Thermodynamics of Binding of Water and Solute to Lysozyme

Using isopiestic vapour pressure technique, extents of binding of water to a globular protein, lysozyme, have been determined in the absence and presence of inorganic salts, sucrose and urea at a fixed temperature. The water vapour adsorption isotherm for lysozyme in the range of water activity varying between zero to-unity is similar to type III BET isotherm. Moles of water adsorbed per mole of lysozyme at unit water activity have been evaluated by extrapolation method and the results support monolayer model for water adsorption by lysozyme under ideal conditions. Using the Bull equation in the integrated form, standard free energies, ΔG⁰_w for water-protein binding interaction at two different temperatures have been evaluated. Based on Clausius-Clapeyron equation in integrated form, the integral enthalpy for water-protein interaction has also been evaluated. Using the isopiestic technique, excess binding of solutes T¹₂ per mole of lysozyme in the presence of different bulk mole-fractions (X₂) of the solutes (LiCl, NaCl, KCl, NaBr, NaI, KSCN, urea and sucrose) has been evaluated in each case from the expression for the Gibbs surface excess. In certain ranges of solute concentration, the plot of Γ²₁ vs X₁ becomes linear so that moles of water and solute bound per mole of lysozyme can be evaluated. X₁ stands for the mole-fraction of the solvent in the bulk phase. Also, using the integrated form of the Gibbs adsorption equation, standard free energy changes(ΔG⁰) for the solute-lysozyme and the solvent-lysozyme interactions for different systems have been computed and the values have been compared critically.