The Effect of Cholesterol on the Dielectric Structure of Lipid Bilayers.

Cholesterol plays an important role in regulating the properties of phospholipid bilayers and many mechanisms have been proposed to explain why cholesterol is so ubiquitous within biological membranes of animals. Here we present the results of studies on the effect of cholesterol on the electrical/dielectric properties of lipid membranes tethered to a solid substrate. These tethered bilayer lipid membranes tBLM were formed on a commercially available chemically modified gold substrate. These lipid bilayers are very robust. Very high-resolution electrical impedance spectroscopy (EIS) was used to determine the dielectric structure of the lipid bilayers and associated interfaces. The EIS data allowed the dielectric substructure of the lipid bilayers to be determined. The results showed that when cholesterol was present in the tethered membranes at a concentration of 10% (mol/mol); the thickness of the tBLMs increased and the membrane conductance decreased. However, when cholesterol was present in the tethered membrane at more than 30% (mol/mol) the effect of cholesterol was dramatically different; the membranes then became thinner and possessed a much larger electrical conductance. The EIS allowed a distinction to be made between a hydrophobic region in the center of the bilayer and another hydrophobic region further out towards the polar head region, in addition to the polar head region itself. Cholesterol was found to have the largest effect on the inner, hydrophobic region, although the outer hydrophobic region was also affected.