Ninhydrin test on aminos

[livingdeadbeat]

Hi everyone,

 

Another question. In writing about ninhydrin and amino acids that form the subunits of proteins i was wondering what it is that makes proline produce a brownish colour?

 

From what i've read i think the purple that it usually gives with primary amino groups is due to the ninhydrin reacting with the N in the amine. I'm guessing, cos proline has a ring structure that makes the anime a secondary the N isn't available for it to reaction and form the purple...

 

Is the above correct and what makes it go brown?

 

Thanks for any help

[mississippichem]

That's right. One molecule of amino acid has to react with two molecules of ninhydrin to form a highly conjugated chromophore that gives the purple color. Proline cannot de-aminate to allow the second molecule of ninhydrin to react. As you stated, proline's amino group is stuck in a pyrrolidine ring.

[livingdeadbeat]

That's right. One molecule of amino acid has to react with two molecules of ninhydrin to form a highly conjugated chromophore that gives the purple color. Proline cannot de-aminate to allow the second molecule of ninhydrin to react. As you stated, proline's amino group is stuck in a pyrrolidine ring.

 

Cool thanks. But why does it turn brownish yellow?

[mississippichem]

This is a guess, but I suspect it is the color of the cyclohemiaminal that will form when the proline nitrogen replaces one of the ninhydrin hemiacetal hydroxyl groups. This is speculative though; I don't see why not, but I have no source or confirmation so take that with a "grain of salt".

[livingdeadbeat]

Ok dude, thanks alot for your help