A new liquid crystalline phase in phosphatidylcholine bilayers as studied by X-ray diffraction.
Model membranes of diacylphosphatidylcholines (CnPC), 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 beta' and the liquid crystalline phase L alpha with differential scanning calorimetry. However, C12PC (dilauroylphosphatidylcholine) shows, as well as the sharp transition at -2 degrees C, a broad peak at 5 degrees C, originally observed by S. Mabrey and J.M. Sturtevant. The broad peak is not artificial: between the two peaks a new phase Lx was predicted for (C12PC) bilayers on the basis of calorimetry (Finegold, Shaw and Singer, Chem. Phys. Lipids 53 (1990), 177-184). The existence of Lx 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 beta'-->new, less ordered liquid crystalline phase Lx-->most disordered liquid crystalline phase L alpha. The phase Lx possibly exists in other CnPCs, and its examination may provide details of the main transition. Because Lx exists at a higher temperature than the main transition from P beta', it promises to be of biological relevance.
Hatta
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Department of Applied Physics, Nagoya University, Japan.
Matuoka
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S
Singer
M A
MA
Finegold
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L
D002152:Calorimetry, Differential Scanning; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D018360:Crystallography, X-Ray; D008051:Lipid Bilayers / Q000737:chemistry*; D008567:Membranes, Artificial; D008956:Models, Chemical; D010713:Phosphatidylcholines / Q000737:chemistry*
D008051:Lipid Bilayers; D008567:Membranes, Artificial; D010713:Phosphatidylcholines; C015153:1,2-dilauroylphosphatidylcholine