Self-association at interfaces

Protein-Protein Interactions in Adsorbed Layers 1.1. Self-Association at Interfaces 

Globular proteins form close-packed monolayers at fluid interfaces. Hence a large contribution to the adsorbed layer viscoelasticity arises from short-range repulsive interactions between hard-sphere particles. In addition to, or instead of, this glass-like5 structure from hard spheres densely packed in two dimensions, many adsorbed proteins can exhibit attractive interactions leading to a more gel-like5 network structure. Hence the mechanical properties of an adsorbed layer depend on many 

The elasticity of the protein layer structure is supposed to act against the tendency of an emulsion or foam to collapse because it allows the stretching of the interface. This behaviour is most commonly observed for globular proteins, which adsorb, partially unfold, and then develop attractive protein-protein interactions (Dickinson, 1999a Wilde, 2000 Wilde et al., 2004). The strength of such an adsorbed layer, reflected in the value of the elastic modulus, and the stress at which the structure breaks down, can be successfully correlated with stability of protein-based emulsions and (more especially) protein-based foams (Hailing, 1981 Mitchell, 1986 Izmailova et al., 1999 Dickinson, 1999a). 

Key points relating to the adsorption behaviour of proteins and their influence on colloid stability are summarized below (Dickinson, 1999a). 

In accord with experiments on emulsions (Husband et al., 1997), the molecular configurations deduced from SCF calculations have demonstrated the crucial role of the cluster ( blob ) of 5 charged phosphoserine residues in p-casein in maintaining the steric stabilizing layer, whilst also preventing interfacial precipitation (multilayers). The mobility of this blob was demonstrated experimentally by P NMR measurements on P-casein-stabilized emulsions (ter Beek et al., 1996). It was inferred that, when the effective charge on the blob is reduced (by dephosphorylation) or screened (by salt addition), the macromolecular spring relaxes 

---