JEE Mains Chemistry · Biomolecules

Peptides and Protein Structure

Amino acids join by peptide (amide) bonds into chains read from the free NH₂ end to the free COOH end; a chain of n residues has n − 1 peptide bonds, and a protein's shape is built in four levels, of which denaturation destroys all but the primary sequence.

All Biomolecules notes JEE Mains Chemistry strategy

Why this matters

Thirty PYQs, twenty-one multiple choice and nine asking for a number, four from 2026. Twelve are counts: peptide bonds, possible sequences, a minimum molar mass, sp² carbons, or how many compounds give the biuret test. Four read a drawn peptide or build a dipeptide from its parts. Fourteen are on the levels of protein structure, the forces that hold them, fibrous against globular proteins, and what denaturation destroys.

Concept 1 of 3: Counting peptide bonds and peptide sequences

A peptide bond forms each time the COOH of one amino acid condenses with the NH₂ of the next and water is lost. A straight chain of n residues therefore has n − 1 such bonds. A chain has a direction, from its free NH₂ end to its free COOH end, so Gly-Ala and Ala-Gly are different peptides; counting sequences is counting ordered arrangements.

Definition

Peptide bond: the amide link $\mathrm{-CO{-}NH-}$ . Dipeptide = 2 residues, tripeptide = 3; oligopeptides have up to ten residues and polypeptides more. NCERT calls a polypeptide with more than a hundred residues and a mass above 10 000 u a protein.
A linear chain of $n$ residues has $n-1$ peptide bonds, so residues minus bonds is always 1.
A name such as alanylglycylvaline lists one residue per part: count the "-yl" parts and add the last one.
Hydrolysis products in mole ratio give the residue count: 2 mol X and 1 mol Y per mol of peptide means 3 residues.
Sequences: $n$ different amino acids, each used once, give $n!$ sequences. With $k$ kinds of amino acid and repetition allowed, $k^n$ chains of $n$ residues. Read the stem to see which applies; the JEE keys count repeats such as Val-Val-Val when the stem does not forbid them.
Minimum molar mass: if the protein contains at least one residue of an amino acid that makes up $p$ % of its mass, $M_{\min} = M \times 100/p$ , where the JEE keys take $M$ as the amino acid's own molar mass.
Biuret test: a violet colour needs at least two peptide bonds, so tripeptides and proteins give it, a dipeptide does not, and biuret itself does.
sp² carbons: each peptide C=O, each COOH carbon and each carbon of an aromatic ring.

Peptide bonds, sequences and minimum molar mass

n_{\text{peptide bonds}} = n - 1 \qquad N_{\text{no repeats}} = n! \qquad N_{\text{repeats allowed}} = k^{\,n} \qquad M_{\min} = \frac{M \times 100}{p}

Worked example

A pentapeptide contains five different amino acids, each once. How many sequences are possible, how many peptide bonds does it have, and what is residues minus peptide bonds?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 2 Apr 2025 · Q119Moderate

Example 1 · Biomolecules · Peptides and Protein Structure

A tetrapeptide "

x

" on complete hydrolysis produced glycine (Gly), alanine (Ala), valine (Val), leucine (Leu) in equimolar proportion each. The number of tetrapeptides (sequences) possible involving each of these amino acids is.

A chain has a direction

Gly-Ala has glycine at the free NH₂ end and Ala-Gly has alanine there; they are different compounds. Count ordered arrangements (n!), not unordered choices.

n residues make n − 1 bonds

Seven residues are held by six peptide bonds, not seven. The last residue's COOH stays free.

The biuret test needs two peptide bonds

Glycine has none and glycylalanine has one, so neither gives the violet colour. A tripeptide and biuret itself do.

Drill 11 more on counting peptide bonds and peptide sequences

Concept 2 of 3: Reading and writing peptide sequences

A peptide is written from the residue with the free NH₂ (the N-terminal) to the one with the free COOH (the C-terminal). To read a drawn peptide, start at the free NH₂, walk along the chain from one α-carbon to the next, and name each residue by its side chain. To make a chosen dipeptide, the COOH end of the first residue must meet the NH₂ of the second.

Definition

Sequence order is N-terminal → C-terminal. In a name every residue but the last ends in "-yl": serylalanine is Ser-Ala, with serine at the free NH₂ end.
Side chains to recognise in a drawing: $\mathrm{-H}$ glycine, $\mathrm{-CH_3}$ alanine, $\mathrm{-CH_2OH}$ serine, $\mathrm{-CH(OH)CH_3}$ threonine, $\mathrm{-CH_2COOH}$ aspartic acid, $\mathrm{-CH_2CH(CH_3)_2}$ leucine, $\mathrm{-CH_2C_6H_5}$ phenylalanine, $\mathrm{-CH_2C_6H_4OH}$ tyrosine.
Building a dipeptide: the acid chloride of the N-terminal amino acid reacts with the free $\mathrm{NH_2}$ of the C-terminal one, losing HCl. $\mathrm{H_2N{-}CH_2{-}COCl}$ with $\mathrm{H_2N{-}CH(CH_3){-}COOH}$ gives Gly-Ala.
Two reactions used to identify the residues come from other chapters, not NCERT Biomolecules: nitrous acid turns an α-amino acid into the α-hydroxy acid with loss of $\mathrm{N_2}$ (alanine gives lactic acid), and glycine on heating loses water to give a cyclic dimer, 2,5-diketopiperazine.

A dipeptide, written N-terminal first

\mathrm{H_2N{-}CH(R_1){-}CO{-}NH{-}CH(R_2){-}COOH} = \text{residue 1-residue 2}

Worked example

Write the condensed formula of the dipeptide Ser-Ala, give its name, and say which residue carries the free

\mathrm{NH_2}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 29 January 2023 · Q129Moderate

Example 2 · Biomolecules · Peptides and Protein Structure

Following tetrapeptide can be represented as (F, L, D, Y, I, Q, P are one letter codes for amino acids)

Read from the free NH₂ end

A drawn peptide read from the COOH end gives the sequence backwards: FLDY would come out as YDLF. Find the free NH₂ first.

The acid chloride belongs to the first residue

To make Gly-Ala, glycine supplies the COCl and alanine the free NH₂. The reverse pairing, glycine's COOH with alanine's COCl, gives Ala-Gly.

Drill 3 more on reading and writing peptide sequences

Concept 3 of 3: Levels of protein structure and denaturation

The primary structure is the sequence, held by covalent peptide bonds. Everything above it, the helix or sheet, the overall fold and the packing of several chains, is held by weaker forces, mainly hydrogen bonds. Heat or a change in pH breaks those weak forces but not the peptide bonds. So the protein unfolds and loses its biological activity, while its sequence survives.

Definition

Fibrous proteins: chains run parallel, held by hydrogen and disulphide bonds; insoluble in water. Keratin (hair, wool, silk), myosin (muscles), collagen.
Globular proteins: chains coil into a sphere; soluble in water. Insulin, albumins.
Denaturation (NCERT): a physical change such as heat or a chemical change such as pH disturbs the hydrogen bonds; globules unfold and helices uncoil, and the protein loses its biological activity. The secondary and tertiary structures are destroyed; the primary structure remains intact.
Examples: coagulation of egg white on boiling; curdling of milk by the lactic acid that bacteria make in it.
A peroxide link ( $\mathrm{-O{-}O-}$ ) plays no part in protein structure.

Level	What it describes	Held by	After denaturation
Primary	The sequence of amino acids in each chain	Peptide (covalent amide) bonds	Intact
Secondary, α-helix	The chain coiled into a right-handed spiral	Hydrogen bonds between the C=O and N–H of peptide bonds on neighbouring turns	Lost; the helix uncoils
Secondary, β-pleated sheet	Chains stretched out and laid side by side	Hydrogen bonds between the C=O and N–H of neighbouring chains	Lost
Tertiary	The overall folding of the chain, which gives the fibrous or globular shape	Hydrogen bonds, disulphide links, van der Waals and electrostatic forces	Lost; globules unfold
Quaternary	The spatial arrangement of two or more polypeptide subunits	The same weak forces acting between the subunits	Lost

Only the primary structure is held by covalent peptide bonds, and only it survives denaturation.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 5 Apr 2026 Shift 2 · Q43Moderate

Example 3 · Biomolecules · Peptides and Protein Structure

Identify the incorrect statement about tertiary structure of proteins.

Quaternary structure is not the overall fold

The overall folding of one chain is the tertiary structure. The quaternary structure is how separate subunits pack together.

Denaturation keeps the peptide bonds

Boiling an egg destroys the secondary and tertiary structures but breaks no peptide bond, so the primary structure stays. A statement that heating breaks the peptide linkages is false.

Fibrous proteins are the insoluble ones

Keratin, collagen and myosin are fibrous and insoluble; albumin and insulin are globular and soluble. Acids denature the soluble globular form, not a soluble fibrous one.

Drill 13 more on levels of protein structure and denaturation

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Counting peptide bonds and peptide sequences
Peptide bonds, sequences and minimum molar mass
$n_{\text{peptide bonds}} = n - 1 \qquad N_{\text{no repeats}} = n! \qquad N_{\text{repeats allowed}} = k^{\,n} \qquad M_{\min} = \frac{M \times 100}{p}$
Reading and writing peptide sequences
A dipeptide, written N-terminal first
$\mathrm{H_2N{-}CH(R_1){-}CO{-}NH{-}CH(R_2){-}COOH} = \text{residue 1-residue 2}$

Reference tables (1)

Levels of protein structure and denaturation5 rows

Level	What it describes	Held by	After denaturation
Primary	The sequence of amino acids in each chain	Peptide (covalent amide) bonds	Intact
Secondary, α-helix	The chain coiled into a right-handed spiral	Hydrogen bonds between the C=O and N–H of peptide bonds on neighbouring turns	Lost; the helix uncoils
Secondary, β-pleated sheet	Chains stretched out and laid side by side	Hydrogen bonds between the C=O and N–H of neighbouring chains	Lost
Tertiary	The overall folding of the chain, which gives the fibrous or globular shape	Hydrogen bonds, disulphide links, van der Waals and electrostatic forces	Lost; globules unfold
Quaternary	The spatial arrangement of two or more polypeptide subunits	The same weak forces acting between the subunits	Lost

Only the primary structure is held by covalent peptide bonds, and only it survives denaturation.

Watch out for (8)

A chain has a direction
→ Counting peptide bonds and peptide sequences
n residues make n − 1 bonds
→ Counting peptide bonds and peptide sequences
The biuret test needs two peptide bonds
→ Counting peptide bonds and peptide sequences
Read from the free NH₂ end
→ Reading and writing peptide sequences
The acid chloride belongs to the first residue
→ Reading and writing peptide sequences
Quaternary structure is not the overall fold
→ Levels of protein structure and denaturation
Denaturation keeps the peptide bonds
→ Levels of protein structure and denaturation
Fibrous proteins are the insoluble ones
→ Levels of protein structure and denaturation

Test yourself on Biomolecules

20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Biomolecules chapter test Drill all 30 questions