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

• 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

Hydrolysing Proteins

• The polypeptide chains in a protein can be broken down into their individual amino acids by prolonged heating with concentrated hydrochloric acid.
• This breaks the peptide bonds between amino acids
• Due to the acidic environment the amino acids formed will have their NH2?groups protonated as?+NH3?groups

Using Chromatography

• The amino acids produced by hydrolysis can be identified using simple chromatography
• Using a suitable solvent, the individual amino acids will rise to different heights on the chromatography paper
• As the amino acids are colourless, the chromatogram is sprayed with a developing agent so that the amino acid positions can be seen
• Once the positions of the amino acids have been established, their Rf?values can be calculated

How chromatography can be used to separate a mixture of amino acids and identify the individual components