Complex formation between bovine serum albumin and strong polyelectrolytes: Effect of polymer charge density

Light scattering and pH titration were used to examine the binding of bovine serum albumin (BSA) to poly(diallyldimethylammonium chloride) (PDADMAC), poly(acrylamidomethylpropyl sulfonate) (PAMPS), poly(methacrylamidopropyltrimethylammonium chloride) (PMAPTAC), and an AMPS-acrylamide random copolymer (PAMPS(80)AAm(20)). The critical protein charge required to induce protein-polyelectrolyte complexation, (Zpr)(c), was found to vary linearly with the square root of the ionic strength (I-1/2), i.e., with the Debye-Huckel parameter (kappa), the proportionality constant being a function of polyelectrolyte chain parameters such as intrinsic stiffness and charge density. This linearity was remarkably continuous through Zpr = 0, with (Zpr)(c) occurring predominantly "on the wrong side" of the isoionic point; i.e., the onset of binding was typically observed when the global protein charge was of the same sign as the polyelectrolyte. Binding of BSA to the lower charge density polyanion (PAMPS(80)AAm(20)) unexpectedly occurred under conditions where binding to the more highly charged homopolyanion (PAMPS) did not. The theoretical treatment of Muthukumar was used to interpret the linearity of (Zpr)(c) vs I-1/2 and the observed influence of polyelectrolyte structural parameters. The apparent applicability of this model to the heterogeneous amphoteric protein surface suggests that binding of polyelectrolytes takes place at "charge patches" whose effective charge densities are different from, but nevertheless linearly dependent on, the global charge density.