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.

The aggregation of beta-lactoglobulin (BLG) at ambient temperature was studied using turbidimetry and dynamic light scattering in the range 3.8

The effect of a polyelectrolyte's chain stiffness on its interaction with an oppositely charged colloid particle was studied by measuring the relative affinity of two polyelectrolytes for (1) mixed cationic/nonionic micelles (DTAB/TX100), and (2) the protein serum albumin. The binding affinity as manifested, respectively, in the critical ionic surfactant mole fraction required for polyelectrolyte-micelle complex formation, and in the critical pH for polyelectrolyte-protein association, was determined by turbidimetric titrations over a range of ionic strengths. Binding was generally weaker for the stiffer chain, hyaluronic acid (HA), relative to the more flexible chain, a copolymer of acrylamidomethylpropanesulfonate (AMPS) and acrylamide (AAm), chosen to have the same linear charge density as HA at neutral pH. In the case of serum albumin, comparisons were also made to AMPS-AAm copolymers of higher charge densities, and to heparin, a highly charged and flexible biopolyelectrolyte. The results are discussed in terms of the ionic strength dependence of the relevant persistence lengths.

No abstract

Highly charged polyelectrolytes adsorbed on oppositely charged colloidal particles are investigated by electrophoresis and dynamic light scattering. The dependence of the adsorbed amount and of the hydrodynamic layer thickness on the molecular mass and the salt level is analyzed. The adsorbed amount increases with increasing salt level and decreases with increasing molecular mass. The hydrodynamic layer thickness is independent of the molecular mass at low salt levels, but increases with the molecular mass as a power law with an exponent 0.10 ± 0.01 at high salt. The same behavior was observed for different polyelectrolytes and substrates and therefore is suspected to be generic. Due to semi-quantitative agreement with computer simulations carried out by Kong and Muthukumar in 1998, the observed behavior is interpreted with conformational changes of single adsorbed polyelectrolyte chains.