Zwitterions

ZWITTERIONS (from German Zwitter—hermaphrodite), hermaphroditic, or dual, amphoteric ions (otherwise amphiions). This name designates an ion bearing an equal amount of free positive and negative electrical charges, which is formed during the electrolytic dissociation of ampholytes (see). In the isoelectric point (see), an ampholyte yields a small and moreover equal amount of anions and cations. Towards the alkaline side from the isoelectric point, the amount of anions yielded by the ampholyte rapidly increases; towards the acidic side, the amount of cations grows accordingly. According to the theory of ampholytes, developed chiefly by Michaelis, at the isoelectric point the overwhelming majority of ampholyte molecules is in an electroneutral, undissociated state. However, Bredig, who first initiated the doctrine of ampholytes, already drew attention to the circumstance that the electroneutral form of an ampholyte can be represented not only by its undissociated molecules. Along with them, at the isoelectric point there must exist molecules that have undergone both dissociations and therefore bear free charges of opposite sign (mutually balancing each other). The structure of such a zwitterion can be expressed by the general formula ⁺NH₃-R-COO^-; obviously, it represents an isomer of the neutral, undissociated molecule NH₂-R-COOH. According to Bjerrum's investigations, in aliphatic amino acids the neutral form consists almost exclusively (over 99%) of zwitterions (in contrast to aromatic and heterocyclic amino acids—tyrosine, proline, tryptophan, histidine—which contain noticeable quantities of undissociated molecules alongside zwitterions at the isoelectric point). When accounting for the zwitter-ionic character of the neutral form of an ampholyte, incomparably greater values are obtained for the acid and alkaline dissociation constants of amino acids than those calculated on the basis of Michaelis's assumption about the almost complete absence of dissociation at the isoelectric point. The newly calculated constants represent values of approximately the same order as the dissociation constants of carboxylic acids (formic, acetic, propionic) on the one hand, and amines (methyl-, ethylamine) on the other. Compared with the latter, those unusually low values that previously had to be attributed to the dissociation constants of the amine and carboxyl in the amino acid molecule appeared completely incomprehensible. The features of the zwitter-ionic theory of ampholyte dissociation are revealed when comparing the scheme it yields with the old conception. Thus, the action of a strong acid on the neutral form of an amino acid was usually expressed by the equation: Alkali acts on the amino group and, by neutralizing it, thereby gives predominance to the acid dissociation that already existed in the neutral (zwitter-ionic) molecule. The presence of zwitterions in an isoelectric ampholyte can also be established by measuring its dielectric constant. Since a zwitterion, as its formula shows, has a large dipole moment, its presence should significantly increase the dielectric constant of the solution. Despite the substantial changes that the theory of ampholyte dissociation thus underwent, the fundamental provisions of the doctrine of the isoelectric point established by Michaelis remain valid. Zwitterions are just as incapable of conducting an electric current and migrating to electrodes as undissociated molecules. Therefore, at the isoelectric point, an ampholyte yields a maximum of electroneutral molecules, whereas with a change in pH to both sides of it, the number of anions or cations of the ampholyte rapidly grows.