Glutaryl phosphatidylcholine effects on pH and composition dependent behavior of liposomes studied by spin-labeling method.

The influence of glutaryl phosphatidylcholine on the molecular organization of phosphatidylcholine liposomes was studied by spin-labeling technique. The ESR signals given by the 5-nitroxide stearic acid label showed that the presence of glutaryl lecithin (i) significantly increased the negative charge density of the polar liposome surface with increasing proton concentration depending on the bulk solution pH, and (ii) apparently decreased the packing (order) of the hydrophobic region close to the surface, essentially in the presence of saturated phospholipids. The spectral information--S (order parameter) and alpha N (isotropic nitrogen coupling constant)--resulted in the location of the probe near or in the polar zone of the membrane or in the hydrophobic region, depending on the protonation/deprotonation of the fatty acid carboxyl group of the probe. The microviscosity of the inner region of the membrane monitored by the 12- and 16-probes was not significantly altered by glutaryl lecithin. On the other hand, glutaryl lecithin has a lesser effect on liposomes containing anionic polar head groups, such as dipalmitoyl phosphatidylglycerol or phosphatidylinositol, the anionic charge of which already had the same effect on protonation of the polar surface. The temperature dependence of dipalmitoyl phosphatidylcholine liposome dynamic behavior indicates that the glutaryl lecithin effect is completely different above and below the gel-to-liquid crystalline phase transition point.