zwitterion

A zwitterion is a molecule that contains both positive and negative charges but carries no net electric charge. The term is commonly applied to amino acids in physiological conditions, where an amino group is protonated and a carboxyl group is deprotonated, producing a positively charged ammonium group (NH3+) and a negatively charged carboxylate (COO−).

This internal charge separation is usually the result of proton transfer within the molecule and depends on pH. At low pH the molecule is fully protonated with net positive charge; at high pH it is deprotonated with net negative charge. At the isoelectric point, the molecule exists predominantly as a zwitterion with zero net charge.

Zwitterions have high polarity and solubility in water, and they possess a significant dipole moment. They play a central role in biochemistry: most amino acids are zwitterionic at physiological pH, influencing protein folding, acid-base buffering, and transport.

The concept extends beyond amino acids to other dipolar ions that contain both positive and negative centers. The term zwitterion is of German origin, from Zwitter, meaning 'hybrid' or 'double,' with the suffix ion.