Calorimetric studies of phosphatidylethanolamines with saturated sn-1 and dienoic sn-2 Acyl chains.

We have semi-synthesized 18 species of mixed chain phosphatidylethanolamines (PE) in which the sn-1 acyl chain is derived from stearic, arachidic, and behenic acids, and the sn-2 acyl chain is originated from cis,cis-octadecadienoic and cis, cis-eicosadienoic acids with the two methylene-interrupted double bonds located at various positions. These PEs constituting the bilayers in the aqueous dispersion were subjected to differential scanning calorimetric experiments. The Tm values associated with the gel-to-liquid crystalline phase transitions for these PEs are found to be significantly smaller than those of the saturated counterparts. Moreover, the magnitude of the Tm-lowering effect of acyl chain diunsaturation depends critically on the positions of the two methylene-interrupted cis double bonds in the sn-2 acyl chain. Specifically, if the sn-2 acyl chain is derived from cis, cis-octadecadienoic acid, the Tm-lowering effect has the following decreasing order: Delta9,12 > Delta6,9 > Delta12,15. For cis, cis-eicosadienoyl acyl chain, the Tm-lowering effect is stronger in the order of Delta10,13 > Delta11,14 > Delta8,11 > Delta5,8 > Delta14,17. Finally, a refined molecular model is presented that can adequately explain the Tm-lowering effect of sn-2 acyl chain diunsaturation. Moreover, this same refined molecular model can also be invoked to better interpret the Tm-lowering effect observed for sn-1 saturated/sn-2 monoenoic PE.