Proteins DLVO theory

Proteins are both colloids and polymers. Therefore, attempts have been made to understand the phenomenon of protein aggregation with the help of models from the polymer and colloid fields such as DLVO theory, describing the stability of colloidal particles, or phase behavior and attraction-repulsion models from polymers (De Young, 1993). For faster progress, more phase diagrams for equilibrium protein precipitation, in both the crystalline and the non-crystalline state, as well as more data on observations of defined protein oligomers or polymers, are required. 

Sauce bearnaise, for example, is an O/W emulsion that is mainly stabilized by egg-yolk protein in an aqueous phase of low pH. Perram et al. [830] describe how this system is primarily stabilized by electrostatic repulsive forces, and show how DLVO theory can be used to describe the effects of pH, surface charge, ionic strength, and temperature, on the stability of this emulsion. 

Some products, like butter and margarine are stabilized by fat crystals. Salad dressings and beverage emulsions are stabilized by other emulsifiers. The stability of non-protein stabilized food emulsions, involving lower molar mass type molecules, tend to be better described by the DLVO theory than are protein-stabilized emulsions. An example of an O/W emulsifier whose emulsions are fairly well described by DLVO theory is sodium stearoyl lactylate [812],... 

An important issue that has to be emphasized is that the experimentally determined dependence ofthe stability ratio on electrolyte concentration, at low ionic strengths, exhibits (at least for NaSCN) a strongly non-DLVO behavior, in a range in which the DLVO theory is considered fairly accurate. Therefore, we are inclined to believe that the electrolyte (even at low ionic strength) induces indeed structural modifications of the adsorbed protein layer at least near the interface. 

The strong discrepancy between experiment and the traditional DLVO theory at low ionic strengths (where the latter theory is considered to be accurate) cannot be explained by additional interactions between ions and surfaces, because they are negligible below 0.01 M. Therefore, we are inclined to believe that the structural modification of the adsorbed protein by the addition of a structure breaking ion, such as SCN" is mainly responsible for the quantitative disagreement between experiment and model calculations. The nonuniformity ofthe colloidal particles may be also responsible for the disagreement. 

In order to elucidate adhesion phenomena occurring among biological cells, a number of studies on membrane adhesion using model membrane systems (such as lipid membranes as well as lipid membranes with incorporated proteins) have recently been carried out. Some of these aggregation phenomena have been analyzed from the viewpoint of the DLVO theory 2 " -" ... 

The principles of colloid stability, particularly including DLVO theory and steric stabilization, can be applied to many food emulsions [80,81]. The applicability of DLVO theory is restricted, however, partly because the primary potential-energy minima are somewhat shallow and partly due to the tendency of adsorbed proteins extend outward from surfaces so far that steric stabilization becomes more important [34,126]. The presence of protein in an adsorption layer can also contribute a viscoelastic restriction to coalescence. Finally, the oils in food colloids are usually triglycerides (of either animal or vegetable origin). These oils may exist in liquid or crystalline states at room temperature frequently both simultaneously. The existence of the crystal form at interfaces contributes yet another stabilizing component [34]. 

Casein or egg-yolk proteins are used as emulsifiers in another category of O/W food emulsions [34,126]. A key difference here is that in these caseinate-stabilized oil emulsions, the casein forms essentially monolayers and there are no casein micelles or any calcium phosphate. Such emulsions are thought to be stabilized more by electrostatic repulsive forces and less by steric stabilization [126]. Similarly, mayonnaise, hollandaise, and beamaise sauces, for example, are O/W emulsions mainly stabilized by egg-yolk protein [34,129], Here, the protein-covered oil (fat) droplets are stabilized by a combination of electrostatic and steric stabilization [129]. Perram et al. [130] described the application of DLVO theory to emulsion stability in sauce beamaise. 

Salad dressings and mayonnaise can be stabilized by ionic surfactants, which provide some electrostatic stabilization as described by DLVO theory, or by nonionic surfactants which provide a viscoelastic surface coating. The protein-covered oil (fat) droplets tend to be mostly stabilized by steric stabilization (rather than electrostatic stabilization) [34,126,129], particularly at very high levels of surface protein adsorption, in which case the adsorption layer can include not just protein molecules but structured protein globules (aggregates). In some cases, lipid liquid crystal layers surround and stabilize the oil droplets, such as the stabilization of O/W droplets by egg-yolk lecithins in salad dressing [34,135]. 

The interaction between proteins can, in the simplest approximation, be described as that between charged hard spheres, and the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (Section 3.5.2) can be used to describe the... 

The thermodynamic model is, like the DLVO theory, only applicable for the adhesion in vitro. Both models are based upon non-specific interactions occurring between particles (cells) and solid surfaces. In vivOr or under in vivo like conditions, specific interactions also have to be taken into account. Such specific interactions have been shown to mediate adhesion between bacteria and natural substrata, such as adhesion of streptococci to dental enamel,29 y adhesion of E. coli to uroepithelial cells.30 Although not clearly demonstrated for the bacterial adhesion to synthetic polymers, it is highly possible that specific interactions, e.g. between bacterial surface proteins and protein layers adsorbed on the polymer surface, play an important role as well. 

An exciting application of colloid aggregation and DLVO theory is shown in the case study Diseases caused by protein aggregation shown next. 

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