A NEW LIQUID-CRYSTALLINE PHASE IN PHOSPHATIDYLCHOLINE BILAYERS AS STUDIED BY X-RAY-DIFFRACTION

Model membranes of diacylphosphatidylcholines (CnPC), with saturated l inear acyl chains of n > 12 carbons, show a single sharp phase transit ion (known as the main transition) between the gel phase P-beta, and t he liquid crystalline phase L(alpha) with differential scanning calori metry. However, C12PC (dilauroylphosphatidylcholine) shows, as well as the sharp transition at -2 degrees C, a broad peak at 5 degrees C, or iginally 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 f or (C12PC) bilayers on the basis of calorimetry (Finegold, Shaw and Si nger, Chem. Phys. Lipids 53 (1990), 177-184). The existence of L(X) ha s now been confirmed by synchrotron X-ray diffraction on samples ident ical to those of the previous work, of similar preparation and at corr esponding scan rates. With temperature, both small-angle (long lamella r) and wide-angle (hydrocarbon chain) spacings show abrupt discontinui ties, and separate broader changes, at temperatures corresponding to t he calorimetric sharp and broad peaks, respectively. All the X-ray dif fraction profiles and spacing results are consistent with the followin g phase scheme with increasing temperature: gel ripple phase P-beta, - -> new, less ordered liquid crystalline phase L(X) --> most disordered liquid crystalline phase L(alpha). The phase L(X) possibly exists in other CnPCs, and its examination may provide details of the main trans ition. Because L(X) exists at a higher temperature than the main trans ition from P(b)eta'), it promises to be of biological relevance.