Interaction of lysozyme with negatively charged flexible chain polymers

The complex formation between the basic protein lysozyme and anionic polyelectrolytes: poly acrylic acid and poly vinyl sulfonic acid was studied by turbidimetric and isothermal calorimetric titrations. The thermodynamic stability of the protein in the presence of these polymers was also studied by differential scanning calorimetry. The lysozyme–polymer complex was insoluble at pH lower than 6, with a stoichiometric ratio (polymer per protein mol) of 0.025–0.060 for lysozyme–poly vinyl sulfonic acid and around 0.003–0.001 for the lysozyme–poly acrylic acid. NaCl 0.1 M inhibited the complex precipitation in agreement with the proposed coulombic mechanism of complex formation. Enthalpic and entropic changes associated to the complex formation showed highly negative values in accordance with a coulombic interaction mechanism. The protein tertiary structure and its thermodynamic stability were not affected by the presence of polyelectrolyte.