CAIE A-Level Chemistry 34.4.2 Formation of Peptide Bonds

Practise drawing dipeptides and tripeptides in the stated residue order with fully displayed peptide bonds.

Exam points

join the carboxyl group of one amino acid to the amino group of the next as –C(=O)–NH–
preserve each side chain and the named N-to-C residue order when drawing a peptide
remove one H2O per peptide bond and distinguish reversed residue order as a different structural isomer

[Maximum number: 4]

Amino acids are molecules that contain $-\mathrm{NH}_{2}$ and -COOH functional groups.
Glycine, $\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}$ , is the simplest stable amino acid.

(a)

Fig. 6.1 shows two syntheses starting with glycine.

Fig. 6.1

[ 4 ]

(i)

State the essential conditions for reaction 1.

[ 1 ]

(ii)

Identify the reagent used in reaction 2 .

[ 1 ]

$\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl} /$ benzoyl chloride / benzoyl anhydride

(iii)

A molecule of phenylalanine, R, can react with a molecule of glycine to form two dipeptides, S and T.

S and T are structural isomers.

Figure for Question (iii) — CAIE A-Level Chemistry A2

Draw the structures of these dipeptides. The peptide bond formed should be shown fully displayed.
S
T

[ 2 ]

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34.4.2—Peptide bond formation question 1

Question (a)

Question (i)

Question (ii)

Question (iii)