Potentiometric and turbidimetric titrations were used to study the interaction between bovine serum albumin (BSA) and poly(diallyldimethylammonium chloride) (PDADMAC). Binding between BSA and PDADMAC, which takes place only above some critical initial pH (pHc), leads to a decrease in the pH of solution, indicating that the interactions enhance the dissociation constant Ka of the ionizable groups on BSA and result in an increase of the number of net negative charge on the protein. The pH difference caused by the interaction, ∆pH, decreases with added salt, which indicates that the effect of the interaction on the pKa of BSA increases with a decrease of ionic strength. Protein binding to PDADMAC imposes a stronger influence on the Ka of the carboxylic groups than on the Ka of the imidazolium and ammonium groups. The fraction of BSA bound (fb ) [BSA]b/Cpr) increases with polymer concentration Cp until all BSA are bound. The rate at which fb increases with added polymer at low Cp depends on the initial pH (pHi), consistent with an increase in the binding constant with pHi. Upon a further increase of pH, phase separation occurs at some well-defined point, pHφ, which increases with ionic strength. pHφ depends strongly on Cp at fixed BSA concentration, but only weakly on Cp at constant r ) Cpr/Cp. Phase separation may also be observed upon addition of polymer to BSA at pH > pHφ. In the range of pHc < pH < pHφ, pH titration and turbidimetry reveal the formation of soluble complexes. However, even when phase separation (coacervation) occurs, no corresponding change in the pH titration curve is observed, indicating that the protein's acid-base equilibria are unperturbed by phase separation.

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