Amino Acids (Cambridge (CIE) A Level Chemistry): Revision Note

Acid / Base Properties of Amino Acids, Zwitterions & the Isoelectric Point

• 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 acid where the amine (-NH₂) group is bonded to the carbon atom next to the -COOH group

• These types of amino acid 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

In most amino acids, the amine group is bonded to the carbon directly adjacent to the carboxylic acid group

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

  • Acidic

  • Basic

  • Neutral

Example amino acids with different R groups

The R group in amino acids can be acidic (aspartic acid), basic (serine) or neutral (alanine)

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

Zwitterion formation within an amino acid

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

The effect of changing pH on zwitterions

An amino acid solution 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 an amino acid

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