LATERAL DISTRIBUTION OF NEGATIVELY CHARGED LIPIDS IN LECITHIN MEMBRANES - CLUSTERING OF FATTY-ACIDS

From ζ-potential measurements on multilamellar liposomes, it is concluded that negatively charged phospholipids incorporated in liquid-crystalline phosphatidylcholine bilayers are randomly distributed in the plane of the bilayer. Furthermore, the distribution of the negatively charged phospholipid between the two halves of the bilayer and over the concentrically arranged bilayers of the multilamellar structure is uniform. The same is true for spin-labeled fatty acids. In contrast, long-chain fatty acids appear to be clustered at neutral pH; i.e., they segregate into patches within the plane of the bilayer. Their carboxyl groups are fully ionized only at a pH ≥ 11, and charge repulsion leads then to a random distribution. At pH <7 the binding of 45Ca2+ to arachidic acid monolayers is insignificant, and only at pH ~ 10 does the amount of Ca2+ bound per fatty acid become comparable to that bound to phosphatidylserine monolayers at neutral pH. Ca2+ binding obviously parallels the deprotonation of the fatty acid carboxyl group in phosphatidylcholine bilayers. The pH dependence of the Ca2+ binding to fatty acid monolayers is similar to the pH-electrophoretic mobility relationship observed with fatty acid containing phosphatidylcholine liposomes. From the similarity in behavior of fatty acid monolayers and fatty acids present in bilayers, it is proposed that in the latter case fatty acids occur as patches of monolayers (clusters). Fatty acids and spin-labeled fatty acids also differ in their effect on the gel to liquid-crystalline transition of dipalmitoylphosphatidylcholine bilayers. The former are stabilizing as evident from an increase in the transition temperature while the latter have a destabilizing effect. Deuterated fatty acids behave like ordinary fatty acids in terms of clustering. The use of probing the structure and dynamics of phospholipid bilayers and membranes with deuterated fatty acids is therefore subject to criticism, particularly when large quantities of fatty acids have to be utilized as in deuteron magnetic resonance experiments. The information thus derived is likely to be affected by probe-probe interactions. © 1979, American Chemical Society. All rights reserved.