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 betwee n the acyl chain asymmetry of mixed-chain phosphatidylcholines and the effe ct 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 differenti al scanning calorimetry (DSC) to characterize the effect of ethanol concent ration on the main phase transition temperature (T-m) of five molecular spe cies of mixed-chain phosphatidylglycerol (PG). For C(14):C(18)PG, C(15):C(1 7)PG, C(16):C(16)PG, and C(17):C(15)PG, a biphasic profile in the T-m versu s [EtOH] plot was observed, and the minimum in the plot for each PG occurre d at 33, 15, 19, and 36 mg/ml, respectively. This biphasic behavior is typi cal 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 et hanol concentration; this effect is characteristic of highly asymmetric pho spholipids. Our DSC results obtained with mixed-chain PG in the presence of ethanol demonstrate that the acyl chain asymmetry of the five lipids studi ed 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 l aboratory that the overall acyl chain structure of PG in the hydrated gel p hase 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 t o that of phosphatidylcholine. (C) 1999 Elsevier Science B.V. All rights re served.