An amino acid contains an acidic group (-COOH) and a basic group (-NH2) in the same molecule. These can react with each other: a proton moves from the carboxylic acid group to the amine group. The result is a zwitterion, an ion with both a positive and a negative charge but no overall charge:H2NCH(R)COOH → +H3NCH(R)COO− Amino acids exist as zwitterions in the solid state. This explains their properties: the zwitterions are held in a lattice by strong ionic attractions, so amino acids are crystalline solids with high melting temperatures (many decompose before melting), and they are soluble in water.The form an amino acid takes in solution depends on the pH:
In alkali (high pH), the NH3+ group loses a proton, giving a negative ion, H2NCH(R)COO−.The pH at which the amino acid exists mainly as the zwitterion, with no overall charge, is its isoelectric point. Each amino acid has its own isoelectric point, which depends on its side chain. For glycine it is about 6.0.Worked example: glycine is placed in a buffer at pH 10. This is above its isoelectric point, so glycine is present mainly as H2NCH2COO− and carries an overall negative charge. At pH 2 it would be mainly +H3NCH2COOH, with an overall positive charge.
In alkali (high pH), the NH3+ group loses a proton, giving a negative ion, H2NCH(R)COO−.The pH at which the amino acid exists mainly as the zwitterion, with no overall charge, is its isoelectric point. Each amino acid has its own isoelectric point, which depends on its side chain. For glycine it is about 6.0.Worked example: glycine is placed in a buffer at pH 10. This is above its isoelectric point, so glycine is present mainly as H2NCH2COO− and carries an overall negative charge. At pH 2 it would be mainly +H3NCH2COOH, with an overall positive charge.