Thermotropic phase behavior of mixed-chain phosphatidylglycerols: implications for acyl chain packing in fully hydrated bilayers

In this communication we report the first systematic investigation of the t hermodynamic properties of fully hydrated mixed-chain phosphatidylglycerols (PG) using high-resolution differential scanning calorimetry (DSC). The cr ystal structure of dimyristoylphosphatidylglycerol shows an acyl chain conf ormation that is nearly opposite to that of phosphatidylcholine (PC). In PC , the sn-1 chain is straight while the sn-2 chain contains a bend; for PG, the sn-1 contains a bend while the sn-2 chain is in the all-trans conformat ion (R.H. Pearson, I. Pascher, The molecular structure of lecithin dihydrat e, Nature, 281 (1978) 499-501; I. Pascher, S. Sundell, K. Harlos, H. Eibl, Conformational and packing properties of membrane lipids: the crystal struc ture of sodium dimyristoylphosphatidylglycerol, Biochim. Biophys. Acta, 896 (1987) 77-88). If the structure of PG found in the single crystal can be e xtrapolated to that in the fully hydrated gel-state bilayer, the observed d ifference in acyl chain conformations implies that modulation of the acyl c hain asymmetry will have an opposite effect on the thermotropic phase behav ior of PG and PC. For example, it is expected, based on the crystal structu res, that C(15):C(13)PG should have a higher main phase transition temperat ure (T-m) than C(14):C(14)PG, and C(13):C(15)PG should have a lower T-m tha n C(14):C(14)PG. However, our DSC studies show clearly that the expectation is not borne out by experimental data. Rather, the T-m values of C(15):C(1 3)PG, C(14):C(14)PG, and C(13):C(15)PG are 18.2 degrees C, 23.1 degrees C, and 24.4 degrees C, respectively. Several other PGs, each with a unique acy l chain composition, have also been studied in this laboratory using high-r esolution DSC; It is shown that the acyl chain conformation of fully hydrat ed PG in general is nearly opposite to that seen in the PG crystal structur e. (C) 1999 Elsevier Science B.V. All rights reserved.