A tetrapeptide has -COOH group on alanine. This produces glycine (Gly), valine (Val), phenyl alanine (Phe) and alanine (Ala), on complete hydrolysis. For this tetrapeptide, the number of possible sequences (primary structures) with group attached to a chiral center is
Answer: 4
Solution
The tetrapeptide contains four amino acids: Gly, Val, Phe, and Ala, with -COOH on Ala (so Ala is at the C-terminus).
Possible sequences: _ _ _ Ala
The three remaining amino acids (Gly, Val, Phe) can be arranged in 3! = 6 ways:
Glycine has no chiral center (H as the side chain). The group is at the N-terminus. For the group to be attached to a chiral center, the N-terminal amino acid must NOT be glycine.
Sequences where Gly is NOT at the N-terminus: sequences 3, 4, 5, 6 = 4 sequences.
Possible sequences: _ _ _ Ala
The three remaining amino acids (Gly, Val, Phe) can be arranged in 3! = 6 ways:
- Gly-Val-Phe-Ala
- Gly-Phe-Val-Ala
- Val-Gly-Phe-Ala
- Val-Phe-Gly-Ala
- Phe-Gly-Val-Ala
- Phe-Val-Gly-Ala
Glycine has no chiral center (H as the side chain). The group is at the N-terminus. For the group to be attached to a chiral center, the N-terminal amino acid must NOT be glycine.
Sequences where Gly is NOT at the N-terminus: sequences 3, 4, 5, 6 = 4 sequences.
JEE Advanced · 2013 · 02 Jun 2013 · Paper 1 · Integer answer