Polyelectrolyte-protein complexes coacervation

Complex coacervation is a phase-separation process, in which two water-soluble polymers are brought to physicochemical conditions leading to their desolvation. The desolvated polymers separate from the equilibrium phase, in the form of a coacervate, that tend to deposit onto dispersed core materials. The polymers forming the coacervate can be two polyelectrolytes susceptible of bearing opposite net charges, and the dispersed cores can be oily droplets of an L/H emulsion. Pectin has been associated to soy proteins to encapsulate propolis, a polyphenol-rich compound produced by bees, for stabilization, water dispersibility, and controlled release in food (Figure 36.9). ... 

Complex coacervates may be formed by synthetic polyelectrolytes, but also by biological polyelectrolytes, such as proteins, polysaccharides, and polynucleotides. The complexation allows for combining two or more desirable properties and/or to provide additional stability for otherwise highly labile functional biopolymers, such as proteins (cf. Chapter 13). 

Polyanions and polycations can co-react in aqueous solution to form polyelectrolyte complexes via a process closely linked to self-assembly processes [47]. Despite progresses in the field of (inter-) polyelectrolyte complexes [47] (IPEC from Gohy et al. [48], block ionomer complexes BIC from Kabanov et al. [49], polyion complex PIC from Kataoka and colleagues [50, 51], and complex coacervate core micelles C3M from Cohen Stuart and colleagues [52], understanding of more complex structures such as polyplexes (polyelectrolyte complexes of DNA and polycations) [53] is rather limited [54]. It has also to be considered that the behavior of cationic polymers in the presence of DNA and their complexes can be unpredictable, particularly in physiological environments due to the presence of other polyelectrolytes (i.e., proteins and enzymes) and variations in pH, etc. 

Several recent reviews deal with the fundamental self-assembly between proteins and natural polyelectrolytes, e.g. DNA and polysaccharides [30, 31, 34, 111], The applications in the food sector of protein and polysaccharide complexes and coacervates are also well covered elsewhere [35,112]. Given these abundant recent reviews, this field is deliberately excluded from the present review. 

In the study of BSA-PDMDAAC complex [34], it was found that the initial complexation pH is insensitive to the concentration of BSA and PDMDAAC. However, the coacervate of the complex formed at phase separation is not reversible at low BSA PDMDAAC ratio. For several protein-polyelectrolyte complexes, Kokufuta [9, 20] found that the amount of polyelectrolyte needed to precipitate a protein is linearly proportional to the amount of the protein in solution. The concentration dependence for the efficiency of protein-polyelec-trolyte phase separation was also reported by Morawetz [46] for several other systems. 

Soluble protein-polyelectrolyte complexes are usually formed at a pH close to the protein lEP, and the soluble complexes aggregate to form coacervates toward phase separation by adjusting pH. Dubin [16, 34] suggested the existence of the primary intrapolymer complexes (in which a single polymer chain is bound by several protein molecules) for the excess protein case. These primary complexes could aggregate to form interpolymer complexes, in which several polymer chains are involved, as shown in Fig. 15.19. The interpolymer complex could also be soluble up to the point of the large scale aggregation. 

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