Fracture Planes in an Ice-bilayer Model Membrane System

Experiments with transferred stearate layers were performed to determine the location of fracture planes in frozen ice-lipid systems. Bilayers and multilayers of carbon-14-labeled stearate were frozen in contact with an aqueous phase and then fractured. The distribution of radioactivity on both sides of the fracture showed that the stearate layers were cleaved apart predominantly in the plane of their hydrocarbon tails. Because bilayers split in this manner, it was possible to measure time-dependent exchange of label between the layers. Exchange occurred with a half-time of 50 minutes in the presence of calcium and 25 minutes in the absence of calcium. Since stearate bilayers and multilayers are models of hydrophobically stabilized structures, the strong influence of their hydrophobic region on the fracture plane provides an explanation of how the freeze-etch technique of electron microscopy can expose inner, hydrophobic faces of cell membranes.