GPAT 2014 Study Material - Pharmacognosy : Alkaloids

Initially defined as nitrogen-containing, basic substances of natural originally and of their nitrogen atom is part of a heterocyclic system and they possess a significant pharmacological activity; they occur only in the vegetable kingdom. They are found as salts, are formed biosynthetically from an amino acid referred to as-True alkaloids. Pseudoalkaloids: Most often have all of the characteristics of the true alkaloids, but they are not derived from amino acids. Most of the known exam-ples are isoprenoids and are referred to as terpenoid alkaloids; mono-terpenoid alkaloids, sesquiterpenoid alkaloids of Nymphaeaceae, diterpenoid alkaloids, such as aconitine from the tubercle of the aconite or steroidal alkaloids. Also known are heterocyclic nitrogen containing substance arising from the metabolism of acetate, for example coniine, the toxic principle of hemlock.

Protoalkaloids: Are simple amines in which the nitrogen atom is not part of a heterocyclic ring; they are basic and are elaborated from amino acids. Simple amines such as serotonin, mescaline from peyote, but also betains. Thus we can state that an alkaloid is an organic compound of natural origin, which contains a nitrogen atom, is more or less basic, is of limited distribution and at low doses, marked pharmacological properties. These compounds have in common some reactions of precipitation with the “general reagents for alkaloids.

Natural occurrence, distribution and localization: Alkaloids Concentration have a wide range of variation From a few ppm anticancer alkaloids of the Madagascan periwinkle to more than 15% of alkaloids in cinchona bark. Only rarely do alkaloids-containing plants contain only one alkaloid; sometimes they do contain virtually only one constituent hyoscyamine from the leaves of bella-donna. In a give plant, the concentration of alkaloids can vary widely from part, and some parts may contain none. One example is quinine, which accum-ulates in the trunk bark of Cinchona, but is completely absent from the leaves;  conessine accumulates in the seeds and the bark of kurchi (Holarrhena pubescens).

Localization: In the plant, alkaloids occur as soluble salts (citrates, malates, tartrates, meconates, isobutyrates, benzoates), or in combination with tannins, alkaloids synthesis takes place at specific sites (growing root, laticiferous cells, chloroplasts);

Function: Some may be involved in plant-predator relationships, by protecting the former against the latter.

Physico-Chemical Properties: Alkaloids have molecular weights ranging from 100 to 900. Although most of the bases that do not contain oxygen atoms are liquid at ordinary temperatures (nicotine, sparteins, coniine), those that do contain oxygen atoms crystallizable solids, the basicity of alkaloids varies greatly, since this property depends on the availability of the lone pair of electrons on the nitrogen atom. The basic character of alkaloids allows the formation of slats with mineral acids(hydrochlorides, sulfates, nitrates) or organic acids (tartrates, sulfamates, maleates). They crystallized salts can be conserved farily well and are the common commercial from of these compounds.

Detection & Characterization: Other reagents are available to characterize Alkaloids, particularly those that give color reactions characteristic of subgroup of alkaloids :

1. p-dimethylaminobenzaldehyde for the ergot alkaloids and pyrrolizidine alkaloids;
2.Cerium and ammonium sulfate, which differentiate indoles (yellow), dihydroindoles (red),β-anilinoacry lates (blue), oxindoles;
3. Ninhydrin for arylalkylamines;
4. The Vitali-Morin reaction for the esters of tropic acid;
5. Reagents containing ferric chloride in the presence of hydrochloride acid (tropolones) or perchloric acid (Rauwolfia). Dragendroff’s reagents, the iodine-iodide solution (or iodine vapors), potassium iodoplatinate, or cerium and ammonium sulfate are commonly used to visualize TLC plates.

Precipitation by Specific Reagents: A good number of alkaloids obtained from various plant sources invariably given a distinct precipitate with certain specific reagents to an extent as small as one microgram. Based on these observation, these alkaloids-precipitating reagents are some times employed for either detecting the presence or absence of alkaloids in : Crude extracts or plant materials, and for ascertaining whether a specific extraction procedure has exhausted completely the alkaloids contents or not.How ever, a negative test i.e., the absence of precipitation, may infer that the alkaloids are absent. It is pertinent to mention here that a positive test may not always indicate the presence of alkaloids, but may also be due to the presence of other plant constituents, such as; purines, proteins, betaines and ammonium salts etc.

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