Hydrocarbon chain packing and molecular motion in phospholipid bilayers formed from unsaturated lecithins. Synthesis and properties of sixteen positional isomers of 1,2-dioctadecenoyl-sn-glycero-3-phosphorylcholine.

Abstract

Isomers of cis-octadecenoic acid, with the double bond in each position in the hydrocarbon chain, were used to synthesize the corresponding 1,2-diacyl-sn-glycero-3phosphorylcholines (lecithins). Differential thermal analysis of the lecithins, as a function of water content, permitted evaluation of the limiting transition temperature (Tc) of each isomer. Values of Tc plotted against double bond position fell on a smooth curve with a minimum at minus 22 degrees for the dioctadec-9'-enoyl compound. The presence of a "pretransition" endotherm in differential thermal analysis of 1,2-dioctadec-15'-and 1,2-dioctadec-16'-enoyl-sn-glycero-3-phosphorylcholine implies the existence of two beta crystalline forms. This was not observed with any of the other lecithins. Enthalpy and entropy data were then obtained from differential scanning calorimetry measurements. Values of delta H were lower (7.6 plus or minus 0.1 kcal mol- minus 1) when the center of unsaturation was near the middle of the hydrocarbon chain than they were (9.6 kcal mol- minus 1) when the center of unsaturation was close to either end of the chain. However, values of delta S showed no consistent variation with double bond position. Four positional isomers of 1-octadec-cis-enoyl-2-octadecanoyl-sn-glycero-3-phosphorylcholine were synthesized. With the double bond near the middle of the chain or close to the terminal group, the Tc values of the mixed acid lecithins were higher than those of the corresponding dioctadecenoyl lecithins. 13-C nuclear magnetic resonance relaxation measurements were used to obtain information about chain motion of selected 1,2-dioctadec-cis-enoyl-sn-glycero-3-phosphorylcholines at a temperature (52 degrees) above the Tc values. Spin-lattice relaxation times of the resolved resonances indicated that location of double bonds near the middle, as compared to either end, of the hydrocarbon chain favors enhanced molecular motion along the length of the chins and especially at the terminal methyl end. In the gel state, the minimum interaction potential energy of hydrocarbon chains in bilayers formed from dioctadecenoyl lipids appears to be minimized by localization of the double bond near the middle of the chains. It is suggested that in the case of homogeneous chains the double bond primarily affects the cooperativity of interactions and has very little steric effect on van der Waals' contacts. By contrast, in bilayers of mixed lecithins, with heterogeneous chains, the steric effect may become dominant, depending on double bond position. These differences in chain packing in the gel state are promulgated beyond the phase transition to the liquid crystalline state as an enhancement of chain motion as the temperature rises above Tc.