Lipid polymorphism and hydrocarbon order

The use of 2H nuclear magnetic resonance for the characterization of the polymorphic behavior of lipids is illustrated. Different lipid phase preferences may be expected to influence the orientational order and its variation along the acyl chains. Several results are presented to support that view. An increase of motional freedom and a redistribution of the order along the acyl chains are observed during the lamellar-to-hexagonal phase transition, showing that the order profile is sensitive to the lipid phase symmetry. In addition, if the preferences for nonlamellar phases are not expressed explicitly, the presence of "nonbilayer" lipids constrained in bilayer environment induces increased hydrocarbon order. This suggests that order parameters of the acyl chains and lipid polymorphic tendencies are intimately related.Key words: lipid, polymorphism, 2H nuclear magnetic resonance, hydrocarbon order.

Cone-shaped lipids increase the susceptibility of phospholipids in bilayers to the action of phospholipases

Addition of cardiolipin or diacylglycerol to dispersions of phosphatidylcholine greatly increased hydrolysis by snake venom or pancreatic phospholipase A2, as well as by a microbial phospholipase. Monogalactosyl diglyceride which, like cardiolipin and diacylglycerol, will form nonbilayer hexagonal II structures also caused an increase in the breakdown of phosphatidylcholine. Addition of digalactosyl diglyceride, a bilayer lipid from the same source, had a much smaller effect on the three phospholipases, indicating that stimulation by the nonbilayer lipids was not due to their fatty acid compositions. Stimulation of the microbial phospholipase by cardiolipin did not require the presence of calcium, leading to the conclusion that the formation of nonbilayer structures was not necessary. The results suggest that cone-shaped lipids increase the accessibility of lipids in bilayers to phospholipases by decreasing the packing of the polar head groups.