Determination of the apparent thermodynamic activities of saturated protein solutions

Although the solubility of a protein is a particularly informative solution parameter, little is known about the thermodynamics of protein solubilization. In these experiments, polyethylene glycol (PEG) is used to decrease the solubility of a number of proteins in a quantifiable manner. Simple thermodynamic considerations show that if the chemical potential of the PEG-induced solid phase is constant and plots of log protein solubility versus PEG concentration are linear, a valid extrapolation of the apparent solubility to zero PEG content can be made. Given the validity of these assumptions, extrapolated values should represent the activity of the protein in saturated solution. Evidence for the validity of this extrapolation includes (a) the experimentally observed linearity of log solubility versus PEG concentration plots, (b) the extrapolation of such plots to correct activities in the situation where protein activities can be experimentally determined, and (c) the independence of the extrapolated activities on protein concentration over a wide range. The utility of the PEG-determined activities, when applied in a comparative manner, is illustrated by application to various hemoglobin solutions. It is found that saturated solutions of the various hemoglobin forms, with the exception of deoxyhemoglobin S, manifest similar activities. In addition, all of the solutions demonstrate an apparent, surprising thermodynamic ideality.