Properties of Amino Acids

Amino acids are colourless, water-soluble crystalline solids. Because each molecule carries both a carboxyl group and an amino group, their acid-base behaviour differs from that of ordinary amines or ordinary carboxylic acids taken alone. The carboxyl group can lose a proton to become negatively charged (-COO⁻), while the amino group can accept a proton to become positively charged (-NH₃⁺), and which of these happens -- and to what extent -- depends on the pH of the surrounding solution. At one specific pH, called the isoelectric point, the number of positive and negative charges on the molecule exactly balance, giving it a net charge of zero. Above the isoelectric point the amino acid carries a net negative charge, and below it, a net positive charge.

In aqueous solution, in fact, the proton from the carboxyl group is transferred internally to the amino group even at neutral pH, leaving the two groups with opposite charges -H₃N⁺ and -COO⁻ on the same molecule at the same time. Despite carrying both a positive and a negative charge, the molecule as a whole is electrically neutral, and this doubly-charged-yet-neutral species is called a zwitterion (from the German for 'hybrid ion'); the amphoteric behaviour of amino acids arises directly from this zwitterionic character. At acidic pH (below the isoelectric point) the amino group is protonated and the carboxyl group is not yet ionised, giving a net charge of +1; at the isoelectric point the classic zwitterion (H₃N⁺-CH(R)-COO⁻) dominates, with a net charge of 0; and at alkaline pH (above the isoelectric point) the amino group has lost its proton and the carboxyl group is ionised, giving a net charge of -1. …