Entering and Exiting the Protein−Polyelectrolyte Coacervate Phase via Nonmonotonic Salt Dependence of Critical Conditions

Abstract

Critical conditions for coacervation of poly(dimethyldiallylammonium chloride) (PDADMAC) with bovine serum albumin were determined as a function of ionic strength, pH, and protein/polyelectrolyte stoichiometry. The resultant phase boundaries, clearly defined with this narrow molecular weight distribution PDADMAC sample, showed nonmonotonic ionic strength dependence, with the pH-induced onset of coacervation (at pH φ ) occurring most readily at 20 mM NaCl. The corresponding onset of soluble complex formation, pH c , determined using highprecision turbidimetry sensitive to changes of less than 0.1% transmittance units, mirrored the ionic strength dependence of pH φ . This nonmonotonic binding behavior is attributable to simultaneous screening of short-range attraction and long-range repulsion. The similarity of pH c and pH φ was explained by the effect of salt on protein binding, and consequently on the number of bound proteins relative to that required for charge neutralization of the complex, a requirement for phase separation. Expansion of the coacervation regime with chitosan, a polycation with charge spacing similar to that of PDADMAC, could be due to either the charge mobility or chain stiffness of the former. The pH φ versus I phase boundary for PDADMAC correctly predicted entrance into and egress from the coacervation region by addition of either salt or water. The ability to induce or suppress coacervation via protein/polyelectrolyte stoichiometry r was found to be consistent with the proposed model. The results indicate that the conjoint effects of I, r, and pH on coacervation could be represented by a three-dimensional phase boundary.

β-Lactoglobulin (BLG) binding to highly charged cationic polymer-grafted magnetic nanoparticles: Effect of ionic strength

“…The obtained results are generally consistent with some previous findings in similar systems. Binding affinitites between BLG and NPs studied by ITC display a non-monotonic trend and a maximmun binding in strength was observed at I = 50 mM slightly higher than previous work [44,47] . The strong electrostatic replusions between cationic NPs as well as the short range attraction and long range repulsion (SALR) effect between protein and NPs were considered to have a significant impact on the non-monotonic ionic strength dependence of interaction simulatenously.…”

“…Although patch is similar to that at I = 50 mM. This anomous ionic strength effect has also been observed in ion-macromolecule interaction [41] , but the phenomena are more common in protein-polyelectrolyte system probably due to the screening of the electrostatic relplusions while not the attarctions between charge patch and polymer as the ionic strength is higher than a plateau which is in the range of 10 -40 mM depending on the protein-polymer pairs but less than 100 mM [18,[42][43][44] . To better understand the maximum binding affinity at 50 mM, the interactions between NPs need to be considered as schemed in Figure 7.…”

Polyelectrolyte complexes: Bulk phases and colloidal systems

“…Somewhat weaker complexes, but important in terms of application, are those between polyelectrolytes and proteins [6][7][8][9][10][11][12][13][14][15][16][17] or even between pairs of oppositely charged proteins [18]. Compaction of (anionic) DNA is driven by the formation of complexes with (cationic) proteins, such as histones [19][20][21].…”

Protein immobilization and separation using anionic/cationic spherical polyelectrolyte brushes based on charge anisotropy

“…33,37 It is still not completely understood why cationic SPBs can selectively adsorb BSA-BLG, but not anionic SPBs. It appears that the anionic SPB is different from the cationic SPB in binding affinity, adsorption amount, architectures, and phase states for proteins, which may arise from the different electrostatic interaction behaviors between SPB and protein "charge patches" 17,[38][39][40] or "charge regulation". 24,25,[41][42][43] Nevertheless, the interactions between protein and anionic SPB as a function of pH and ionic strength have not been systematically compared with those for cationic SPBs in both qualitative and quantitative ways.…”