Freeze Fracture

Freeze fracture is a preparative procedure for visualizing the distribution of proteins within the lipid bilayer of membranes by transmission electron microscopy. The membrane samples are rapidly frozen to a very low temperature and then fractured with a knife blade. Fracturing occurs along planes where the intramolecular forces are weakest. In membranes, these planes occur along the center of the phospholipid bilayer, where the apposing fatty acid chains of the monolayer leaflets interact. Freeze fracturing thus exposes the nonpolar interior of the membrane. Integral membrane proteins that cross the bilayer interrupt the fracturing process and are carried along with either monolayer leaflet, leaving pits in the apposed monolayer. The fracture faces are shadowed with metal, and the proteins appear as small particles. The technique may be combined with immunolabeling to visualize the location of specific proteins (1).  

Freeze etching utilizes freeze-drying of the fractured frozen samples to remove water, thereby exposing the components of the surfaces of the fractured samples; the surfaces are then shadowed for viewing in the electron microscope.

References

1. P. Pinto da Silva (1984) In Immunolabeling for Electron Microscopy (J. M. Polack and I. M. Varndell, eds.), Elsevier, Amsterdam, pp. 179–188.