Number of cyclic tripeptides formed with 2 amino acids A and B is :

Solution

Chemistry ma’am · 3 steps

In a ring, every rotation is the same peptide, so only the composition of the three units matters.

1. Why rings differ. A cyclic tripeptide has three amino acid units joined in a closed ring by peptide bonds. It has no free N-terminus or C-terminus, so it has no starting point. A–A–B, A–B–A and B–A–A are rotations of one ring, so they are one peptide.
2. Direction check. With two identical units and one different unit, reading the ring the other way round gives B–A–A. That is just another rotation, so the order of the units creates no new peptide here.

   Units used	Ring	Distinct peptides
   three A	cyclo(A–A–A)	1
   three B	cyclo(B–B–B)	1
   two A, one B	cyclo(A–A–B)	1
   one A, two B	cyclo(A–B–B)	1

3. So. The four compositions give 1 + 1 + 1 + 1 = 4 cyclic tripeptides, which is option (d).

The idea to keep. For cyclic peptides, first list the possible compositions, then remove the arrangements that are only rotations of the same ring. With three units of two kinds, each composition gives one ring.

Answer from the NTA final answer key. Solution by PrepWiser.