Amino Acids: Zwitterions and pH Changes

• This page explores the reactions of amino acids to pH changes induced by adding acids or alkalis.
• The information provided applies to amino acids containing a single -NH2 and -COOH group.

Amino Acids as Zwitterions

• Amino acids possess both a basic amine group and an acidic carboxylic acid group.
• A hydrogen ion internally transfers from the -COOH group to the -NH2 group.
• Subsequently forming a zwitterion, which has both a negative and positive charge.
• Even in solid form, amino acids exist as zwitterions.
• A simple solution resulting from dissolving an amino acid in water also contains this zwitterion.
• A zwitterion is a compound that has no net electrical charge, but contains separate positively and negatively charged parts.

Adding an Alkali to Amino Acid Solution

• Adding hydroxide ions increases the pH of an amino acid solution, leading to the removal of a hydrogen ion from the -NH3+ group.
• Electrophoresis can confirm that the amino acid is now a negative ion.
• In electrophoresis, a drop of amino acid solution is placed on moistened filter paper with a crocodile clip at each end attached to a battery.
• Ninhydrin is sprayed on the paper to reveal the location of the colorless amino acid solution.
• After drying and gentle heating, the amino acid appears as a colored spot.
• The amino acid will migrate towards the anode (positive electrode).

Adding an Acid to Amino Acid Solution

• Adding acid lowers the pH of an amino acid solution, causing the -COO- part of the zwitterion to accept a hydrogen ion.
• In electrophoresis, the amino acid moves toward the cathode (negative electrode).
• Starting with the ion produced under acidic conditions and slowly adding alkali.
• The hydrogen in the -COOH group is removed before the -NH3+ group, reverting back to the zwitterion.
• The isoelectric point is the pH at which the amino acid has no net charge and doesn't move during electrophoresis.
• The isoelectric point varies between different amino acids.
• Further addition of hydroxide ions results in the removal of a hydrogen ion from the -NH3+ group.
• Adding an acid reverses the above process.
• The -NH2 group picks up hydrogen ions first, leading back to the zwitterion.

Why the Isoelectric Point is Not at pH 7

• Zwitterions interact with water molecules, acting as both an acid and a base.
• As an acid, the -NH3+ group donates a hydrogen ion to a water molecule, but the equilibrium lies to the left because it is a weak acid.
• As a base, the -COO- group takes a hydrogen ion from a water molecule, but the equilibrium also lies to the left (it is a weak base).
• The positions of the two equilibria aren't identical and depend on the influence of the "R" group.
• For simple amino acids, the first equilibrium lies further to the right than the second.
• There will be more of the negative ion from the amino acid in the solution than the positive one.
• Without modification, electrophoresis would show a slight drift of the amino acid towards the anode.
• Adding a small amount of acid to the solution reduces the amount of the negative ion until concentrations of the two ions are identical.
• The pH typically needs to be lowered to about 6 to achieve this.
• For glycine, the isoelectric point is pH 6.07; for alanine, 6.11; and for serine, 5.68.

Explore the reactions of amino acids to pH changes. Understand how amino acids form zwitterions with both positive and negative charges. Learn about the effect of adding alkalis to amino acid solutions.