DMPC/DPPC mixtures

Figure 25. DSC curves of DMPC/DPPC mixtures as a function of composition (molar ratios). The dotted lines are the simulated curves calculated with the model described in the text [90].

Figure 26. Left Phase diagram obtained for DMPC/DPPC mixtures. The points for onset and end of melting obtained by the usual empirical procedure are indicated by open and filled dots, the triangles were obtained from the simulation of the DSC curves. The solid lines are fit curves through the triangles using the four parameter model described in the text. Right Non-ideality parameters as a ftinc-tion of composition as obtained from the simulation of phase diagrams of DMPC/DPPC and DMPE/DPPE (not shown) [90].

Fig. 13. Temperature-composition phase diagrams of the binary mixtures a) DMPC/DPPC (di-CiVdi-Ci6>, b) DMPC/DSPC (di-Ci4/di-C g), and c) DLPC/DSPC (di-Cu/di-Cig) dispersed in excess water (g, gel phase f, fluid phase x, mole fraction).

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

DSC studies of lipid-surfactant mixtures in the regime of low surfactant concentrations provide some insight into the partition coefficient of the surfactant molecule between water and the bilayer. We have systematically studied the be-havior of DMPC/octylglucoside (DMPC/OG) mixtures by ITC and DSC. The results of the ITC experiments will be described below. Figure 31 shows as an example some DSC curves of DPPC/OG mixtures as a function of total OG concentration. First, a decrease of the transition temperature due to preferential OG partitioning into the L,t-phase bilayers is observed. 

Supported lipid bilayers on planar silicon substrates have been formed using S-layer protein from B. coagulans E38/vl and from B. sphaericus CCM 2177 as support onto which l,2-dimyristoyl-OT-glycero-3-phosphocholine (DMPC) (pure or mixtures with 30% cholesterol) or DPPC bilayers were deposited by the Langmuir-Blodgett-technique (Pig. 

More recently, MD simulations under constant pressure involving full representation of phospholipids and cholesterol have been carried out. They dealt either with DMPC- [70, 71] or DPPC-cholesterol mixtures [72, 73]. The results of these four simulations were generally in agreement but differed in some aspects. 

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).

P. Hoyrup, O.G. Mouritsen, and K. Jorgensen. Phospholipase A2 activity towards vesicles of DPPC and mixtures of DMPC and DSPC with small amounts of SMPC. Biochim. Biophys. Acta, 2001,... 

The thermotropic phase transition temperature of a vesicle composed of a mixture of dipalmitoyl and dimyristoyl phosphatidylcholine (DPPC and DMPC, respectively) is intermediate between the phase transition temperatures of the single lipid vesicles and reflects the relative concentrations of the two lipids in the vesicle. This can be used to determine the rate of exchange of phosphatidylcholine between two unilamellar vesicles of initially pure phospholipid. 

Both aqueous organic solvent mixtures and differently charged micelles can mimic only roughly the environment of natural cell membranes. In order to analyze in more appropriate model systems possible interactions of gastrin and CCK with cell membranes and to determine their conformational states in lipid bilayers, we have recently investigated in detailed manner this aspect using liposomes. The similarity betwen liposomes and natural membranes is extensively exploited both in vitro and in vivo because of the ability of liposomes to mimic the behaviour of natural membranes. Moreover, the value of liposomes as model membrane systems derives from the fact that they can be constructed with natural constituents. In our approach, we selected as model membranes those formed with the zwitterionic lipids di-myristoylphosphatidylcholine (DMPC) and di-palmitoylphosphatidylcholine (DPPC) as these lipids constitute the major components of most cell membranes. Moreover, in order to operate with a simple system, small unilamellar vesicles (SUVs) were used, i.e. with a diameter between 25 and 250 nm as resulting by rod-type sonication or by extrusion (51). 

With binary mixtures such as DPPC-PS (50-50), the shift of Tm was from 32 to 35°C in the presence of 5 mM calcium. Blood clotting factors II, X and IX induced a further small shift. With the DMPC-DPPS binary mixture, calcium shifts the Tm from 37.5 C down to 33.5 C, this temperature being again decreased by 1 to 2 C in the presence of blood clotting factors (Fig. 6b). 

These effects are quite weak when compared to those induced by CTX on the same lipid mixtures an increase of 8°C for DPPC-PS and a decrease of 9°C for DMPC-DPPS (25). For this protein, which only binds to PS, the Tm observed is that of a PC-enriched phase. 

Nonideal miscibility - in ideal mixtures, the components do not interact with each other, thus the properties of the mixture depend linearly on the corresponding property of each component, and on the composition of the mixture. However, in practice, two-component lipid mixtures behave nonideally, and this gives rise to anomalies. For example the phosphatidylcholines dimyristoyl PC (DMPC, C14 saturated fatty acids), dipalmitoyl PC (DPPC, C16 saturated fatty acids) and distearoyl PC (DSPC, Cl 8 saturated fatty acids) have their main... 

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... 

DMPC/PEO-lipid mixtures this deviation was attributed to the presence of the poly(ethylene oxide) chain which hindered the estabhshment of a close contact between the film forming molecules. For DPPC/CnCONH- -CD mixtures the deviation from linearity was surmized to be due to an interaction at the level of hydrophobic chains protruding into the air phase. The thermodynamic relationship given by Joos and Demel (BBA 183 447-457) analysing miscibilities in monolayers according to their collapse pressures, confirmed the deviation from ideality for both systems and indicated that the effect was much more pronounced for DPPC/CiiCONH-)S-CD system. 

Fig. 5 Surface pressure-area isotherms of DMPC (1), PEO-lipid Fig. 4 Surface pressure-area isotherms of DPPC (1), CuCONH-jS- (12,13) (5), and their mixtures, 7 3 (2), 1 1 (3) and 2 8 (4)...

---