Mobility Shift Affinity Capillary Electrophoresis at High Ligand Concentrations: Application to Aluminum Chlorohydrate–Protein Interactions

The active ingredients in antiperspirant products are aluminum chlorohydrates (ACHs) that, when interacting with proteins present in sweat and sweat duct walls, lead to the obstruction of the sweat ducts and thus reduce delivery of sweat at the skin surface. This study is aimed at developing a methodology based on affinity capillary electrophoresis (ACE) to obtain a quantitative ranking of the interaction between ACHs and proteins under experimental conditions close to those of industrial applications. Usually, in ACE, the metal ligand is introduced at typically μM to mM concentrations in a background electrolyte (BGE) containing a buffering agent that sets the pH and ionic strength. In this work, ACE was implemented in a range of ACH (ligand) concentrations up to 50 g/L (0.2 M in Al­(H₂O)₆·3Cl) in the absence of other buffering agents, to mimic as much as possible the conditions encountered in the production of antiperspirant products. Under such electrophoretic conditions, the challenge is to extract quantitative information about the interaction from the electrophoretic mobility of the protein, knowing that many effects (including Joule heating, viscosity, pH, ionic strength of the BGE, and distribution of the ligands) vary with the concentration of ACH. With relevant corrections on the effective mobility, it has been possible to observe and quantify a much stronger interaction of ACH components with bovine serum albumin compared to lysozyme.