Question

The condensation of two amino acids, glycine and $(\pm )$ alanine yields total product X. Assume that only 1 peptide bond is formed. Find the value of X.

Answer 1

Gly: $N{H}_2-C{H}_2-COOH$
Ala: $C{H}_3-C^{\ast }H\left(N{H}_2\right)-COOH$
From structures, we know that Glycine has no chiral centre while Alanine has 1 chiral centre.
Since the order matters in peptide formation, we can form 4 types of products:
1. Gly-Ala (1 chiral centre)
2. Ala-Gly (1 chiral centre)
3. Gly-Gly (0 chiral centreS)
4. Ala-Ala (2 chiral centres)
Total number of stereoisomers = $2^n$; n= no. of chiral centres
Hence, no. of total products $=2^1+2^1+2^0+2^2=2+2+1+4=9$