Time-resolved x-ray diffraction and calorimetric studies at low scan rates: I. Fully hydrated dipalmitoylphosphatidylcholine (DPPC) and DPPC/water/ethanol phases

摘要: The phase transitions in fully hydrated dipalmitoylphosphatidylcholine (DPPC) and DPPC/water/ethanol phases have been studied by lowangle time-resolved x-ray diffraction under conditions similar to those employed calorimetry (scan rates 0.05–0.5°C/min uniform temperature throughout the samples). This approach provides more adequate characterization of equilibrium transition pathways allows for close correlations between structural thermodynamic data. No coexistence rippled gel (P β' ) liquid-crystalline (L α was found main DPPC; rather, a loss correlation lamellar structure, observed as broadening reflections, takes place narrow range ∼100 mK at midpoint. Formation long-living metastable phase, denoted P (mst), differing from initial cooling direction both calorimetry. direct conversion (mst) into occurs over 24 h but only way sequence → L . According differential scanning (DSC), enthalpy (mst)-L is ∼5% lower than that -L transition. effects ethanol (Rowe, E. S. 1983. Biochemistry. 22:3299–3305; Simon, A., T. J. McIntosh. 1984. Biochim. Biophys. Acta 773:169–172) on mechanism reversibility DPPC were clearly visualized. At concentrations inducing formation interdigitated proceeds through final finite range. During subtransition recorded scan rate 0.3°C/min, smooth monotonic increase spacing its subgel c value place. width reflections remains unchanged during this transformation. grounds propose "sequential" relaxation subgel-gel which not accompanied growth domains within one.