Yellow Result From Ninhydrin Test?

The Ninhydrin Test is used to test for the presence of amino acids (NOT proteins).

Ninhydrin degrades amino acids into aldehydes, ammonia, and CO₂  (carbon dioxide) through a series of reactions; the net result is ninhydrin in a partially reduced form hydrindantin:

 (reaction 1)

Ninhydrin then condenses with ammonia and hydrindantin to produce a blueish-purple pigment.

(reaction 2)

The typical positive result is a blueish-purple color, as seen in the image below:

(Image credit: pdecell)

But, there is an amino acid that gives a yellow result. This amino acid is PROLINE.

(Image Credit)

As you can see, proline has a ring structure (secondary amino acid).

The blueish-purple result is usually associated with primary amino acids. In these amino acids, the N is free to react with ninhydrin. However, in proline, the N is not available for reaction as it is locked in the ring structure. Therefore no ammonia is produced, so no blue color is presented (see “reaction 2” above).

(Click here for reference and more on the Ninhydrin Test)