Binary phospholipid mixtures

This leads to phase diagrams, which end in angular points on both sides and a somewhat reduced width of the coexistence range in between. However, reducing the width of the transition range of mixtures by this procedure assumes that the cooperativity of the transition simply scales with the composition in linear fashion. This is unlikely, as theoretical calculations show. In mixtures, the cooperativity is likely to be much lower than in the pure compounds [84]. 

We have therefore devised a new method for the determination of the temperatures for onset and end of melting, which is based on a simulation of the experimental DSC-curves. This procedure uses a regular solution model for nonideal mixing in both, the ordered and the liquid-crystalline phase, and incorporates the additional broadening by assuming a simple two-state transition of limited cooperativity, with the cooperative unit size c.u. as an adjustable parameter. This model is still a simplification of the real situation as it is based on the assumption that the cooperativity does not depend on temperature [84]. Nevertheless, this procedure seems to be more reliable for the determination of the phase boundaries as the arbitrariness in detennining the onset and end of melting temperatures is replaced by a more objective procedure. 

The method for calculation of the heat capacity curves will now be outlined in a condensed fonn. The reader is referred to the original publications for further details [80,85-87]. 

For a binary system with components A and B the molar enthalpy H in the phase transition region from the gel (g) to the liquid-crystalline (I) phase can be 

For non-ideal mixing in the two phases we can fonnulate two equations for the enthalpies and H as a function of composition introducing an excess enthalpy ah  

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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... 

Bagatolli LA, Qratton E (2000) Two-photon fluorescence microscopy of coexisting lipid domains in giant unilamellar vesicles of binary phospholipid mixtures. Biophys J 78 290-305... 

Figure 24, Simplified schematic diagram of possible types of phase diagrams observed for pseudo-binary phospholipid mixtures.

In almost all cases of pseudo-binary phospholipid mixtures, we found that the non-ideal mixing is also non-symmetric, so that this four parameter model gave the best fits to the experimental data. An illustration of how the shape of the phase diagram can change, when the asyimnetry of mixing is increased due to increased and pn values for the gel and liquid-crystalline phase, respeetively, is shown in Figure 27. 

As indicated in my report, we now know the rates of lateral diffusion of phospholipids in lipid bilayers in the fluid state, and in a few cases the rates of lateral diffusion of proteins in fluid lipids are also known. At the present time nothing is known about the rates of lateral diffusion of phospholipids in the crystalline, solid phases of the substances. As mentioned in my report, there are reasons to suspect that the rates of lateral diffusion of phospholipids in the solid solution crystalline phases of binary mixtures of phospholipids may be appreciable on the experimental time scale. Professor Ubbelohde may well be correct in pointing out the possibility of diffusion caused by defects. However, such defects, if present, apparently do not lead to significant loss of the membrane permeability barrier, except at domain boundaries. 

Fig. 5 HPLC profile of the Spectralipid SN standard mixture monitored by ELSD after HPLC separation of the phospholipid components on a Nucleosil 100-7 Diol stationary phase. The mobile phase included hexane, 2-propanol, butanol, tetrahydrofuran, iso-octane, and water in a binary gradient. (Reprinted from Ref. 69 with the kind permission of the authors and of Elsevier Science Publishers.)...

The interaction of soluble cations with the phospholipid phosphate groups has been investigated on a mixture of DPPC and 1,2-dipalmitoyl-sn-glycero-3-phosphoserine DPPS as a function of surface pressure and Ca2+ ion presence [51], The presence of Ca2+ in the subphase induce an acyl chain ordering at all surface pressures in both components of the binary mixture that was not observed in the case of pure DPPC alone. Unlike the bulk phase mixture of... 

Although studies of the thermotropic phase behavior of singlecomponent multilamellar phospholipid vesicles are necessary and valuable, these systems are not realistic models for biological membranes that normally contain at least several different types of phospholipids and a variety of fatty acyl chains. As a first step toward understanding the interactions of both the polar and apolar portions of different lipids in mixtures, DSC studies of various binary and ternary phospholipid systems have been carried out. Phase diagrams can be constructed by specifying the onset and completion temperatures for the phase transition of a series of mixtures and by an inspection of the shapes of the calorimetric traces. A comparison of the observed transition curves with the theoretical curves supports... 

In an earlier review [3], mixed micelles formed by bile salts were classified into those with (i) non-polar lipids (e.g., linear or cyclic hydrocarbons) (ii) insoluble amphiphiles (e.g., cholesterol, protonated fatty acids, etc.) (iii) insoluble swelling amphiphiles (e.g., phospholipids, monoglycerides, acid soaps ) and (iv) soluble amphiphiles (e.g., mixtures of bile salts with themselves, with soaps and with detergents) and the literature up to that date (1970) was critically summarized. Much recent work has appeared in all of these areas, but the most significant is the dramatic advances that have taken place in our understanding of the structure, size, shape, equilibria, and thermodynamics of bile salt-lecithin [16,18,28,29,99-102,127, 144,218,223,231-238] and bile salt-lecithin-cholesterol [238,239] micelles which are of crucial importance to the solubihty of cholesterol in bile [1]. This section briefly surveys recent results on the above subclasses. Information on solubilization, solubilization capacities or phase equilibria of binary, ternary or quaternary systems or structures of liquid crystalline phases can be found in several excellent reviews [5,85,207,208,210,211,213,216,217] and, where relevant, have been referred to earlier. 

Abstract The miscibility of two phospholipids dipalmitoyl-phosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) possessing both a choline head group, with per-(6-dodecanoylamino-6-deoxy) j8-cyclodextrin (CnCONH-/ -CD) and poly(ethylene oxide)-bearing lipid (PEO-lipid), respectively, has been assessed by surface pressure measurements of binary monolayers under dynamic conditions. Although the four studied amphiphiles had similar hydrophobic moieties constituted of hydrocarbon units with the number of carbons ranging from 12 to 16, PEO-lipid markedly differed from other amphiphiles due to its bulky poly(ethylene oxide) chain containing 13 ethylene oxide units totally immersed in the aqueous subphase. The additivity rule applied to these binary mixtures clearly showed that molecular areas for both systems deviated from linearity. For... 

Drop Impact on Monolayers with Different Binary Mixtures of Either Unsaturated (DOPC) or Saturated (DPPC) Phospholipids with Cholesterol... 

A large number of phase diagrams for binary mixtures combining cholesterol with different saturated and unsaturated phosphatidylcholines have been established. Cholesterol at different bilayer concentrations can promote or suppress lateral segregation of phospholipids of differing acyl chain length. 

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