Kinetics of Symmetric and Asymmetric Phospholipid Transfer between Small Sonicated Vesicles Studied by High-Sensitivity Differential Scanning Calorimetry, NMR, Electron Microscopy, and Dynamic Light Scattering

The transfer of Laphosphatidylcholines of different acyl chain length between small unilamellar vesicles via the aqueous phase was studied by using high-sensitivity differential scanning calorimetry (hs-DSC),1H NMR, dynamic light scattering, and freeze-fracture electron microscopy. Two cases of lipid transfer were studied: (1) the asymmetric lipid transfer between phosphatidylcholine vesicles with different acyl chain lengths of the lipids; (2) the symmetric lipid transfer between phosphatidylcholine vesicles of the same type of lipid by using acyl chain deuteriated and protonated lipid analogues. From the hs-DSC data the off rates of the lipids were calculated for both types of transfer as a function of incubation temperature of the vesicles and the acyl chain lengths of the lipids by using a kinetic model. Significant differences between the two types of lipid transfer were found. In the asymmetric case a net transfer of lipids from the vesicle fraction with the lower lipid chain melting temperature (donor vesicles) to those with the higher one (acceptor vesicles) was observed even while the latter were in the gel state. Additionally, the transfer kinetics was strongly dependent on the proportion between donor and acceptor vesicles. In the symmetric case the lipid transfer between the two vesicle populations was nearly equal (i.e., lipid transfer in a 1:1 ratio between the vesicles) and occurred only if all lipids were in the liquid-crystalline state. A further characteristic of the asymmetric lipid transfer was an acceleration of the flip-flop rate of the acceptor vesicles by at least 1 order of magnitude in the initial stage of transfer. The dependence of the transfer kinetics on the incubation temperature, the acyl chain lengths of the lipids, and the total lipid concentration was found to be similar for both types of lipid transfer, provided that all lipids were in the fluid state. It is further shown that lipid transfer is a characteristic of small sonicated vesicles, whereas larger vesicles prepared by detergent dialysis exchanged lipids mainly by vesicle fusion. © 1988, American Chemical Society. All rights reserved.

Duke Scholars

Author Christoph F. Schmidt Physics