Abstract
Models for the organization of sterols into regular arrays within phospholipid bilayers have been proposed previously. The existence of such arrays in real systems has been supported by the fact that concentration-dependent sterol properties show discontinuities at the cholesterol mole fractions corresponding to regular lattice arrangements. Experimental results presented here are based on a surface plasmon resonance assay that was used to analyze rates of cyclodextrin-mediated removal of cholesterol from adsorbed liposomes at cholesterol mole fractions up to χC = 0.55. Two kinetic pools of cholesterol were detected; there was a fast pool present at χC > 0.25, and a slow pool, with a removal rate that was dependent on the initial χC but that did not vary as χC decreased during the course of one experiment. The cholesterol activity therefore seems to be affected by sample history as well as local concentration, which could be explained in terms of the formation of superlattices that are stable for relatively long times. We also describe a variation on the traditional lattice models, with phosphatidylcholine (PC) being treated as an arrangement of hexagonal tiles; the cholesterol is then introduced at any vertex point, without increasing the total area occupied by all the lipid molecules. This model is consistent with Langmuir trough measurements of total lipid area and provides a simple explanation for the maximum solubility of cholesterol in the PC bilayer.
Highlights
Biological membranes typically consist of bilayers of amphipathic molecules, oriented so that their hydrophilic head groups are towards the aqueous surroundings, and their longer hydrophobic tails are in the membrane interior
The experimental format was that solutions of βCD were added to layers of adsorbed liposomes with a range of χC values; the drop in the surface plasmon resonance (SPR) signal was monitored as a means of following the rate of cholesterol removal
Our model is compatible with other results that have been presented in support of cholesterol lattice formation within phospholipid bilayers, in the sense that regular arrays can be produced at defined mole fractions
Summary
Biological membranes typically consist of bilayers of amphipathic molecules, oriented so that their hydrophilic head groups are towards the aqueous surroundings, and their longer hydrophobic tails are in the membrane interior. Phospholipids, which are the most abundant of these membrane molecules, have a cylindrical shape, with the cross-sectional areas of the hydrophilic and hydrophobic portions being roughly equal. This permits the phospholipids to pack into the flat sheets of the bilayers. Cholesterol has approximately half the cross-sectional area of phosphatidylcholines [1]; this size difference has been taken into account in traditional lattice models by considering the cholesterol as being in a lattice of acyl chains [2,3,4].
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