The surface lipid layer of human low density lipoprotein probed by dipyrenyl phospholipids

Properties of the surface lipid-protein layer of human low density lipoprotein (LDL) have been studied with fluorescent phosphatidylcholine analogues containing a pyrenyl fatty acid of variable length at both sn-1 and sn-2 position of the glycerol moiety. Only intramolecular excimer formation takes place at low concentrations, as indicated by the independence of the ratio of excimer to monomer fluorescence intensities (E/M) on the amount of the incorporated dipyrenyl phospholipid. The E/M parameter which depends on the fluidity of the probe's environment were measured for a series of dipyrenyl phospholipids in three systems, i.e. in LDL, LDL-like lipid particles (LDp) and small unilamellar phosphatidylcholine/sphingomyelin/cholesterol vesicles (SUV). The data indicate that the fluidity of the phospholipid acyl chain region decreases in the order: SUV greater than LDp greater than LDL. This suggests that interactions with both the core lipids and the protein moiety (apoB-100) contribute to the rigidity of the surface lipid layer of LDL. Dipyrenyl phospholipids also detect the thermotropic transition of the core lipids of both LDL and LDp, suggesting that this transition influences the fluidity of the surface lipid layer.