Influence of the positions of cis double bonds in the sn-2-acyl chain of phosphatidylethanolamine on the bilayer's melting behavior

In an attempt to examine the effects of different numbers and positions of cis double bonds in the sn-2-acyl chain of phosphatidylethanolamine (PE) on the bilayer's melting behavior, 21 molecular species of PE were first semisynthesized, and their T-m and Delta H values were subsequently determined by high resolution differential scanning calorimetry, In the plot of T-m versus the number of the cis double bond, some characteristic profiles were observed for the various series of PEs, For instance, if the cis double bond was first introduced into the sn-2-acyl chain of C(20):C(20)PE at the Delta(5)-position, the T-m was observed to reduce drastically. Subsequent stepwise additions of up to five cis double bonds at the methylene-interrupted positions toward the methyl end resulted in a progressive yet smaller decrease in T-m. If, on the other hand, the cis double bonds were introduced sequentially at the Delta(11)-, Delta(11,14)-, and Delta(11,14,17)-positions along the sn-2-acyl chain of C(20):C(20)PE, the T-m profile in the T-m versus the number of the cis double bond showed a down-and-up trend. Most interestingly, for positional isomers of C(20):C(20:3 Delta(5,8,11))PE,C(20):C(20:3 Delta(8,11,14))PE, and C(20):C(20:3 Delta(11,14,17))PE) an inverted bell-shaped T-m profile was detected in the plot of T-m against the position of the omega-carbon for these isomers. Similar T-m profiles were also observed for C(18):C(20)PE, C(20):C(18)PE, and their unsaturated derivatives. This work thus demonstrated that both the positions and the numbers of cis double bonds in the sn-2 acyl chain could exert noticeable influence on the gel-to-liquid crystalline phase transition behavior of the lipid bilayer. Finally, a molecular model was presented, with which the behavior of the gel-to-liquid crystalline phase transition observed for lipid bilayers composed of various sn-1-saturated/sn-2-unsaturated lipids can be rationalized.