Synthesis of melatonin MCQs With Answer is a focused study guide for B. Pharm students covering both the biochemical and chemical routes to produce melatonin. This concise introduction explains key precursors, enzymes (tryptophan hydroxylase, AADC, AANAT, HIOMT/ASMT), cofactors like acetyl-CoA and SAM, common laboratory reagents (acetic anhydride, methylating agents) and industrial considerations such as yields, purification (HPLC, recrystallization), safety and green enzymatic approaches. Emphasis is placed on regulatory quality tests, analytical characterization and formulation challenges for a pharmaceutical perspective. The content is ideal for exam preparation and deeper understanding of pathway control, synthetic strategies and process optimization. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which amino acid is the primary precursor for melatonin biosynthesis?
- L-tyrosine
- L-tryptophan
- L-phenylalanine
- L-histidine
Correct Answer: L-tryptophan
Q2. Which enzyme catalyzes the hydroxylation of tryptophan to 5-hydroxytryptophan in melatonin biosynthesis?
- Tryptophan decarboxylase
- Tryptophan hydroxylase
- Aromatic L-amino acid decarboxylase
- Hydroxyindole O-methyltransferase
Correct Answer: Tryptophan hydroxylase
Q3. Which enzyme converts 5-hydroxytryptophan to serotonin by decarboxylation?
- Arylalkylamine N-acetyltransferase (AANAT)
- Tryptophan hydroxylase
- Aromatic L-amino acid decarboxylase (AADC)
Correct Answer: Aromatic L-amino acid decarboxylase (AADC)
Q4. What is the immediate enzymatic N-acetylation product of serotonin in melatonin biosynthesis?
- 5-Hydroxytryptamine
- N-acetylserotonin
- 5-Methoxytryptamine
- 5-Hydroxyindoleacetic acid
Correct Answer: N-acetylserotonin
Q5. Which enzyme catalyzes the O-methylation of N-acetylserotonin to form melatonin?
- Monoamine oxidase
- Hydroxyindole O-methyltransferase (HIOMT/ASMT)
- Aromatic L-amino acid decarboxylase
Correct Answer: Hydroxyindole O-methyltransferase (HIOMT/ASMT)
Q6. Where is the majority of endogenous melatonin synthesized in the human body?
- Adrenal cortex
- Hypothalamus
- Pineal gland
- Thyroid gland
Correct Answer: Pineal gland
Q7. Which neural structure primarily times melatonin synthesis via circadian signaling?
- Locus coeruleus
- Suprachiasmatic nucleus (SCN)
- Ventral tegmental area
- Hippocampus
Correct Answer: Suprachiasmatic nucleus (SCN)
Q8. In chemical synthesis routes, which reagent is commonly used for N-acetylation of an aminoethyl side chain to form the amide of melatonin?
- Acetic anhydride
- Sodium borohydride
- Pyridinium chlorochromate
- m-Chloroperbenzoic acid
Correct Answer: Acetic anhydride
Q9. Which methylating agent is frequently used for O-methylation of 5-hydroxy groups in laboratory syntheses?
- Dimethyl sulfoxide (DMSO)
- Methyl iodide (iodomethane)
- Sodium methylate
- Hydrogen peroxide
Correct Answer: Methyl iodide (iodomethane)
Q10. Which protecting group is commonly used for amine protection during multi-step chemical syntheses of tryptamine derivatives?
- Tert-butyloxycarbonyl (Boc)
- Methyl ester
- Benzoyl peroxide
- Hydroxyl protecting group
Correct Answer: Tert-butyloxycarbonyl (Boc)
Q11. What is the cofactor that supplies the acetyl group for AANAT-mediated acetylation?
- ATP
- Acetyl-CoA
- S-adenosylmethionine (SAM)
- Nicotinamide adenine dinucleotide (NAD+)
Correct Answer: Acetyl-CoA
Q12. What methyl donor cofactor is required by HIOMT/ASMT for O-methylation?
- FAD
- NADPH
- S-adenosylmethionine (SAM)
- Acetyl-CoA
Correct Answer: S-adenosylmethionine (SAM)
Q13. Which enzymatic step is generally considered rate-limiting in the biosynthesis of melatonin?
- Hydroxylation of tryptophan
- N-acetylation by AANAT
Correct Answer: N-acetylation by AANAT
Q14. Which intermediate is the direct substrate for HIOMT/ASMT in melatonin biosynthesis?
- Serotonin
- 5-Hydroxytryptophan
- N-acetylserotonin
- 5-Hydroxyindoleacetic acid
Correct Answer: N-acetylserotonin
Q15. For industrial chemical synthesis of melatonin, which substituted indole is a common synthetic starting material?
- 5-Methoxyindole
Correct Answer: 5-Methoxyindole
Q16. Which green chemistry strategy can be used to perform O-methylation more sustainably?
- Use of dimethyl sulfate without controls
- Enzymatic methylation with SAM recycling systems
- Excess methyl iodide under prolonged reflux
- Ozone oxidation
Correct Answer: Enzymatic methylation with SAM recycling systems
Q17. Which analytical technique is preferred for quantifying melatonin in bulk drug and finished dosage forms?
- Thin layer chromatography
- High-performance liquid chromatography (HPLC)
- Paper chromatography
- Flame photometry
Correct Answer: High-performance liquid chromatography (HPLC)
Q18. Which spectral method gives definitive structural information (functional group connectivity) for melatonin?
- Infrared spectroscopy only
- Mass spectrometry only
Correct Answer: Nuclear magnetic resonance (NMR) spectroscopy
Q19. What is the approximate molecular weight of melatonin (C13H16N2O2)?
- 150 g/mol
- 232 g/mol
- 350 g/mol
- 412 g/mol
Correct Answer: 232 g/mol
Q20. Which functional groups are present in the melatonin molecule?
- Phenol, ether and ketone
- Indole, amide and methoxy
- Carboxylic acid, ester and aldehyde
- Thiol, imine and nitro
Correct Answer: Indole, amide and methoxy
Q21. Which solvent profile best describes melatonin solubility for formulation work?
- Highly water soluble, insoluble in organic solvents
- Only soluble in supercritical CO2
Correct Answer: Soluble in organic solvents, sparingly soluble in water
Q22. Which analytical technique is most sensitive and specific for measuring melatonin in plasma samples?
- Colorimetric assay
- LC-MS/MS (liquid chromatography–tandem mass spectrometry)
- Paper chromatography
- Simple UV spectrophotometry
Correct Answer: LC-MS/MS (liquid chromatography–tandem mass spectrometry)
Q23. Which of the following reagents is classified as highly hazardous and carcinogenic commonly used historically for methylation reactions?
- Methanol
- Dimethyl sulfate (DMS)
Correct Answer: Dimethyl sulfate (DMS)
Q24. Which molecule provides the methyl group during O-methylation catalyzed by HIOMT/ASMT in vivo?
- Acetyl-CoA
- Formate
- S-adenosylmethionine (SAM)
- ATP
Correct Answer: S-adenosylmethionine (SAM)
Q25. Which neurotransmitter released from sympathetic nerve terminals stimulates melatonin production in the pineal gland?
- Serotonin
- Norepinephrine
Correct Answer: Norepinephrine
Q26. At what time of day is endogenous melatonin secretion typically highest?
- Midday
- Early morning
- Night / during dark period
- Late afternoon
Correct Answer: Night / during dark period
Q27. Which retinal cells containing melanopsin mediate light information that suppresses melatonin synthesis?
Correct Answer: Melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs)
Q28. Which intracellular second messenger increases AANAT activity and promotes melatonin synthesis?
- cGMP
- cAMP
- IP3
- Calcium alone
Correct Answer: cAMP
Q29. In a chemical acetylation step converting serotonin to N-acetylserotonin, which side product control is critical during scale-up?
Correct Answer: Acylation of aromatic ring and over-acetylation
Q30. Which laboratory base is commonly used with methyl iodide to perform O-methylation on phenolic or hydroxyindole substrates?
- Sodium borohydride
Correct Answer: Potassium carbonate (K2CO3)
Q31. What is the systematic chemical name commonly used for melatonin?
- N-acetyl-5-methoxytryptamine
- 5-Hydroxytryptamine
- 5-Methoxyindoleacetic acid
- N-methyltryptamine
Correct Answer: N-acetyl-5-methoxytryptamine
Q32. Which excipient class is commonly used to improve aqueous solubility of poorly water-soluble melatonin in oral formulations?
Correct Answer: Cyclodextrins or solubility enhancers
Q33. Which analytical parameter is essential for specification of melatonin active pharmaceutical ingredient (API) according to pharmacopeial standards?
Correct Answer: Content uniformity / assay
Q34. Which purification method is typically preferred for large-scale melatonin production to ensure cost-effective removal of impurities?
Correct Answer: Recrystallization
Q35. Which environmental factor most strongly degrades melatonin during storage?
Correct Answer: Exposure to light and oxidation
Q36. Is melatonin a chiral molecule?
- Yes, it has one stereocenter
- No, it is achiral
- Yes, it is racemic
- Only at extreme pH ranges
Correct Answer: No, it is achiral
Q37. Which commonly used chromatographic mobile phase is suitable for HPLC analysis of melatonin?
Correct Answer: Acetonitrile-water mixture (with buffer or formic acid)
Q38. Which impurity control is particularly important for methylation steps during melatonin synthesis?
Correct Answer: Unreacted hydroxy precursor and over-methylated by-products
Q39. Which sleeping disorder drug is a melatonin receptor agonist and structurally related to melatonin?
Correct Answer: Ramelteon
Q40. For biosynthetic regulation, which molecule from the retina inhibits melatonin production during light exposure?
Correct Answer: Signal via retinal ganglion cells to the SCN leading to reduced norepinephrine release
Q41. Which laboratory transformation converts a 5-hydroxy group on an indole ring to a 5-methoxy group?
Correct Answer: O-methylation
Q42. During scale-up, which class of tests is essential to monitor residual solvents after methylation steps?
- Microbial limit tests
Correct Answer: Residual solvent analysis (headspace GC)
Q43. Which chromatographic detector is commonly used with HPLC for melatonin quantitation in quality control?
Correct Answer: UV-Vis detector
Q44. Which reagent would most directly convert 5-hydroxyindole to 5-methoxyindole under SN2 conditions?
- Methyl iodide with base
Correct Answer: Methyl iodide with base
Q45. Which approach helps minimize toxic methylation reagents in an industrial melatonin synthesis?
- Use excess dimethyl sulfate without containment
- Enzymatic methylation or safer alkylating agents with containment and scavenging
- Perform methylation at open bench without ventilation
Correct Answer: Enzymatic methylation or safer alkylating agents with containment and scavenging
Q46. Which quality test would best detect structural isomers or closely related impurities in melatonin API?
- Melting point only
Correct Answer: Comprehensive HPLC with appropriate chromatographic conditions (related substances)
Q47. Which statement about chemical vs enzymatic synthesis of melatonin is correct?
- Chemical synthesis always yields no impurities
Correct Answer: Enzymatic routes can offer regioselectivity and milder conditions than chemical methylation
Q48. Which excipient consideration is important for melatonin tablet formulation due to its physicochemical properties?
Correct Answer: Use of solubility enhancers or surfactants to improve dissolution rate
Q49. Which storage recommendation is appropriate for finished melatonin API to preserve stability?
Correct Answer: Protect from light, store in cool and dry conditions
Q50. What is the correct enzymatic sequence from tryptophan to melatonin?
- Tryptophan hydroxylase → AADC → AANAT → HIOMT
Correct Answer: Tryptophan hydroxylase → AADC → AANAT → HIOMT
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