Coacervation in Biopolymers

Abstract

Polyelectrolyte charge; Ionic strength; Surface patch binding; Viscoelastic behavior and internal structure IntroductionCoacervation: Coacervation is a thermodynamic transition which allows a homogeneous solution of charged macroions to undergo liquidliquid phase separation, giving rise to a polymer-rich dense phase coexisting with its supernatant. These two liquid phases are immiscible but are thermodynamically compatible. The polymer-rich dense phase is often called the coacervate. Structurally it lies between the crystalline and liquid phases. Thus, it can bear intermediate range structural order and can be referred to as a mesophase. Coacervation has been mostly studied in aqueous solutions of charged synthetic or biological macromolecules in the past. In particular, protein-polysaccharide and protein-protein coacervates have attracted much attention because of their inherent potential in generating new biomaterials. In addition, such studies provide basic understanding of specific and non-specific interactions operating between complementary polyelectrolytes [1-6] or polyelectrolyte-polyampholyte [7][8][9][10][11][12][13][14][15] pairs. Normally, polysaccharides are strong polyelectrolytes whereas proteins, in addition, can be polyampholytes. Hence, the association problem reduces to that of the general study of interaction between polyelectrolyte (PE) and polyampholyte (PA) molecules [6,16]. A recent review encapsulates many of the anomalous as well as the salient features of protein-polyelectrolyte interactions [1] The phenomenon of protein based coacervates, formed of strong electrostatic interactions, has been reported for β-lactoglobulin-gum Arabic [7,8] [17]. The diversity of material properties associated with coacervates can be gauged from the fact that β-lactoglobulin-gum arabic coacervates were found to be associated with vescicular to sponge-like internal structure whereas whey protein-gum arabic coacervate was observed to be a highly concentrated (melt-like) phase. In contrast, *Corresponding author: Kamla Rawat, Special Center for Nanosciences, Jawaharlal Nehru University, New Delhi 110067, India, Tel: +91 11 2670 4699; Fax: +91 11 2674 1837; E-mail: kamla.jnu@gmail.com β-lactoglobulin-pectin coacervates were found to be a heterogeneous phase comprising of pectin networks with protein domains forming the junction points [17]. It has been shown that a polyelectrolyte, DNA and a polyampholyte, gelatin can undergo associative interaction and form complex coacervates with interesting thermal properties [15]. Further, it has been realized that in a class of systems coacervation transition is governed by surface selective patch binding even though both the polyions carry similar net charge [11,18,19]. In particular, in SPB interactions complementary polyions (normally a PA-PE pair) seek oppositely charged patches to bind overcoming the repulsion occurring between similarly charged surface patches. This is often referred to as binding on the wrong side of pH.The following provides a brief structural i...

Complex coacervate‐based materials for biomedicine

“…Characterization of coacervation has commonly been performed using methods such as turbidimetry and/or light scattering that take advantage of the light scattered by the presence of coacervate droplets in solution. Turbidimetry, in particular, is amenable for high‐throughput experiments designed to examine the effect of multiple variables such as charge stoichiometry, ionic strength, and pH on coacervate phase behavior . These types of measurements are typically paired with observations via optical microscopy to determine whether the observed phase separation is the result of liquid complex coacervation or the formation of a solid precipitate, as well as verifying the location of observed phase boundaries.…”

Gelatin-Alginate Complexes for EGF Encapsulation: Effects of H-Bonding and Electrostatic Interactions

“…Once protein drugs are encapsulated in the coacervate phase, they could be protected from the external environment and maintain their bioactivity [6,7,8,9,10,11,12]. Coacervation is a spontaneous formation of a dense liquid phase from interactions of complementary macromolecular species [13,14]. For complex coacervation to occur by electrostatic interactions, the reaction pH should be adjusted so that the net charges of cationic and anionic polyelectrolytes are neutralized to form a polymer-rich liquid phase [13,15,16].…”

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.…”