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Pharmacognosy II: Tryptophan Derived Alkaloids & Their Applications

Tryptophan Derived Alkaloids

  • L-Tryptophan: A neutral heterocyclic amino acid and precursor for many indole alkaloids, contains two nitrogen atoms (2N) in the indole and a second nitrogen two carbons away from the β-position of the indole ring.

    • Rearranged to convert into quinoline-ring, leading to quinolone alkaloids.

Classification of Tryptophan-Derived Alkaloids

  • Simple Indole Alkaloids

  • Simple β-carboline Alkaloids

  • Terpenoid Indole Alkaloids

  • Quinoline Alkaloids

  • Pyrroloindole Alkaloids

  • Ergot Alkaloids

Simple Indole Alkaloids

  • Examples: Serotonin, psilocin, psilocybin, N,N-dimethyltryptamine.

  • Serotonin:

    • Synonyms: 5-Hydroxytryptamine (5-HT), Enteramine.

    • Source: Root bark of Gossypium hirsutum L., found in mushrooms and fruits (25-400 mg/kg in walnut; 3-30 mg/kg in plantains, pineapple, etc.).

    • Character: White powder, m.p. 121-122 °C, slightly soluble in H2O.

    • Uses: Potent vasoconstrictor, neurotransmitter in CNS; important in sleep-walking cycles.

Simple β-Carboline Alkaloids

  • Example: Harmine

    • Source: Seeds of Peganum harmala L., Banisteria caapi, Passiflora incarnata L.

    • Character: Slender orthorhombic prisms from methanol, m.p. 261°C; sublimation properties.

    • Uses: CNS stimulant, narcotic hallucinogen.

Terpenoid-Indole Alkaloids

  • Major group (> 3000 compounds). Found in Apocynaceae, Loganiaceae, Rubiaceae.

  • Contains tryptamine and a C9 or C10 monoterpene residue.

  • Examples: Ajmalicine, tabersonine, strychnine, brucine, reserpine.

Strychnine

  • Source: Seeds of Strychnos nux-vomica L.; many varieties.

  • Character: Colourless cubes; intensely bitter, m.p. 275-285°C.

  • Uses: Very toxic, CNS stimulant, vermin killer, causes convulsions.

Reserpine

  • Synonyms: Crystoserpine.

  • Source: Roots of Rauwolfia serpentina; Catharanthus roseus.

  • Uses: Hypotensive; alleviates mild anxiety.

Isolation of Reserpine

  • Process includes soaking roots in NaHCO3, solvent extraction, evaporation, and crystallization.

Ibogaine

  • Alkaloid from Tabernanthe iboga, acts as a hallucinogen and cholinesterase inhibitor.

Yohimbine

  • Derived from Corynanthe johimbe; acts as an alpha2-adrenergic receptor antagonist, and has aphrodisiac properties.

Quinoline Alkaloids

  • Modifications of terpenoid-indole alkaloids. Notable examples: Quinine, quinidine.

    • Quinine: Derived from Cinchona species; antimalarial uses and properties.

Pyrrolo-Indole Alkaloids

  • Small group of derivatives. Example: Physostigmine.

  • Uses: Antidote for tricyclic antidepressants; reduces intraocular pressure.

Ergot Alkaloids

  • Derived from Claviceps purpurea; includes ergotamine.

  • Uses: Anti-migraine; regulates vascular headaches.

Miscellaneous Alkaloids

  • Purine Alkaloids: Found in tea, coffee (e.g., caffeine), weak bases.

  • Imidazole Alkaloids: Derived from histidine, main alkaloid is pilocarpine, used as a cholinergic drug.

Vinca Alkaloids

  • Catharanthus roseus produces vinblastine and vincristine.

    • Uses: Treats various cancers by inhibiting cell division.

Steroid and Piperidine Alkaloids

  • Steroidal alkaloids include conessine; piperidine alkaloids include lobeline.

Summary of Specific Colour Reactions for Alkaloids

  • Murexide test for caffeine; for tropane alkaloids Vitali-Morin test; and others for classification of specific alkaloids.

  • This content encapsulates a broad spectrum of alkaloids derived from plants, their biological sources, characteristics, uses, and their medicinal significance.