Protein electrostatic factor

The rate constants for the association of proteins with one another and with other macromolecules are profoundly influenced by the geometry of the interaction and by electrostatic factors. Only a small part of each protein may be involved in the formation of a protein-protein complex, which imposes a bad steric factor on the reaction. Accordingly, protein-protein association rate constants may be as low as 104 s 1 M x (Table 4.1). But there is very fast association at > 5 X 109 s-1 AT 1 at low ionic strength for proteins that have complementary charged surfaces, such as bamase with its polypeptide inhibitor barstarfwhose, properties are discussed in Chapter 19), thrombin with its polypeptide inhibitor hirudin, and ferricytochrome c with ferrocytochrome b5. 

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). 

Some differences should be noted in the behavior of proteins and synthetic polymers. First, proteins differ in their selectivity toward different dispersed phases during flocculation. This selectivity is determined by electrostatic factors and by specific binding by sulfhydryl and amino groups of amino acid residues. Second, the biopolymer s native structure is of prime importance for promoting sorption. For... 

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. 

Retention in HIC can be described in terms of the solvophobic theory, in which the change in free energy on protein binding to the stationary phase with the salt concentration in the mobile phase is determined mainly by the contact surface area between the protein and stationary phase and the nature of the salt as measured by its propensity to increase the surface tension of aqueous solutions [331,333-338]. In simple terms the solvopbobic theory predicts that the log u ithn of the capacity factor should be linearly dependent on the surface tension of the mobile phase, which in turn, is a llne2u function of the salt concentration. At sufficiently high salt concentration the electrostatic contribution to retention can be considered constant, and in the absence of specific salt-protein interactions, log k should depend linearly on salt concentration as described by equation (4.21)... 

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