Diagnose → Learn → Master: Every study-note section includes 15 free MCQs — start with
5 Diagnostic MCQs to identify gaps in your understanding, then complete
10 Mastery MCQs near the end to check what you have learned.
Proteins may look much more complicated than the small organic molecules you have met so far, but their underlying chemistry is surprisingly simple. At this level, the key is to recognise one repeating idea: α-amino acids join together through peptide bonds to build a chain.
Once you can see the –CONH– link, the rest becomes a matter of following what happens to that bond. Condensation makes it by removing water; hydrolysis reverses the process by using water to break it. The main examination challenge is then to predict the correct form of the amino-acid products under acidic or alkaline conditions.
Diagnostic Quiz
What Do You Already Know About Proteins?
Try these five questions before reading the chapter. Focus on how amino acids join and how peptide bonds are broken.
5 diagnostic questions
Structures and reaction ideas where useful
Instant explanation for every answer option
Question 1 of 5
Quiz complete
How did you do?
Which structural unit is the peptide bond in a protein chain?
–COO–
–COO– is associated with an ester linkage, not a peptide bond.
–CONH–
Correct. A peptide bond is the amide linkage –CONH– between amino-acid residues.
–NHNH–
A peptide bond contains a carbonyl group directly bonded to nitrogen, so –NHNH– is not the correct linkage.
–COOCO–
This does not represent the amide linkage that connects amino-acid residues in proteins.
The defining link to recognise in a polypeptide chain is –C(=O)–NH–, usually written more compactly as –CONH–.
What type of reaction joins two α-amino acids to form a peptide bond?
Glycine
Alanine
Addition
Addition reactions do not eliminate a small molecule when the new bond forms.
Substitution
The overall formation of the peptide link is classified as condensation, because H₂O is eliminated.
Condensation
Correct. The amino acids join and one molecule of H₂O is eliminated when each peptide bond forms.
Oxidation
Peptide-bond formation is not described as an oxidation reaction.
The carboxylic acid group of one amino acid and the amino group of another combine, forming a peptide bond while eliminating H₂O.
Which conditions can be used to hydrolyse peptide bonds in a protein in the laboratory?
Aqueous acid or aqueous alkali, with heating
Correct. Peptide bonds can be hydrolysed by heating with aqueous acid or aqueous alkali, commonly under reflux.
Dry hydrogen chloride at room temperature
Hydrolysis requires water, so dry conditions are unsuitable.
Concentrated sulfuric acid with no water
Hydrolysis requires water; strongly dehydrating conditions are not the required conditions here.
Aqueous bromine at room temperature
Aqueous bromine is not the reagent used to hydrolyse peptide bonds.
Peptide hydrolysis is the reverse of condensation. Water is needed, and acid or alkali together with heat provides suitable conditions for cleavage of the peptide bond.
Which functional group is present on the nitrogen-containing end of an amino-acid product after acidic hydrolysis?
–NH₂
In acidic solution, the basic amino group is protonated rather than remaining mainly as –NH₂.
–NH₃⁺
Correct. The amino group accepts H⁺ in acidic solution and is present as –NH₃⁺.
–NO₂
Hydrolysis does not convert the amino group into a nitro group.
–CN
A nitrile group is not formed during peptide hydrolysis.
Acidic hydrolysis produces amino-acid-derived species in which the amino group is protonated, so the nitrogen-containing group is –NH₃⁺.
Which carboxyl-containing group is present after alkaline hydrolysis of a protein?
–CHO
Hydrolysis does not convert the carboxylic acid group into an aldehyde.
–COCl
An acyl chloride group is not formed in aqueous alkaline conditions.
–COOH
In alkaline solution, a carboxylic acid group is deprotonated, so it is present as a carboxylate ion.
–COO⁻
Correct. Under alkaline conditions, the carboxylic acid group exists as the carboxylate ion –COO⁻.
In alkaline hydrolysis, the amino-acid-derived products are present as carboxylate salts, so the carboxyl group is written as –COO⁻ with the appropriate positive counterion.
1. Proteins are condensation polymers of α-amino acids
You have already met α-amino acids as molecules containing both an amino group and a carboxylic acid group. For protein chemistry, the important feature is that these two functional groups allow one amino acid to join to another repeatedly.
In a general α-amino acid, the amino group, –NH₂, and the carboxylic acid group, –COOH, are attached to the same carbon atom. The side chain, R, can vary from one amino acid to another, but the functional groups involved in peptide formation are the same.
Protein chemistry at this level is mainly about making and breaking the –CONH– peptide bond.
The peptide bond
A peptide bond forms when the carboxylic acid group of one amino acid reacts with the amino group of another. The carbonyl carbon becomes directly bonded to nitrogen, producing the amide linkage –CONH–.
Example dipeptide containing one peptide bond
In the structure above, the sequence –C(=O)–NH– is the peptide bond. Because it is an amide functional group, the terms peptide bond and amide linkage refer to the same structural feature in this context.
Before condensation: one amino acid provides a –COOH group and another provides an –NH₂ group.
After condensation: the two residues are connected by –CONH– and one molecule of H₂O has been eliminated.
From a dipeptide to a polypeptide
When two amino acids join, the product is a dipeptide. If condensation continues, more amino-acid residues are added and a polypeptide chain is formed. Proteins are built from such polypeptide chains.
Each new peptide bond formed in a linear chain corresponds to one condensation step and the elimination of one H₂O molecule. For example, joining three amino acids into one tripeptide requires two peptide bonds, so two H₂O molecules are eliminated.
A quick way to count is to focus on the links rather than the monomers. A linear chain containing four amino-acid residues has three peptide bonds; therefore three H₂O molecules were eliminated when that chain formed.
2. Forming peptide bonds by condensation
The formation of a peptide bond is a condensation reaction because two molecules join while a small molecule, water, is removed.
–COOH + H₂N– → –CONH– + H₂O
The equation is only a shorthand for the reacting functional groups. The rest of each amino-acid molecule remains attached on either side of the newly formed peptide bond.
Glycine + alanine
H₂NCH₂COOH + H₂NCH(CH₃)COOH
condensation, −H₂O
→
One possible dipeptide
H₂NCH₂CONHCH(CH₃)COOH
Where does the water come from?
To see the condensation clearly, look only at the two groups that react. An –OH is lost from the carboxylic acid group, while an H is lost from the amino group. These combine to form H₂O, and the carbonyl carbon then remains bonded to nitrogen.
Part of the reacting groups
What happens during condensation?
Result
–COOH
Contributes –OH
Part of H₂O is removed
–NH₂
Contributes H
Part of H₂O is removed
Carbonyl carbon and nitrogen
Remain bonded together
Peptide bond –CONH– forms
3. Hydrolysis breaks peptide bonds
Hydrolysis is the reverse of condensation. Instead of removing water to make the peptide bond, water is used to break the peptide bond and regenerate the functional groups of the amino-acid residues.
Peptide bonds are not simply broken by adding cold water. In the laboratory, proteins are hydrolysed by heating with aqueous acid or aqueous alkali, often under reflux.
–CONH– + H₂O → –COOH + H₂N–
This equation shows the underlying bond-breaking idea. However, the actual form of the amino-acid products depends on whether the hydrolysis takes place in acidic or alkaline solution.
Acidic hydrolysis
When a protein is heated with an aqueous acid such as HCl(aq), the peptide bonds are hydrolysed. The amino groups of the products are basic, so in the acidic solution they are protonated to form –NH₃⁺ groups.
H₂N–CH(R)–COOH + H⁺ → ⁺H₃N–CH(R)–COOH
Therefore, when drawing products directly from acidic hydrolysis, do not leave the amino group as neutral –NH₂. Show the protonated form –NH₃⁺. If hydrochloric acid is used, Cl⁻ is the counterion of the ammonium salt.
Alkaline hydrolysis
When a protein is heated with aqueous alkali such as NaOH(aq), peptide bonds are again hydrolysed. This time, the carboxylic acid groups of the products are deprotonated by OH⁻, so they are present as carboxylate ions.
H₂N–CH(R)–COOH + OH⁻ → H₂N–CH(R)–COO⁻ + H₂O
Thus, after alkaline hydrolysis, draw the carboxyl group as –COO⁻, together with the appropriate positive counterion such as Na⁺ if NaOH was used.
Hydrolysis conditions
Nitrogen-containing group in product
Carboxyl-containing group in product
Aqueous acid, heat
–NH₃⁺
–COOH
Aqueous alkali, heat
–NH₂
–COO⁻
Always separate two ideas: hydrolysis breaks the peptide bond; the pH of the reaction mixture determines whether the resulting amino and carboxyl groups are protonated or deprotonated.
4. How to work out the products of protein hydrolysis
In an examination question, a polypeptide may look intimidating because several amino-acid residues are joined together. The safest method is to ignore the overall size at first and locate every –CONH– peptide link.
Step 1: identify each peptide bond
Look for the repeating sequence –C(=O)–NH–. Each occurrence marks a point where the chain can be hydrolysed.
Step 2: split the chain at each peptide bond
Complete hydrolysis breaks all peptide bonds. The fragments therefore return to individual amino-acid units rather than remaining as shorter peptides.
Step 3: adjust the products for the reaction conditions
Under acidic conditions, write the amino group as –NH₃⁺. Under alkaline conditions, write the carboxyl group as –COO⁻. This final acid-base step is where many otherwise correct hydrolysis answers lose marks.
Suppose a dipeptide contains glycine and alanine residues. Complete acidic hydrolysis gives the glycine and alanine units with protonated amino groups. Complete alkaline hydrolysis gives the corresponding carboxylate salts instead. The carbon skeletons of the original amino-acid residues are unchanged; only the peptide link is broken and the acid-base form of each product depends on the medium.
Common Mistakes
Common mistake
Why it is wrong
Better approach
Calling –COO– a peptide bond
A peptide bond is an amide linkage containing nitrogen: –CONH–.
Look specifically for C(=O) directly bonded to N.
Forgetting that H₂O is eliminated during peptide formation
Peptide formation is a condensation reaction.
Remember: –OH from –COOH and H from –NH₂ combine to form H₂O.
Drawing –NH₂ after acidic hydrolysis
The amino group is protonated in acidic solution.
Write –NH₃⁺ for the nitrogen-containing group under acidic conditions.
Drawing –COOH after alkaline hydrolysis
The carboxylic acid group is deprotonated by the alkali.
Write –COO⁻ and include the appropriate positive counterion where required.
What you should be able to do
Recognise the monomers
Identify α-amino acids as the monomers from which proteins are built.
Identify the peptide bond
Recognise and draw the –CONH– linkage in a peptide or protein chain.
Explain condensation
Explain how amino acids join while eliminating H₂O to form peptide bonds.
Relate peptides to proteins
Describe how repeated condensation produces polypeptide chains.
State hydrolysis conditions
State that peptide bonds can be hydrolysed by heating with aqueous acid or aqueous alkali.
Predict hydrolysis products
Draw the correct amino-acid-derived products after acidic or alkaline hydrolysis, including the appropriate charges.
Mastery Quiz
Can You Apply Protein Chemistry?
These ten questions test whether you can apply peptide-bond formation and hydrolysis to structures and unfamiliar examples.
10 mastery questions
Structures and reaction ideas where useful
Instant explanation for every answer option
Question 1 of 10
Quiz complete
How did you do?
A linear tripeptide contains three amino-acid residues. How many peptide bonds does it contain?
1
One peptide bond would join only two residues into a dipeptide.
2
Correct. Three residues joined in one linear chain require two links between them.
3
The number of peptide bonds is one less than the number of residues in a linear chain.
4
Four peptide bonds would require more than three residues in a simple linear chain.
A linear chain of n amino-acid residues contains n − 1 peptide bonds. Therefore a tripeptide contains 2 peptide bonds.
How many H₂O molecules are eliminated when four amino acids form one linear tetrapeptide?
4
Four residues do not require four links; there are only three links between adjacent residues.
2
Two condensation steps would only create two peptide bonds.
3
Correct. A tetrapeptide has three peptide bonds, and each forms with elimination of one H₂O molecule.
1
One H₂O corresponds to formation of only one peptide bond.
Four residues joined linearly require three condensation reactions, so three molecules of H₂O are eliminated.
Which statement best describes complete hydrolysis of a peptide bond?
Water is used to break the –CONH– linkage and regenerate amino-acid-derived functional groups.
Correct. Hydrolysis reverses peptide-bond formation by using water to split the linkage.
Water is eliminated to break the –CONH– linkage.
Eliminating water describes condensation, not hydrolysis.
The nitrogen atom is removed from the molecule.
Nitrogen remains part of the amino-acid-derived product; the peptide linkage is broken.
The carbonyl group is reduced to an alcohol.
Hydrolysis is not a reduction of the carbonyl group.
Condensation removes water to make a peptide bond. Hydrolysis does the reverse: it uses water to break the bond.
The dipeptide shown is completely hydrolysed by heating with excess HCl(aq). Which description of the products is correct?
Dipeptide
The peptide remains unchanged because HCl only protonates the terminal amino group.
Heating with aqueous acid hydrolyses the peptide bond as well as protonating amino groups.
Two amino-acid products form, each containing –NH₂ and –COO⁻.
That acid-base form corresponds to neither acidic hydrolysis nor alkaline hydrolysis.
Two amino-acid products form, each containing –NH₂ and –COOH.
The peptide bond is correctly hydrolysed, but in strongly acidic solution the amino groups are protonated to –NH₃⁺.
Two amino-acid products form, each containing –NH₃⁺ and –COOH.
Correct. Complete acidic hydrolysis splits the dipeptide and protonates the amino groups.
Acidic hydrolysis breaks the peptide bond and gives amino-acid-derived ammonium species. If HCl is used, Cl⁻ ions act as counterions.
A protein is completely hydrolysed by heating with excess NaOH(aq). Which form of each amino-acid-derived product is expected?
⁺H₃N–CH(R)–COOH
This is the protonated form expected in acidic solution, not strongly alkaline solution.
H₂N–CH(R)–COO⁻
Correct. In alkaline solution, the carboxylic acid group is present as a carboxylate ion.
H₂N–CH(R)–COOH
In excess alkali, the carboxylic acid group is deprotonated to –COO⁻.
⁺H₃N–CH(R)–COO⁻ only
A zwitterionic representation is not the correct form to show for a product remaining in excess strong alkali.
Alkaline hydrolysis produces carboxylate salts. The amino group is written as –NH₂ and the carboxyl group as –COO⁻ under these conditions.
Why can a peptide bond also be described as an amide linkage?
Because it contains an –OH group bonded directly to nitrogen.
The defining feature is a carbonyl carbon bonded to nitrogen, not an –OH bonded to nitrogen.
Because it contains a C=C bond next to nitrogen.
A peptide bond does not require a carbon-carbon double bond.
Because it contains the –C(=O)–N– arrangement characteristic of an amide.
Correct. The peptide bond is chemically an amide linkage.
Because every peptide bond contains an –NH₂ group.
Within the linkage, nitrogen is part of –CONH–, not necessarily a free –NH₂ group.
The peptide linkage has a carbonyl carbon directly bonded to nitrogen, which is the structural pattern of an amide functional group.
Which change is the reverse of peptide-bond formation?
Hydrolysis of –CONH– using water
Correct. Hydrolysis uses water to reverse the condensation that formed the peptide bond.
Further condensation with elimination of H₂O
Further condensation would form additional peptide bonds rather than break them.
Oxidation of the side chain
Changing a side chain is not the reverse of peptide-bond formation.
Protonation of the amino group only
Protonation changes charge but does not break the peptide bond.
Condensation and hydrolysis are opposite processes: condensation removes water to form the linkage, while hydrolysis uses water to break it.
How many amino-acid residues and peptide bonds are present in the structure shown?
Peptide structure
1 residue and 1 peptide bond
The structure contains two amino-acid-derived portions joined together.
2 residues and 2 peptide bonds
Two residues in a linear dipeptide are connected by only one peptide bond.
3 residues and 2 peptide bonds
The displayed structure is a dipeptide, not a tripeptide.
2 residues and 1 peptide bond
Correct. Two amino-acid residues are linked by one –CONH– group.
Count the amino-acid-derived units on either side of the –CONH– link. This structure contains two residues connected by one peptide bond.
A linear polypeptide contains six amino-acid residues. Complete hydrolysis breaks every peptide bond. How many amino-acid-derived units are obtained?
5
Five is the number of peptide bonds in the original six-residue chain, not the number of residues released.
6
Correct. Complete hydrolysis releases the six original amino-acid-derived residues as separate products.
10
Hydrolysis separates the existing residues; it does not create extra carbon skeletons.
12
The chain contains only six amino-acid-derived units before hydrolysis.
Complete hydrolysis breaks all five peptide bonds in the six-residue chain, leaving six separate amino-acid-derived products.
Which statement correctly compares acidic and alkaline hydrolysis of a protein?
Only acidic conditions can break peptide bonds.
Both aqueous acid and aqueous alkali can hydrolyse peptide bonds when heated.
Both conditions give neutral H₂N–CH(R)–COOH as the only product form.
The ionic form of the products depends on the pH of the reaction mixture.
Correct. The peptide bonds are broken in both cases, but the acid-base form of the products differs.
Alkaline hydrolysis converts every amino group into –NH₃⁺.
–NH₃⁺ is favoured in acidic rather than alkaline solution.
The cleavage step is hydrolysis in both media. The important difference is the final protonation state: –NH₃⁺ in acid and –COO⁻ in alkali.
Frequently asked questions
Common questions about proteins and peptide bonds
The chemistry becomes much easier when you keep the peptide bond at the centre of the topic. These questions clear up the points students most often confuse when moving between amino acids, peptides and hydrolysis products.
Is a peptide bond the same as an amide bond?
In proteins, yes. The peptide bond is an amide linkage containing the structural unit –C(=O)–NH–. The term “peptide bond” is used because this amide linkage joins amino-acid residues together.
Why are proteins called condensation polymers?
They are formed when many α-amino acid monomers join together and each new peptide bond forms with elimination of a small molecule, H₂O. Repeated condensation therefore builds the polypeptide chain.
Why is water needed for hydrolysis if acid or alkali is also present?
Hydrolysis means bond cleavage involving water. The aqueous acid or alkali provides the reaction conditions that allow the peptide bond to be broken, while water supplies the components needed to regenerate the groups on the two fragments.
Why do acidic and alkaline hydrolysis give differently charged products?
The peptide bond is broken in both cases, but the surrounding pH changes the acid-base form of the products. In acid, amino groups are protonated to –NH₃⁺. In alkali, carboxylic acid groups are deprotonated to –COO⁻.
How can I avoid mistakes when drawing hydrolysis products?
First locate every –CONH– peptide bond and split the chain at those positions. Then reconstruct the amino-acid-derived units. Only after that should you adjust the charges for the conditions: –NH₃⁺ in acidic solution or –COO⁻ in alkaline solution.
Proteins become much less intimidating once you learn to read them as chains of amino-acid residues connected by one recurring functional group. Follow the –CONH– bond: condensation forms it, hydrolysis breaks it, and the reaction conditions determine the final ionic form of the amino-acid products.
