Hello teacher, I have some problems about my experiment to solve: We have an exp

Question

Hello teacher, I have some problems about my experiment to solve:

We have an experiment of Sakaguchi test, a chemical test used for detecting the presence of arginine in proteins. The -naphthol will react with the guanidine group in amino acid.

This is the photo of my experiment result:

(The leftmost sample is tyrosine, and the second one is tryptophan, the third one is phenylalanine, 4th is cysteine, 5th is proline, 6th is arginine, 7th is methionine, 8th is histidine, 9th is BSA, and the rightmost one is Gelatin protein.)

Most of them seems normal, but the tyrosine, tryptophan, phenylalanine in left seems to have some problem. There are no guanidine group in it at all, means that it mustn't have any reaction to occur. But they still turn to yellow! What happened?(Our tutor says this is atypical reaction. So what reactions occur in it?)

The most ridiculous one as below:(reference group, only ddH2O in it. There are no any amino acid.)

What’s going on? Why only pure water can still react with -naphthol? The ddH2O seems no problem in it, and our experiment progress seems to be OK too. It must be something mistakes in it, but I have no idea. (HELP!!) Could you help me to point out witch part might be wrong? Thank you very much!

Explanation / Answer

6th test tube is arginine (shown in the picture).

The Sakaguchi test is a chemical test used for detecting the presence of arginine in proteins. Sakaguchi reagent consists of 1-Naphthol and a drop of sodium hypobromite. The guanidine group in arginine reacts with Sakaguchi reagent to form a red-coloured complex. The -naphthol will react with the guanidine group in arginine (amino acid).  

Tyrosine, tryptophan, phenylalanine (first 3 test tubes on the left) turn yellow with the Sakaguchi test eventhough there are no guanidine group in it at all, means that it mustn't have any reaction to occur. There is a possibility that the test tubes which you have used have traces of amino acid-arginine and therefore answering for Sakaguchi test.

Probably the ddH2O test tube also may contain traces of amino acid-arginine and therefore answering for Sakaguchi test.

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