Hydrocarbon Thickness Dictates Cholesterol's Location, Orientation and Motion in a Phospholipid Bilayer

Abstract

The lateral sequestration of lipids with polyunsaturated fatty acid (PUFA) chains into membrane domains depleted of cholesterol has been hypothesized to have an important role in neurological function. This effect has long been attributed to a strong aversion of the disordered polyunsaturated fatty acid (PUFA) chains to the rigid smooth alpha face of cholesterol. Previously, we have performed neutron diffraction studies of deuterated cholesterol incorporated into bilayers composed of diarachidonoyl phosphatidylcholine (di-20:4 PC, DAPC), a lipid with two omega-6 PUFA chains. It was found that cholesterol sequestered at the bilayer midplane, in contrast to its usual upright orientation with the hydroxyl group located near the lipid/water interface. To date it remains unclear whether or not cholesterol's aversion to PUFA chains is a universal property of PUFAs, a behavior unique to omega-6 PUFAs, or the product of membrane disorder.Using different physical characterization techniques (i.e., neutron diffraction, NMR) and MD simulations, we obtained detailed structural data that rationalize much of the previously inexplicable data regarding cholesterol's behaviour in PUFA bilayers. We are able to demonstrate that cholesterol's mass distribution in the center of a PUFA bilayer is the same for both omega-6 and omega-3 PUFAs. In addition, neutron, 2H NMR and MD data suggest cholesterol's sequestration into the bilayer center may in fact be driven by hydrophobic thickness mismatch, and not necessarily membrane disorder.