4 ALKALOIDS IN CHEMOSYSTEMATICS
Broadly speaking the alkaloids invariably occur across the entire plant kingdom. In reality, alkaloids are usually found more abundantly in plants specifically belonging to the Dictoyledones than in either the Monocotyledones or the non-flowering plants. However, it has been observed that amongst the Pteridophyta and Gmnospermae, only the Lycopodiaceae family happens to synthesize these compounds to any reasonable extent. The Lycopodium alkaloids are essentially the quinolizidinederivatives; and usually have the ring structure as displayed by Lycopodine below:
(15R)-15-Methyllycopodan-5-one; (C16H25NO).
One school of thought believes that the alkaloids are confined rarely in dicotyledon orders classified before the Centrospermae, thereby establishing linkage of these simplier flowering plants with the Gymnospermae. The distribution and occurrence of alkaloids is found to be quite uneven among the remaining orders. Thus, an order rich in such compounds could be preceded and followed by an order wherein alkaloids are not synthesized at all. Such an erratic distribution may be noticed even more distinctly and apparently in certain plant families also. Therefore, one may safely conclude that alkaloids by themselves are absolutely spinless in establishing phylogenetic relationships either between orders or between families within an order. Of course, there exists some exceptionally few cases, for instance: Centrospermae which has been duly illustrated below.
Such natural orders that are found to be rich in alkaloids are: Centrospermae, Gentianales, Magnoliales and Ranunculaloes belonging to the class of Dicotylendones; and Liliales, and Orchidales belonging to the category of Monocotylendones. It has been reported duly tht both Papaveraceae and the absolutely unrelated Apocynaceae contain the largest number of these secondary metabolites. Surprisingly, each and every species of Papaveraceae thoroughly studied till date comprises of alkaloids; whereas, the Apocynaceae contains a rather more prominent diversity of complex indole alkaloids. There are a host of other plant families in which alkaloids usually occur more predominantly. and frequently are, namely: Amaryllidaceae, Compositae, Leguminosae, Liliaceae, Loganiaceae, Orchidaceae, Ranunculaceae, Rubiaceae, Rutaceae and Solanaceae. However, the Amaryllidaceae alkaloids are found to be specific only to that family, that contains no other variety.
A broad survey of literature has adequately proved the fact that while applying the chemosystematics to the classification of plants, the biosynthetic pathway is certainly more vital and important than that of the end-product. It may be further examplified by considering, quinine, a quinoline derivative obtained in Rubiaceae, is biosynthesized from tryptophan by a biosynthetic pathway very similar to that forming the complex indole alkaloidal characteristic features of the family. Besides, quinoline derivatives are also found in the Rutaceae, but in this particular instance they are biosynthesized from anthranilic acid; and also by a pathway that is very specific to this family.
Lastly, one may draw an inference that the strategic application of alkaloid biosynthesis to classification is made rather complicated by both convergence and divergence. Some typical examples of convergence and divergence are given below:
(a) Examples of Convergence: The synthesis of the tropane alkaloids from hygrine is onaccount of convergence. It normally occurs in a plethora of unrelated plant families, such as: Convolvulaceae (Convolvulus species), Cruciferae (Cochleavia arctica); Erythroxylaceae (Erythroxylum coca); Euphorbiaceae (Phyllanthus discoidens); and Solanaceae (includes several genera).
(b) Examples of Divergence: Papillionoideae exhibit several examples of divergence; because, members of this subfamily synthesise virtually a wide spectrum of secondary metabolites.
The Papillionoideae alkaloids are found to exhibit no apparent relationship either in molecular structure or in their respective biosynthetic pathways. Hence, the spiroamine alkaloids found in Erythrina species have virtually little structural relationship to the pyrrolizidine alkaloids that are characteristic of Crotolaria species, and are biosynthesized from tyrosine; whereas, the pyrolizidine alkaloids are normally found to originate from ornithine.
