Electrophoretic Mobilities of Protein-Coated Hexadecane Droplets at Different pH

Electrophoretic mobilities of hexadecane droplets in 10 mM potassium phosphate solutions (pH between 3.0 and 7.0) and in phosphate buffered saline (pH 7.0) were measured during adsorption of bovine (BSA) and human (HSA) serum albumin, immunoglobulin G (IgG), and fibrinogen (Fg) from single-protein solutions as well as during protein adsorption from binary HSA/IgG and HSA/Fg mixtures and from diluted plasma. Electrophoretic mobilities became less negative upon adsorption of proteins within 1.5 min after the initiation of adsorption. Only for IgG, was a time-dependent change of the electrophoretic mobilities of the protein-hexadecane complex observed. In phosphate buffered saline, less negative electrophoretic mobilities were measured than in the potassium phosphate solution. Iso-electric points of the protein-hexadecane complexes in 10 mM potassium phosphate were located at pH 5.0 for albumin, at pH 5.5 for Fg, and at pH 6.6 for IgG, i.e., about the iso-electric points of the pure unadsorbed proteins. This confirms that the net charge addition upon protein adsorption, which is positive below the iso-electric point of the proteins, at low protein concentrations determines the effects on the final electrophoretic mobilities of the protein-hexadecane complexes. As a name for the methodology applied, we propose PATH (protein adsorption to hydrocarbons), in analogy to the well-known MATH (microbial adhesion to hydrocarbons) method. The major advantage of PATH is that it represents an in situ method to study protein adsorption, without artifactual rinsing steps, while furthermore the hydrocarbon phase can be replaced by organic solvents to study the role of acid-base interactions in protein adsorption. In combination with drop-shape analysis techniques, PATH also enables us to determine in situ effects of protein adsorption on interfacial tensions. Copyright 1998 Academic Press.