zwitterionilisation

Zwitterionilisation refers to the process by which a molecule containing both acidic and basic functional groups, such as an amino acid, can exist as a zwitterion. A zwitterion is a neutral molecule that has an equal number of positive and negative electrical charges distributed within its structure. This state is typically achieved in aqueous solutions at a specific pH value known as the isoelectric point (pI). At the isoelectric point, the molecule carries no net charge, and its solubility is often at its minimum.

The process of zwitterionilisation involves the internal transfer of a proton. For example, in an amino acid like glycine, the carboxyl group (-COOH) is acidic, while the amino group (-NH2) is basic. In a neutral solution, the carboxyl group can donate a proton, becoming a negatively charged carboxylate ion (-COO-), and the amino group can accept a proton, becoming a positively charged ammonium ion (-NH3+). The resulting molecule, H3N+-CH2-COO-, is a zwitterion. The pH of the surrounding environment dictates the extent of protonation and deprotonation of these groups, thus influencing whether the molecule exists as a cation (at low pH), an anion (at high pH), or a zwitterion (at the isoelectric point). This phenomenon is crucial in understanding the behavior and properties of amino acids and other amphoteric compounds in biological and chemical systems.