Mixed Monolayers Of Cholesterol Research Articles

Thermodynamic quantification of sodium dodecyl sulfate penetration in cholesterol and phospholipid monolayers

Sodium dodecyl sulfate (SDS) is one of the extensively used surfactants in bioprocesses. Present work analyzes the penetration of SDS in pure and mixed Langmuir monolayers consisting of 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol. A series of isotherms are obtained by compressing the monolayers over subphase containing varying concentrations of SDS. The extent of penetration of SDS is quantified in terms of its mole percentage in the monolayer by applying various thermodynamic equations derived from Gibbs adsorption equation. Analysis of pure monolayers shows that SDS penetration is directly correlated with the monolayer fluidity and the lipid packing density. We find that POPC monolayers, which do not pack closely on compression due to a kink in one of the acyl chains, exhibit higher SDS penetration than DPPC monolayers which are more rigid due to two saturated acyl chains that pack very efficiently. The SDS penetration is also analyzed in mixed lipid monolayers in which the lipids are present in molar proportions similar to that in the plasma membranes of mammalian cells. Thermodynamic quantification shows that mixed monolayers of DPPC with POPC (in 2:1 molar ratio) exhibit penetration similar to pure POPC monolayers indicating dominating effect of unsaturated phospholipids on membrane packing. We find lower SDS penetration in mixed cholesterol-POPC monolayers than pure POPC monolayers at all surface pressures and for all SDS subphase concentrations used. This confirms with the well-known condensation effect of cholesterol on POPC monolayers. Cholesterol is known to have a rigidifying effect on DPPC monolayers at low surface pressures and a fluidizing effect at high surface pressures. Consistent with these experimental observations, our thermodynamic quantification predicts lower SDS penetration at low pressures and higher penetration at high pressures when compared to pure DPPC monolayers. Thus, thermodynamic theories are able to accurately predict the experimentally observed trends for penetration in mixed monolayers of cholesterol with POPC and DPPC. Our work demonstrates that thermodynamic quantification is a reliable technique to estimate extent of penetration of various additives in pure and mixed lipid monolayers.

Polar head group interactions in mixed Langmuir monolayers

We have investigated the miscibility in the mixed monolayers of cholesterol (Ch)-octyl cyano biphenyl (8CB) and cholesteryl acetate (ChA)-8CB using surface manometry and epifluorescence microscopic techniques. The main skeleton is the same both in Ch and ChA, whereas the polar head group is alcohol in Ch and ester in ChA. The 8CB molecule has a polar cyano as a terminal group and we probe its interaction with the polar group of Ch or ChA molecules in the mixed monolayers. Both Ch-8CB and ChA-8CB mixed monolayers exhibit two collapse pressures. In the case of the Ch-8CB mixed monolayer, the lower collapse pressure varies after 0.6 mole fraction (MF) of Ch in 8CB and the higher collapse pressure is nearly independent of composition. In ChA-8CB mixed monolayer, the lower collapse pressure varies continuously with the composition of ChA while the higher collapse pressure is nearly independent of the composition of ChA. In both these mixed monolayers, above the lower collapse pressure, 8CB gets squeezed out of the monolayer and forms multilayers. We find that in the case of Ch-8CB there is a phase separation in the monolayer occurring in the range of 0.15-0.9 MF of Ch. However, in ChA-8CB, the monolayer phase is miscible in all the proportions (except at very high concentration of ChA) below the lower collapse pressure. We attribute this better miscibility in the ChA-8CB compared to the Ch-8CB to the role played by the ester and cyano polar head group interactions.

Local control of antibody binding to hapten‐presenting interfaces: Steric and electrostatic interaction

AbstractThe binding of labeled antibodies to hapten substituted monolayers at the air/water interface has been studied by means of fluorescence microscopy. Haptens with various spacer lengths between the epitope and a hydrocarbon chain, anchoring the molecule to the interface, have been synthesized. With DMPC,a unspecific binding has been shown to predominate over specific binding due to electrostatic interactions. At high surface pressures the bound antibody is detached because of steric interference with the lipid head groups. Due to a reduction of electrostatic interactions, no unspecific binding is observed to monolayers of cholesterol, which carries a small dipole moment. Mixed monolayers of cholesterol and DMPC separate into two fluid phases, with preferential antibody binding to the cholesterol‐enriched phase.

The action of polysilicic acid on mixed monolayers of cholesterol and lecithins

Silicic acid produces marked expansion in dipalmitoyl lecithin films and in mixed cholesterol-dipalmitoyl lecithin films with more than 50 % phospholipid. In the presence of silicic acid, these films no longer exhibit a transition region between the liquid-expanded and liquid-condensed states. Mixed cholesterol-dilauroyl lecithin.films are also expanded by silicic acid. In the absence of silicic acid, the addition of cholesterol produces greater condensation in dilauroyl lecithin films than in films of dipalmitoyl lecithin, but the reverse is the case if the substrate contains silicic acid. These results have been interpreted in terms of a possible electrostatic interaction between negatively charged dissociated silicate ions and the positively charged trimethyl ammonium group of the choline fragment.

Mixed monolayers of cholesterol and hexadecyl—trimethylammonium bromide

Cholesterol monolayers were spread over solutions of hexadecyl—trimethylammonium bromide (HDTAB) with extensive precautions to avoid contamination. The force-area curves for the cholesterol-HDTAB monolayers were not dependent upon the HDTAB concentration. The primary effects of the HDTAB were to make the cholesterol monolayers more gaseous, and to increase the area per cholesterol molecule from 40 Å 2 to 70 Å 2 at the collapse point. The stabilizing effect of HDTAB on cholesterol black lipid membranes is discussed. The kinetics of diffusion of HDTAB from monolayers to bulk was observed to obey first order approach toward equilibrium.

Cholesterol as liquifier in phospholipid membranes studied by surface viscosity measurements of mixed monolayers

The surface viscosity of cholesterol, distearoyl lecithin, oleoyl-stearoyl lecithin monolayers and mixed monolayers of cholesterol with both lecithins were measured. The presence of cholesterol causes the surface viscosity of the phospholipid monolayer to be reduced, so cholesterol acts as a liquifier. This liquifaction of the mixed monolayer is a pure physical phenomena and follows from the rate theory of Eyring.

Longitudinal capillary waves. Part 2.—Experiments

It has been shown theoretically that longitudinal waves can easily be studied at an interface with a high elastic modulus. Measurements on mixed monolayers of cholesterol and dimyristoyl lecithin show that the behaviour of longitudinal waves is in accordance with predictions from theory. The elastic modulus at this interface as obtained from the wavelength of longitudinal waves at 200 c/sec, is several times the “equilibrium” modulus obtained from the slope of force-area curves.