Question 20.4: Assign IUPAC names to each of the following small peptides. ......

a. The three amino acids present are glutamic acid, serine, and alanine. Alanine, the C-terminal residue (on the far right), keeps its full name. The other amino acid residues in the peptide receive “shortened” names that end in -yl. The -yl replaces the -ine or -ic acid ending of the amino acid name. Thus

glutamic acid becomes glutamyl
serine becomes seryl
alanine remains alanine

The IUPAC name, which lists the amino acids in the sequence from N-terminal residue to C-terminal residue, becomes glutamylserylalanine.

b. The four amino acids present are glycine, tyrosine, leucine, and valine. Proceeding as in part a, we note that

glycine becomes glycyl
tyrosine becomes tyrosyl
leucine becomes leucyl
valine remains valine

Combining these individual names gives the IUPAC name glycyltyrosylleucylvaline.
The complete name for a small peptide should actually include a handedness designation (\scriptsize{\text{L}}-designation; Section 20.3) before the name of each residue. For example the dipeptide serylglycine (Ser–Gly) should actually be \scriptsize{\text{L}}-seryl–\scriptsize{\text{L}}-glycine (\scriptsize{\text{L}}-Ser–\scriptsize{\text{L}}–Gly). The \scriptsize{\text{L}}-handedness designation is, however, usually not included because it is understood that all amino acids present in a peptide, unless otherwise noted, are \scriptsize{\text{L}} enantiomers.