The term alkaloids (or alkali-like) was first and foremost proposed by the pharmacist, W. Meissner, in 1819, for the basic nitrogen-containing compounds of plant origin.
Some characteristic features of alkaloid :
(a) Complex molecular structure, and
(b) Significant pharmacological activity.
Nomenklature The major characteristic of the nomenclature of alkaloids is the lack of any agreed
systematic prevailing system. Therefore, by a general agreement, the chemical rules designate that the names of all alkaloids must end with the suffix (ine). The latin names end with (ina).
Thus, the names of the alkaloids are usually obtained in a number of ways, namely: (a) From the generic name of the plant producing them: Examples: Atropine from Atropa belladona Linn., (Solanaceae); and Hydrastine from
Hydrastis canadenisis L. (Ranunculaceae). (b) From the specific name of the plant yielding them: Examples: Belladonine from Atropa belladona L. (Solanaceae); and Cocaine
from Erythroxylum coca Lam. (Erythroxylaceae). (c) From the common name of the drug producing them: Example: Ergotamine from Claviceps purpurea (Er.) Tul. (Hypocreales) commonly known as
ergot. (d) From their specific physiological activity: Examples: Emetine from Hedera helix L. (Araliaceae) called Ivy; Narcotine from Papaver
somniferum L. (Papaveraceae) known as Opium Poppy; and Morphine from P. somniferum L. (e) From the name of the discoverer: Example: Pelletierine from the barks of Puniea granatum Linn., (Punicaceae). (f ) From their physical property: Example: Hygrine from the roots of Withania somniferum (L.) Dunal (Solanaceae) called
Ashwagandha (Hygro = moist).
McKee* (1962) reported that about 1000 alkaloids, which are known, belong to almost 100 families, 500 genera and spread over 1200 species. However, it has been observed beyond reasonable doubt that the alkaloids are not evenly distributed amongst the plant kingdom. They have been found to be absent in Algae and in the lower groups of plants with the exception of one or two families of the fungi. The glaring examples of fungal alkaloids include those of ergot alkaloids.
However, in the plant kingdom, the alkaloids generally, seem to get confined to a certain families and genera with regard to their distribution. For instance, amongst the angiosperms the families which have been recognized as outstanding for alkaloidal-yielding plants are, namely: Apocynaceae, Berberidaceae, Papaveraceae, Ranunculaceae, Rubiaceae and Solanaceae.
Monocotyledons, generally do not produce alkaloids, but investigation have revealed that two of the most promising families viz., Amaryllidaceae and Liliaceae do contain alkaloid-containing plants.
Dicotyledons, mostly contain the alkaloids. It has been observed that neither Labiatae nor Rosaceae contain any alkaloid. Furthermore, almost 15% of all vascular plants contain alkaloids.
Alkaloids may occur in various parts of the plant. It may, however, be pointed out that in a particular species, normally only one or two specific organs and not all organs, essentially afford the function of alkaloidal formation.
For instance, the alkaloids of the tobacco plant, Nicotiana tabacum Linn., (Solanaceae), are produced in the root and are subsequently translocated to the leaves where they usually accumulate, whereas the seeds are completely devoid of alkaloid.
In another glaring example the opium poppy, Papaver somniferum, the alkaloids solely occur in the fresh latex of the fruit, while the seeds of poppy are virtually devoid of alkaloids.
Likewise, the colchicum** corm. Colchicum autumnale Linn. (Liliaceae), the alkaloids are found both in the seed and in the corm.
Interestingly, the bark of cinchona tree, Cinchona officinalis Linn., (Rubiaceae) contain the alkaloids (viz., quinine) exclusively.
Salts of Alkaloids It has been found that a plethora of alkaloids occurring in various plant species are in the form of salts of organic acids, such as: acetic acid, malic acid, oxalic acid, succinic acid, tartaric acid, tannic acid or some other specific plant acids. In certain instances, the alkaloids are found to be in combination with special plant acids
Function of Alkaloids in Plants
(a) As strategically located poisonous agents in plants thereby protecting them either against herbivorous animals or insects,
(b) As probable by-products of various detoxification reactions representing a metabolic lockingup of compounds, otherwise harmful or detrimental to the plant,
(c) As pronounced regulatory growth factors, and (d) As reserve substances in plant capable of
supplying nitrogen or other necessary elements to its economy.
A plethora of alkaloids contain one or more asymmetric carbon atoms in the molecule, and hence exhibit optical activity. It has been observed that in the majority of instances only the ()- isomer (i.e., the levorotatory component) has appreciable and distinct pharmacological activity than the corresponding (+)-isomer (i.e., the dextrorotatory, component) of the same alkaloidal species.
In fact, the optical activity is invariably associated with the alkaloids and their respective salts. However, the optical activity and the specific rotation usually varies with the solvent used, the temperature, the wave length of light and other minor factors.
Relative pressor activities* of D()-ephedrine and D(+) ephedrine: The relative pressor activities of D()-ephedrine is found to be 36 with regard to its D(+)-ephedrine isomer at ll i.e., the former is almost 3 times more active than the latter
Antimigraine activity of ()-ergotamine and (+)-ergotamine: It has been observed that the antimigraine activity of ()-ergotamine possesses 3-4 times more activity than its corresponding (+)-ergotamine isomer
Showing both ()-and (+)-forms active pharmacologically: In certain alkaloids, the () form as well as the (+) form are medicinally useful. Examples: The ()-Quinine is primarily employed as a potent antimalarial agent; whereas the (+)-Quinine, also known as quinidine, is solely used in restoring cardiac arrythmia to normal rythm
General Characteristics of Alkaloids
The general characteristics of alkaloids may be grouped together in two categories, namely:
(a) Physical characteristics, and
(b) Chemical characteristics.
Physical Characteristics : Solubitily
However, it has been observed that the free alkaloid bases as such are invariably found to be fairly soluble in organic solvents, such as: either, chloroform, relatively non-polar solvents (hexane, benzene, petroleum ether), immiscible solvent, lower alcohols (methanol, ethanol); but they are either practically insoluble or very sparingly soluble in water.
Interestingly, the alkaloidal salts are almost freely soluble in water, relatively less soluble in alcohol and mostly either insoluble or sparingly soluble in organic solvents
Examples Atropine sulphate and morphine hydrochloride are much more soluble in water than their corresponding bases i.e., atropine and morphine
Physical Characteristics : Solubitily
However, there are a few exceptions to the above stated generalizations, namely:
(i) Certain alkaloid bases are water soluble, but these may be solely regarded as exceptions rather than any specific rule, such as: ephedrine, colchicine, pilocarpine; the quaternary alkaloid-base like berberine and tubocurarine; caffeine-base readily extracted from tea with water.
(ii) Narceine and pilocarpine are insoluble in organic solvents, whereas morphine is sparingly soluble in organic solvents viz., solubility in either 1:5000.
(iii) Certain alkaloidal salts, for instance: lobeline hydrochloride and apoatropine hydrochloride are found to be soluble in organic solvent like chloroform.
(iv) Some alkaloidal salts are sparingly soluble in water whereas others are extremely watersoluble, such as: Quinine sulphate-soluble in 1:1000 parts of water, Quinine hydrochloride soluble in 1:1 part of water.
Chemical Characteristics : basicity
Chemical Characteristics : Salient Features
1. The weaker bases, i.e., alkaloids having low pKa values, shall require a more acidic medium to form their respective salts with the corresponding acid.
2. The strongly basic alkaloids i.e., those possessing high pKa values, shall require comparatively low acidic medium to form their respective salts with the acid.
3. The alkaloids are usually neutrallized with acids to form salts that may b