Thermodynamic Studies of Aqueous Binary and Ternary Solutions Involving Lysozyme and Urea at 298.15 K

Abstract

Measurements for osmotic pressure and densities for lysozyme in aqueous and aqueous solutions containing urea have been reported at 298.15 K using a vapor pressure osmometer and vibrating tube digital density meter, respectively. The partial molar volumes and activity coefficients of both the components and the osmotic pressure of the solutions have been calculated. The molar mass and second virial coefficient of lysozyme have been obtained using the McMillan–Mayer theory of solutions and are compared with the values obtained in aqueous salt solutions. The Guggenheim–Stokes equation for osmotic coefficient has been used to obtain the protein molecular diameter in an aqueous medium. The estimated diameter of lysozyme is similar in magnitude to that in the pure state, confirming the retaining of the native state of the protein in aqueous solutions over the studied concentration region. These studies have been extended to aqueous–urea (∼2.0 mol·kg–1) solutions at 298.15 K. Data of water activity and activity coefficient of all the three components in ternary solutions as a function of lysozyme concentration have been determined by the methodology developed by us earlier. The data have been further analyzed to obtain Gibbs free energy changes of transfer of lysozyme as well as that urea from corresponding aqueous binary to aqueous ternary solutions through which information about the binding of denaturant to protein can be derived. The analysis further reveals that the binding of urea to the protein molecules occurs through the breakdown of protein–water H–bonds leading to the opening of a coiled structure of lysozyme. All these results are discussed in terms of protein hydration, protein–protein hydrophobic interactions, and protein denaturant binding equilibria for the solutions of lysozyme.