A new liquid crystalline phase in phosphatidylcholine bilayers as studied by X-ray diffraction

Abstract

Model membranes of diacylphosphatidylcholines (C nPC), with saturated linear acyl chains of n > 12 carbons, show a single sharp phase transition (known as the main transition) between the gel phase P β, and the liquid crystalline phase L α with differential scanning calorimetry. However, C12PC (dilauroylphosphatidylcholine) shows, as well as the sharp transition at −2°C, a broad peak at 5°C, originally observed by S. Mabrey and J.M. Sturtevant. The broad peak is not artificial: between the two peaks a new phase L x was predicted for (C12PC) bilayers on the basis of calorimetry (Finegold, Shaw and Singer, Chem. Phys. Lipids 53 (1990), 177–184). The existence of L x has now been confirmed by synchrotron X-ray diffraction on samples identical to those of the previous work, of similar preparation and at corresponding scan rates. With temperature, both small-angle (long lamellar) and wide-angle (hydrocarbon chain) spacings show abrupt discontinuities, and separate broader changes, at temperatures corresponding to the calorimetric sharp and broad peaks, respectively. All the X-ray diffraction profiles and spacing results are consistent with the following phase scheme with increasing temperature: gel ripple phase P β′ → new, less ordered liquid crystalline phase L x → most disordered liquid crystalline phase L α. The phase L x possibly exists in other C nPCs, and its examination may provide details of the main transition. Because L x exists at a higher temperature than the main transition from P β′, it promises to be of biological relevance.