Proteins and Other Macromolecules

 

The charge on a macromolecule or protein will arise from the ionization of the functional groups that are contained by the molecule. In proteins these groups can give rise to either positive or negative charges. In a medium of acid pH the net charge on a protein is usually be positive (and will, thus, the protein molecule will behave as cations), whereas if the medium is basic then the charge on the protein will be predominantly basic (and thus, the protein molecule will behave as behave a anion). At a certain pH the protein will have no net charge and this is called the isionic point. This is not the same and will not necessarily coincide with zero electrophoretic mobility, which is called the isoelectric pont. This is because there is usually an adsorption of free ions from solution onto the molecular surface of the protein, which modifies the net effective charge. The adsorption of ions from bulk liquid onto the surface of the macromolecules forms a double layer, the two layers being oppositely charged.

 

The ion surface adsorption results in the net surface charges on many dispersive (hydrophobic ) substances which have no natural charge such as hydrocarbon droplets or air bubbles. Most surface charges that result from adsorption give a negative property to the material as it is due to the adsorption of anions, however, the converse can occur. For example colloidal silver iodide particles in the presence of excess negative iodide ions acquires a net negative charge, whereas if surrounded by an excess of positively charged silver ions assume a positive charge.

 

The formation of the classic double layer is depicted in figure 1.

 

 

Figure 1. The Formation of a Charged Double Layer on the Surface of Macromolecule

 

It is seen in figure 1, that the movement of a macromolecule induced by the effect of an electric field on the surface charges on the molecule (the inner layer of the double layer), is retarded by two other processes. There is the opposing effect of the electric field on the outer layer of the double layer and the viscous drag on the molecule through the electrolyte.