Decoration of specific sites on freeze-fractured membranes.

Fracturing under ultrahigh vacuum (UHV, P less than or equal to 10(-9) Torr) produces membrane fracture faces devoid of contamination. Such clean surfaces are a prerequisite for studies of interactions between condensing gases and distinct regions of a surface. For the study of water condensation, a device has been developed which enables production of pure water vapor and controlled variation of its partial pressure in an UHV freeze-fracture apparatus. Experiments with yeast plasmalemma fracture faces, produced at -196 degrees C and exposed to pure water vapor before replication, resulted in a "specific decoration" with ice crystals of those pits in the extraplasmic face where the corresponding particles of the plasmic face had been removed. Because water condenses as discrete ice crystals which resemble intramembrane particles, ice crystals might easily be misinterpreted as actual membrane structures. At low specimen temperature (T less than or equal to 110 degrees C) the structural features of membrane fracture faces produced under high vacuum (P approximately 10(-6) Torr) should, therefore, be interpreted with caution.