Equilibrium and dynamic surface pressure-area measurements on protein films at air-water and oil-water interfaces

The interfacial pressure-area per molecule (pi-A) isotherms of spread films of the proteins beta-lactoglobulin (beta-L) and bovine serum albumin(BSA) have been measured at the air-water (A-W) and oil-water (O-W) interface using a novel Langmuir trough. The films are considerably more expanded at the O-W interface compared to the A-W interface at low interfacial pressure (pi), implying a greater extent of unfolding at the O-W interface. Rates of desorption of beta-L have been measured at both interfaces at constant pi. Desorption rates show the strong dependence on pi expected of protein monolayers, increasing markedly above a certain pi, though all desorption rates are quite low, even on approaching the equilibrium pi for adsorbed films. Desorption rates appear to be higher at the O-W interface at a given pi, compared with the A-W interface, but integration of the pi-A curve reveals that the rate of desorption as a function of the free energy barrier to desorption is similar for both interfaces. Protein films have also been subjected to a rapid step change in area and the recovery to the equilibrium rr measured. The results show that the typical dominant relaxation times of an O-W monolayer and an A-W monolayer are 60 and 20 s, respectively. Thus the contribution of the ''in-film'' relaxation (involving changes in molecular conformation at the interface) to the change in pi for dilatational experiments on adsorbed protein films has an unexpectedly fast time-scale. (C) 1997 Elsevier Science B.V.