SURFACE PRESSURE ISOTHERMS OF CALCIUM-BINDING PROTEIN MONOLAYERS AT THE AIR-WATER-INTERFACE

We present results of an air/water interfacial study of the calcium-bi nding protein, calbindin D-9k. The isotherm displays two inflection po ints, One at low surface pressures earmarks the limit of the gaseous s tate of the monolayer and represents the coexistence of two phases, Th e second inflection point appears at higher surface pressures and is l ikely associated with the expulsion of segments from the interface, Re sults found with a film formed with a mutant protein (three charged am ino acids are replaced by neutral groups) suggest that the monolayers are not greatly expanded at the pH of the experiments, However, with t he mutant protein monolayer both inflection points are substantially d iminished. In this paper we also demonstrate the utility of plotting t he area derivative of the surface pressure in the interpretation of mo nolayer behavior. (C) 1995 Academic Press, Inc.