Phase behaviour and complex coacervation of aqueous polypeptide solutions

Abstract

Mixing of oppositely charged polyelectrolytes in aqueous solutions may result in the formation of polyelectrolyte complexes (PEC). Phase separation and complex coacervation of polypeptides are investigated in this study. Polypeptides have identical backbones and differ only in their charged side groups, making them attractive model systems for this work. All experiments are conducted using equal chain lengths of polycation and polyanion in order to isolate and highlight effects of the interactions of the charged groups during complexation. Complex coacervation is strongly affected by the polypeptide mixing ratio (stoichiometry), ionic strength (salt concentration), total polymer concentration, pH, and temperature. To examine the effect of these parameters on complex formation we use sample turbidity as an indicator, and optical microscopy to discriminate between coacervate and precipitate. We establish phase diagrams as a function of polybase content and salt concentration using the critical salt concentrations required to reach the coacervate to solution boundary. Additionally, we examine the effect of molecular weight on the complex formation for the P(L-Lysine) (PLys)/P(L-Glutamic acid) (PGlu) system and establish a phase diagram. By determining the water content of the coacervate phase under various conditions we find that the salt content and stoichiometry of the mixed polyelectrolytes have a significant effect on the coacervate composition.

pH and Salt Effects on the Associative Phase Separation of Oppositely Charged Polyelectrolytes

“…In particular, they did not observe a single-phase solution for ( ) ≤ 5 M for the pH = 5 case, which bears similarity to the low pH behavior we observe for the PAA-PDMAEMA-KCl and PAA-PDADMAC-KCl systems. However, Priftis and Tirrell [9] observed a somewhat different trend in their experiments on polypeptide complexation. They observed that the critical salt concentration always increased with an increase in degree of dissociation of polyelectrolytes, as we find theoretically in the absence of hydrophobicity effects ( = = 0) in Figure 4a.…”

“…They observed that the critical salt concentration always increased with an increase in degree of dissociation of polyelectrolytes, as we find theoretically in the absence of hydrophobicity effects ( = = 0) in Figure 4a. Further, all these studies [2,3,9] have reported a precipitate → coacervate transition with increase in salt concentration. The high critical salt concentrations at low pH observed in our experiments also shows resemblance to the increased stability of multilayer films containing weak polycarboxylic acids at low pH observed in an earlier experimental study [45], where the critical salt concentration required to dissolve the multilayers was found to increase dramatically at low pH.…”

“…Oparin's hypothesis has however been dismissed in modern theories of the origin of life, particularly after the discovery of DNA as the genetic material [7]. Most of the earlier studies on polyelectrolyte complexation focused on coacervation of natural polymers such as albumin and gelatin, but recent studies [2,3,[8][9][10][11][12] have extended this to the complexation of synthetic polyelectrolytes. Renewed scientific interest in polyelectrolyte complexation is due, in part, to its use in polyelectrolyte assemblies and multilayer films [13,14], which have many technological applications [15,16].…”

Microencapsulation of oil droplets using cold water fish gelatine/gum arabic complex coacervation by membrane emulsification

“…The ions released in the solution decrease the Debye length and reduce the ability of the oppositely charged polypeptides to interact (Weinbreck et al, 2003, Priftis & Tirrell, 2012.…”

Linear viscoelasticity of complex coacervates

“…The qualitative nature of turbidity-style measurements allows a phenomenological characterization of coacervate phase behavior, rather than a more direct quantification of the binodal phase space [8,15,71,[73][74][75][76][77]83,88,[91][92][93]104,105,108,112,114,. Typical characterization experiments include evaluation of the stoichiometric ratio of polycation to polyanion, the effect of increasing salt concentration, and the effect of variable pH.…”