zwitterione

A zwitterion is a molecule that contains both positive and negative charges but is overall electrically neutral. The charges are usually localized on different atoms within the same molecule. Zwitterions are especially common for amino acids at physiological pH, where the amino group is protonated to form an ammonium group (−NH3+) and the carboxyl group is deprotonated to form a carboxylate (−COO−), creating an internal salt.

The isoelectric point (pI) is the pH at which a molecule carries no net charge. For amino acids, the zwitterionic form dominates near the pI; below this pH the molecule tends to be positively charged, and above it tends to be negatively charged. Zwitterions are highly polar, which gives them high water solubility and relatively low volatility, and they often have limited ability to cross lipid membranes. In solid form, many amino acids exist predominantly as zwitterions, and in solution the equilibrium favors the zwitterionic form around physiological pH.

Zwitterions are not restricted to amino acids. They also occur in other classes of compounds such as amino alcohols, betaines, and certain detergents. The internal charge distribution can influence acidity and basicity, conformational preferences, and intermolecular interactions, affecting reactivity and stability. Zwitterionic character is exploited in analytical techniques like electrophoresis and in materials science through zwitterionic polymers and electrolytes, illustrating how internal salt formation modulates solubility, interactions, and function in chemical and biological systems.