Amino Acids (Oxford AQA International A Level Chemistry)

Amino Acids

• Amino acids are the building blocks of proteins

• They are organic compounds that contain two functional groups:

  • A basic amino (-NH₂) group

  • An acidic carboxylic acid (-COOH) group

General structure of an amino acid

• 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

Examples of 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

The structure of 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

• 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

Structures of amino acids in acidic and basic conditions

The isoelectric point