how does casein react to the different amino acid test? (ex of test: biuret, ninhydrin,etc.)?

Question

not just for the biuret & ninhydrin, but also for the others like sakaguchi's test, diacetyl test etc... i need the all...thank you

Answer 1

Casein is a water soluble protein, so it should give a positive test result (light blue to deep purple) when reacted with Biuret reagent.

Ninhydrin tests for amino acids not proteins (or peptide bonds). The reaction is usually negative with casein.

The Xanthoproteic test uses nitric acid to produce an intense yellow color in samples containing proteins. It works by nitrating (adding a NO2 group to) the aromatic ring in certain amino acid groups (tyrosine, tryptophan, and phenylalanine). Casein contains amino acids with these rings and would give a positive result.

The bicinchoninic acid (BSA) assay indicates positive protein by changing from green to purple. Casein will test positive, in fact, a deep purple.

The Lowery test uses Copper (II) with Folin-phenol reagent and also reacts with most soluble proteins. Casein turns the reagent blue, which is positive.

The Bradford test which uses Coomassie dye would also be positive due to the arginine present.

The Sakaguchi reaction tests for the presence of the guanidino functional group. It should be positive.

The diacetyl test is usually used to test for Urea. I do not know what results you would get with undigested Casein in water.

Answer 2

Biuret's tests for amount of protein in a sample. Casein is a milk protein and will test positive, ie. violet color, since the reagent binds with peptide bonds.

Ninhydrin is used to detect free amino groups, so unless you degrade the casein first, you will get a negative since the protein is bonded covalently (peptide bonds) and has only one free amino group at the N-terminus (disregarding any possible amino groups of the variable chain, ie R-group).

Answer 3

The biuret reagent is a dilute solution of copper (II) ions. These ions are a light blue color in water. Nitrogen containing compounds form complex ions with the copper ions and usually give intense purple or dark blue colors. Since amino acids and peptide bonds contain nitrogen they form a darker blue or purple color when the nitrogen complexes with the copper ions.

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