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

Ans. Sakaguchi test gives positive result (indicated by appearance of red color) with arginine and proteins (because all proteins would contain arginine).

The test is highly sensitive and gives negative result (not appearance of red color) for all amino acids except arginine.

# Note that RED COLOR is POSITVE result, all other colors being negative result.

# The appearance of yellow color is observed with many amino acids. The yellow color is till the negative result.

The appearance of yellow color with some amino acids is possibly due to formation of amino-naphthol derivative of the amino acids. This complex is yellow-colored.

# Conclusion: There is nothing wrong with your experiment. It’s usual to get yellow color with some of the amino acids in Sakaguchi test.

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