7.6.9 Amino Acids

• Amino acids are organic compounds that contain two functional groups:
  • A basic amino (-NH₂) group
  • An acidic carboxylic acid (-COOH) group

• Due to the presence of both a basic and acidic group in amino acids, they are said to be amphoteric
  • They can act as both acids and bases

Naturally occurring amino acids

• 2-aminocarboxylic acids are a type of amino acids in which the amine (-NH₂) group is bonded to the carbon atom next to the -COOH group
• These type of amino acids form the ‘building blocks’ that make up proteins
• There are 20 naturally occurring amino acids with the general structural formula of RCH(NH₂)COOH

General structural formula of amino acids

• The R group varies in different amino acids and can be:
  • Acidic
  • Basic
  • Neutral

The R group varies in different amino acids

Acid / base properties of amino acids

• Amino acids will undergo most reactions of amines and carboxylic acids including acid-base reactions of:
  • Amines with acids
  • Carboxylic acids with bases

• However, they can also interact intramolecularly (within themselves) to form a zwitterion
• A zwitterion is an ion with both a positive (-NH₃⁺) and a negative (-COO^-) charge
• Because of these charges in a zwitterion, there are strong intermolecular forces of attraction between amino acids
  • Amino acids are therefore soluble crystalline solids

An amino acid molecule can interact within itself to form a zwitterion

Isoelectric point

• A solution of amino acids in water will exist as zwitterions with both acidic and basic properties
• They act as buffer solutions as they resist any changes in pH when small amounts of acids or alkali are added
• If an acid is added (and thus the pH is lowered):
  • The -COO^- part of the zwitterion will accept an H⁺ ion to reform the -COOH group
  • This causes the zwitterion to become a positively charged ion

• If a base is added (and thus the pH is raised):
  • The -NH₃⁺ part of the zwitterion will donate an H⁺ ion to reform the -NH₂ group
  • This causes the zwitterion to become a negatively charged ion

A solution of amino acids can act as a buffer solution by resisting any small changes in pH

• The pH can be slightly adjusted to reach a point at which neither the negatively charged or positively charged ions dominate and the amino acid exists as a neutral zwitterion
  • This is called the isoelectric point of the amino acid

The isoelectric point of amino acids is the pH at which the amino acid exists as a neutral zwitterion