X-ray diffraction studies of oriented dilauroyl phosphatidylcholine bilayers in the L delta and L alpha phases

X-ray diffraction studies on oriented multilayers of dilauroyl phosphatidylcholine in the lyotropic liquid-crystalline L alpha phase and a not previously reported mono-domain three-dimensional L delta phase at two temperatures (293 and 343 K) and various relative humidities (0-100%) are described. Absolute one-dimensional electron-density profiles of the different structural phase bilayers were constructed to a resolution of 4 A using direct methods (e.g. swelling and triplet structure-invariant relationships) to solve for the phase problem. The absolute electron-density distributions clearly demonstrate differences between the two structural phases of dilauroyl phosphatidylcholine bilayers. In addition, the various structural properties of the two different phases have been quantified. In the case of the L delta phase, the structural quantities (e.g. volumes of the terminal methyl group and headgroup, and the number of waters) are examined for the first time.