Build a tripeptide and count its peptide bonds
Three amino-acid residues condense to form a tripeptide with two peptide bonds and two water molecules eliminated.
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
How many peptide bonds are in a linear tripeptide?
What you will learn
- Count residues and peptide bonds in a tripeptide
- Deduce the products of sequential condensation
- Preserve the order of amino-acid residues
Link three residues in sequence
A tripeptide contains three amino-acid residues joined in a chain. Each peptide bond links the carboxyl group of one residue to the amino group of the next.
Three residues require two peptide bonds, so two water molecules are eliminated during formation from three free amino acids.
Count links, not terminal groups
For n amino-acid residues in one unbranched peptide, there are n−1 peptide bonds. A tripeptide therefore has two peptide bonds and retains one free amino terminus and one free carboxyl terminus.
The order of residues matters. Write the sequence from N-terminus to C-terminus and keep the side chains on the correct α-carbons.
Pause and explain
How many water molecules are eliminated when three amino acids form one tripeptide?
Worked example
How many peptide bonds and water molecules are involved in making one tripeptide from three amino acids?
Show the worked solution
- Connect residue 1 to residue 2.
- Connect residue 2 to residue 3.
- Each condensation removes one water molecule.
Answer Two peptide bonds form and two H₂O molecules are eliminated.
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Test this topic →From guided practice to a new situation
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.
A tripeptide is formed from glycine, alanine and serine in that order. State the number of residues, peptide bonds and water molecules lost.
Give me a hint
A chain of n residues has n−1 links.
Compare my reasoning
- Count three residues.
- Two links connect the three residues.
- One water is lost per link.
Three residues, two peptide bonds and two H₂O molecules eliminated.
Look for these in your work
- Residues
- Bond count
- Water count
How many peptide bonds are in a linear peptide of six amino-acid residues?
Give me a hint
Use n−1 for an unbranched chain.
Compare my reasoning
- There are six residues.
- Subtract one for the number of joins.
- The chain has five peptide bonds.
Five peptide bonds.
Look for these in your work
- Residue count
- Rule
- Answer
A student says a tripeptide has three peptide bonds because it contains three amino acids. Correct the count and explain.
Give me a hint
Visualise a chain: adjacent pairs form the links.
Compare my reasoning
- Residue 1 links to residue 2.
- Residue 2 links to residue 3.
- There are two adjacent joins.
A linear tripeptide has two peptide bonds; three residues require two joins.
Look for these in your work
- Adjacent pairs
- Count
- Reason
Common mistakes
- Counting three peptide bonds for three residues
- Removing only one water molecule
- Reversing the stated residue sequence
- Removing the terminal –NH₂ or –COOH groups
What can you explain now?
How many peptide bonds are in a linear tripeptide?
Compare with the explanation
Two
Three residues need two links to form one chain.
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Make it stick
Why does a tripeptide have two peptide bonds rather than three?
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.