Effects of polyelectrolyte charge distribution and chain stiffness on polyelectrolyte-protein complex formation and coacervation
Crucial parameters affecting protein-polyelectrolyte complexation include protein charge anisotropy, chain flexibility and polyelectrolyte (PE) charge sequence distribution. PE chain flexibility was found to affect the colloid-binding affinity: stiffer PE's binding more strongly than flexible PEs. However, the definition of chain stiffness should not be conflated with polyelectrolyte persistence length especially in the cases corresponding to the resistance of the chain to bending. A Monte Carlo study of the PE binding site coupled with protein electrostatic potential modeling has further clarified these issues by identifying the nonspecific polyelectrolyte binding site on serum albumin at conditions corresponding to experiments. Examination of the binding between serum albumin and decamers of acrylamidopropanesulfonate and acrylamide of different sequences has shown that the bound decamer retains much of its configurational entropy.
Polyelectrolyte stiffness and charge sequences have profound effects on the formation of polyelectrolyte-protein coacervates, as shown by comparison of coacervates made with chitosan vs. those made with a more flexible and fully charged synthetic PE of the same structural charge density. The coacervates with chitosan differ markedly in rheology and dynamic light scattering, and SANS. These differences are explained in the context of a model in which coacervates contain protein-rich dense domains with sizes > than a few hundred nanometers. This model has been supported by fluorescence recovery after photobleaching, CryoTEM and pulsed field gradient NMR. In the context of this model, chain flexibility and the charges of ca, 50 nm polyelectrolyte-protein aggregates have been shown to affect the connectivity and size of the dense domains, which behave as transient obstacles to protein diffusion within the coacervates.