Hydrophobic Barriers of Lipid Bilayer Membranes Formed by Reduction of Water Penetration by Alkyl Chain Unsaturation and Cholesterol

Abstract

The hydrophobicity profiles across phosphatidylcholine (PC)-cholesterol bilayer membranes were estimated in both frozen liposome suspensions and fluid-phase membranes as a function of alkyl chain length, unsaturation, and cholesterol mole fraction. A series of stearic acid spin labels, with the probe attached to various positions along the alkyl chain, cholesterol-type spin labels (cholestane and androstane spin labels), and Tempo-PC were used to examine depth-dependent changes in local hydrophobicity, which is determined by the extent of water penetration into the membrane. Local hydrophobicity was monitored primarily by observing the z component of the hyperfine interaction tensor (Az) of the nitroxide spin probe in a frozen suspension of the membrane at -150 degrees C and was further confirmed in the fluid phase by observing the rate of collision of Fe(CN)6(3-) with the spin probe in the membrane using saturation recovery ESR. Saturated-PC membranes show low hydrophobicity (high polarity) across the membrane, comparable to 2-propanol and 1-octanol, even at the membrane center where hydrophobicity is highest. Longer alkyl chains only make the central hydrophobic regions wider without increasing the level of hydrophobicity. Introduction of a double bond at C9-C10 decreases the level of water penetration at all locations in the membrane, and this effect is considerably greater than the cis configuration than with the trans configuration. Incorporation of cholesterol (30 mol %) dramatically changes the profiles; it decreases hydrophobicity (increases water penetration) from the polar headgroup region to a depth of approximately C7 and C9 for saturated- and unsaturated-PC membranes, respectively, which is about where the bulky rigid steroid ring structure of cholesterol reaches in the membrane. Membrane hydrophobicity sharply increases at these positions from the level of methanol to the level of pure hexane, and hydrophobicity is constant in the inner region of the membrane. Thus, formation of effective hydrophobic barriers to permeation of small polar molecules requires alkyl chain unsaturation and/or cholesterol. The thickness of this rectangular hydrophobic barrier is less than 50% of the thickness of the hydrocarbon regions. Results obtained in dioleoyl-PC-cholesterol membranes in the fluid phase are similar to those obtained in frozen membranes. These results correlate well with permeability data for water and amino acids in the literature.

Application of Electron Paramagnetic Resonance Spectroscopy for Validation of the Novel (AN+DN) Solvent Polarity Scale

“…This finding with this solvent parameter is indeed expected as according to several reports in the literature, it is dependent on the class of solvent [31][32][33]. The dielectric constant varies when the solvent form or not hydrogen bonds and/or van der Waals interactions with the N-O moiety of the spin probes, inducing redistribution of the electrons in the medium.…”

The Overall Partitioning of Anthracyclines into Phosphatidyl‐Containing Model Membranes Depends Neither on the Drug Charge Nor the Presence of Anionic Phospholipids

“…Our data also show that the anthracycline environment is more polar when drug is embedded in cholesterol-containing LUV. However, this can be explained by the fact that incorporation of cholesterol increases water penetration (Subczynski et al, 1994) which puts the anthracycline molecules, with no modification of their position, in an environment less hydrophobic than in the absence of cholesterol.…”

Cholesterol effects on the phospholipid condensation and packing in the bilayer: a molecular simulation study

“…In e¡ect, the surface density of the membrane, which is the total mass of all membrane lipids divided by the surface area of the membrane, increases by 10%, from 1.87U10 37 g/cm 2 in the DMPC bilayer to 2.04U10 37 g/cm 2 in the DMPC^Chol bilayer. At the same time, the density of the whole system (lipids and water), which is the total mass of all membrane molecules divided by the simulation box volume, increases by much less than 1%, from 1.135 þ 0.0003 g/cm 3 in the DMPC bilayer to 1.138 þ 0.0003 g/cm 3 in the DMPC^Chol bilayer.…”