Thermotropic properties of bilayers containing branched-chain phospholipids. Calorimetric, Raman, and 31P NMR studies.

Diisopalmitoylphosphatidylcholine (DIPPC), -phosphatidylethanolamine (DIPPE), and -phosphatidylglycerol (DIPPG) have been synthesized, and the structures of aqueous dispersions of these lipids have been examined by high-sensitivity differential scanning calorimetry, 31P nuclear magnetic resonance, and Raman spectroscopy. DIPPC at temperatures below 23.1 degrees C readily forms a gel phase with the acyl chains packed in an orthorhombic subcell. Above this temperature, this "orthorhombic" phase converts directly to the liquid-crystalline phase. The phase diagram for the system DIPPC--dipalmitoyl-PC (DIPPC-DPPC) shows that the gel phases formed by either lipid can accommodate only limited amounts of the other species and suggests that the low-temperature orthorhombic phase of DIPPC is distinct in its structure from the "subgel" phase of DPPC. DIPPE forms a well-ordered gel phase only in samples that are equilibrated at low temperatures for long times (approximately days to weeks) or at very high lipid concentrations. However, this lipid readily forms an "intermediate" phase with a very disordered acyl chain packing upon cooling from the liquid-crystalline state. Mixtures of DIPPE with DIPPG exhibit similar thermotropic properties. Hydrated DIPPE appears to be stable in the lamellar phase up to at least 98 degrees C, while di-cis- and di-trans-9-hexadecenoyl-PE convert to the hexagonal II phase at 43.5 and 92.5 degrees C, respectively. We discuss the relevance of these results to the structure and stability of bacterial membranes containing branched-chain acyl lipids.