A study of mixed phospholipid/beta-casein monolayers at the water verticalbar air surface

The compression/expansion properties of monolayers of dimyristoylphosphatid ylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC) and distearoylphosp hatidylcholine (DSPC) mixed with beta-casein at the water/air interface are investigated by the film balance technique. An increase of the area per li pid molecule at constant surface pressure was found as the result of the in corporation of protein molecules into the lipid monolayers. When the films are compressed to a sufficiently high surface pressure the isotherms of the mixed protein-lipid monolayers coincide with those of pure lipid monolayer s; i.e. the protein is "squeezed out". Quantitative recording of consecutiv e cyclic compressions/expansions reveals that the processes in the mixed mo nolayers are reversible. The analysis suggests phase separation between pro tein and lipid. (C) 1998 Elsevier Science B.V. All rights reserved.