Ionic strength dependence of protein-polyelectrolyte interactions

The effect of univalent electrolyte concentration on protein-polyelectrolyte complex formation has been measured by frontal analysis continuous capillary electrophoresis (FACCE) and turbidimetry for the interaction of bovine serum albumin (BSA) with a synthetic hydrophobically modified polyacid, for BSA with (porcine mucosal) heparin (Hp), a highly charged polyanion, and for Hp and insulin. All three highly diverse systems display maxima or plateaus in complex formation in the range of ionic strength 5 < I < 30 mM, confirmed in the case of BSA-Hp by multiple techniques. Similar maxima are reported in the literature, but with little discussion, for BSA-poly(dimethyldiallylammonium chloride), lysozyme-hyaluronic acid, and lysozyme-chondroitin sulfate, always in the I range 5-30 mM. While inversion of salt effect has been discussed specifically for the interaction of gelatin and sodium polystyrenesulfonate with gelatin(28) and with beta-lactoglobulin,(10) the general nature of this phenomenon, regardless of polyelectrolyte origin, molecular weight, and charge sign has not been recognized. The position of the maxima and their occurrence when protein and polyelectrolyte have the same net charge imply that they arise when Debye lengths extend, at low I, beyond half the protein diameter so that addition of salt screens repulsions, as well as attractions. This appears to be a general effect caused by electrostatic repulsions that can coexist simultaneously with hydrophobic interactions. Modeling of protein electrostatics via Delphi is used to visualize this effect for BSA, lysozyme, insulin, and beta-lactoglobulin.