Phase transition sequence between fluid liquid-crystalline and interdigitated lamellar gel phases in mixed-chain diacyl phosphatidylcholine

Abstract

The phase transition behavior of asymmetric phosphatidycholine with palmitic acid at the sn-1 position and capric acid at the sn-2 position (C16C10PC) has been examined by simultaneous differential scanning calorimetry and synchrotron X-ray diffraction complemented by electron spin resonance. The experiments were performed for a sequence of cooling, heating and recooling scans of aqueous dispersions of C16C10PC. The results showed that a single exothermic peak centered at 5°C appeared on the initial cooling. It coincided with a transition of a liquid-crystalline lamellar phase (d-spacing, 6.4 nm) directly into an interdigitated lamellar gel phase (d-spacing, 5.2 nm). An immediate reheating scan showed a single broad endothermic transition peak in the temperature range 3–10°C over which there was a direct transition from the interdigitated lamellar gel phase to the fluid liquid-crystalline lamellar phase with coexistence of the two phases in the region of the transition. Coexistence of phases was also inferred from ESR evidencing two different environments for 16 doxyl stearic acid probe. Unlike non-interdigitated gel phases of symmetric phospholipids, the interdigitated gel phase of C16C10PC is typified by a high value of the magnetic hyperfine coupling constant. This constant (16 Gauss) indicates around the nitroxide group a viscous highly ordered environment with hydrogen bonding within the interdigitated bilayer. The subsequent cooling scan was markedly different from the initial cooling scan since the thermogram showed two exothermic peaks. The first peak at 7°C, representing 30% of the overall enthalpy change, coincided with the conversion of fluid liquid-crystalline phase into non-interdigitated lamellar gel phase (d-spacing, 6.32 nm). The second peak at 5°C is observed when the interdigitated lamellar gel phase (d-spacing, 5.2 nm) takes place. This behavior is consistent with the buildup of extended lamellar sheets with low curvature induced by cooling down vesicles with high radius of curvature fusing when the interdigitated lamellar gel phase takes place beyond the transition temperature. Time-resolved DSC and X-ray diffraction show differences with ESR at the transition temperature which support the occurrence of a metastable non-interdigitated gel between the fluid and the interdigitated lamellar gel in the transition sequence.