AQA A Level Chemistry

Revision Notes

7.7.1 Amino Acids

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

Nitrogen Compounds - General Structural Formula of Amino Acids, downloadable AS & A Level Chemistry revision notes

General structural formula of amino acids

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

 

Nitrogen Compounds - Different Types of Amino Acids, downloadable AS & A Level Chemistry revision notes

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

Nitrogen Compounds - Zwitterion, downloadable AS & A Level Chemistry revision notes

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

Nitrogen Compounds - Buffer Solution of Amino Acids, downloadable AS & A Level Chemistry revision notes

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

 

Nitrogen Compounds - Isoelectric Point of Amino Acids, downloadable AS & A Level Chemistry revision notes

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

Exam Tip

Naturally occurring amino acids are usually referred to by a traditional name and a three letter code as the IUPAC systematic names can be rather complicated. For example, aspartic acid, serine and alanine are abbreviated to Asp, Ser and Ala, respectively.

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