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

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.

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?

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".

Ok dude, thanks alot for your help