Influence of cis double bonds in the sn-2 acyl chain of phosphatidylethanolamine on the gel-to-liquid crystalline phase transition.

We have semisynthesized 19 species of mixed-chain phosphatidylethanolamines (PEs) in which the sn-1 acyl chain is derived from saturated fatty acids with varying chain lengths and the sn-2 acyl chain has different chain lengths but contains 0, 1, and 2 cis double bond(s). The gel-to-liquid crystalline phase transition temperatures (Tm) of lipid bilayers prepared from these 19 mixed-chain PEs were determined calorimetrically. When the Tm values are compared with those of saturated and monounsaturated counterparts, a common Tm profile is observed in the plot of Tm versus the number of cis double bonds. Specifically, a marked stepwise decrease in Tm is detected as the number of cis double bonds in the sn-2 acyl chain of the mixed-chain PE is successively increased from 0 to 1 and then to 2. The large Tm-lowering effect of the acyl chain unsaturation can be attributed to the increase in Gibbs free energy of the gel-state bilayer as a result of weaker lateral chain-chain interactions. In addition, we have applied molecular mechanics calculations to simulate the molecular structure of dienoic mixed-chain C(X):C(Y:2 delta n,n+3)PE in the gel-state bilayer, thus enabling the three independent structural parameters (N, delta C, and LS) to be calculated in terms of X, Y, and n, which are intrinsic quantities of C(X):C(Y:2 delta n,n+3)PE. When the Tm values and the corresponding N and delta C values of all dienoic mixed-chain PEs under study are first codified and then analyzed statistically by multiple regressions, the dependence of Tm on the structural parameters can be described quantitatively by a simple and general equation. The physical meaning and the usefulness of this simple and general equation are explained.