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Biophysical Journal 56: 757-768 (1989)
© 1989 the Biophysical Society
Department of Applied Physics, School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan
ABSTRACT
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 in calorimetry (scan rates 0.05-0.5°C/min and uniform temperature throughout the samples). This approach provides more adequate characterization of the equilibrium transition pathways and allows for close correlations between structural and thermodynamic data. No coexistence of the rippled gel (Pß') and liquid-crystalline (L
) phases was found in the main transition of DPPC; rather, a loss of correlation in the lamellar structure, observed as broadening of the lamellar reflections, takes place in a narrow temperature range of 
100 mK at the transition midpoint. Formation of a long-living metastable phase, denoted by Pß'(mst), differing from the initial Pß' was observed in cooling direction by both x-ray diffraction and calorimetry. No direct conversion of Pß'(mst) into Pß' occurs for over 24 h but only by way of the phase sequence Pß'(mst) 
Lß' Pß'. According to differential scanning calorimetry (DSC), the enthalpy of the Pß'(mst)-L transition is by 5% lower than that of the Pß'-L transition. The effects of ethanol (Rowe, E. S. 1983. Biochemistry. 22:3299-3305; Simon, S. A., and T. J. McIntosh. 1984. Biochim. Biophys. Acta 773:169-172) on the mechanism and reversibility of the DPPC main transition were clearly visualized. At ethanol concentrations inducing formation of interdigitated gel phase, the main transition proceeds through a coexistence of the initial and final phases over a finite temperature range. During the subtransition in DPPC recorded at scan rate 0.3°C/min, a smooth monotonic increase of the lamellar spacing from its subgel (Lc) to its gel (Lß') phase value takes place. The width of the lamellar reflections remains unchanged during this transformation. This provides grounds to propose a "sequential" relaxation mechanism for the subgel-gel transition which is not accompanied by growth of domains of the final phase within the initial one.
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