You are here

Ninhydrin Reaction

Hazards

The ninhydrin solution will react with the proteins of skin. Wear gloves or you'll end up with purple hands.

Chemicals and Solutions

  • Ninhydrin solution (0.25% in water)
  • Methionine
  • Hydroxyproline

Materials

  • Two 100 mL cylinders
  • Two stirring bars
  • Two magnetic stirrers
  • Two spatulas

Procedure

  1. Fill the two cylinders half full with ninhydrin solution.
  2. Add a stirring bar to each cylinder and begin stirring the solutions on the magnetic stirrers.
  3. To one cylinder add methionine and to the other add hydroxyproline. Allow the solutions to stir for about 5 to 10 minutes.
  4. After about 5 minutes the cylinder containing hydroxyproline will show the formation of a gold colored solution.
  5. Several minutes later, the cylinder containing methionine will show the formation of a purple colored solution. (Husky Colors).

Discussion

Ninhydrin is the hydrate of indane-1,2,3-trione. With the exception of proline and hydroxy-proline, all the alpha-amino acids found in proteins react with ninhydrin to give the same intensely colored purple anion (570 nm). The purple anion consists of two ninhydrin molecules, each minus their hydroxy groups and bonded together by a central nitrogen. The nitrogen is the only portion of the purple anion that is derived from the amino acid.

the equation of the ninhydrin reaction using structural formulae

Proline and hydroxy-proline do not react with ninhydrin in the same way as the other amino acids because their alpha amino group is part of a five membered ring.

Share