Cholesterol, monolayer mixtures

The influence of CHL and Q3P on film morphology in monolayer mixtures with DPPC has been studied. Monolayers of DPPC as well as it mixtures tvith cholesterol, transferred by HP method, showed a molecularly smooth structure of uniform thickness. The addition of Q3P or CHL to DPPC, as investigated by AFM phase measurements, showed that a marked phase separation occurs in DPPC/Q3P mixtures or DPPC/SM/CHL films at small concentration of the alcohols, proving raft domain formation in the case of DPPC/SM/CHL films. 

Cholesterol monolayers are also used to model dmg-membrane interactions. Figure 6.14 shows the surface pressure-area isotherms for equimolar mixtures of valinomycin... 

The properties of monolayers of the cyclodepsipeptide, valinomydn (Fig. 4.12) have been the subject of recent investigations. Ries and Swift [105] have determined a molecular area of 3.70 nm from extrapolations of pressure-area isotherms for monolayers of valinomydn from which they infer a horizontal orientation of molecules within the monolayer. Mixtures of valinomydn and cholesterol (which has a vertical orientation in monolayers) have been investigated in view of their similarity to biological membranes and the possibility of their use as models for naturally occurring membranes. Monolayers of valinomydn are also of interest because of the ability of valinomycin to stimulate the transport of ions across mitochondrial and red blood cell membranes [106]. Several workers have studied the interaction of electrolytes with valinomycin monolayers [106-108]. These studies have shown a specific interaction of... 

There has been extensive activity in the study of lipid monolayers as discussed above in Section IV-4E. Coexisting fluid phases have been observed via fluorescence microscopy of mixtures of phospholipid and cholesterol where a critical point occurs near 30 mol% cholesterol [257]. 

Our present ideas about the nature of biological membranes, which are so fundamental to all biochemical processes, are based on the Singer-Nicholson mosaic model. This model of the membrane is based on a phospholipid bilayer that is, however, asymmetrical. In the outside monolayer, phosphatidylcholine (lecithin) predominates, whereas the inner monolayer on the cytoplasmic side is rich in a mixture of phos-phatidylethanolamine, phosphatidylserine, and phosphatidylinositol. Cholesterol molecules are also inserted into the bilayer, with their 3-hydroxyl group pointed toward the aqueous side. The hydrophobic fatty acid tails and the steran skeleton of cholesterol... 

Discontinuities are seen in the relationship between increase in film pressure, An, and lipid composition following the injection of globulin under monolayers of lecithin-dihydro-ceramide lactoside and lecithin-cholesterol mixtures. The breaks occur at 80 mole % C 16-dihydrocaramide lactoside and 50 mole % cholesterol. Between 0 and 80 mole % lactoside and between 0 and 50 mole % cholesterol the mixed films behave as pure lecithin. Two possible explanations are the formation of complexes, having molar ratios of lecithin-lactoside 1 to 4 and lecithin-cholesterol 1 to 1 and/or the effect of monolayer configurations (surface micelles). In this model, lecithin is at the periphery of the surface micelle and shields the other lipid from interaction with globulin. 

Although glycosphingolipids are the specific lipid components in the antigen-antibody complex, their activity is markedly enhanced by other (auxiliary) lipids such as lecithin and lecithin-cholesterol mixtures (15). The present study deals with the effect of lipid composition on the penetration of lactoside—cholesterol and lactoside—lecithin monolayers by rabbit y-globulin. We also investigated the lecithin-cholesterol system. Furthemore, since criteria for the existence of lipid-lipid complexes in monolayers are still few (8, 17), we have used infrared spectroscopy to examine lipid mixtures for the presence of complexes. 

The infrared evidence for hydrogen bonding between cholesterol and lecithin in chloroform solution is no evidence of a similar complex in the monolayer but suggests such a possibility. It does not exclude the hydrophobic bonding suggested by Chapman from NMR studies of the aqueous suspensions of equimolar mixtures of cholesterol and lecithin (3). 

Since cholesterol is an important component of many biological membranes mixtures of polymerizable lipids with this sterol are of great interest. In mixed monolayers of natural lipids with cholesterol a pronounced condensation effect , i.e. a reduction of the mean area per molecule of phospholipid is observed68. This influence of cholesterol on diacetylenic lecithin (18, n = 12), however, is not very significant (Fig. 32). Photopolymerization indicates phase separation in this system. Apparently due to the large hydrophobic interactions between the long hydrocarbon chains of... 

The lipid lamellae within the stratum corneum are thought to consist of a complex mixture of compounds but to contain predominantly cholesteryl sulfate (5%), free fatty acids (15%), cholesterol (25%), and ceramides (50%) (Abraham and Downing, 1990). Unlike DPPC monolayers, those formed from stratum corneum lipids do not undergo any obvious phase transitions during compression) therefore the information available from the resulting isotherms is more limited. The behavior of monolayers consisting of these types of compounds has been investigated in the presence and absence of Azone. 

A mixed monolayer consisting of stearic acid (9.9%), palmitic acid (36.8%), myristic acid (3.8%), oleic acid (33.1%), linoleic acid (12.5%), and palmitoleic acid (3.6%) produces an expanded area/pressure isotherm on which Azone has no apparent effect in terms of either expansion or compressibility (Schuckler and Lee, 1991). Squeeze-out of Azone from such films was not reported, but the surface pressures measured were not high enough for this to occur. The addition of cholesterol (to produce a 50 50 mixture) to this type of fatty acid monolayer results in a reduction of compressibility. However, the addition of ceramide has a much smaller condensing effect on the combined fatty acids (ratio 55 45), and the combination of all three components (free fatty acids/cholesterol/ceramide, 31 31 38) produces a liquid condensed film of moderate compressibility. The condensed nature of this film therefore results primarily from the presence of the membrane-stiffening cholesterol. In the presence of only small quantities of Azone (X = 0.025), the mixed film becomes liquid expanded in nature, and there is also evidence of Azone squeeze-out at approximately 32 mN m. ... 

The chemical structures of the lipid studied are shown in the Fig. 1. There are two different types of behavior for monolayers from those lipid mixtures (i) solid film formation with relatively low pressure of a collapse (cholesterol rich monolayers) and (ii) viscous elastic film formation, when the concentration of CHL is less 30% in the monolayer. The character of the behavior of a monolayer is quite similar upon replacing of CHL by Q3P, though the LB film morphology is essentially different. 

Contrary to the case of Q3P, cholesterol only slightly disordered the structure of the film in the mixed monolayers with DPPC or SM. No brightly expressed phase separation was found despite of essential variations (from 60 to 20%) in molar ratio of the alcohol in DPPC/CHL or SM/CHL mixtures. 

This micromembrane was also used to investigate the spontaneous formation of microdomains, when the distal lipid monolayer is made up of a lipid mixture. Microdomains are in the gel state when they consist primarily of glycolipids and sphingohpids, in a liquid-ordered state (so-called hpid rafts ) when they also contain cholesterol, and in a hquid-disordered state when they consist primarily... 

Figure 23. Changes in lamellar structures with temperature in mixtures of DPPE with cholesterol. The sequence, during which the area remained constant at 54 and the pH was 11.4, begins (upper left-hand corner) at 15°C. The monolayer is then cooled to 5.2°C and heated back to the starting temperature. It is seen that the lamellar thickness is a characteristic of each temperature but that the pattern is not reproduced. From Heckl and Mohwald. ...

Thermodynamics of Monolayer Solutions of Lecithin and Cholesterol Mixtures by the Surface Vapor Pressure Method... 

The dependence of the ttv values on the composition of the vapor and condensed states for DML-CHOL, DOL-CHOL, and DOL-DML mixtures is shown in Figure 6. The upper curve is the surface vapor pressure as a function of the mole fraction of the liquid-expanded film the lower curve is for the dependence of irv on the composition of the gaseous phase. Ideal mixing behavior is given by the linear dotted line which joins the 7ry° points for each of the pure compounds. In all cases there was complete miscibility of the components as represented by the continuous function of 7rv with x. In the cholesterol mixtures positive deviations from Raoult s law are observed for the mixture of lecithins, ideal mixing is observed. These results confirm those obtained with lipid mixtures—i.e., cholesterol mixed with liquid-expanded lipid films forms rion-ideal mixtures with positive deviations for mixtures of lipids which are in the same monolayer state, as in the case of the liquid-expanded DOL-DML mixtures, ideal mixing results (8). 

In addition, it is generally observed for mixtures of aliphatic hydrocarbons that if there is a difference in molecular size, a negative excess volume of mixing results (21). For the lecithin-cholesterol mixtures molecular area decreases (Figure 7), but it does not decrease with the mixture of the lecithins which forms an ideal mixture. Thus, the mixing of cholesterol and lecithin monolayers is consistent with bulk hydrocarbon mixtures with respect to both positive excess heats and negative excess volumes of mixing. 

---