Amino acids have a carboxyl group, amine group, hydrogen and a side chain bound to a central carbon (in proline, the amine group and the side chain have also formed a ring). Because the pKa
of carboxylic acids tends to be less than 7 while the pKa
of amines tends to be greater than 7, at neutral pH, the carboxyl group of an amino acid will strongly favor the base form (negatively charged carboxylate) while the amine group will tend to be in the acid form (positively charged ammonium). For this reason, the typical amino acid with a neutral side chain will exist as a zwitterion at neutral pH, the carboxylate group negatively charged and the ammonium group positively charged. A zwitterion is a chemical compound that is electrically neutral but carries formal positive and negative charges on different atoms.