The likelihood of aggregation during protein renaturation can be assessed using the second virial coefficient

Protein aggregation is commonly observed during protein refolding. To better understand this phenomenon, the intermolecular interactions experienced by a protein during unfolding and refolding are inferred from second virial coefficient (SVC) measurements. It is accepted that a negative SVC is indicative of protein-protein interactions that are attractive, whereas a positive SVC indicates net repulsive interactions. Lysozyme denatured and reduced in guanidinium hydrochloride exhibited a decreasing SVC as the denaturant was diluted, and the SVC approached zero at approximately 3 M GdnHCl. Further dilution of denaturant to renaturation conditions (1.25 M GdnHCl) led to a negative SVC, and significant protein aggregation was observed. The inclusion of 500 mM L-arginine in the renaturation buffer shifted the SVC to positive and suppressed aggregation, thereby increasing refolding yield. The formation of mixed disulfides in the denatured state prior to refolding also increased protein solubility and suppressed aggregation, even without the use of L-arginine. Again, the suppression of aggregation was shown to be caused by a shift from attractive to repulsive intermolecular interactions as reflected in a shift from a negative to a positive SVC value. To the best of our knowledge, this is the first time that SVC data have been reported for renaturation studies. We believe this technique will aid in our understanding of how certain conditions promote renaturation and increase protein solubility, thereby suppressing aggregation. SVC measurements provide a useful link, for protein folding and aggregation, between empirical observation and thermodynamics.