PROPERTIES OF BETA-CASEIN AT THE AIR WATER INTERFACE AS SUPPORTED BY SURFACE RHEOLOGICAL MEASUREMENTS/

The properties of air/water adsorbed and spread monolayers of native a nd dephosphorylated beta-casein were monitored using surface pressure (Langmuir trough) and surface rheology (ring trough) techniques. Two s tages of rearrangement are observed for native beta-casein at surface areas of about 1.0-1.3 and 0.7 m(2) mg(-1). The first accounts for dis tinct surface elasticity changes in the film, which are probably due t o the expulsion of the most hydrophilic segments of the protein chain. The second accounts for the collapse of the monolayer. The experiment s on dephosphorylated beta-casein monolayers show that dephosphorylati on changes the surface elasticity behavior of the monolayer, in partic ular between 1 and 1.3 m(2) mg(-1). We calculated a two-dimensional Fl ory exponent, v, for both proteins. This exponent is constant over a ( semi-) dilute range of surface tensions, maximally up to a surface are a of around 1.3 m(2) mg(-1). The adsorption of native beta-casein is s hown to be diffusion-limited up to a surface area of around 1 m(2) mg( -1). Experiments at high ionic strength show the importance of charge on the typical surface elasticity behavior of beta-casein. Experiments with enzymatically treated beta-casein show the importance of the pre sence of a hydrophilic section in the molecule on the surface elastici ty behavior. It is assumed and shown that, at surface concentrations b elow monolayer collapse and at given solvent conditions, native beta-c asein and dephosphorylated beta-casein show irreversible (air/water) a dsorption behavior. Furthermore, the proteins in the monolayer are ver y flexible (i.e. quick relaxations).