Studies of a weak polyampholyte at the air-buffer interface: The effect ofvarying pH and ionic strength

We have carried out experiments to probe the static and dynamic interfacial properties of beta -casein monolayers spread at the air-buffer interface, and analyzed these results in the context of models of weak polyampholytes. Measurements have been made systematically over a wide range of ionic stre ngth and pH. In the semidilute regime of surface concentration a scaling ex ponent, which can be linked to the degree of chain swelling, is found. This shows that at pH close to the isoelectric point, the protein is compact. A t pH away from the isoelectric pH the protein is extended. The transition b etween compact and extended states is continuous. As a function of increasi ng ionic strength, we observe swelling of the protein at the isoelectric pH but contraction of the protein at pH values away from it. These behaviors are typical of a those predicted theoretically for a weak polyampholyte. Di lational moduli measurements, made as a function of surface concentration e xhibit maxima that are linked to the collapse of hydrophilic regions of the protein into the subphase. Based on this data we present a configuration m ap of the protein configuration in the monolayer. These findings are suppor ted by strain (surface pressure) relaxation measurements and surface quasie lastic light scattering measurements which suggest the existence of loops a nd tails in the subphase at higher surface concentrations. (C) 2001 America n Institute of Physics.