Binding of polyanions to carrier ampholytes in isoelectric focusing.

The binding of carrier ampholytes to polyanions is markedly pH-dependent: it is very strong at pH 3, rather weak at pH 5 and abolished at pH 7. Binding is affected by the type of negative charge, its density and spatial orientation on the polyanion. On the basis of the type of negative charge, the binding strength decreases in the following order: polyphosphate greater than polysulphate greater than polycarboxylate. Given the same type of negative charge, the binding is dependent on charge density and its space orientation: thus polyglutamic acid forms stronger complexes than polygalacturonic acid. The minimum length of the polyanion eliciting a measurable binding appears to be of the order of about six negative charges, as demonstrated with hexametaphosphate.