Characteristic Behaviour of Amino Acids (Edexcel International A Level Chemistry)

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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

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

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

Nitrogen Compounds - Formation of Peptide Bonds, downloadable AS & A Level Chemistry revision notes

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

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.