Thermodynamic Properties of Supersaturated Protein Solutions

Abstract

The purpose of this work is to investigate water and protein (lysozyme) activities in supersaturated aqueous protein solutions. An electrodynamic levitation trap (ELT) is used to suspend a single charged droplet of aqueous protein solution without a container. This technique allows investigation of homogeneous nucleation and measurements of water activity deep into the metastable zone of a supersaturated droplet. The system was treated as a ternary one (protein + solvent + “ideal” salt), and the protein activity is calculated employing the Gibbs−Duhem equation. The trend of logarithm ( a protein / ) with protein concentration shows initially a sharp increase, but then at higher protein concentrations the trend starts to level off. The same trend is seen in salt-water systems and organic−water systems and follows the expected behavior of supersaturated solutions. As the system is brought closer and closer to the spinodal curve, the second derivative of the Gibbs free energy of the solution with respect to protein concentration moves toward zero. Due to the small values of mole fractions of protein used for the integration of the Gibbs−Duhem equation as well as the unknown dependence of salt activity on protein concentration, the results show exceptionally high values for a protein / .