Ninhydrin and secondary amine

[dttom]

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, 2010 by dttom

[UC]

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

[dttom]

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)?