Why pH titration in lysozyme suspensions follow a Hofmeister series

We present theoretical results that provide new insights into the Hofmeister effects observed in protein suspensions. With a buffered solution at a supposedly fixed pH, measurements of that pH with glass electrodes in protein suspensions depend strongly on both ionic species and concentration of background salt and protein. The observed Hofmeister series cannot be explained with standard electrostatic theories. While purely electrostatic limiting laws can be used to obtain partial understanding of some nonspecific trends in buffer and protein solutions, it has long been clear that they fail to explain such ion specificity. The reasons, as explored in a number of our previous papers, have to do with the neglect in these theories of electrodynamic fluctuation (dispersion) forces between ions and proteins. We here use a Poisson–Boltzmann cell model that takes these ionic dispersion potentials between ions and protein into account. The observed ion specificity can then be accounted for.