Question #382fd

1 Answer
Jul 24, 2017

Proline gives a yellow colour because it is a secondary amine.

Most amino acids are primary amines with the general structure "H"_2"NCHRCOOH".

Except for proline and hydroxyproline, all the α-amino acids are oxidized by ninhydrin to give the same intensely colored purple anion.

The central "N" atom is the only portion of the purple anion that comes from the amino acid.

NinhydrinNinhydrin
(Adapted from Biochemistry is a good thing - WordPress.com

Proline reacts differently because it is a secondary amine.

The product is a yellow-coloured zwitterion.