Phase separation protein + surfactant

Effect of surfactant type and concentration An increase in surfactant concentration results in an increase in the number of micelles rather than any substantial change in size, and this enhances the capacity of the reverse micelle phase to solubilize proteins. Woll and Hatton [24] observed increasing protein solubilization in the reverse micelle phase with increasing surfactant concentration. In contrast, Jarudilokkul et al. [25] found that at low minimal concentrations (6-20 mmol dm AOT), reverse mieelles eould be highly seleetive in separating very similar proteins from... 

Though CPE has many advantages [10], some problems remain to be solved such as (1) limited number of surfactants, (2) high cloud-point, and (3) strong hydrophobic nature. In an attempt to overcome some of these limitations, Tani et al. [281] proposed a new method that involves solubilization of hydrophobic membrane proteins into aqueous micellar solutions of alkylglucosides, followed by phase separation induced by the addition of a water-soluble polymer such as poly(ethylene)glycol (PEG) and dextran T-500. Using this approach they could carry out the whole procedure from solubilization to phase separation at 0 °C. 

Figure 8.9 Phase separation in a mixed layer of protein + surfactant from Brownian dynamic simulation. In the picture are cross-linked protein-like particles (black) and surfactant-like displacer particles (grey). Reproduced from Wijmans and Dickinson (1999b) with permission.

In Part Four (Chapter eight) we focus on the interactions of mixed systems of surface-active biopolymers (proteins and polysaccharides) and surface-active lipids (surfactants/emulsifiers) at oil-water and air-water interfaces. We describe how these interactions affect mechanisms controlling the behaviour of colloidal systems containing mixed ingredients. We show how the properties of biopolymer-based adsorption layers are affected by an interplay of phenomena which include selfassociation, complexation, phase separation, and competitive displacement. 

Based upon the use of nonionic surfactant systems and their cloud point phase separation behavior, several simple, practical, and efficient extraction methods have been proposed for the separation, concentration, and/or purification of a variety of substances including metal ions, proteins, and organic substances (429-441. 443.444). The use of nonionic micelles in this regard was first described and pioneered by Watanabe and co-workers who applied the approach to the separation and enrichment of metal ions (as metal chelates) (429-435). That is, metal ions in solution were converted to sparingly water soluble metal chelates which were then solubilized by addition of nonionic surfactant micelles subsequent to separation by the cloud point technique. Table XVII summarizes data available in the literature demonstrating the potential of the method for the separation of metal ions. As can be seen, factors of up to forty have been reported for the concentration effect of the separated metals. 

Gunning, A.R, Mackie, A.R., Kirby, A.R., and Morris, V.J. Scanning near-field optical microscopy of phase separated regions in a mixed interfacial protein (BSA) surfactant (Tween 20) film, Langmuir, 17, 2013, 2001. 

Reversed Micellar Extraction This scheme involves use of microscopic water-in-oil micelles formed by surfactants and suspended within a hydrophobic organic solvent to isolate proteins from an aqueous feed. The micelles essentially are microdroplets of water having dimensions on the order of the protein to be isolated. These stabilized water droplets provide a compatible environment for the protein, allowing its recovery from a crude aqueous feed without significant loss of protein activity [Ayala et al., Biotechnol. and Bioeng., 39, pp. 806-814 (1992) and Bordier, J. Biolog. Chem., 256(4), pp. 1604-1607 (February 1981)]. Also see the discussion of ultrafiltration membranes for concentrating micelles in Liquid-Liquid Phase Separation Equipment. ... 

Figure 8 Schematic of some possible displacement reactions of proteins from an interface. In (a) one type of protein is displaced by a second the displaced protein may be either denatured or in a form close to native (b) displacement by small-molecule surfactant -the displaced protein may adopt one of several forms, including ones in which a complex is formed with the surfactant (c) as some protein is displaced, there is phase separation on the interface between adsorbed protein and the displacing surfactant.

The possibility of the formation of nonisotropic surfaces on some types of emulsion droplets has been recently demonstrated. On interfaces of protein which have been treated with small-molecule emulsifiers, the protein is displaced. However, when insuffi dent emulsifier is added to cause desorption of all of the protein, there is a tendency for the different surfactants to form regions (i.e., to phase separate on the interface) (122). Clearly, such an interface offers the opportunity for directed aggregation because of the anisotropy of the surface. However, it depends on the presence of at least two surfactants. 

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