PHASE-TRANSITION SEQUENCE BETWEEN FLUID LIQUID-CRYSTALLINE AND INTERDIGITATED LAMELLAR GEL PHASES IN MIXED-CHAIN DIACYL PHOSPHATIDYLCHOLINE

The phase transition behavior of asymmetric phosphatidycholine with pa lmitic acid at the sn-l position and capric acid at the sn-2 position (C16C10PC) has been examined by simultaneous differential scanning cal orimetry and synchrotron X-ray diffraction complemented by electron sp in 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 degrees C a ppeared on the initial cooling. It coincided with a transition of a li quid-crystalline lamellar phase (d-spacing, 6.4 nm) directly into an i nterdigitated lamellar gel phase (d-spacing, 5.2 nm). An immediate reh eating scan showed a single broad endothermic transition peak in the t emperature range 3-10 degrees C over which there was a direct transiti on from the interdigitated lamellar gel phase to the fluid liquid-crys talline lamellar phase with coexistence of the two phases in the regio n 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 th e magnetic hyperfine coupling constant. This constant (16 Gauss) indic ates around the nitroxide group a viscous highly ordered environment w ith hydrogen bonding within the interdigitated bilayer. The subsequent cooling scan was markedly different from the initial cooling scan sin ce the thermogram showed two exothermic peaks. The first peak at 7 deg rees C, representing 30% of the overall enthalpy change, coincided wit h the conversion of fluid liquid-crystalline phase into non-interdigit ated lamellar gel phase (d-spacing, 6.32 nm). The second peak at 5 deg rees C is observed when the interdigitated lamellar gel phase (d-spaci ng, 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. Tim e resolved DSC and X-ray diffraction show differences with ESR at the transition temperature which support the occurrence of a metastable no n-interdigitated gel between the fluid and the interdigitated lamellar gel in the transition sequence. (C) 1997 Elsevier Science Ireland Ltd .