ensemble as described in Sect. 2. Section 3 reviews the evidence for the polymerization-initiated phase separation of polymeric domains from the unpolymerized lipids. 

One of the major goals of these many investigations of lipids is, of course, a better understanding of the in - vivo behavior of membranes. Beyond studies of binary lipid mixtures, as mentioned above, a further step which is necessary is the incorporation of proteins into the layers. In many respects, this increase in the complexity of the bilayer systems resembles that encountered in the use of synthetic surfactants in "real - world" situations, where blends, rather than single, surfactants are used. Surfactant blends in aqueous solutions are often further modified in use by the solubilization of oily organic compounds, as in the cases of detergency or cosmetic formulation. 

The properties of lipid bilayers formed from mixtures of lipids are very relevant to the understanding of the lipid bilayers that form the basis of biological membranes. Detailed studies have been performed on bilayers formed from binary lipid mixtures, and some reports in the recent literature describe phase diagrams of lipid bilayers prepared from ternary mixtures that include cholesterol. Figure 4 (24-31) shows some phase diagrams of lipid bilayers formed from binary and ternary mixtures of lipids. The general observation is that lipids in a bilayer are not very... 

L.A. Bagatolli and E. Gratton. Two-photon fluorescence microscopy of coexisting lipid domains in giant unilamellar vesicles of binary lipid mixtures. Biophys. 

Fig. 15. T -phase diagram of the equimolar binary lipid mixtures DMPC/DPPC and DMPC/DSPC in excess water as a function of pressure.

Observations of thermotropic phase transitions even in simple lipid mixtures are complicated by factors such as nonideal miscibility, coexistence of different phases under certain conditions, and specific interactions between lipids, leading to complex formation. A well-studied, yet incompletely understood, binary lipid mixture that exemplifies these difficulties is the one formed by phosphatidylcholine and cholesterol. 

Recording surface pressure/area isotherms is a relatively simple method for describing the miscibility behavior of a two component lipid mixture. The mean area per molecule of a binary mixture can be calculated using the following equation ... 

Fig. 5 (a) TE of DNA complexes of binary DOTAP/DOPC lipid mixtures (black circles). Their TE increases over several orders of magnitude with increasing molar fraction of monovalent DOTAP (lipid mixtures with constant <1>dotap = 0.3. Different symbol shapes correspond to different choi (cf. legend), (b) The TE of the DNA complexes of ternary DOTAP/DOPC/Chol lipid mixtures (empty circles) plotted against aM significantly deviates from the universal bell shaped curve observed for binary systems [21]. Reprinted with permission from [27]. Copyright 2009 American Chemical Society... 

Three sets of two-chamber diffusion cell experiments were conducted 1) ethanol/saline in the donor chamber and saline in the receiver chamber, 2) saline in the donor and ethanol/saline in the receiver chamber, and 3) with ethanol/saline in both chambers. The results were shown to deviate enormously from the classical lipid barrier model. A new model, based on diffusion across a binary solvent mixture, was used to analyze the data. A good agreement was observed between experimental data and theoretical results. 

Figure 4 Some phase diagrams for lipid bilayers in excess water prepared from binary and ternary lipid mixtures, a) Multibilayer lipid vesicles prepared from binary mixtures of DMPC and DPPC (24) b) Multibilayer lipid vesicles prepared from binary mixtures of DMPC and DSPC [adapted by Reference (25) from data for perdeuterated lipids published by Knoll et al. (26)] c) Multibilayer lipid vesicles prepared from binary mixtures of diCi/.QPC and C22 oCi2 oPC (27) d) Multibilayer lipid vesicles prepared from binary mixtures of DMPC and cholesterol (28) e) Multibilayer lipid vesicles prepared from ternary mixtures of palmitoyl sphingomyelin, POPC, and cholesterol [adapted by Reference (29), from data published by De Almeida et al. (30)] Lipid bilayers prepared from ternary mixtures of DSPC, DOPC, and cholesterol (31).

Figure 4 Order parameter profiles (39) for the lipid hydrocarbon (a) sn-1 and (b) sn-2 chains in a binary membrane mixture of DPPC and cholesterol. The cholesterol concentrations are 0mol% (open circle),...

Fig. 24 Plots of ln(D) vs l/af for DMPC and the requirements for molecules obeying the free area model, for pure DMPC and 70mol%DMPC + 30 mol%DSPE-PEG2000. The straight solid line indicates that DMPC obeys the free area model. In case of the binary lipid/lipopolymer mixture, the free area model is only valid between points a-c, but not between c-d [39] (reproduced with permission from the American Chemical Society)...

In order to observe the concentration fluctuations caused by the gel-fluid phase coexistence in the above-mentioned binary phospholipid mixtures, SANS studies in combination with the H/D substitution technique were performed. Under these so-called matching conditions, no SANS signal is obtained for homogeneously mixed lipids in the all-gel or all-fluid phases, since then the scattering length density is constant over the whole sample. However, in the case of gel-fluid phase heterogeneities, SANS occurs due to the different compositions and... 

Approximately twenty different lipid systems have been examined by the method of TRXRD. The list includes single lipid species as well as binary lipid, lipid-small molecule and lipid-protein mixtures and isolated membranes and membrane lipid extracts (Table 1). A total of thirty, nominally distinct phase transition types are described in the table. The actual number of disparate phase transitions is expected to be less than this because of redundancies arising from an inadequate nomenclature and/or incomplete phase characterization. 

Baglioni P —> Carretti E Baptista ALF, Coutinho PJG, Real Oliveira MECD, Rocha Gomes JIN Lipid interaction with textile fibres in dyeing conditions 88 Barbosa EFG —> Santos MSCS Bechinger C —> Brunner M Behr J-P Lleres D Bergstrom M, Eriksson JC Synergistic effects in binary surfactant mixtures 16... 

Another important class of materials which can be successfiilly described by mesoscopic and contimiiim models are amphiphilic systems. Amphiphilic molecules consist of two distinct entities that like different enviromnents. Lipid molecules, for instance, comprise a polar head that likes an aqueous enviromnent and one or two hydrocarbon tails that are strongly hydrophobic. Since the two entities are chemically joined together they cannot separate into macroscopically large phases. If these amphiphiles are added to a binary mixture (say, water and oil) they greatly promote the dispersion of one component into the other. At low amphiphile... 

In binary mixtures of water, surfactants, or lipids the most common structure is the gyroid one, G, existing usually on the phase diagram between the hexagonal and lamellar mesophases. This structure has been observed in a very large number of surfactant systems [13-16,24—27] and in the computer simulations of surfactant systems [28], The G phase is found at rather high surfactant concentrations, usually much above 50% by weight. 

Once synthesized several factors influence the particular leaflet of the membrane lipid bilayer where the lipids reside. One is static interactions with intrinsic and extrinsic membrane proteins which, by virtue of their mechanism of biosynthesis, are also asymmetric with respect to the membrane. The interaction of the cytoplasmic protein, spectrin with the erythrocye membrane has been the subject of a number of studies. Coupling of spectrin to the transmembrane proteins, band 3 and glycophorin 3 via ankyrin and protein 4.1, respectively, has been well documented (van Doit et al, 1998). Interaction of spectrin with membrane lipids is still somewhat conjectural but recent studies have characterized such interactions more precisely. O Toole et al. (2000) have used a fluorescine derivative of phosphatidylethanolamine to investigate the binding affinity of specttin to lipid bilayers comprised of phosphatidylcholine or a binary mixture of phosphatidylcholine and phosphatidylserine. They concluded on the basis... 

Jenning, V, Mader, K. and Gohla, S. H., Solid lipid nanoparticles (SLN) based on binary mixtures of liquid and solid lipids a H-NMR study. Int. J. Pharm., 205, 15-21, 2000. 

Domain formation in binary mixtures of a polymerizable lipid and non-polymerizable lipid is well established for diacetylenic lipids. The rigid diacetylenic unit facilitates the formation of enriched domains in the condensed phase of monolayers or the solid-analogous phase of bilayers. Since diacetylenes polymerize most readily in solid-like states, most studies have focused on conditions that favor domain formation. Only in the case of a mixture of a charged diacetylenic lipid and a zwitterionic PC was phase separation not observed. Ringsdorf and coworkers first reported the polymerization of a phase-separated two-dimensional assembly in 1981 [33], Monolayer films were prepared from mixtures consisting of a diacetylenicPC (6) (Fig. 5) and a nonpolymerizable distearoyl PE (DSPE). 

Figure 2 shows a spin-label-derived phase diagram for binary mixtures of (II) and (IV), dipalmitoylphosphatidylcholine and dielaidoylphosphatidylcholine. It will be seen that the diagram describes miscibility of these two lipids in both the solid and solution phases. (Other binary mixtures of lipids show immiscibility in the solid as well as the fluid phases.45,54)... 

Brulet and McConnell4 have observed the 13C choline methyl nuclear resonance spectra of 50 50 binary mixtures of dielaidoylphosphatidyl-choline and dipalmitoylphosphatidylcholine. In one set of experiments one lipid was I3C enriched in the choline methyl group, and in the other... 

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