Chloride binding effects on lysozyme solubility

Lysozyme salting-out phase equilibria were examined in ammonium-sulfate solutions in the presence of low concentrations of either of two anions (chloride or trichloroacetate) that associate with lysozyme.Addition of either anion enhances lysozyme partitioning to the dense phase.Trichloroacetate increases partitioning more than chloride.At pH > 4, 0.1 m chloride enhances lysozyme partitioning in concentrated ammonium-sulfate solutions; however, at lower pH, chloride decreases lysozyme partitioning.The pH -dependent enhancement of partitioning by chloride is attributed to competition between chloride and sulfate binding.Ion binding may increase the effective protein diameter and thereby increase protein partitioning to the dense phase.35CI NMR spectroscopy in the presence of Au(CNh-(which competes for chloride binding sites) provide a quantitative measure of chloride ions bound to lysozyme.These ionbinding and salting-out experiments suggest that, at constant ionic strength, pH-~ependent ion binding to proteins may be responsible for the pH dependence of protein salting-out phase equilibria.