A tetrapeptide has -$\mathrm{COOH}$ group on alanine. This produces glycine (Gly), valine (val), phenyl alanine (Phe) and alanine (Al), on complete hydrolysis. For this tetrapeptide, the number of possible sequence (primary structure) with-${\mathrm{NH}}_2$ group attached to a chiral centre is .. . .

PLAN The hydrolysis of a peptide bond releases two free amino acids.

Four amino acids are connected by three peptide linkages to form the chiral carbon tetrapepetide C'.

- The COOH group is on the alanine component, therefore in each combination it is fixed at the C-terminal position.

Glycine cannot be on the N-terminal side because it is optically inactive. the following are potential combinations:

Phe-Val-Gly-Ala, Phe-Gly-Val-Ala

Val-Phe-Gly-Ala, Val-Gly-Phe-Ala

Thus, it is feasible to use all four combinations.