Protein-protein interactions in aqueous ammonium sulfate solutions. Lysozyme and bovine serum albumin (BSA)

Osmotic pressures have been measured to determine lysozyme-lysozyme, BSA-BSA, and lysosyme-BSA interactions for protein concentrations to 100 g-L-1 in an aqueous solution of ammonium sulfate at ambient temperature, as a function of ionic strength and pH. Osmotic second virial coefficients for lysozyme, for BSA, and for a mixture of BSA and lysozyme were calculated from the osmotic-pressure data for protein concentrations to 40 g-L-1. The osmotic second virial coefficient of lysozyme is slightly negative and becomes more negative with rising ionic strength and pH. The osmotic second virial coefficient for BSA is slightly positive, increasing with ionic strength and pH. The osmotic second virial cross coefficient of the mixture lies between the coefficients for lysozyme and BSA, indicating that the attractive forces for a lysozyme-BSA pair are intermediate between those for the lysozyme-lysozyme and BSA-BSA pairs. For protein concentrations less than 100 g-L-1, experimental osmotic-pressure data compare favorably with results from an adhesive hard-sphere model, which has previously been shown to fit osmotic compressibilities of lysozyme solutions.