Localization of Cholesterol within Supported Lipid Bilayers Made of a Natural Extract of Tailor-Deuterated Phosphatidylcholine

Cholesterol is an essential component of mammalian membranes that is known to induce a series of physicochemical changes in the lipid bilayer. Such changes include the formation of liquid ordered phases with an increased thickness and configurational order as compared to liquid disordered phases. For saturated lipid membranes, cholesterol molecules localise close to the lipid head group-tail interface. However, the presence of polyunsaturated lipids was recently shown to promote the relocation of cholesterol towards the inner interface between the two bilayer leaflets. Here, neutron reflection is used to study the localisation of cholesterol within supported lipid bilayers composed of a natural mixture of phosphatidylcholine (PC). The lipids were produced in a genetically
modified strain of E. coli and grown under specific deuterated conditions to give an overall neutron scattering length density (or level of deuteration) of the lipids matching that of D2O. Besides the traditional solvent contrast variation approach, both hydrogenated and per-deuterated cholesterol were used for additional contrast. This approach shows that cholesterol is located closer
to the lipid head group - tail interface in this natural PC extract rather than in the centre of the core of the bilayer as for very thin or polyunsaturated membranes. Moreover, cholesterol preferentially enriches the outer leaflet, possibly due to an asymmetric leaflet distribution in the vesicles used for the supported lipid bilayer formation. Thus, here we report the localisation of cholesterol is studied in bilayers composed of a deuterated, natural phosphatidylcholine extract and a combination of hydrogenated and perdeuterated cholesterol.