Two experiments were performed on a peptide sample ‘X’. In experiment 1, treatment of ‘X’ with dithiothreitol (DTT), followed by blocking of free sulphydryl groups, yielded two polypeptides whose amino acid sequences are as shown below:

I. Ala-Phe-CysA³-Met-Tyr-CysA⁶-Leu-Trp-CysA⁹-Asn
II. Val-CysB²-Trp-Val-Ile-Phe-Gly-CysB⁸-Lys

In experiment 2, ‘X’ was treated with chymotrypsin, a protease that cleaves the carboxy-terminal of aromatic residues. The amino acid composition of five peptides obtained from this experiment are shown below:

I. ([Ala], [Phe])
II. ([Asn], 2[Cys], [Met], [Tyr])
III. ([Cys], [Gly], [Lys])
IV. (2[Cys], [Leu], 2[Trp], [Val])
V. ([Ile], [Phe], [Val])

Based on the above results, which cysteine/s are linked by disulfide bond in peptide ‘X’?

Correct option is B

Explanation-

Experiment 1 (with DTT - reducing agent):
Revealed two polypeptides after breaking disulfide bonds:
Polypeptide I:
Ala–Phe–CysA³–Met–Tyr–CysA⁶–Leu–Trp–CysA⁹–Asn
Polypeptide II:
Val–CysB²–Trp–Val–Ile–Phe–Gly–CysB⁸–Lys

So, we have Cys residues at  A³, A⁶, A⁹ and B², B⁸
These cysteines are potential candidates for disulfide bond formation before DTT treatment.
Experiment 2 (Chymotrypsin digestion):
Chymotrypsin cleaves after aromatic residues (Phe, Tyr, Trp).
Peptide fragments obtained:
1.  [Ala, Phe]
2.  [Asn], 2[Cys], [Met], [Tyr]
3.  [Cys], [Gly], [Lys]
4.  2[Cys], [Leu], 2[Trp], [Val]
5.  [Ile], [Phe], [Val]

Step-by-step interpretation:
Map the fragments from chymotrypsin digestion onto the original sequences to find linked cysteines.
Fragment II: [Asn], 2[Cys], [Met], [Tyr]
Contains Asn and two Cys residues — likely from A³–Met–Tyr–CysA⁶–Leu–Trp–CysA⁹–Asn
Suggests a fragment involving CysA⁶ and CysA⁹, which are not cleaved apart, hence likely linked by a disulfide bond.
Fragment IV: 2[Cys], [Leu], 2[Trp], [Val]
Corresponds well with CysA³–Leu–Trp–CysA⁹ from polypeptide I and CysB²–Trp–Val from polypeptide II
Again indicates inter-chain disulfide bonding, possibly CysA³–CysB²
So from the above:
1.  CysA⁶–CysA⁹ are linked
2.  CysA³–CysB² are also likely linked

Correct Option: b - A3–A9 and B2–A6
This matches the disulfide bond pattern deduced.