Correlating aggregation kinetics and stationary diffusion in protein-sodium salt systems observed with dynamic light scattering.

This paper compares two manifestations of electrolyte-mediated interaction between globular proteins. Salt-induced protein aggregation is studied with dynamic light scattering (DLS) in solutions of lysozyme and bovine serum albumin (BSA) containing different types of sodium salts. The same types of ions are used in a second measurement series assessing the effect of more dilute electrolytes on protein diffusivity in non-aggregating protein dispersions. Both aggregation and stable diffusion exhibit strong ion specificity along the lines of the Hofmeister series: chaotropic counterions act as the strongest coagulants and, in stable protein solutions, lead to the lowest "protein interaction parameter", evaluated as the slope of protein diffusivity versus protein concentration. Within this common qualitative trend, lysozyme and BSA solutions show marked differences, including the sign of the interaction parameter for most of the tested solution compositions. Despite the different nature of lysozyme and BSA, a strong correlation is found in both cases between the ion-specific interaction parameter and the proteins' aggregation tendency as indicated by the salt concentration required for fast aggregation. The interaction parameter, available via quick and easy DLS measurements on stable protein solutions, may thus serve as a predictor of ion-specific aggregation trends.