# Question #382fd

Jul 24, 2017

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

Most amino acids are primary amines with the general structure $\text{H"_2"NCHRCOOH}$.

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

The central $\text{N}$ atom is the only portion of the purple anion that comes from the amino acid.

(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.