The biuret test is a chemical test used to detect the presence of proteins in a sample. The samples you mentioned have different properties that influence the biuret test results.
Among the options:
- a. Glycine: Glycine is an amino acid, and since it does not contain peptide bonds (as it is not part of a polypeptide chain), it does not produce a positive reaction in the biuret test. Therefore, glycine gives a negative biuret test.
- b. Tyrosine: Tyrosine is also an amino acid. Similar to glycine, it does not form peptide bonds as a free amino acid and does not yield a positive biuret test. Hence, tyrosine will also show a negative result.
- c. Gelatin: Gelatin is a protein derived from collagen. Despite being a protein, when it is denatured, its structure may not effectively react with the biuret reagent depending on its state of hydrolysis. If the gelatin is too hydrolyzed or tested in an improper way, it may show a weak or negative biuret test.
- d. Egg albumin: Egg albumin is actually a protein and should give a positive biuret test. It is composed of long polypeptide chains that contain peptide bonds.
- e. Casein (milk protein): Casein is also a protein, and it normally gives a positive biuret test due to its polypeptide structure.
To summarize, glycine and tyrosine definitely provide negative biuret tests, while gelatin may give a negative result under certain conditions. On the other hand, egg albumin and casein will yield positive results as they are proteins with sufficient peptide bonds to react with the biuret reagent.