Ethanol-Phosphatidylcholine Interactions: A Real Time X-Ray Diffraction Study

Abstract

Dipalmitoylphosphatidylcholine has been previously shown to have a biphasic response to the presence of ethanol-water solutions as a function of ethanol concentration (E.S. Rowe, Biochim. Biophys. Acta 22 (1985) 3299-3305). Subsequently, Simon and Mclntosh (Biochim. Biophys. Acta 773 (1984), 169-172) showed that DPPC bilayers formed an interdigitated multilamellar phase when the ethanol concentration was greater than 50 mg ethanol/ml water. We have used scanning calorimetry and real time x-ray diffraction to examine DPPC bilayers hydrated with 10 and 150 mg ethanol per ml water. When the lower concentration ethanol solution was present, DPPC bilayers formed subgel, gel, rippled, and liquid crystalline bilayer phases as the sample temperature was increased continuously. However, when the higher ethanol solution was used, DPPC bilayers formed subgel, gel (interdigitated), and liquid crystalline bilayer phases when driven by a temperature scan. In both cases, the subgel phase was not directly reversible upon cooling. Arguments are presented for the interpretation that both sub-gel phases consist of bilayers with interdigitated acyl chains. The transitions between the subgel and gel phases in both sample systems, and the pre-transition for DPPC in 150 mg ethanol/ml of water was shown to be second order while others proceeded via first order or two state mechanisms.