Influence of Chain Stiffness on the Interaction of Polyelectrolytes with Oppositely Charged Micelles and Proteins

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.