We have carried out a systematic study of specific ion effects on lysozyme solutions using static light scattering to characterize protein-protein interactions (in terms of the osmotic second virial coefficient, B22) and electrophoretic light scattering to approximate the diffuse layer potential of lysozyme. Lysozyme has long been used as a model system for specific ion effects in part because its solubility behavior follows the reverse Hofmeister series, where chaotropes rather than kosmotropes are more effective salting-out agents. A key question we address is how anion binding alters protein-protein interactions (and protein solubility). Recent computational work has suggested ion binding will mainly alter electrical double layer forces between proteins. B22 has been measured for lysozyme solutions as a function of pH and salt concentration for various salts with different anions spanning the range of the Hofmeister series. For the same solution conditions, we measured the electrophoretic mobility for lysozyme. We found that under low ionic strength conditions, the measured zeta-potential does not always correlate with the trends in protein-protein interaction measurements. In particular, with increasing ionic strength, protein-protein interactions become more attractive in chaotropic anion solutions versus those containing sulfate, although the zeta potential remained less positive in sulfate solutions over all ionic strength. The results demonstrate that chaotropic anion binding to proteins induces short-ranged attractive forces between proteins of non-electrostatic origin, rather than changing only double layer forces.

Combined electrophoretic and light scattering approach to elucidate the molecular origin of reverse Hofmeister series effects on protein solutions

253rd National Meeting (2017)

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