amino acid zwitterion

An amino acid carries two opposite chemical personalities in one small molecule: an acidic carboxylic acid group (-COOH) and a basic amine group (-NH2), both attached to the same central carbon. With an acid and a base sharing a molecule, the obvious thing happens — the acid hands its proton to the base. The result is the zwitterion: a single molecule that is positively charged at one end and negatively charged at the other, yet electrically neutral overall. 'Zwitterion' comes from the German for 'hybrid ion.'

Here is the proton transfer in plain steps. The -COOH group is a reasonable acid (its proton wants to leave); the -NH2 group is a reasonable base (it wants to grab a proton). Rather than reaching out to the solvent, the carboxyl simply donates its H+ to its own amine neighbour. The carboxyl becomes -COO- (negative) and the amine becomes -NH3+ (positive). So an amino acid in water or in the solid state is not really the neutral H2N-CHR-COOH you draw on paper; it is the zwitterion +H3N-CHR-COO-. Which charges are present depends on pH: in strong acid both ends are protonated (net positive), in strong base both are deprotonated (net negative), and at one special pH in between — the isoelectric point — the molecule is exactly the balanced, net-neutral zwitterion.

This dual-ion character explains the otherwise puzzling physical behaviour of amino acids and is pure acid-base chemistry you already know. Because they are effectively internal salts, amino acids are high-melting crystalline solids that dissolve well in water and poorly in nonpolar solvents — they behave like ionic compounds, not like ordinary small organic molecules. The same -COOH and -NH2 groups are exactly the partners that join into the peptide (amide) bond, so this little acid-base molecule is the monomer from which every protein is built.