Small phosphatidylcholine vesicles appear to be faceted below the thermal phase transition

Summary We have studied the main thermal transition in dipalmitoyl phosphatidylcholine (DPPC) multilayers and a similar transition in small (300 Å diameter), single-walled vesicles by X-ray diffraction. As judged by the large-angle diffraction, the transition in the multilayers is narrow; aside from small tails, the transition occurs over a range of 0.5°C. In contrast, the transition in the vesicles is quite broad; the range is about 7°C. These observations are in agreement with recently published data.Referring to the small vesicles below the thermal transition, a bilayer structure in which the C₁₆ chains are all straight and pointed radially is inconsistent with the large-angle diffraction. Assuming instead that the chains are packed in a regular, planar array, it is clear from their small size that the vesicles can have only limited regions of planar packing. The X-ray data indicate that the planar regions are 75 Å across on the average. In view of the 75-Å size and the average vesicle diameter of about 300 Å, we propose that the small vesicles are faceted below the transition, i.e., that the vesicles are polygonal. The small-angle diffraction pattern from the vesicles below the transition provides support for the faceted structure.