Lipid bilayer and water proton magnetization transfer: effect of cholesterol

Magnetization transfer between macromolecules and water can be a significant factor contributing to tissue water 1H relaxation. Using saturation transfer techniques, the degree of magnetization transfer between the macromolecular matrix and bulk water 1H can be directly measured and magnetization transfer contrast (MTC) can be generated in MR images. A significant degree of MTC has been observed in tissues with high plasma membrane content such as kidney and brain. The purpose of this study was to establish whether lipid bilayers, as models for cell membranes, could exchange magnetization with the water solvent and whether this effect could contribute to MTC observed in intact tissues. Magnetization transfer was measured in aqueous dispersions of egg phosphatidylcholine (EPC) in the presence and absence of cholesterol. It was found that neither EPC bilayers nor cholesterol by themselves significantly exchanged magnetization with bulk water 1H. However, as the concentration of cholesterol was increased, the pseudo-first-order magnetization exchange rate increased to a maximum value of approximately 1 s-1. The cholesterol-induced 1H magnetization exchange may be related either to longer correlation times of the lipid or to an increase in the number of water molecules associated with the bilayer. These results indicate that EPC-cholesterol bilayers exchange 1H magnetization with bulk water. These results are consistent with lipid bilayer contributions to bulk water relaxation and MTC in intact biological tissues.