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Friday, 30 November 2012

Alkaloids Derived from Tryptophan

2.8 Alkaloids Derived from Tryptophan L-Tryptophan is a neutral heterocyclic amino acid containing essentially an indole ring system. It has been observed that it serves as a precursor for a wide spectrum of indole alkaloids. However, there exists an ample concrete evidence that major rearrangement reaction may convert the predominant indole-ring system into a quinoline-ring system thereby enhancing further the overall ability of tryptophan to act broadly as an alkaloid precursor. The various alkaloids derived from tryptophan are conveniently classified into the following categories, namely: (i) Simple Indole Alkaloids; (ii) Simple b-Carboline Alkaloids; (iii) Terpenoid Indole Alkaloids; (iv) Quinoline Alkaloids; (v) Pyrroloindole Alkaloids; (vi) Ergot Alkaloids. These aforesaid categories of alkaloids shall be discussed separately with typical important examples followed by the possible biosynthetic pathways, wherever necessary. 2.8.1 Simple Indole Alkaloids L-Tryptophan (i.e., a-aminoindole-3-propanoic acid) on decarboxylation yields tryptamine. The Nmethyl and N, N-dimethyl derivatives of the latter are broadly distributed in the plant kingdom as serotonin—a simple hydroxylated derivative. Sequential biotransformation viz., decarboxylation, N-methylation and hydroxylation gives rise to the formation of psilocin; whereas, phosphorylation of the OH group in psilocin yields psilocybin. The three alkaloids, namely: serotonin, psilocin and psilocybin shall be discussed in the sections that follow:
A. Serotonin Synonyms 5-Hydroxytryptamine; 5-HT; Enteramine; Thrombocytin; Thrombotonin; Biological Sources The root bark of Gossypium hirsutum L. (Malvaceae) (American Unplanted Cotton) contains serotonin. Chemical Structure
3-(2-Aminoethyl)-1H-indol-5-ol; (C10H12N2O).