Principle of Biuret test

The Biuret test is given by compounds containing at least two peptide bonds (or structures equivalent to biuret functionality), which form a violet complex with alkaline $\text{Cu}^{2+}$.

So, let us check each compound:

Evaluate each compound

(i) Glycine

Glycine is a single amino acid: $\mathrm{H_2N-CH_2-COOH}$ It has no peptide bond.

$\Rightarrow$ Biuret test: Negative

(ii) Glycylalanine

Glycylalanine is a dipeptide. A dipeptide contains one peptide bond.

Since Biuret test requires at least two peptide bonds, it does not give a positive test.

$\Rightarrow$ Biuret test: Negative

(iii) Tripeptide

A tripeptide contains three amino acids joined by two peptide bonds.

Therefore, it satisfies the condition for Biuret test.

$\Rightarrow$ Biuret test: Positive

(iv) Biuret

Biuret itself has the structure: $\mathrm{H_2N-CO-NH-CO-NH_2}$ It contains the required linkage and gives the Biuret test (the test is named after this compound).

$\Rightarrow$ Biuret test: Positive

Count positive cases

Positive compounds are:

• Tripeptide
• Biuret

Therefore, total number of compounds giving positive Biuret test is $2$