INFLUENCE OF THE POSITIONS OF CIS DOUBLE BONDS IN THE SN-2-ACYL CHAIN OF PHOSPHATIDYLETHANOLAMINE ON THE BILAYER'S MELTING BEHAVIOR

Abstract In an attempt to examine the effects of different numbers and positions of cis double bonds in thesn-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 ΔHvalues 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 Δ5-position, the T m was observed to reduce drastically. Subsequent stepwise additions of up to fivecis double bonds at the methylene-interrupted positions toward the methyl end resulted in a progressive yet smaller decrease inT m. If, on the other hand, the cisdouble bonds were introduced sequentially at the Δ11-, Δ11,14-, and Δ11,14,17-positions along thesn-2-acyl chain of C(20):C(20)PE, theT 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(20:3Δ5,8,11)PE, C(20):C(20:3Δ8,11,14)PE, and C(20):C(20:3Δ11,14,17)PE, an inverted bell-shapedT m profile was detected in the plot ofT m against the position of the ω-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 ofcis 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 varioussn-1-saturated/sn-2-unsaturated lipids can be rationalized.