Zwitterform

Zwitterform is a term used to describe a zwitterionic form of a molecule, in which functional groups carry both positive and negative charges but the overall molecular charge is neutral. This internal salt-like state often arises in amphoteric compounds such as amino acids, where proton transfer between acidic and basic groups creates a positively charged site and a negatively charged site within the same molecule.

In amino acids, the zwitterionic form is common in aqueous solution at physiological pH. The amino group is protonated to form NH3+, while the carboxyl group is deprotonated to form COO−, yielding a net zero charge. This form is stabilized by intramolecular interactions and extensive hydrogen bonding with solvent and neighboring molecules. The pH at which the molecule carries no net charge is called the isoelectric point, which depends on the pKa values of the ionizable groups.

Zwitterforms are influenced by the environment. Solvent polarity, temperature, and ionic surroundings can shift the balance between the zwitterionic form and other tautomeric or neutral forms. In solid state and many biological contexts, the zwitterionic form often dominates, affecting solubility, crystallinity, and intermolecular interactions. Zwitterions tend to be highly soluble in water due to their ionic character, though their permeability and transport across membranes can differ from nonionic forms.

Examples extend beyond amino acids to other amphoteric molecules that harbor both positive and negative centers, such as certain amino alcohols and betaines. In biochemistry and pharmacology, the zwitterionic form is a key consideration for understanding reactivity, binding, and pharmacokinetic properties.