ChemistryCBSE Class 12 · Biomolecules

Proteins: Amino Acids, Peptide Bond and Structure

Updated 15 October 2026 · Fact-checked

Proteins are polymers of α-amino acids joined by peptide bonds (–CO–NH–), formed when the –COOH of one amino acid reacts with the –NH₂ of the next and water is lost. Their structure has four levels: primary (sequence), secondary (α-helix or β-pleated sheet), tertiary (overall folding) and quaternary (arrangement of subunits). Heat or pH change denatures it.

Understand Proteins: Amino Acids, Peptide Bond and Structure

Amino acids. Proteins are built from α-amino acids, in which an amino group and a carboxyl group are attached to the same (α) carbon: $H_{2} N - C H R - C O O H$ . The side chain R varies; for glycine R = H, for alanine R = $C H_{3}$ . Amino acids are classified as neutral (one $- N H_{2}$ and one $- C O O H$ , like glycine), acidic (more –COOH, like aspartic and glutamic acid) or basic (more –NH₂, like lysine). Those the body can make are non-essential; those that must come from the diet are essential (NCERT lists valine, leucine, isoleucine, arginine, lysine, threonine, methionine, phenylalanine, tryptophan and histidine).

Properties. Amino acids are colourless, crystalline, water-soluble solids with high melting points. They behave like salts because in aqueous solution the carboxyl group loses a proton and the amino group gains one, forming a dipolar ion, the zwitter ion: $H_{3} N^{+} - C H R - C O O^{-}$ . Because the zwitter ion can react with both acids and bases, amino acids are amphoteric. Except glycine, all naturally occurring α-amino acids are optically active (the α-carbon is chiral), and most have the L-configuration.

Peptide bond. The –COOH of one amino acid condenses with the –NH₂ of another, losing water and forming an amide link, $- C O - N H -$ , called the peptide bond. Two amino acids give a dipeptide: glycine + alanine → glycylalanine (Gly-Ala), $H_{2} N - C H_{2} - C O - N H - C H (C H_{3}) - C O O H$ . A chain of $n$ amino acids contains $n - 1$ peptide bonds. Chains of more than ten amino acids are polypeptides; a polypeptide with more than a hundred amino acids (molar mass above about 10 000 u) is a protein.

Fibrous and globular proteins. In fibrous proteins, chains run parallel and are held together by hydrogen and disulphide bonds, forming fibres that are generally insoluble in water: keratin (hair, wool, silk) and myosin (muscle). In globular proteins, chains coil into a roughly spherical shape; they are usually water soluble: insulin and albumins.

Levels of structure.

Primary: the sequence of amino acids in each chain. Any change in sequence creates a different protein.
Secondary: the shape the chain adopts because of hydrogen bonds between the >C=O and –NH– groups of peptide bonds. In the α-helix, the chain twists into a right-handed screw, with each –NH– hydrogen-bonded to a C=O of a nearby turn. In the β-pleated sheet, chains lie side by side, almost fully extended, and are held by hydrogen bonds between neighbouring chains, like the pleats of a curtain.
Tertiary: the overall folding of the chain, giving fibrous or globular shapes; stabilised by hydrogen bonds, disulphide linkages, van der Waals forces and electrostatic attractions.
Quaternary: the spatial arrangement of two or more polypeptide subunits in a protein that has them.

Denaturation. A native protein has a particular shape and biological activity. A change in temperature or pH disturbs the hydrogen bonds, so the globules unfold and helices uncoil, and activity is lost. This is denaturation: the 2° and 3° structures are destroyed but the primary structure is unchanged. Coagulation of egg white on boiling and curdling of milk (by lactic acid formed by bacteria) are everyday examples.

Proteins: Amino Acids, Peptide Bond and Structure: formulas and key facts

General α-amino acid: $H_{2} N - C H R - C O O H$; R = H (glycine), $C H_{3}$ (alanine), $C H (C H_{3})_{2}$ (valine).
Zwitter ion: $H_{2} N - C H R - C O O H ⇌ H_{3} N^{+} - C H R - C O O^{-}$; Explains high m.p., solubility and amphoteric behaviour.
Peptide bond formation: $H_{2} N C H_{2} C O O H + H_{2} N C H (C H_{3}) C O O H \to H_{2} N C H_{2} C O - N H C H (C H_{3}) C O O H + H_{2} O$; Gly-Ala; peptide bond is –CO–NH–.
Counting bonds and mass: $peptide bonds = n - 1; M_{p e pt i d e} = \sum M_{amin o a c i d s} - 18 (n - 1)$; Gly (75) + Ala (89) − 18 = 146 g mol⁻¹.
Levels of structure: $1° sequence; 2° α -helix, β -pleated sheet; 3° folding; 4° subunits$; Denaturation destroys 2° and 3°; 1° stays.

How to solve Proteins: Amino Acids, Peptide Bond and Structure questions

Protein questions are definitions, comparisons and structure-drawing, with a few counting problems on peptides. Use exact NCERT terms.

1For an amino acid, identify the α-carbon, the side chain R and whether it is neutral, acidic or basic.
2For zwitter ion questions, move H⁺ from –COOH to –NH₂ and show the charges.
3To write a peptide, join –COOH of the first (N-terminal) amino acid to –NH₂ of the next; remove $H_{2} O$ ; mark the –CO–NH–.
4Name a dipeptide from the N-terminal residue: Gly-Ala means glycine's –COOH is in the peptide bond.
5For counts: $n$ amino acids → $n - 1$ peptide bonds; molar mass = sum − 18 per bond; different sequences of $k$ distinct amino acids used once = $k!$ .
6For structure questions, state the bonding responsible at each level (peptide bonds; H-bonds; H-bonds, disulphide, van der Waals, ionic; subunit arrangement).
7For denaturation, name the cause (heat, pH), what is lost (2°, 3°, activity) and what remains (1°), with an everyday example.

Proteins: Amino Acids, Peptide Bond and Structure in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Denaturation by change in pH

Suppose your mother adds a few spoons of lemon juice to hot milk to make paneer. The milk separates into white curds and a clear greenish liquid (whey).

Explain what happens to the milk proteins in terms of protein structure. Which level of structure is unaffected?

Try it first, then show the solutionSolution

Lemon juice lowers the pH; heat adds energy.
The change in pH and temperature disturbs the hydrogen bonds (and ionic attractions) that hold the protein chains in their folded shape.
The protein molecules unfold and clump together, separating as curds: this is denaturation (coagulation).
Peptide bonds are not broken, so the sequence of amino acids (primary structure) is unchanged; secondary and tertiary structures are lost.

Answer: The milk proteins are denatured by acid and heat: their folded 2° and 3° structures collapse and they coagulate; the primary structure is unchanged.

Memory hook: Heat or acid unties a protein's folds but cannot cut its peptide chain.

Tricks to remember Proteins: Amino Acids, Peptide Bond and Structure

Short memory aids. Say each one aloud once, then use it on a question the same day.

Essential ten: 'PVT TIM HALL'
Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine.
Use it when: Listing essential amino acids.
1-2-3-4 = Sequence, Shape, Fold, Team
Primary: sequence; secondary: local shape (helix/sheet); tertiary: whole fold; quaternary: team of subunits.
Use it when: Describing structure levels.
Glycine is the odd one
Glycine's R is H, so its α-carbon has two H: the only common amino acid that is not optically active.
Use it when: MCQs on chirality of amino acids.

Proteins: Amino Acids, Peptide Bond and Structure: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: The achiral amino acid · 1 mark · MCQBoard level

Which of the following amino acids is optically inactive?
(a) Alanine (b) Glycine (c) Valine (d) Lysine

Try it first, then show the solutionSolution

Glycine is $H_{2} N C H_{2} C O O H$ ; its α-carbon has two hydrogens, so it is not chiral.

Answer: (b) Glycine.

Example 2: Zwitter ions · 2 marks · Very short answerBoard level

What is a zwitter ion? Explain why amino acids are amphoteric, with the structure of glycine as an example.

Try it first, then show the solutionSolution

In water the –COOH of an amino acid donates a proton to its own –NH₂, forming a dipolar ion: $H_{3} N^{+} - C H_{2} - C O O^{-}$ for glycine (1 mark).
The $- C O O^{-}$ part can accept a proton from acids and the $- N H_{3}^{+}$ part can donate one to bases, so amino acids react with both: they are amphoteric (1 mark).

Answer: A dipolar ion $H_{3} N^{+} C H_{2} C O O^{-}$ ; it reacts with both acids and bases.

Example 3: A dipeptide · 3 marks · Short answerHarder board level

(a) Write the structure of the dipeptide Gly-Ala and mark the peptide bond. (b) Calculate its molar mass. (c) How does Ala-Gly differ from Gly-Ala? (Glycine 75, alanine 89 g mol⁻¹)

Try it first, then show the solutionSolution

(a) $H_{2} N - C H_{2} - \underline{C O - N H} - C H (C H_{3}) - C O O H$ ; the peptide bond is –CO–NH– (1 mark).
(b) $75 + 89 - 18 = 146$ g mol⁻¹ (one molecule of water is lost) (1 mark).
(c) In Ala-Gly, alanine's –COOH forms the peptide bond and glycine carries the free –COOH: $H_{2} N C H (C H_{3}) C O - N H C H_{2} C O O H$ . The two are different compounds (structural isomers) (1 mark).

Answer: (a) $H_{2} N C H_{2} C O N H C H (C H_{3}) C O O H$ . (b) 146 g mol⁻¹. (c) The sequence is reversed; they are different dipeptides.

Example 4: Levels of protein structure · 3 marks · Short answerHarder board level

Distinguish between the primary, secondary, tertiary and quaternary structures of proteins, naming the forces involved at each level. Give two differences between the α-helix and the β-pleated sheet.

Try it first, then show the solutionSolution

Primary: sequence of amino acids, held by peptide (covalent) bonds. Secondary: α-helix or β-pleated sheet, held by hydrogen bonds between >C=O and –NH– of peptide groups (1 mark).
Tertiary: overall folding (fibrous or globular), held by hydrogen bonds, disulphide links, van der Waals and electrostatic forces. Quaternary: arrangement of two or more subunits (1 mark).
α-Helix: a single chain coils into a right-handed spiral with hydrogen bonds within the chain. β-Pleated sheet: extended chains lie side by side, with hydrogen bonds between neighbouring chains (1 mark).

Answer: 1° sequence; 2° helix/sheet by H-bonds; 3° folding by several forces; 4° subunit arrangement. Helix: intra-chain H-bonds, coiled; sheet: inter-chain H-bonds, extended.

Example 5: Denaturation and peptide counting · 5 marks · Long answerHOTS / competency

(a) When an egg is boiled, the clear, soluble albumin becomes white and insoluble. Name this change and explain which levels of structure are affected. (b) Why does an enzyme lose its activity when heated strongly? (c) How many different tripeptides can be formed from glycine, alanine and valine if each is used once? (d) Calculate the molar mass of Gly-Ala-Val. (Gly 75, Ala 89, Val 117 g mol⁻¹)

Try it first, then show the solutionSolution

(a) Denaturation (coagulation). Heat breaks hydrogen bonds; the globular protein unfolds and the chains aggregate. Secondary and tertiary structures are destroyed; the primary structure remains (1½ marks).
(b) Enzymes are globular proteins whose activity depends on their exact 3-D shape (active site). Heating denatures them, changing that shape (1 mark).
(c) $3! = 6$ sequences: GAV, GVA, AGV, AVG, VGA, VAG (1 mark).
(d) Two peptide bonds, so two water molecules are lost: $75 + 89 + 117 - 2 (18) = 245$ g mol⁻¹ (1½ marks).

Answer: (a) Denaturation; 2° and 3° lost, 1° intact. (b) The enzyme's shape is destroyed. (c) 6. (d) 245 g mol⁻¹.

Where students lose marks in Proteins: Amino Acids, Peptide Bond and Structure

The slips examiners see most often, and what to write instead.

Mistake: Writing the peptide bond as –CO–O– or –NH–NH–
Do this: The peptide bond is an amide: –CO–NH–.
Usually costs: 1 mark.
Mistake: Saying denaturation breaks peptide bonds
Do this: Denaturation disrupts H-bonds and other weak forces; peptide bonds (primary structure) remain.
Usually costs: 1 mark.
Mistake: Forgetting to subtract water in peptide masses
Wrong working
Gly-Ala = 75 + 89 = 164
Right working
Gly-Ala = 75 + 89 − 18 = 146 g mol⁻¹
Do this: Subtract 18 for every peptide bond formed.
Usually costs: 1 mark.
Mistake: Calling glycine optically active
Do this: Glycine's α-carbon has two H atoms; it is achiral.
Usually costs: 1 mark.

Exam tips

Write the zwitter ion with charges clearly: $H_{3} N^{+}$ and $C O O^{-}$ .
In dipeptide questions, underline or label the –CO–NH– bond.
Learn at least four essential and two non-essential amino acids by name.
Use 'hydrogen bonds between >C=O and –NH– of peptide bonds' when describing secondary structure.
For denaturation, always name what is lost and what stays.

Practice questions on Proteins: Amino Acids, Peptide Bond and Structure

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of the following is an essential amino acid?
(a) Glycine (b) Alanine (c) Valine (d) Glutamic acid
Hint
It must come from the diet.
Answer
(c) Valine.
Q2 · 2 marks · Very short answerBoard level
Distinguish between fibrous and globular proteins with one example each.
Hint
Shape and solubility.
Answer
Fibrous: chains lie parallel, held by H-bonds and disulphide bonds, forming fibres; insoluble in water (keratin, myosin). Globular: chains coil into a spherical shape; usually soluble in water (insulin, albumins).
Q3 · 1 mark · Very short answerHarder board level
A protein consists of a single chain of 120 amino acids. How many peptide bonds does it have, and how many molecules of water are released when it is formed from free amino acids?
Hint
$n - 1$ .
Answer
119 peptide bonds; 119 molecules of water are released.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Amino acids have high melting points and are soluble in water.
Reason (R): Amino acids exist as dipolar zwitter ions and behave like salts.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Ionic character.
Answer
(a).
Q5 · 3 marks · Short answerHarder board level
What is denaturation of proteins? Which structure remains intact? Give two examples from daily life.
Hint
Heat or pH.
Answer
Loss of the biological activity of a protein when a change in temperature or pH disrupts the hydrogen bonds, unfolding the globules and uncoiling the helix. The primary structure (sequence) remains intact; secondary and tertiary structures are lost. Examples: coagulation of egg white on boiling; curdling of milk by lactic acid.
Q6 · 2 marks · Very short answerHarder board level
Classify the following amino acids as neutral, acidic or basic: glycine, lysine, aspartic acid, alanine.
Hint
Count –NH₂ and –COOH.
Answer
Glycine and alanine: neutral (one –NH₂, one –COOH). Lysine: basic (two –NH₂). Aspartic acid: acidic (two –COOH).
Q7 · 3 marks · Short answerHOTS / competency
How many different tripeptides can be made from glycine and alanine if each position may be either amino acid (repeats allowed)? List them, and explain why Gly-Ala-Gly and Gly-Gly-Ala are different molecules.
Hint
Two choices for each of three positions.
Answer
$2^{3} = 8$ : GGG, GGA, GAG, AGG, GAA, AGA, AAG, AAA. Gly-Ala-Gly and Gly-Gly-Ala have the same composition but a different sequence (primary structure), so they are different compounds with different properties.
Q8 · 4 marks · Case-basedHOTS / competency
Read and answer.
Suppose a hair-care laboratory studies keratin, the main protein of hair. Keratin chains are long and lie parallel, held together by hydrogen bonds and by disulphide (–S–S–) links between cysteine units. Wet hair can be stretched, and hair treated with chemicals that break disulphide bonds can be reshaped.
(a) Is keratin a fibrous or globular protein? Give a reason. (1 mark)
(b) Name two types of bonds that maintain its structure. (1 mark)
(c) Which level(s) of protein structure do hydrogen bonds stabilise? (1 mark)
(d) Does breaking disulphide bonds change the primary structure? Explain. (1 mark)
Hint
Primary structure = peptide-bonded sequence.
Answer
(a) Fibrous: its chains lie parallel to form fibres and it is insoluble in water. (b) Hydrogen bonds and disulphide bonds. (c) Secondary (α-helix/β-sheet) and tertiary structure. (d) No; the sequence of amino acids joined by peptide bonds is unchanged; only the cross-links that fix the 3-D shape are broken.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is a peptide bond?Show answer
The amide link –CO–NH– between two amino acids.
2.Example of a fibrous protein?Show answer
Keratin (or myosin).
3.What holds the α-helix together?Show answer
Hydrogen bonds between >C=O and –NH– groups of the chain.
4.Which structure survives denaturation?Show answer
The primary structure.
5.Molar mass of Gly-Ala?Show answer
146 g mol⁻¹.
6.Which amino acid is not optically active?Show answer
Glycine.

Proteins: Amino Acids, Peptide Bond and Structure: frequently asked questions

What are proteins made of in Class 12 Chemistry?

Proteins are polymers of α-amino acids joined by peptide bonds (–CO–NH–).

What is the difference between essential and non-essential amino acids?

Non-essential amino acids can be made by the body; essential amino acids cannot and must be obtained from food.

What is a zwitter ion?

A dipolar ion, H_3N^+\text{–}CHR\text{–}COO^-, formed when an amino acid's carboxyl group transfers a proton to its amino group.

What are the four levels of protein structure?

Primary (sequence), secondary (α-helix or β-pleated sheet), tertiary (overall folding) and quaternary (arrangement of subunits).

What happens during denaturation?

Heat or a pH change disrupts the hydrogen bonds; the protein unfolds and loses its biological activity, but its primary structure is unchanged.