Lysophosphatidylcholine-cholesterol complex.

Lysophosphatidylcholine (lysoPC) and cholesterol at 1:1 molar ratio form multilamellar and, on sonication, unilamellar liposomes in water. Calorimetric scannings of varied mixtures in water give evidence for the existence of a 1:1 complex of the lipids. The permeability of the 1:1 bilayer against glycerol, at 22-42 degrees C, is lower than that of phosphatidylcholine-cholesterol bilayers; the energy of activation of permeation is 73% higher. This implies a low groundstate of the entropy of activation of permeation. Unilamellar lysoPC-cholesterol 1:1 liposomes, isolated by gel exclusion chromatography, are able to incorporate no more than 8 to 10% excess lysoPC and no additional cholesterol at all. Their physical parameters such a outer radius (14.8 nm) and bilayer thickness (4.2 nm) are between those reported for phosphatidylcholine vesicles and phosphatidylcholine-cholesterol vesicles. The outside-inside distribution of lysoPC in the small vesicles (determined by 31P-NMR) is 2.0. A comparison of 13C-NMR spectra of lysoPC (in 2H2O) and lysoPC-cholesterol vesicles shows that in the vesicles the signals for the carboxyl carbon of lysoPC as well as those for carbons 1 and 2 (and, partly, 3) of motional restriction in this region of the molecule. The low groundstate of the entropy of activation of permeation, and the immobilization of the glycerol moiety of the lysoPC, argue for a high degree of structural organization in the "hydrogen belt" regions of the lysoPC-cholesterol bilayer, and for lipid-lipid complexing via hydrogen bonding in these regions.