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 dyna mic surface tension response to a sawtooth-shaped variation in the sur face area. Four systems were studied: (a) bovine serum albumin (BSA) a queous solution; (b) dimethyl sulfoxide (DMSO) added to the BSA aqueou s solution; (c) ethyl alcohol (ethanol) added to the BSA aqueous solut ion; and (d) hepoxilin methyl ester, a lipidic hydroxy epoxide derived from arachidonic acid, dissolved in DMSO and then added to the BSA aq ueous solution. In the mixtures of the solutions, competitive adsorpti on to the surface is revealed between the smaller molecules and the pr otein molecules, through the change of the pattern in the dynamic surf ace tension response to the surface area change. In the initial stage of the adsorption, the smaller organic molecules (DMSO, ethanol) diffu se to the surface at a faster rate than the protein molecules, impedin g protein adsorption. However, in the cases of DMSO/BSA and ethanol/BS A solutions, the experiment indicates that the small organic molecules are squeezed out of the surface because of the compression/dilation p rocess. After steady state is established, the surface tension pattern shows a skewed shape suggesting a conformational change in the protei n. When hepoxilin, a naturally occurring biologically active C20 fatty acid containing a hydroxyl epoxide functionality, is present in the B SA 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 wit h the protein, leading to a stabilization of one conformation of the p rotein molecules at the surface.