Solubility and Zwitterion Structure

Like the constituent amino acids, proteins and peptides are amphoteric and exist in the solid state as zwitterions (10.40) since they contain terminal amino and carboxylate groups. In solution the state of ionisation is pH dependent. At very acid pH the molecules are in form (10.40a), at very alkaline pH they are in form (10.40c), and at intermediate pH values mixtures of (a), (b) and (c) are present. 

Molecules in form (10.40b) are isoelectric and have equal numbers of + and - charges making them electrically neutral. At a particular pH value, called the isoelectric point (pi), which is characteristic of each type of amino acid, the systan will behave as if it was electrically neutral and no migration will occur in an electric field. This concept is frequently extended to proteins which are said to have an average isoelectric point where their net charge becomes zero. 

Like the individual amino acids, proteins are least soluble at their (average) isoelectric points. This means that soluble or dispersed proteins may precipitate or aggregate when the pH of their environment becomes equal to that of the isoelectric point (Table 10.9). 

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