Ninhydrin and Buiret Tests

Ninhydrin Reaction

  • Tests for amino acids
  • reacts to form a purple coloured imino derivative
  • Most amino acids are colourless
  • not all may give a purple colour
  • Proline and hydroxyproline, secondary amino acids give a yellow colour  due its cyclic structure which results in the reduction of flexibility of polypeptide regions. The ninhydrin reaction as this reagent requires free alpha amino group (-NH2) .  Therefore the amino acids which have a free -NH2 (amino) group, are positive for the ninnydrin test but because proline only has -NH (imino) group it is negative.

Buiret Test

  • Tests for proteins
  • the buiret reagent is light blue in colour as it containd copper(II) ions
  • turns purple in the presence of proteins
  • the purple colour is formed when the copper ions in the buiret reagent reacts with peptide bonds on the polypeptide chains to form a complex