5.5.7 Characteristic Behaviour 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

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

Reactions of the amine group

• The amine group is basic and reacts with acids to make salts
• For example, a general amino acid reacts with hydrochloric acid to form the ammonium salt:

H₂NCHRCOOH + HCl ⇌ H₃N⁺CHRCOOH + Cl^- 

Reactions of the carboxylic acid group

Reaction with aqueous alkalis

• An amino acid reacts with aqueous alkali such as sodium or potassium hydroxide to form a salt and water
• For example, a general amino acid reacts with sodium hydroxide to form a sodium salt:

H₂NCHRCOOH + NaOH ⇌ H₂NCHRCOO^- Na⁺ + H₂O

Esterification with alcohols

• Amino acids, like carboxylic acids, can be esterified by heating with alcohol in the presence of concentrated sulfuric acid 
• The carboxylic acid group is esterified whilst the basic amine group is protonated due to the acidic conditions:

H₂NCHRCOOH + C₂H₅OH + H⁺ ⇌ H₃N⁺CHRCOOC₂H₅  + H₂O

Optical activity

• Almost all 2-amino acids contain a chiral centre (the C of the CH group), and so are optically active
  • The only exception is glycine, which has a CH₂ group instead
• Aqueous solutions of the enantiomers rotate the plane of polarisation of plane-polarised light
  • Dextrorotatory (+)
  • Laevorotatory (-)
• If an amino acid is synthesised in the lab, a racemic mixture is formed

Peptide bonds

• Each amino acid contains an amine (-NH₂) and carboxylic acid (-COOH) group
• The new amide bond between two amino acids is also called a peptide link or peptide bond
• • The -NH₂ group of one amino acid can react with the -COOH group of another amino acid in a condensation reaction to form a dipeptide
• Since this is a condensation reaction, a small molecule (in this case H₂O) is eliminated
• The dipeptide still contains an -NH₂ and -COOH group at each end of the molecule which can again participate in a condensation reaction to form a tripeptide

A peptide bond is an amide bond between two amino acids

• A polypeptide is formed when many amino acids join together to form a long chain of molecules