Dynamic surface tension responses to surface area change of mixed solutions of a protein and small or medium-sized organic molecules

Axisymmetric Drop Shape Analysis has been employed to measure the dynamic surface tension response to a sawtooth-shaped variation in the surface area. Four systems were studied: (a) bovine serum albumin (BSA) aqueous solution; (b) dimethyl sulfoxide (DMSO) added to the BSA aqueous solution; (c) ethyl alcohol (ethanol) added to the BSA aqueous solution; and (d) hepoxilin methyl ester, a lipidic hydroxy epoxide derived from arachidonic acid, dissolved in DMSO and then added to the BSA aqueous solution. In the mixtures of the solutions, competitive adsorption to the surface is revealed between the smaller molecules and the protein molecules, through the change of the pattern in the dynamic surface tension response to the surface area change. In the initial stage of the adsorption, the smaller organic molecules (DMSO, ethanol) diffuse to the surface at a faster rate than the protein molecules, impeding protein adsorption. However, in the cases of DMSO/BSA and ethanol/BSA solutions, the experiment indicates that the small organic molecules are squeezed out of the surface because of the compression/dilation process. After steady state is established, the surface tension pattern shows a skewed shape suggesting a conformational change in the protein. When hepoxilin, a naturally occurring biologically active C20 fatty acid containing a hydroxyl epoxide functionality, is present in the BSA solution, a rather symmetric shape is found in the dynamic surface tension changes, different from the skewed shape found in that of the pure BSA solution. This suggests that hepoxilin molecules interact with the protein, leading to a stabilization of one conformation of the protein molecules at the surface.