Effect of glycerol on the interactions and solubility of bovine pancreatic trypsin inhibitor

The effects of additives used to stabilize protein structure during crystallization on protein solution phase behavior are poorly understood. Here we investigate the effect of glycerol and ionic strength on the solubility and strength of interactions of the bovine pancreatic trypsin inhibitor. These two variables are found to have opposite effects on the intermolecular forces; attractions increase with [NaCl], whereas repulsions increase with glycerol concentration. These changes are mirrored in bovine pancreatic trypsin inhibitor solubility where the typical salting out behavior for NaCl is observed with higher solubility found in buffers containing glycerol. The increased repulsions induced by glycerol can be explained by a number of possible mechanisms, all of which require small changes in the protein or the solvent in its immediate vicinity. Bovine pancreatic trypsin inhibitor follows the same general phase behavior as other globular macromolecules where a robust correlation between protein solution second virial coefficient and solubility has been developed. This study extends previous reports of this correlation to solution conditions involving nonelectrolyte additives.