Hydrocarbon chain packing and the effect of ethanol on the thermotropic phase behavior of mixed-chain phosphatidylglycerols

Previous studies in this laboratory have delineated the relationship between the acyl chain asymmetry of mixed-chain phosphatidylcholines and the effect of ethanol concentration ([EtOH]) on their melting behavior (Li et al., Biophys J., 70 (1996) 2784-2794). This present investigation extends these findings to another phospholipid family by using high-resolution differential scanning calorimetry (DSC) to characterize the effect of ethanol concentration on the main phase transition temperature (T-m) of five molecular species of mixed-chain phosphatidylglycerol (PG). For C(14):C(18)PG, C(15):C(17)PG, C(16):C(16)PG, and C(17):C(15)PG, a biphasic profile in the T-m versus [EtOH] plot was observed, and the minimum in the plot for each PG occurred at 33, 15, 19, and 36 mg/ml, respectively. This biphasic behavior is typical of phospholipids whose acyl chain asymmetry is fairly small. For C(18):C(14)PG, only a linear decrease in the T-m was observed as a function of ethanol concentration; this effect is characteristic of highly asymmetric phospholipids. Our DSC results obtained with mixed-chain PG in the presence of ethanol demonstrate that the acyl chain asymmetry of the five lipids studied can be ranked as follows: C(15):C(17)PG < C(16):C(16)PG < C(14):C(18)PG < C(17):C(15)PG < C(18):C(14)PG, confirming independent results from this laboratory that the overall acyl chain structure of PG in the hydrated gel phase is opposite to that detected in the single crystals. In fully hydrated bilayers, the acyl chain conformation of PG is most likely to be similar to that of phosphatidylcholine. (C) 1999 Elsevier Science B.V. All rights reserved.