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Phosphatidylcholines, sphingomyelins, cholesterol, and cholesterol esters were enriched with 3IC by chemical synthesis in specific positions of their hydrophilic groups and aliphatic chains. Their spin-lattice relaxation times were determined in organic solvents. The substances were organized as liposomes and recombined with total human high density apolipoproteins and the two separated main components, apolipoprotein A-I (apoLp-Gln-I) and apolipoprotein A-II (apoLp-Gln-II). The enrichment of '3C in the hydrophilic and hydrophobic moiety of these lipids increases the sensitivity of the method and allows the determination of T, times of special regions of these molecules in reasonable periods of time. It should give insight into hydrophilic and hydrophobic interactions of lipidprotein complexes.A readily accessible system for the study of lipid-lipid and lipid-protein interactions in their natural molecular organization is the high density lipoprotein (HDL) fraction of human serum. Detailed information is available on the lipid and protein composition of these particles. Two apoproteins, apoA-I and apoA-II, comprise almost 90% of the molecule, with a high degree of a-helical structure (61-70%) (3)(4)(5)(6)(7)(8)(9). The aminoacid sequence of the latter has been elucidated (10). The apoproteins are assembled with an approximately equal amount of lipids. The lipid components are mainly phosphatidylcholine, sphingomyelin, cholesterol esters, and cholesterol. The acyl residues of the ester lipids are predominantly unsaturated fatty acids (oleic and linoleic acid).The molecular organization of the molecules in the HDL particle relates closely to other important questions concerning the structure and interactions of lipids and proteins in more complex lipoprotein aggregates, such as biological membranes.Several models of the HDL structure have been proposed (7,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). There is no general agreement on any one of these structures, most likely because of insufficient knowledge of the lipid-protein interactions and the tertiary structure of the proteins.

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