Cholesterol films

Molecular domains have been observed using the AFM analyses of LB films of a collapsed state (Birdi, 1997, 2003). For example, cholesterol films showed half-but-terfly-shaped domains (each domain consisting of 107 molecules Figure 10.4). This quantity was estimated from the following data the height of the domains was 90 A, which corresponds with six layers of the cholesterol molecule (length of the molecule is found to be 15 A from molecular models). AFM image analysis is capable... 

Figure 4. Permeability of a phospholipid/cholesterol film for different solutes ascorbic acid (1), tyrosine (2), uric acid (3), acetaminophen (4), cysteine (5), desipramine (6), perphenazine (7), trimipramine (8), promethazine (9), and chlorpromazine (10). (Adapted from ref. 13.)...

Bianco et al. examined the electrochemical properties of lipid films containing c-type of cytochromes [41-43], and investigated pH effects. Cytochrome c, cytochrome C3, and cytochrome C553 incorporated into phosphatidylcholine cholesterol films doped with lauric add gave nearly reversible voltammograms. 

The ion-sulfur protein ferrdoxin from spinach in phosphatidylcholine-cholesterol films doped with dodecyl-amine or DODAB gave reversible CVs [44]. Chlorella... 

Especially, for monolayers consisting of more than one substance, there is an immense amonnt of information contained in the pressure-area isotherm. For example, a mixed behenic acid/cholesterol film gives what seems at first sight to be a very noisy isotherm, bnt in fact it contains characteristic oscillation freqnencies, as can readily be seen from the Fourier transform of the derivative (with respect to time the monolayer was compressed at a uniform rate, hence in effect making area and time equivalent) of the isotherm (Figure 5). The origin of these oscillations has been traced to spatial structming. ... 

Cholesterol crystallisation is thought to be the first step in the formation of gallstones in the human biliary system and the process of cholesterol nucleation remains incompletely understood. GIXD revealed a phase transition from a monolayer to a highly crystalline rectangular bilayer phase (165). The presence of the phospholipid DPPC in the cholesterol film inhibited cholesterol crystallisation [165]. AFM provided complementary information on the thickness and morphology of the cholesterol films transferred to a solid support The cholesterol monolayer thickness was 13 2 A and in the bilayer phase the presence of elongated faceted crystallites of pure cholesterol about 10 layers thick could be observed [165]. 

Fig. IV-20. Film pressure-area plots for cerebronic acid (a long-chain a-hydroxy carboxylic acid) and cholesterol (see insert) and for an equimolar mixture. At low pressures the r-a plot is close to that of the average (dashed line), an unanticipated kink then appears, and finally, the horizontal portion probably represents ejection of the cholesterol. (From Ref. 239.)...

Some fairly typical results, obtained by LaMer and co-workers [275] are shown in Fig. IV-24. At the higher film pressures, the reduction in evaporation rate may be 60-90%—a very substantial effect. Similar results have been reported for the various fatty acids and their esters [276,277]. Films of biological materials may offer little resistance, as is the case for cholesterol [278] and dimyristoylphosphatidylcholine (except if present as a bilayer) [279]. 

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. 

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]. 

When liposomes are prepared from a molecular mixture of lipid components it is important that all lipids be homogeneously dissolved in an organic solvent in order to obteiin bilayers with evenly distributed lipids after hydration. For example, the solubilities of phosphatidylcholine and cholesterol in chloroform are similar their solubility in benzene differs. Upon removal of benzene from the lipid solution an inhomogeneous lipid film is formed on the glass wall and... 

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. 

The lipid layer, which consists of cholesterol esters, phospholipids, and triglycerides, prevents and regulates aqueous evaporation from the tear film. 

X.C. Tan, MJ. Li, RX. Cai, LJ. Luo, and X.Y. Zou, An amperometric cholesterol biosensor based on multiwalled carbon nanotubes and organically modified sol-gel/chitosan hybrid composite film. Anal. Biochem. 337, 111-120 (2005). 

The interfacial barrier theory is illustrated in Fig. 15A. Since transport does not control the dissolution rate, the solute concentration falls precipitously from the surface value, cs, to the bulk value, cb, over an infinitesimal distance. The interfacial barrier model is probably applicable when the dissolution rate is limited by a condensed film absorbed at the solid-liquid interface this gives rise to a high activation energy barrier to the surface reaction, so that kR kj. Reaction-controlled dissolution is somewhat rare for organic compounds. Examples include the dissolution of gallstones, which consist mostly of cholesterol,... 

Figure 14 Effect of temperature on the hydration amount W (O) and the initial hydration rate v (Z) of 10 layers of Cholesterol LB films on each side of the QCM...

We observed also the morphological pictures of cholesterol LB films. The small disordered (white) area was frequently observed in the prepared cholesterol LB films and they were largely decreased after aging at 70 °C for 1 h (photographs are not shown). This is consistent with the hydration behavior that the hydration of cholesterol LB films was largely decreased with increasing temperatures (see Figure 14). 

From the frequency measurements of the LB-film-deposited QCM plate in water, the behavior of phospholipid LB films can be classified into three types (i) phospholipids having relatively hydrophilic head groups such as DPPC and DPPG are hydrated and then easily flaked from the substrate in the fluid liquid crystalline state above Tc (ii) DPPE and DPPS having the less hydrophilic head groups are hydrated only near their Tc (iii) cholesterol LB films show relatively large hydration behavior even at low temperatures due to the water penetration into the structure defects in the membrane. 

Figure 1 Cryo-TEM microscopy of DSPC DSPG CHOL liposomes obtained by thin-film hydration method and extruded through 0.2-pm polycarbonate membranes. Abbreviations. DSPC, l,2-distearoyl-s -glycerol-3-phosphocholine DSPG, 2,2-distearoyl-5 -glycerol-3[phosphor-rac-(l-glycerol)] CHOL, cholesterol.

Liposomes (SUVs) were prepared by probe sonication according to standard procedures (31) in the presence of STPP. A mixture of lecithin, cholesterol, and STPP (PC/Ch/STPP = 65/15/20, molar ratio final total lipid 25 mg/ mL) was dissolved in chloroform followed by removal of the organic solvent using a rotary evaporator. After adding 5 mM HEPES (pH 7.4) to the dry lipid film, the sample was probe sonicated with a Sonic Dismembrator (Model 100, Fischer Scientific) at a power output of approximately 10 W for 30 minutes. To remove any titanium particles, which have been shed from the tip of the probe during sonication, the sample was centrifuged for 10 minutes at 3000 X g. The formed liposomes were separated from free, i.e., nonincorporated, STPP by gel filtration chromatography on a Sephadex G-15 column. 

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). 

LB films with multilayers (varying from a few layers to thousands) could be made, and the adsorption monitored by measuring the decrease in fl on each stroke. If no adsorption takes place, then no change is observed. There are some lipids, such as cholesterol, which do not form LB films. There are also other methods, such as... 

The different behavior of 7 and 8 is probably due to the charged head group in 7. Phase separation to form enriched domains of this lipid in mixed monolayers would be inhibited by electrostatic repulsion. Interestingly, mono-layer films of 7 mixed with the biologically important molecule cholesterol did exhibit phase separation at all compositions provided the temperature was maintained below the Tm of 7. Presumably the significantly different shapes of the two molecules promotes the phase separation and overcomes the electrostatic barrier. 

The mixed lipid-cholesterol monolayers are unstable. Recent studies of these systems show that for molecular fractions of cholesterol larger than 30% the cholesterol separates from the film, as if there were a limited misability in two dimensions. 

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