Effects of phospholipid acylchain structure on thermotropic phase properties. 2: Phosphatidylcholines with unsaturated or cyclopropane acyl chains

Abstract

Phosphatidylcholines (PCs) have been synthesized from several cis- and trans-alkenoic acids and from the corresponding racemic cyclopropane fatty acids in which a methylene group is inserted across the double bond, and the thermotropic behavior of the hydrated lipids has been studied by differential thermal analysis. The cyclopropane fatty acyl PCs resemble the corresponding unsaturated PCs in the dependence of their transition temperature ( T c) on the acyl chain length and in their mixing properties with dipalmitoyl PC. Methylenation of the double bonds in a PC with cis-unsaturated acyl chains raises T c by about 15°, while the same modification of a trans-unsaturated PC raises T c by at most a few degrees. Our findings generally indicate that cyclopropane acyl chains behave quite like the corresponding unsaturated acyl chains in a membrane bilayer. The methylenation of cis-unsaturated membrane lipid acyl chains by bacteria entering the stationary phase is thus unlikely to significantly change the transition temperature or the fluidity of the membrane lipids but instead probably serves to maintain these properties while altering some other property, such as the chemical or lamellar phase stability, of the lipids.