Question

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 $N{H}_2$ group attached to a chiral centre is:

• A. 4
• B. 3
• C. 6
• D. 2

Answer 1

Answer: (A)

Since $-COOH$ group is attached to alanine, one terminal of the tetrapeptide contains alanine. Thus, its position is fixed.

Glycine is achiral whereas valine and phenylalanine are chiral. Thus, $-N{H}_2$ group is attached to either valine or phenylalanine.

Thus following are the four possible combinations of the tetrapeptide.

$H_{2}N-Val-Gly-Phe-Ala-COOH$

$H_{2}N-Val-Phe-Gly-Ala-COOH$

$H_{2}N-Phe-Gly-Val-Ala-COOH$

$H_{2}N-Phe-Val-Gly-Ala-COOH$

Therefore, the correct option is A.