EXPERIMENTAL-ANALYSIS OF PROTEIN PRECIPITATION BY POLYETHYLENE-GLYCOL AND COMPARISON WITH THEORY

We have been attempting for some time to develop a fundamental theory of protein precipitation by non-ionic polymer, the goal being to predict protein solubilities as a function of various process parameters. As a supplement to these efforts, we present in this paper an experimental analysis of the solubility of three globular proteins, bovine serum albumin, catalase and thyroglobulin in solutions containing polyethylene glycol (PEG). These systems were chosen because they represent a wide. range in protein sizes. The variation in the precipitation behavior is studied by systematically altering the polymer molecular weights, pH and ionic strength. Our aim is to isolate the effect of each of these process variables on protein solubility. Four different molecular weights of PEG are used. Solubility curves are generated at three different pHs and two ionic strengths for each protein-polymer pair. We verify the reported trends in solubility with both protein and polymer size, and pH and the ionic strength. We find also that the effect of the ionic strength is diminished as the pH approaches the isoelectric point. These results are in accord with the predictions of our theory. Finally, we compare the experimental data with theoretical predictions and find qualitative agreement.