amino acids, amides and chirality

• α amino acid


  Amino acid where both NH2 and COOH groups are joined to the same C
• General structure of an α amino acid

  • NH2
  • CH
  • CO2H
  • R (R group can be a variety of different things depending on what amino acid it is)
• Simplest amino acid


  • Glycine, where the R is an H
• Acidic or basic amino acids

  • Have an extra carboxylic acid or an amine group on the R group
• Naming amino acids


  You do not need to know any common names for the 20 essential amino acids, but should be able to name given amino acids using IUPAC organic naming
• Zwitterion


  The no charge form of an amino acid never occurs. The amino acid exists as a dipolar zwitterion.
• Acidity and Basicity


  The amine group is basic and the carboxylic acid group is acidic.
• Amino acids act as weak buffers


  Only gradually change pH if small amounts of acid or alkali are added to the amino acids
• Isoelectric point


  An amino acid exists as a zwitterion at a pH value called the isoelectric point
• Factors affecting isoelectric point

  • If the side R group of an amino acid contains an acidic or basic group then the pH value of the isoelectric point will be different
  • An amine group on the R group may make the isoelectric point be pH > 10
  • An carboxylic acid group on the R group may make the isoelectric point be pH < 3
• Reactions of amino acids

  The carboxylic acid group and amine group in amino acids can undergo the usual reactions of these functional groups
• Esterification reaction


  • C H2N CO2H H CH3 + CH3OH H+ C N H3+ C H CH3O O CH3 + H2O
• Amides


  Add –amide to the stem name
• Secondary and tertiary amides

  • N-methylpropanamide
  • N,N-dimethylpropanamide
  • N,N,2-trimethylpropanamide
• Hydrolysis of di-peptides/proteins


  1. If proteins are heated with concentrated acid or alkali they can be hydrolysed and split back in to their constituent amino acids

  2. The composition of the protein molecule may then be deduced by using TLC chromatography
• Optical isomerism


  Occurs in carbon compounds with 4 different groups of atoms attached to a carbon (called an asymmetric carbon)
• Chiral (asymmetric) carbon atom

  • A carbon atom that has four different groups attached
• Enantiomers


  Two compounds that are optical isomers of each other
• Optical activity


  Optical isomers rotate plane polarised light in different directions
• Optical isomerism vs EIZ isomerism

  Optical isomerism and EIZ isomerism are different types of stereoisomerism, which is defined as the same structural formula but a different spatial arrangement of atoms