Cholesterol monolayers

DOrc monolayers, due to the unsaturation, i.e. kinks of the alkyl chains, are in the liquid expanded phase, which is a fluid phase at all film pressures FI [3,13,15]. At 21 °C and T1 >25 mN m DPPC monolayers are in the solid analogous phase [3,13,16], which is highly incompressible and condensed [13,16]. Shah and Schulman [13] show that the effect of cholesterol on either saturated or unsaturated phospholipids is strikingly different. Cholesterol increases the surface elasticity, the dilational and the shear viscosity of unsaturated phospholipid monolayers [3,13,14,17]. In saturated monolayers cholesterol disturbs the order between phospholipid molecules fluidifying the solid monolayer [13,14,18] and lowering its shear viscosity [18]. Pure cholesterol monolayers are liquid [13] and have very low surface shear viscosities which are hardly detectable [18]. 

In mixed monolayer, cholesterol and phosphatide molecules are in close contact, and therefore, should interact at least part of the time. The question whether such interactions lead to complexes, i.e., associations of appreciably longer half-life than associations between like molecules, would seem to depend on whether the van der Waals interactions generated by complex formation are appreciably higher than those generated between like molecules. Hence the half-life of complexes may be altered by the nature of acyl chains (length, position, unsaturation). 

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]. 

Fig. IV 23. Penetration of cholesterol monolayers by CTAB (hexadecyl-trimethylam-monium bromide. [From D. M. Alexander, G. T. Barnes, M. A. McGregor, and K. Walker, Phenomena in Mixed Surfactant Systems, in J. F. Scamehom, ed., ACS Symposium Series 311, p. 133, 1986 (Ref. 269). Copyright 1986, American Chemical Society.]...

Barnes and co-workers have studied mixed-monolayer systems [278,281,283,284] and found some striking nonidealities. Mixed films of octadecanol and cholesterol, for example, show little evaporation resistance if only 10% cholesterol is present [278] apparently due to an uneven granular microstructure in films with cholesterol [284]. Another study of cellulose decanoate films showed no correlation between holes in the monolayer and permeation rate [285]. Polymerized surfactants make relatively poor water evaporation retarders when compared to octadecanol [286]. There are problems in obtaining reproducible values for r [287] due to impurities in the monolayer material or in the spreading solvent. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

A study by Bames and co-workers of the equilibrium spreading behavior of dimyristol phosphatidylcholine (DMPC) reconciles the differences between spreading of bulk solids and dispersions of liposomes [41]. This study shows the formation of multibilayers below the monolayer at the air-water interface. An incipient phase separation, undetectable by microscopy, in DMPC-cholesterol... 

Fig. XV-8. Fluorescence micrographs of crystalline domains of an S-DPPC monolayer containing 2% cholesterol and compressed to the plateau region. [From H. McConnell, D. Keller, and H. Gaub, J. Phys. Chetn., 40, 1717 (I486) (Ref, 49). Copyright 1986, American Chemical Society.]...

Langmuir-Blodgett films (LB) and self assembled monolayers (SAM) deposited on metal surfaces have been studied by SERS spectroscopy in several investigations. For example, mono- and bilayers of phospholipids and cholesterol deposited on a rutile prism with a silver coating have been analyzed in contact with water. The study showed that in these models of biological membranes the second layer modified the fluidity of the first monolayer, and revealed the conformation of the polar head close to the silver [4.300]. 

In a class of reahstic lattice models, hydrocarbon chains are placed on a diamond lattice in order to imitate the zigzag structure of the carbon backbones and the trans and gauche bonds. Such models have been used early on to study micelle structures [104], monolayers [105], and bilayers [106]. Levine and coworkers have introduced an even more sophisticated model, which allows one to consider unsaturated C=C bonds and stiffer molecules such as cholesterol a monomer occupies several lattice sites on a cubic lattice, the saturated bonds between monomers are taken from a given set of allowed bonds with length /5, and torsional potentials are introduced to distinguish between trans and "gauche conformations [107,108]. 

Figure 26 shows the redox potential of 40 monolayers of cytochrome P450scc on ITO glass plate in 0.1 KCl containing 10 mM phosphate buffer. It can be seen that when the cholesterol dissolved in X-triton 100 was added 50 pi at a time, the redox peaks were well distinguishable, and the cathodic peak at -90 mV was developed in addition to the anodic peak at 16 mV. When the potential was scanned from 400 to 400 mV, there could have been reaction of cholesterol. It is possible that the electrochemical process donated electrons to the cytochrome P450scc that reacted with the cholesterol. The kinetics of adsorption and the reduction process could have been the ion-diffusion-controlled process. 

FIG. 26 Cyclic voltammograms of 40 monolayers of Langmuir-Schaefer films of cytochrome P450SCC on indium-tin oxide glass plate (ITO) in 10 mM phosphate buffer at a scan rate of 20 mV/s between 0.4 and —0.4 V vs. Ag/AgCl. LS films on ITO worked as the working electrode, platinum as the counter, and Ag/AgCl as the reference electrode. Cholesterol dissolved in X-triton 100 was added 50 p.1 at a time (1) with cholesterol, (2) 50 p.1 of cholesterol, (3) 100 p.1 cholesterol, and (4) 150 p.1 of cholesterol. 

Lipoproteins. A lipoprotein is an endogenous macromolecule consisting of an inner apolar core of cholesteryl esters and triglycerides surrounded by a monolayer of phospholipid embedded with cholesterol and apoproteins. The functions of lipoproteins are to transport lipids and to mediate lipid metabolism. There are four main types of lipoproteins (classified based on their flotation rates in salt solutions) chylomicrons, very-low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL). These differ in size, molecular weight, and density and have different lipid, protein, and apoprotein compositions (Table 11). The apoproteins are important determinants in the metabolism of lipoproteins—they serve as ligands for lipoprotein receptors and as mediators in lipoproteins interconversion by enzymes. 

Solvents used to increase solubility for compounds during screening of permeability across the cell monolayers, together with commonly used excipients for formulations, can also affect the barrier as they contain ingredients which enhance drug absorption [100, 151]. There are different mechanisms by which these compounds can modulate the barrier [4, 149, 150] for example, they may increase the tight junctional pathway inhibiting carrier-mediated transport, or cholesterol... 

J.C. Vidal, J. Espuelas, E. Garda-Ruiz, and J.R. Castillo, Amperometric cholesterol biosensors based on the electropolymerization of pyrrole and the electrocatalytic effect of Prussian-Blue layers helped with self-assembled monolayers. Talanta 64, 655 (2004). 

K.E. Schmid, W.S. Davidson, L. Myatt, and L.A. Woollett. Transport of cholesterol across a BeWo cell monolayer Implications for net transport of sterol from maternal to fetal circulation. J Lipid Res. 44 1909-1918 (2003). 

The ability of a chiral molecule to distinguish between the enantiomers of a second (different) chiral molecule was defined in Sect. II as a diastereomer discrimination. This phenomenon may be observed in a mixed monolayer of two chiral surfactants and may also occur when a chiral substance is dissolved in the aqueous subphase under the monolayer of a second chiral substance. As before, examples of such chiral discrimination would not include those whose difference in monolayer behavior results only from the gross structural differences of diastereomers such as the different force-area characteristics exhibited by mixed monolayers of l-oleoyl-2-stearoyl-3-s -phospha-tidylcholine with epimeric steroids (120). The relevant experiment, that of comparing the monolayer behavior of mixed monolayers of cholesterol with enantiomeric phospholipids, has been reported (121). As might be anticipated from our previous discussion of... 

Evidence from studies on the penetration of cholesterol monolayers by nonionic surfactants of two Brij series suggests penetration occurs at extremely low concentrations (22), the C] 2 compounds interacting at lower concentrations than C] 0 compounds. (Figure 7). 

Lipoproteins are an important class of serum proteins in which a spherical hydrophobic core of triglycerides or cholesterol esters is surrounded by an amphipathic monolayer of phospholipids, cholesterol and apolipoproteins (fatbinding proteins). Lipoproteins transport lipid in the circulation and vary in size and density, depending on their proteindipid ratio (Figure 7.3). Lipoprotein metabolism is adversely affected by obesity low-density lipoprotein (LDL)-cholesterol and plasma triglyceride are increased, together with decreased high-density lipoprotein (HDL)-cholesterol concentrations. 

The measurements of n versus A isotherms generally exhibit, when compressed, a sharp break in the isotherms that has been connected to the collapse of the mono-layer under given experimental conditions. The monolayer of some lipids, such as cholesterol, is found to exhibit an unusual isotherm (Figure 4.7). The magnitude of FI increases very little as compression takes place. In fact, the collapse state or point is the most useful molecular information from such studies. It has been found that this is the only method that can provide information about the structure and orientation of amphiphile molecules at the surface of water (Birdi, 1989). 

This value of Aco corresponds to the cholesterol molecule oriented with the hydroxyl group pointing toward the water phase. Atomic force microscope (AFM) studies of cholesterol in Langmuir-Blodgett (LB) films has shown that there exist domain structures (see Chapter X). This has been found for different collapse lipid monolayers (Birdi, 2003). Different data have provided much information about the orientation of lipid on water (Table 4.1). 

FIGURE 4.7 n versus A isotherm of cholesterol monolayer on water. 

The spread mixed lipid monolayer studies provide information about the packing and orientation of such molecules at the water interface. These interfacial characteristics affect many other systems. For instance, mixed surfactants are used in froth flotation. The monolayer surface pressure of a pure surfactant is measured after the injection of the second surfactant. From the change in n, the interaction mechanism can be measured. The monolayer method has also been used as a model biological membrane system. In the latter BLM, lipids are found to be mixed with other lipidlike molecules (such as cholesterol). Hence, mixed monolayers of lipids + cholesterol have been found to provide much useful information on BLM. The most important BLM and temperature melting phenomena is the human body temperature regulation. Normal body temperature is 37°C (98°F), at which all BLM function efficiently. 

When a surfactant is injected into the liquid beneath an insoluble monolayer, surfactant molecules may adsorb at the surface, penetrating between the monolayer molecules. However it is difficult to determine the extent of this penetration. In principle, equilibrium penetration is described by the Gibbs equation, but the practical application of this equation is complicated by the need to evaluate the dependence of the activity of monolayer substance on surface pressure. There have been several approaches to this problem. In this paper, previously published surface pressure-area Isotherms for cholesterol monolayers on solutions of hexadecy1-trimethyl-ammonium bromide have been analysed by three different methods and the results compared. For this system there is no significant difference between the adsorption calculated by the equation of Pethica and that from the procedure of Alexander and Barnes, but analysis by the method of Motomura, et al. gives results which differ considerably. These differences indicate that an independent experimental measurement of the adsorption should be capable of discriminating between the Motomura method and the other two. 

For interpreting thesedata, and as a first step towards formulating a model for monolayer penetration, it is clearly desirable to calculate the amount of surfactant that has penetrated the monolayer. This has proved to be a difficult theoretical problem, but in recent years some limited solutions and a general solution have been found. In this paper we examine data for the penetration of cholesterol monolayers by hexadecy1-trimethyl-ammonium bromide (CTAB) (7) and compare the penetration or adsorption values calculated from the different treatments. 

Numerical data are available from our earlier penetration work for a number of monolayer/surfactant systems. The simplest of these systems was selected for this initial analysis the penetration of cholesterol monolayers by hexadecyl-trimethyl-ammonium bromide (CTAB) J). Cholesterol monolayers at 298 K exhibit a single, highly incompressible, condensed phase with the transition to a gaseous phase occurring at a negligibly low surface pressure. CTAB does not appear to undergo surface hydrolysis (10) and the gaseous-to-expanded phase transition occurs at a low concentration (0.043 mmol kg ) and a low surface pressure (1.0 mN m l). 

FIG. 1. Penetration of cholesterol monolayers by CTAB calculated by the equation of Pethlca and the procedure of Alexander and Barnes. 

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