The peptide bond trap you need to fix: NEET regularly asks how many peptide bonds form when a given number of amino acids polymerise. Students who answer "N" instead of "N − 1" lose marks on what should be a free recall question.
α-Amino acids are organic compounds carrying both an amino group (−NH₂) and a carboxyl group (−COOH) bonded to the same carbon — the α-carbon. NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 420, defines them as the building blocks of proteins. Except glycine (R = H), all α-amino acids have a chiral α-carbon, making them optically active. Naturally occurring amino acids in proteins are L-configuration.
Peptide bond formation is a condensation reaction: the −COOH of one amino acid reacts with the −NH₂ of the next, releasing one water molecule and forming an amide (−CO−NH−) linkage. This bond is:
For a linear chain of N amino acid residues, exactly N − 1 peptide bonds form and N − 1 water molecules are released (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 422).
NEET watch-out: The question may phrase it as "dipeptide" (1 bond), "tripeptide" (2 bonds), or give a residue count and ask for bonds or water molecules lost. The answer is always residue count minus one.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
An α-amino acid is defined as an amino acid in which the amino group is attached to the:
Answer: C. By definition (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 420), α-amino acids have the −NH₂ group on the α-carbon — the carbon directly bonded to the −COOH group.
Why A is wrong: A is wrong because β-carbon is the second carbon from −COOH; amino group there would make it a β-amino acid, not α.
Why B is wrong: B is wrong because nitrogen in an amide bond is part of the peptide linkage formed after condensation, not the free amino group that defines the monomer.
Why D is wrong: D is wrong because the side chain (R group) varies among amino acids and does not carry the defining −NH₂ in α-amino acids.
Which α-amino acid lacks a chiral centre and is therefore optically inactive?
Answer: B. Glycine has R = H, so the α-carbon bears two identical substituents (two H atoms effectively — NH₂, COOH, H, H), making it achiral (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 420).
Why A is wrong: A is wrong because alanine has R = −CH₃; the α-carbon has four different groups (−NH₂, −COOH, −H, −CH₃) and is chiral.
Why C is wrong: C is wrong because valine has R = −CH(CH₃)₂; four different substituents on the α-carbon make it chiral.
Why D is wrong: D is wrong because leucine has R = −CH₂CH(CH₃)₂; four different groups on the α-carbon make it optically active.
The naturally occurring amino acids found in proteins predominantly belong to which stereochemical configuration?
Answer: C. Proteins in living organisms are built exclusively from L-amino acids (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 420).
Why A is wrong: A is wrong because D-amino acids are rare in nature and not incorporated into ribosomal protein synthesis in humans.
Why B is wrong: B is wrong because a racemic mixture would contain equal amounts of D and L forms; biological proteins use only L-form selectively.
Why D is wrong: D is wrong because meso forms require an internal plane of symmetry with two chiral centres — single amino acids with one α-carbon cannot be meso.
A peptide bond is formed between two amino acids by:
Answer: A. Peptide bond formation is a condensation (dehydration) reaction: the carboxyl group of one amino acid reacts with the amino group of the next, eliminating one water molecule and forming a −CO−NH− amide linkage (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 422).
Why B is wrong: B is wrong because removing H₂ from two amino groups would be an oxidative coupling of amines, not peptide bond formation; the reaction requires −COOH and −NH₂.
Why C is wrong: C is wrong because disulfide bridges (−S−S−) form between cysteine residues and stabilise tertiary/quaternary structure — they are not peptide bonds.
Why D is wrong: D is wrong because ionic (salt bridge) interactions between charged groups are non-covalent and occur in folded proteins; the peptide bond itself is a covalent amide linkage.
How many peptide bonds are present in a tripeptide?
Answer: B. A tripeptide contains 3 amino acid residues linked by (3 − 1) = 2 peptide bonds. Each bond forms between consecutive residues with loss of one water molecule.
Why A is wrong: A is wrong because 3 would equal the residue count; peptide bonds = N − 1, not N. This is the common off-by-one error.
Why C is wrong: C is wrong because 1 bond connects only 2 residues (dipeptide), not 3.
Why D is wrong: D is wrong because 4 bonds would require 5 residues (pentapeptide); tripeptide has only 2 bonds.
A polypeptide chain contains 50 amino acid residues. How many water molecules are released during its complete synthesis from free amino acids?
Answer: D. Each peptide bond formation releases one H₂O. For 50 residues: number of bonds = 50 − 1 = 49, so 49 water molecules are released.
Why A is wrong: A is wrong because using N instead of N − 1 is the standard off-by-one trap; 50 water molecules would require 51 residues.
Why B is wrong: B is wrong because N + 1 = 51 has no chemical basis; it reverses the subtraction direction.
Why C is wrong: C is wrong because 48 = N − 2 implies missing a bond; in a linear polypeptide every consecutive pair forms exactly one bond, giving N − 1.
Which of the following statements about the peptide bond is correct?
Answer: A. The lone pair on nitrogen delocalises into the adjacent C=O, giving the C−N bond partial double-bond character. This restricts rotation and makes the peptide bond planar (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 422).
Why B is wrong: B is wrong because free rotation applies to ordinary C−N single bonds; the peptide bond's resonance restricts rotation, making the six atoms (Cα, C, O, N, H, Cα) coplanar.
Why C is wrong: C is wrong because the peptide bond is a covalent amide linkage (−CO−NH−), not an electrostatic interaction between charged groups.
Why D is wrong: D is wrong because denaturation disrupts non-covalent interactions (H-bonds, hydrophobic, ionic); the covalent peptide bond remains intact and requires hydrolysis to break.
Complete hydrolysis of an oligopeptide into its constituent amino acids uses 7 water molecules. The number of amino acid residues in this oligopeptide is:
Answer: D. Complete hydrolysis breaks all peptide bonds; 7 water molecules consumed means 7 peptide bonds. Since bonds = N − 1, the residue count N = 7 + 1 = 8. (This is the reverse application of the condensation formula.)
Why A is wrong: A is wrong because 6 = 7 − 1 reverses the formula incorrectly; if there were 6 residues, only 5 peptide bonds would exist.
Why B is wrong: B is wrong because 7 residues would have only 6 peptide bonds and release/consume only 6 water molecules, not 7.
Why C is wrong: C is wrong because 9 residues would have 8 peptide bonds, requiring 8 water molecules for hydrolysis — one more than stated.
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Given
A linear polypeptide is synthesised from 20 amino acid residues.
Required
(a) Number of peptide bonds formed.
(b) Number of water molecules released.
Concept
Peptide bond formation is a condensation reaction. Each bond connects two consecutive residues with loss of one H₂O. For a linear (non-cyclic) chain, the number of bonds equals one less than the residue count.
Formula
Peptide bonds = N − 1
Water molecules released = N − 1
Substitution
N = 20
Peptide bonds = 20 − 1
Water released = 20 − 1
Calculation
Peptide bonds = 19
Water released = 19
Note on exact constants: N = 20 is a counting integer (exact number of residues); the "−1" is a structural fact (the first residue has no preceding partner). Neither introduces rounding considerations.
Final answer
19 peptide bonds are formed and 19 water molecules are released.
Common trap
The off-by-one error: answering "20" by equating bonds to residue count. Remember — it takes two residues to make the first bond, so the count is always one fewer.
Similar NEET-style question
"A polypeptide on complete hydrolysis yields 35 amino acids. How many peptide bonds were present in the original polypeptide?"
Answer: 35 − 1 = 34.
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Building blocks of proteins. R-CH(NH₂)-COOH structure. Classification: essential (must be in diet) vs non-essential. Acid-base: zwitterionic at physiological pH; isoelectric point (pI).
-- NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, p. 421Peptide bond (-CO-NH-) formed between -COOH of one amino acid and -NH₂ of next. Polymer of amino acids = polypeptide / protein.
-- NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, p. 422More in Biomolecules: 4 exam traps and mistakes · 3 formulas from its other lessons.
2 questions from NEET 2020, 2026. Answers verified against NTA official keys.
Which of the following is a basic amino acid ?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
swapped classes
Tempts surface-level recall.
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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