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A tetrapeptide has -COOH group on alanin...

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 `-NH_(2)` group attached to a chiral centre is :

A

12

B

18

C

4

D

none of these

Text Solution

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The correct Answer is:
C
(D) As-COOH group is in alanine, it forms the C- end of peptide chain. Further `-NH_(2)`group is attached to a chiral centre, hence glycine (which is the only amino acid not optically active) is not at the N-end of the chain.
Fixing alamine at S, we are left wtih their amino acids
P: Val or Phe, (two ways)
Q: include glycine but exclude the amino acid at P (two ways)
R: Include glycine but exclude the amino acids at P and Q (one way)
S: Ala (one way)
`:.` no. of tetrapeptides possible
=2 x 2 x 1 x 1 =4.
`H_(2)N-P-Q-R-S-CO OH`
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