Form a dipeptide by making a peptide bond
Join two amino acids by condensation between one carboxyl group and the next amino group, eliminating water and forming a peptide bond.
Section 34 · Nitrogen compounds
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 34. Use the stated reagents, conditions and product scope. Written practice is self-checked This online lesson cannot demonstrate supervised laboratory skills.
Before you begin
- Recall nucleophilic substitution, reduction, acyl-chloride addition–elimination, acid–base equilibria and covalent bonding.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What bond links two amino-acid residues in a peptide?
What you will learn
- Identify the groups that react
- Draw the peptide linkage
- Account for water loss and chain direction
Condense two amino acids
A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of another. The –OH from –COOH and an H from –NH₂ combine to form H₂O.
The remaining parts join as –C(=O)–NH–, an amide (peptide) bond. Two amino-acid residues form a dipeptide with one peptide bond.
Keep the residue order
The two amino acids can be joined in either order; the sequence of residues defines a different dipeptide.
After condensation, one end retains a free amino group (N-terminus) and the other a free carboxyl group (C-terminus). Track the order shown in the question rather than rearranging the side chains.
Pause and explain
What small molecule is eliminated when two amino acids form a peptide bond?
Worked example
Show the peptide linkage formed when glycine and alanine join in that order.
Show the worked solution
- Use glycine's –COOH and alanine's –NH₂.
- Remove OH and H as H₂O.
- Join the residues through –C(=O)–NH–.
Answer H₂NCH₂COOH + H₂NCH(CH₃)COOH → H₂NCH₂CONHCH(CH₃)COOH + H₂O.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Join alanine then glycine into a dipeptide and show the peptide bond.
Give me a hint
React alanine's carboxyl group with glycine's amino group.
Compare my reasoning
- Keep the residue order alanine–glycine.
- Form –C(=O)–NH– between them.
- Remove H₂O.
H₂NCH(CH₃)CONHCH₂COOH + H₂O (alanine–glycine dipeptide plus water).
Look for these in your work
- Order preserved
- Peptide linkage
- Water eliminated
Identify the peptide bond in H₂NCH₂CONHCH(CH₃)COOH.
Give me a hint
Look for the carbonyl carbon directly bonded to nitrogen.
Compare my reasoning
- Find –C(=O)–NH–.
- This linkage connects two residues.
- It is an amide bond called a peptide bond.
The peptide bond is –C(=O)–NH– between the glycine and alanine residues.
Look for these in your work
- Carbonyl found
- C–N adjacency
- Name
A student joins two amino acids by linking their α-carbons and removes H₂O. Correct the condensation site.
Give me a hint
The syllabus peptide bond forms between carboxyl and amino groups.
Compare my reasoning
- Use –COOH on one residue.
- Use –NH₂ on the next residue.
- Join through –C(=O)–NH– and eliminate water.
The residues join at carboxyl C and amino N to form –CONH–; the α-carbons are not directly joined.
Look for these in your work
- Groups selected
- Correct atoms linked
- Water loss
Common mistakes
- Writing –COO–NH– instead of –CONH–
- Removing an entire amino acid side chain
- Forgetting water is eliminated
- Treating the peptide bond as a C–C bond
What can you explain now?
What bond links two amino-acid residues in a peptide?
Compare with the explanation
–C(=O)–NH–
The peptide bond is an amide linkage.
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Make it stick
Which atoms join to make a peptide bond, and which atoms leave as water?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.