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The binding affinities of polyanions for bovine serum albumin in NaCl solutions from I ) 0.01-0.6 M, were evaluated on the basis of the pH at the point of incipient binding, converting each such pH c value into a critical protein charge Z c . Analogous values of critical charge for mixed micelles were obtained as the cationic surfactant mole fraction Y c . The data were well fitted as Y c or Z c ) KI a , and values of K and a were considered as a function of normalized polymer charge densities (τ), charge mobility, and chain stiffness. Binding increased with chain flexibility and charge mobility, as expected from simulations and theory. Complex effects of τ were related to intrapolyanion repulsions within micelle-bound loops (seen in the simulations) or negative protein domainpolyanion repulsions. The linearity of Z c with I at I < 0.3 M was explained by using protein electrostatic images, showing that Z c at I < 0.3 M depends on a single positive "patch"; the appearance of multiple positive domains I > 0.3 M (lower pH c ) disrupts this simple behavior.

Electrostatic interactions between synthetic polyelectrolytes and proteins can lead to the formation of dense, macroion-rich liquid phases, with equilibrium microheterogeneities on length scales up to hundreds of nanometers. The effects of pH and ionic strength on the rheological and optical properties of these coacervates indicate microstructures sensitive to protein-polyelectrolyte interactions. We report here on the properties of coacervates obtained for bovine serum albumin (BSA) with the biopolyelectrolyte chitosan and find remarkable differences relative to coacervates obtained for BSA with poly(diallyldimethylammonium chloride) (PDADMAC). Coacervation with chitosan occurs more readily than with PDADMAC. Viscosities of coacervates obtained with chitosan are more than an order of magnitude larger and, unlike those with PDADMAC, show temperature and shear rate dependence. For the coacervates with chitosan, a fast relaxation time in dynamic light scattering, attributable to relatively unrestricted protein diffusion in both systems, is diminished in intensity by a factor of 3-4, and the consequent dominance by slow modes is accompanied by a more heterogeneous array of slow apparent diffusivities. In place of a small-angle neutron scattering Guinier region in the vicinity of 0.004 Å -1 , a 10-fold increase in scattering intensity is observed at lower q. Taken together, these results confirm the presence of dense domains on length scales of hundreds of nanometers to micrometers, which in coacervates prepared with chitosan are less solidlike, more interconnected, and occupy a larger volume fraction. The differences in properties are thus correlated with differences in mesophase structure.