Abstract
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 Tm and DeltaH values were subsequently determined by high resolution differential scanning calorimetry. In the plot of Tm 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 Delta5-position, the Tm 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 Tm. If, on the other hand, the cis double bonds were introduced sequentially at the Delta11-, Delta11,14-, and Delta11,14,17-positions along the sn-2-acyl chain of C(20):C(20)PE, the Tm profile in the Tm versus the number of the cis double bond showed a down-and-up trend. Most interestingly, for positional isomers of C(20):C(20:3Delta5,8,11)PE, C(20):C(20:3Delta8,11,14)PE, and C(20):C(20:3Delta11,14,17)PE, an inverted bell-shaped Tm profile was detected in the plot of Tm against the position of the omega-carbon for these isomers. Similar Tm 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.
Highlights
In recent years, there has been increasing evidence suggesting the existence of a close correlation between some vital functions of cells and the various degrees of unsaturation in the sn-2-acyl chains of membrane phospholipids [1,2,3]
In the first differential scanning calorimetry (DSC) heating scan, a single, relatively sharp transition peaked at 26.4 °C is observed; the same sample exhibits a broader transition centered at 22.4 °C upon immediate reheating (Fig. 1)
A similar thermal history-dependent phase transition behavior is exhibited by the sample of C(20):C(20:3⌬11,14,17)PE with a single, sharp transition centered at 27.2 °C in the first DSC heating scan and a broader transition peaked at 23.3 °C in the second DSC heating scan (Fig. 1)
Summary
There has been increasing evidence suggesting the existence of a close correlation between some vital functions of cells and the various degrees of unsaturation in the sn-2-acyl chains of membrane phospholipids [1,2,3]. The first detailed and systematic studies of the thermotropic phase behavior of phospholipids containing various numbers of cis double bonds in their sn-2-acyl chains were performed by Keough and co-workers (4 – 6). By using two series of phosphatidylcholines (PCs) and by applying the differential scanning calorimetry (DSC) techniques, they have shown that the introduction of a cis double bond into the sn-2-acyl chain at or near the center of the chain has a marked reducing effect on the phase transition temperature (Tm) associated with the gel-toliquid crystalline phase transition of the lipid bilayer. We have studied systematically the effect of different positions of single cis double bond in the sn-2-acyl chain on the phase transition behavior of lipid bilayers composed of sn-1 saturated/sn-2-monounsaturated PCs or phosphatidylethanolamines (PEs) by high resolution DSC and computer-based molecular mechanics (MM) simulations (9 –11). Based on the results of MM simulations, we have developed a molecular model that can explain the characteristic Tm profile observed for monoenoic phospholipids [10, 11]
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