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 thermodynamic properties of fully hydrated mixed-chain phosphatidylglycerols (PG) using high-resolution differential scanning calorimetry (DSC). The crystal structure of dimyristoylphosphatidylglycerol shows an acyl chain conformation 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 conformation (R.H. Pearson, I. Pascher, The molecular structure of lecithin dihydrate, Nature, 281 (1978) 499-501; I. Pascher, S. Sundell, K. Harlos, H. Eibl, Conformational and packing properties of membrane lipids: the crystal structure of sodium dimyristoylphosphatidylglycerol, Biochim. Biophys. Acta, 896 (1987) 77-88). If the structure of PG found in the single crystal can be extrapolated to that in the fully hydrated gel-state bilayer, the observed difference in acyl chain conformations implies that modulation of the acyl chain asymmetry will have an opposite effect on the thermotropic phase behavior of PG and PC. For example, it is expected, based on the crystal structures, that C(15):C(13)PG should have a higher main phase transition temperature (Tm) than C(14):C(14)PG, and C(13):C(15)PG should have a lower Tm than C(14):C(14)PG. However, our DSC studies show clearly that the expectation is not borne out by experimental data. Rather, the Tm values of C(15):C(13)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 acyl chain composition, have also been studied in this laboratory using high-resolution DSC. It is shown that the acyl chain conformation of fully hydrated PG in general is nearly opposite to that seen in the PG crystal structure.