Protein molecules, electrostatic repulsion

With large solute molecules (surfactants, proteins), mutual electrostatic repulsion of interfaces may also have an effect. Solid particles can also have a substantial influence. In some reacting, boiling or biological systems, foaming can readily occur. 

In recent years, it has been shown (Douzou and Maurel, 1976) that some proteins can behave as polyanions or polycations, and the stability of their solid state might be endangered at lower salt concentration due to repulsive forces between protein molecules. Much more important is the problem of enzyme activity in crystals suspended in cooled mixed solvents as a consequence of cosolvent- and temperature-induced changes in salt concentration and therefore in electrostatic potentials. 

The solubility at the isolelectric point is lower than the solubility at pH values away from pi. One way of explaining this is that the electrostatic repulsion between different protein molecules is at a minimum at pi. 

Based on a recent isothermal mixing calorimetric study (IFin et al., 2005) it was postulated that, when both surfactant micelles and protein carry a rather high like net charge, then the micelles cannot take part in interactions with protein on account of the strong electrostatic repulsion between them. Hence, in this case, the interactions occur only between the protein and the individual surfactant molecules which are in equilibrium with their micelles. This behaviour is reflected in the lack of any change in the thermodynamic character (heat effects in the case of iso-... 

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. ... 

High ionic strength, adjusted by the addition of 1M MgCl2 at pH 5.0, was reported to increase significantly the adsorption of native and mutated CBH I to cellulose, whereas the addition of 1M NaCl only affected the adsorption of native CBHI with intact CBD. The activity of the CBD-less CBH I (CBH core protein) on crystalline cellulose was also increased (Reinikainen et al., 1995). The improved binding was explained by reduced electrostatic repulsion between protein molecules, which possibly was masked by the salt ions. 

Flocculation is an entirely colloidal phenomena where the inter-ictlon Bitween protein molecules is determined by the balance between electrostatic repulsion due to the electric double layer and van der Waals attraction (2). 

At the second critical pH (pH,, ), which is usually below the protein isoelectric point, strong electrostatic interaction between positively charged protein molecules and anionic polysaccharide chains will cause soluble protein/polysaccharide complexes to aggregate into insoluble protein/polysaccharide complexes. For negatively charged weak acid-based (e.g., carboxylic acid) polysaccharides like pectin, with the decrease of pH below the pKa of the polysaccharide, protein (e.g., bovine serum albumin (BSA))/polysaccharide (e.g., pectin) insoluble complexes may dissociate into soluble complexes, or even non-interacted protein molecules and polysaccharide chains, due to the low charges of polysaccharide chains as well as the repulsion between the positively charged proteins (Dickinson 1998). 

What is the effect on these repulsive electrostatic interactions of changing from an aqueous to a nonaqueous solvent Does the result contribute to the stabilization, the further ordering, or the disordering of the native aqueous conformation of a protein molecule It is important to emphasize that we wish to compare at this point the native conformation of the protein molecule in the aqueous and the nonaqueous solvents. Since, as will be shown subsequently, the native conformation is disrupted in most nonaqueous solvents, it is a hypothetical state not experimentally attainable. Nevertheless, the question just asked can be explored theoretically. 

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