To solve the problem, we need to determine the number of possible sequences (primary structures) of a tetrapeptide that includes the amino acids Glycine (Gly), Valine (Val), Phenylalanine (Phe), and Alanine (Ala), with the condition that the -NH2 group is attached to a chiral center.
### Step-by-Step Solution:
1. **Identify the Amino Acids**:
The tetrapeptide consists of four amino acids: Glycine (Gly), Valine (Val), Phenylalanine (Phe), and Alanine (Ala).
2. **Understand Chirality**:
A chiral center is a carbon atom that is attached to four different groups. In this case, Alanine has a chiral center because it has -NH2, -COOH, -CH3 (from the R group), and a hydrogen atom attached to its alpha carbon.
3. **Determine the Fixed Position**:
According to the problem, the -COOH group is fixed on Alanine. This means that Alanine will be at one end of the tetrapeptide.
4. **List the Remaining Amino Acids**:
The remaining amino acids to arrange are Glycine (Gly), Valine (Val), and Phenylalanine (Phe).
5. **Check for Chirality of Glycine**:
Glycine is not chiral because it has two hydrogen atoms attached to its alpha carbon. Therefore, it cannot occupy the chiral center position.
6. **Arrange the Remaining Amino Acids**:
We can arrange Valine and Phenylalanine in the first three positions of the tetrapeptide. The possible arrangements of Valine and Phenylalanine are:
- Valine, Phenylalanine
- Phenylalanine, Valine
7. **Count the Possible Sequences**:
Since Glycine cannot be at the chiral center, we can only arrange Valine and Phenylalanine in the first two positions, while Alanine remains fixed at the end. The sequences can be:
- Valine, Phenylalanine, Alanine
- Phenylalanine, Valine, Alanine
Thus, we have two arrangements of the first three amino acids (Valine, Phenylalanine, and Glycine) with Alanine fixed.
8. **Final Count**:
Therefore, the total number of possible sequences (primary structures) of the tetrapeptide, with the -NH2 group attached to a chiral center, is **2**.
