I was told that ninhydrin could react with secondary amine to form a colored complex of iminium salt. However, in an experiment I treat N-acetylglucosamine with ninhydrin, it gave negative result. N-acetylglucosamine contains a secondary amino group, but why it gave negative result?

But ninhydrin gives colored complex with proline, which is also a secondary amine...

Is it just different iminium salts have different absorption spectrum?

Edited April 12, 201015 yr by dttom

You do know the difference between an amine and an amide, right? Have a closer look at the structures.

Oh I see, I miss that, thanks.

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By the way, I got a reddish brown complex when proline is treated with ninhydrin with heating, this indicates an iminium salt formation (usually brown in color). I also got such a brown appearance when I treated histidine. His got 2 nitrogen atom in the side group, one N doubly bonded to a carbon on one hand and singly bonded to another carbon on the other hand, this N atom is basic while another one is not by resonance stabilisation. I want to ask if it is because this basic N atom which behaves as a secondary amine that give rise to the brown complex (probably iminium salt)?