Chemical nature of amino acids MCQs With Answer

Chemical nature of amino acids MCQs With Answer is an essential topic for B. Pharm students studying biochemistry and pharmaceutical chemistry. This introduction covers amino acid structure, acid–base behavior, stereochemistry, side-chain classifications, pKa values, zwitterionic forms, peptide bond chemistry, and analytical tests, all in concise, exam-focused language. These keyword-rich MCQs emphasize concepts like isoelectric point, Henderson–Hasselbalch calculations, resonance in peptide bonds, and reactions used in amino acid analysis, helping students build strong foundational knowledge for drug design and pharmaceutical applications. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which statement best describes the zwitterionic form of amino acids at physiological pH?

• The amino group is neutral and the carboxyl group is neutral
• The amino group is protonated and the carboxyl group is deprotonated
• Both amino and carboxyl groups are deprotonated
• Both amino and carboxyl groups are protonated

Correct Answer: The amino group is protonated and the carboxyl group is deprotonated

Q2. Which amino acid is achiral and lacks an optical isomer?

• Alanine
• Glycine
• Valine
• Proline

Correct Answer: Glycine

Q3. Which amino acid contains a secondary amine (imino group) in its side chain?

• Proline
• Histidine
• Lysine
• Serine

Correct Answer: Proline

Q4. Which of the following amino acids is aromatic and strongly absorbs UV at 280 nm?

• Alanine
• Phenylalanine
• Tryptophan
• Leucine

Correct Answer: Tryptophan

Q5. Which pair of pKa values is most relevant for calculating the isoelectric point (pI) of a neutral amino acid?

• Side-chain pKa and COOH pKa
• COOH pKa and NH3+ pKa
• NH3+ pKa and side-chain pKa
• Any two pKa values

Correct Answer: COOH pKa and NH3+ pKa

Q6. What is the approximate pKa of the α-carboxyl group of most amino acids?

• ~2.2
• ~4.5
• ~7.4
• ~10.5

Correct Answer: ~2.2

Q7. Which amino acid side chain has a pKa near physiological pH (~6), enabling it to act as a proton donor/acceptor in enzyme active sites?

• Histidine
• Lysine
• Tyrosine
• Cysteine

Correct Answer: Histidine

Q8. Which reaction forms a peptide bond between two amino acids?

• Hydrolysis
• Oxidation
• Condensation (dehydration) reaction
• Isomerization

Correct Answer: Condensation (dehydration) reaction

Q9. What is the major resonance contributor that explains partial double-bond character of the peptide bond?

• Alpha carbon resonance
• Carbonyl oxygen radical
• Delocalization of lone pair on nitrogen into the carbonyl
• Pyrrole-like aromaticity

Correct Answer: Delocalization of lone pair on nitrogen into the carbonyl

Q10. Which reagent gives a purple color (Ruhemann’s purple) with most free amino acids during qualitative analysis?

• Benedict’s reagent
• Ninhydrin
• Biuret reagent
• Xanthoproteic reagent

Correct Answer: Ninhydrin

Q11. Which amino acid gives a yellow-orange color instead of purple with ninhydrin?

• Proline
• Glycine
• Alanine
• Serine

Correct Answer: Proline

Q12. Which class of amino acids is characterized by side chains that can form hydrogen bonds but are not charged at physiological pH?

• Hydrophobic amino acids
• Polar uncharged amino acids
• Aromatic amino acids
• Acidic amino acids

Correct Answer: Polar uncharged amino acids

Q13. Which amino acid contains a sulfur atom in a thioether linkage and is nonpolar?

• Cysteine
• Methionine
• Tyrosine
• Histidine

Correct Answer: Methionine

Q14. Which amino acid side chains are negatively charged at physiological pH?

• Aspartate and Glutamate
• Lysine and Arginine
• Serine and Threonine
• Phenylalanine and Tyrosine

Correct Answer: Aspartate and Glutamate

Q15. For an acidic amino acid like aspartic acid, the isoelectric point (pI) is calculated by averaging which pKa values?

• COOH and NH3+ pKa
• COOH pKa and side-chain COOH pKa
• NH3+ pKa and side-chain pKa
• Any two pKa values

Correct Answer: COOH pKa and side-chain COOH pKa

Q16. Which technique separates amino acids based on their isoelectric point and electric charge?

• Size-exclusion chromatography
• Isoelectric focusing
• Gas chromatography
• Distillation

Correct Answer: Isoelectric focusing

Q17. What is the role of pyridoxal phosphate (PLP) in amino acid metabolism?

• Acts as a protease inhibitor
• Serves as a coenzyme in transamination reactions
• Hydrolyzes peptide bonds
• Methylates amino acids

Correct Answer: Serves as a coenzyme in transamination reactions

Q18. Which amino acid side chain can form disulfide bonds in proteins?

• Methionine
• Cysteine
• Tryptophan
• Serine

Correct Answer: Cysteine

Q19. Which amino acid is classified as basic due to a positively charged side chain at physiological pH?

• Aspartate
• Lysine
• Phenylalanine
• Serine

Correct Answer: Lysine

Q20. Which chemical property explains why peptide bonds are planar and restrict rotation?

• Steric hindrance of side chains
• Hydrogen bonding between backbones
• Partial double-bond character due to resonance
• Ionic interactions with solvent

Correct Answer: Partial double-bond character due to resonance

Q21. Which amino acid side chain is hydrophobic and branched-chain, commonly found in aliphatic regions of proteins?

• Leucine
• Aspartate
• Histidine
• Tyrosine

Correct Answer: Leucine

Q22. Edman degradation is used to:

• Hydrolyze all peptide bonds simultaneously
• Sequence proteins by removing N-terminal residues stepwise
• Identify post‑translational modifications by mass
• Colorimetrically quantify amino acids

Correct Answer: Sequence proteins by removing N-terminal residues stepwise

Q23. Which amino acid(s) contribute most to absorbance at 280 nm used for protein quantitation?

• Arginine and Lysine
• Tryptophan and Tyrosine
• Glycine and Alanine
• Serine and Threonine

Correct Answer: Tryptophan and Tyrosine

Q24. Which test specifically detects aromatic amino acids by nitration of aromatic rings producing a yellow color?

• Biuret test
• Xanthoproteic test
• Ninhydrin test
• Benedict’s test

Correct Answer: Xanthoproteic test

Q25. Which statement about the α-carbon of most amino acids is correct?

• It is sp hybridized
• It is a stereocenter (chiral) except in glycine
• It always carries a double bond to nitrogen
• It is always planar

Correct Answer: It is a stereocenter (chiral) except in glycine

Q26. Which amino acid side chain can act as a nucleophile in enzyme catalysis due to its thiol group?

• Tyrosine
• Cysteine
• Proline
• Phenylalanine

Correct Answer: Cysteine

Q27. Which property best explains why amino acids exist predominantly as zwitterions in solid state and in aqueous solution?

• High temperature stability
• Internal proton transfer between amino and carboxyl groups
• Complete ionization of side chains
• Hydrophobic collapse

Correct Answer: Internal proton transfer between amino and carboxyl groups

Q28. Which amino acid is most likely to be found in the interior hydrophobic core of globular proteins?

• Serine
• Valine
• Asparagine
• Glutamate

Correct Answer: Valine

Q29. The Henderson–Hasselbalch equation is used to relate pH, pKa, and:

• Temperature
• Ionic strength
• Ratio of conjugate base to acid
• Molecular weight

Correct Answer: Ratio of conjugate base to acid

Q30. Which amino acid’s side chain can be phosphorylated as a common post-translational modification affecting protein function?

• Phenylalanine
• Tyrosine
• Leucine
• Methionine

Correct Answer: Tyrosine

Q31. Which reagent is typically used to label the N-terminal amino acid for identification (Sanger method)?

• Phenyl isothiocyanate (PITC)
• 2,4-Dinitrofluorobenzene (DNFB)
• DNPH (2,4-dinitrophenylhydrazine)
• Ninhydrin

Correct Answer: 2,4-Dinitrofluorobenzene (DNFB)

Q32. Which amino acid contains an imidazole side chain that can stabilize or destabilize charges in enzyme active sites?

• Histidine
• Lysine
• Aspartate
• Proline

Correct Answer: Histidine

Q33. Which of the following is not a characteristic of peptide bonds?

• Planarity
• Rotation around the C–N bond is restricted
• They are readily hydrolyzed under physiological conditions without catalysts
• They have partial double-bond character

Correct Answer: They are readily hydrolyzed under physiological conditions without catalysts

Q34. Which amino acid is classified as polar uncharged and contains a hydroxyl group that can hydrogen bond?

• Threonine
• Leucine
• Phenylalanine
• Arginine

Correct Answer: Threonine

Q35. Which amino acid side chain is positively charged and contains a guanidinium group?

• Lysine
• Arginine
• Histidine
• Glutamate

Correct Answer: Arginine

Q36. In a titration curve of a neutral amino acid, the buffering regions correspond to:

• Points where pH rapidly changes
• Regions near pKa values where acid and base forms are present
• The isoelectric point only
• Only the side-chain pKa

Correct Answer: Regions near pKa values where acid and base forms are present

Q37. Which statement about the isoelectric point (pI) of an amino acid is correct?

• It is the pH at which the amino acid is most soluble in organic solvents
• It is the pH at which the net electrical charge on the amino acid is zero
• It is always equal to pH 7
• It indicates the pH at which the amino acid is fully protonated

Correct Answer: It is the pH at which the net electrical charge on the amino acid is zero

Q38. Which analytical technique separates amino acids based on polarity and size using a stationary phase and mobile phase?

• Isoelectric focusing
• Paper or thin-layer chromatography
• Mass spectrometry
• X-ray crystallography

Correct Answer: Paper or thin-layer chromatography

Q39. Which amino acid has a phenolic hydroxyl group capable of hydrogen bonding and can be ionized at high pH?

• Tyrosine
• Phenylalanine
• Valine
• Glycine

Correct Answer: Tyrosine

Q40. Which process converts an amino acid’s amino group into a keto group, playing a central role in amino acid catabolism?

• Transamination
• Amidation
• Saponification
• Carboxylation

Correct Answer: Transamination

Q41. Which amino acid is most likely to disrupt an α-helix due to its unique cyclic structure?

• Alanine
• Proline
• Leucine
• Glutamine

Correct Answer: Proline

Q42. Which two functional groups are directly involved in peptide bond formation?

• Side-chain hydroxyl and side-chain amine
• α-Carboxyl group and α-amino group
• Backbone carbonyl and backbone hydroxyl
• Side-chain thiol and side-chain carboxyl

Correct Answer: α-Carboxyl group and α-amino group

Q43. What is the primary reason serine, threonine, and tyrosine are common phosphorylation sites?

• They are hydrophobic residues
• They contain hydroxyl groups that can be phosphorylated
• They have acidic side chains
• They are aromatic only

Correct Answer: They contain hydroxyl groups that can be phosphorylated

Q44. Which property of amino acids is most directly responsible for their behavior as buffers in solution?

• Their ability to form peptide bonds
• The presence of ionizable groups with specific pKa values
• Their molecular weight
• Their optical activity

Correct Answer: The presence of ionizable groups with specific pKa values

Q45. Which side chain is most likely to form hydrogen bonds with water and other polar molecules?

• Phenylalanine
• Serine
• Leucine
• Isoleucine

Correct Answer: Serine

Q46. Which amino acid has the highest basicity among common amino acids (highest side-chain pKa)?

• Lysine (pKa ~10.5)
• Arginine (pKa ~12.5)
• Histidine (pKa ~6.0)
• Glutamate (pKa ~4.2)

Correct Answer: Arginine (pKa ~12.5)

Q47. During protein biosynthesis, peptide bonds are formed by ribosomes between:

• Free amino acids and nucleotides
• Amino acids carried by tRNA at the ribosome’s A and P sites
• Two mRNA molecules
• Two tRNA molecules without ribosome involvement

Correct Answer: Amino acids carried by tRNA at the ribosome’s A and P sites

Q48. Which test detects peptide bonds and gives a violet or purple color in the presence of proteins?

• Ninhydrin test
• Biuret test
• Xanthoproteic test
• Bromine water test

Correct Answer: Biuret test

Q49. Which amino acid contains a hydroxyl group and often participates in hydrogen bonding and active site catalysis?

• Alanine
• Serine
• Valine
• Phenylalanine

Correct Answer: Serine

Q50. In Henderson–Hasselbalch form, when pH = pKa for an ionizable group, what is the ratio of [A−] to [HA]?

• 10:1
• 1:10
• 1:1
• 0:1

Correct Answer: 1:1

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

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