• 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