Protein electrostatic interaction factor

The expected value of the electrostatic interaction factor w is given by Eq. (4). To estimate the radius R of the sphere representing the protein molecule, we can use an experimental value of the partial specific volume V and a reasonable estimate for the bound water, 5i grams per gram of protein (usually 6i 0.2 is chosen). The volume per protein molecule becomes (Tanford, 1961b)... 

If Ml is the true molecular weight of the protein, Wi the corresponding value of the electrostatic interaction factor, and Zi the corresponding charge at any pH, then, for any titratable group with given pKint,... 

Because protein-ba sed foams depend upon the intrinsic molecular properties (extent and nature of protein-protein interactions) of the protein, foaming properties (formation and stabilization) can vary immensely between different proteins. The intrinsic properties of the protein together with extrinsic factors (temperature, pH, salts, and viscosity of the continuous phase) determine the physical stability of the film. Films with enhanced mechanical strength (greater protein-protein interactions), and better rheological and viscoelastic properties (flexible residual tertiary structure) are more stable (12,15), and this is reflected in more stable foams/emulsions (14,33). Such films have better viscoelastic properties (dilatational modulus) ( ) and can adapt to physical perturbations without rupture. This is illustrated by -lactoglobulin which forms strong viscous films while casein films show limited viscosity due to diminished protein-protein (electrostatic) interactions and lack of bulky structure (steric effects) which apparently improves interactions at the interface (7,13 19). 

It is well known the tendency of polysaccharides to associate in aqueous solution. These molecular associations can deeply affect their function in a particular application due to their influence on molecular weight, shape and size, which determines how molecules interact with other molecules and water. There are several factors such as hydrogen bonding, hydrophobic association, an association mediated by ions, electrostatic interactions, which depend on the concentration and the presence of protein components that affect the ability to form supramolecular complexes. 

The electrostatic interaction between oppositely charged protein and polysaccharide can be utilized for encapsulation and delivery of hydro-phobic nutraceuticals. As a result of this interaction, we may have either complex coacervation (and precipitation) or soluble complex formation, depending on various factors, such as the type of polysaccharide used (anionic/cationic), the solution pH, the ionic strength, and the ratio of polysaccharide to protein (see sections 2.1, 2.2 and 2.5 in chapter seven for more details) (Schmitt et al, 1998 de Kruif et al., 2004 Livney, 2008 McClements et al, 2008, 2009). The phenomenon of complex... 

Fat absorption of protein additives has been studied less extensively than water absorption and consequently the available data are meager. Although the mechanism of fat absorption has not been explained, fat absorption is attributed mainly to the physical entrapment of oil (7). Factors affecting the protein-lipid interaction include protein conformation, protein-protein interactions, and the spatial arrangement of the lipid phase resulting from the lipid-lipid interaction. Non-covalent bonds, such as hydrophobic, electrostatic, and hydrogen, are the forces involved in protein-lipid interactions no single molecular force can be attributed to protein-lipid interactions ( ). 

Another property of proteins which is important in the understanding of the limits of their catalytic activity, as well as being useful in their recovery, is solubility. The solubility of globular proteins in aqueous solution is enhanced by weak ionic interactions, including hydrogen bonding between solute molecules and water. Therefore, any factor which interferes with this process must influence solubility. Electrostatic interactions between protein molecules will also affect solubility, since repulsive forces will hinder the formation of insoluble aggregates08. ... 

With more and more crystal structures becoming available, opinions arise that electrostatic interactions are an important factor conferring thermostability on proteins. This opinion is supported by the increasing number of salt bridges found in many thermostable and hyperthermostable proteins (Table 3.3) (Karshikoff, 2001) comparison of the capsid surface residues in lumazine synthase from Bacillus subtilis (mesophilic) and Aquifex aeolicus (hyperthermophilic) show significantly more charged residues than polar residues in the hyperthermophilic protein. 

---