A polymer having COOH group is coupled with a polystyrene bead having primary amine group, then ninhydirn test carried out in ethanol solvent. Absence of a peak at around 580 nm shows that the primary amine group absent and after the coupling reaction between primary amine and acid. But a new peak seen around 544 nm!! How can I identify this peak? What is this peak representating? Any suggestion or literature will be highly appreciated. Thank you in advance.
Ranjet
Did you use blank sample as background reference？
You have to use everything (all the reagents) except the amine sample to run the test as your blank to set the background as “0”。
@low reading， the background may cause （hide/shift）abs max 570nm to lower wavelength, cause the change of your spectrum too, those will affect your test results. You also need to do a standard curve of ninhydrin test with a well know primary amine@know concentration.
Hope this helps!
Good luck!
This article might help http://vlab.amrita.edu/?sub=3&brch=63&sim=156&cnt=1
Generally to get Ruhemann's purple (at 570 nm) you need 2 molecules of ninhydrin per 1 molecule of amine, which might not be the case in your experiment. This may be the cause of variation.
I am unsure on nature of your amine, but this seems similar to imino acid reaction, which gives a lower-energy band.
https://iubmb.onlinelibrary.wiley.com/doi/pdf/10.1016/0307-4412%2878%2990153-X
Quote: " The reaction of imino acids with ninhydrin proceeds initially in a similar manner to that of a-amino acids. Imino acids, e.g. proline and hydroxyproline, react with ninhydrin to give a yellow color. At higher temperatures ("~100°C), the yellow compound (X) is transformed to the purple-red compound (XI) "
Dear Mikhail, thank you very much for the prompt reply. It is indeed helpful. To provide you more clear understanding the situation here, Case (1): I had polystyrene beads with primary amines on its surface. I have carried out the ninhydrin test in EtOH at 75'C for 45 min and found the deep blue color which produced UV-vis peak at around 588 nm (Ruhemann's product). Case (2): Through a coupling reaction I have coupled a polymer with this bead which I expect resulted through the amide bond formation and the bead has now many such amide (-CONH-) bonds. Thereafter, I have carried out the ninhydrin test and didn't receive the UV-vis peak around 588 nm (or 570 nm) but found a new peak around 544 nm. I want to know what this peak is representing. Thank you once again. (See the attached file too). And yes, thank you for the links you had sent. Those are helpful.
In my experience, amides do react with ninhydrin with intense heating. Maybe activation via phenyl can also help reaction. Unfortunately, I was unable to find specifics of such reaction, but I suspect it's something like this.
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