A hexapeptide has the composition `Ala,Gly,Phe,Val.` Both the `N-` terminal and `C-` teminal units are Val. Cleavage of the hexapeptide by chymotypsin gives two different tripeptides, both having Val as the N-terminal group. Among the products of random hydrolysis is a `Ala-Val` dipeptide fragment . What is the primary structure of the hexapeptide?

Options

A. Val-Gly -Phe -Val-Ala-Val
B. Val-Ala-Phe-Val-Gly-Val
C. Val-Gly-Ala-Val-Phe-Val
D. Val-Phe-Val-Ala-Gly-Val

Answer

To determine the primary structure of the hexapeptide with the given conditions, we will follow a systematic approach.

### Step-by-Step Solution:

1. **Understanding the Composition**:
The hexapeptide consists of the amino acids: Alanine (Ala), Glycine (Gly), Phenylalanine (Phe), and Valine (Val). The total number of amino acids is six, and both the N-terminal and C-terminal units are Valine (Val).

2. **Setting Up the Structure**:
Given that both terminals are Valine, we can represent the hexapeptide as:
- Valine - X - Y - Z - A - Valine
where X, Y, Z, and A are the remaining amino acids (Ala, Gly, Phe).

3. **Chymotrypsin Cleavage**:
Chymotrypsin cleaves at the carboxyl side of aromatic amino acids (like Phe) and leucine. Since we have Phe in our sequence, we can expect that the cleavage will occur at this position.

4. **Identifying Possible Tripeptides**:
After cleavage, we need to form two different tripeptides, both having Val as the N-terminal group. The possible combinations of amino acids can be:
- If we assume the structure as Val - Ala - Phe - Gly - Val, cleavage would yield:
- Tripeptide 1: Val - Ala - Phe
- Tripeptide 2: Val - Gly - Val
- If we assume the structure as Val - Gly - Phe - Ala - Val, cleavage would yield:
- Tripeptide 1: Val - Gly - Phe
- Tripeptide 2: Val - Ala - Val

5. **Checking for Dipeptide Formation**:
The question states that among the products of random hydrolysis, an Ala-Val dipeptide fragment is formed. We need to check which of the above combinations can yield this dipeptide:
- From Val - Ala - Phe, we can hydrolyze to get Ala - Val.
- From Val - Gly - Phe, we cannot form Ala-Val.
- From Val - Gly - Ala, we can hydrolyze to get Ala - Val.

6. **Final Structure**:
The only structure that satisfies all conditions (both terminal Val, cleavage by chymotrypsin yielding two tripeptides with Val as N-terminal, and formation of Ala-Val dipeptide) is:
- Val - Ala - Phe - Gly - Val

### Conclusion:
The primary structure of the hexapeptide is:
**Val-Ala-Phe-Gly-Val**