Cholesterol formate film

Fig. 21. (a) Rate constant for the hydrolysis on 0.1 N HCl of a cholesterol formate monolayer (50a). The incorporation into the film of a little long-chain sulfate (C22H46-SOi ) greatly accelerates the reaction. The calculated increases in reaction rate according to the Gouy (33) and Donnan equations are shown (b) For hydrolysis on 0.66 N HCl incorporation of CisH37N(CH3)+ into the film retards reaction because hydrogen ions are repelled from the surface (50a). 

This method of finding the concentration of ions near the surface was applied by Davies (49,21) to the hydrolysis of ionized films of the ester monocetyl succinate. Table IX shows that the rate constant for this hydrolysis, which increased 300% if calculated using bulk concentrations of the catalytic hydroxyl ion, varied by not more than 36% when evaluated using the surface concentrations deduced from (xxv) and (xxvi). Figure 19 shows a similar effect for the addition of neutral salt, the marked catalysis by which is thus demonstrated to be due entirely to electrostatic effects. The acceleration in the rate of hydrolysis of a film cholesterol formate if the surface bears a negative charge can be predicted on the basis of the Donnan equations (xxv) and (xxvi). Values of 5 of 6 A. and 8 A. have been used, the results being compared with experiment in Fig. 21a. The calculated retardation is shown in Fig. 21b. 

Using an automated film balance the behavior of mixed monomolecular films exhibiting deviations from ideality was studied. Particular attention was paid to condensation effects obtained when cholesterol is mixed with a more expanded component. The deviations at various film pressures are discussed in terms of the partial molecular areas of the film components. Slope changes in these plots are caused by phase transitions of the expanded monolayer component and do not indicate the formation of surface complexes. In addition, the excess free energies, entropies, and enthalpies of mixing were evaluated, but these parameters could be interpreted only for systems involving pure expanded components, for which it is clear that the observed condensation effects must involve molecular interactions. 

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. 

The present paper is aimed to construct and characterize the lipid multicomponent films onto a solid surface by a Langmuir-Blodgett (LB) technology. The goal of the work was to compare the interaction of cholesterol or quercetin-3-O-palmitate (Q3P) with membrane lipids at different alcohol concentrations to prove lateral domains formation as a result of molecular association and to understand preferable cholesterol affinity. 

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. 

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. 

The formation of liposomes [or better arsonoliposomes (ARSL)], composed solely of arsonolipids (Ars with R=lauric acid (C12) myristic acid (C14) palmitic acid (C16) and stearic acid (C18) (Fig. 1) have been used for ARSL construction), mixed or not with cholesterol (Choi) (plain ARSL), or composed of mixtures of Ars and phospholipids (as phosphatidylcholine [PC] or l,2-distearoyl- -glyceroyl-PC [DSPC]) and containing or not Choi (mixed ARSL), was not an easy task. Several liposome preparation techniques (thin-film hydration, sonication, reversed phase evaporation, etc.) were initially tested, but were not successful to form vesicles. Thereby a modification of the so called one step or bubble technique (8), in which the lipids (in powder form) are mixed at high temperature with the aqueous medium, for an extended period of time, was developed. This technique was successfiil for the preparation of arsonoliposomes (plain and mixed) (9). If followed by probe sonication, smaller vesicles (compared to those formed without any sonication [non-sonicated]) could be formed [sonicated ARSL] (9). Additionally, sonicated PEGylated ARSL (ARSL that contain polyethyleneglycol [PEG]-conjugated phospholipids in their lipid bilayers) were prepared by the same modified one-step technique followed by sonication (10). 

Notably, liposomes composed of 3 -[N- N N -dimethylaminoethane)carbamoyl)cholesterol (DC-Chol) together withdioleoylphosphatidylethanolamine (DOPE) (DC-Chol/ DOPE liposome) have been classified as one of the most efficient vectors for the transfection of DNA into cells (8-10) and in clinical trials (11, 12). It has been demonstrated that a 3 2 or 1 1 molar ratio of DC-Chol/DOPE liposome results in high transfection efficiency (10). In these cases, liposomes are mostly prepared by the dry-film method. To further improve the transfection efficiency, it is necessary to evaluate DC-Chol/DOPE liposome from formulation and preparation method of liposome to formation method of their lipoplex. 

Figure 2.9a shows the lipid molecule DMPC. Two layers contacted via the hydrophobic tails lead to spontaneous formation of a double-layer biomimetic membrane that can be transferred to a single-crystal ultraplanar electrochemical Au(lll) surface. The hydrophilic head groups contact the electrode surface via an intermediate water film. Due to the structurally very well-defined assembly, not only AFM and in situ STM but also neutron reflectivity. X-ray diffraction, and infrared reflection absorption spectroscopy (IRRAS) have been employed to support the direct visual in situ STM. Electrochemically controlled structural changes, phase transitions, and the effects of the common membrane component cholesterol (Figure 2.9b) and peptide drugs have been investigated in this way. 

Figure 12. Epifluorescence (fluorescent probe, 23) photomicrograph of a mono-molecular film of the phospholipid dipalmitoyl phosphatidyl choline (10, R = R = n-CisHsi) at the air-water interface. The black regions are composed of solid-phase lipid, and the white (fluorescent) regions are fluid-phase lipid containing about 1 mol% of a fluorescent lipid probe. (Top) Micrograph showing the onset of solid phase formation bar, 50 pm. Middle) Micrograph showing formation of chiral solid domains when the phospholipid is one of the enantiomeric forms (R) bar, 50 pm. Bottom) Micrograph showing spiral forms of enantiomeric lipid when 2 mol% of cholesterol is included in the monolayer so as to reduce the line tension bar, 30 pm. Reproduced from ref. 146 (McConnell and Keller, Proc. Natl. Acad. Sci. USA 1987, 84,4706) with permission of the Academy of Sciences of the USA.

Exposure of octadecanethiol monolayer to the small phospholipid vesicles leads to their unrolling and adsorption of a single lipid layer on top of the octadecanethiol film. Unrolling of vesicles on the surface of the hydrophilic monolayer, however, leads to the formation of the lipid bilayer on top of the original film. This principle was used by Evans and coworkers to create lipid bilayers supported by cholesterol moieties present in a mixed gold-thiol monolayer (Figure 28)430. 

Zull et al. (1968) have used the ATR technique (see Chapter 3) to obtain spectra of solid films of egg lecithin-cholesterol mixtures. In films cast from organic solvents, cholesterol can form a sterol phospholipid (2 1) complex which involves interaction of the cholesterol —OH with lecithin polar groups. Association with the quaternary nitrogen of lecithin was ruled out, and the formation of a hydrogen bond between... 

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]. 

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