Alkaloids

Alkaloids 

nitrogen-containing organic compounds, predominantly of vegetable origin, which have the properties of bases. They are physiologically active.

The discovery of alkaloids in plants was of great significance, since even in the beginning of the 19th century it was believed that vegetable compounds—as distinct from those of animal origin—contain no nitrogen. The first alkaloids discovered were the components of opium (1803) in the form of a mixture of crystalline substances, from which morphine was separated in 1806. Other discoveries of the 19th century were strychnine, quinine, caffein, atropine, and ephedrine. By the 1960’s more than 1,000 alkaloids, both natural and synthetic, were known.

The majority of alkaloids are colorless crystalline substances which by their chemical nature represent heterocyclic compounds with nitrogen atoms in the cycle. The simplest alkaloids contain up to ten carbon atoms; the more complex ones, more than 50. Alkaloids form crystalline salts with acids (for example, sulfuric, hydrochloric, salicylic, and oxalic); these reactions are employed in the extraction and purification of alkaloids.

The establishment of alkaloid structure is often a very complex operation; it is usually accomplished by determining the nature of the groups containing oxygen and nitrogen atoms and by splitting the alkaloid molecule into fragments of known structure. The correctness of the conclusions reached regarding structure is then verified by synthesis. Great progress has been made in the determination of alkaloid structure. The structures of such complex substances as morphine, strychnine, and reserpine are known, and structural formulas have been proposed for aconite and related alkaloids.

The great structural diversity of alkaloids has hindered the classification of these compounds. Most often the main criterion used in classification is the structure of the nitrogen-containing part of the alkaloid molecule (as in the case of derivatives of pyrrolidine, pyridine, quinoline, indole, and so on). Substances of unestablished structure are classified according to the systematic principle, placing all alkaloids from similar plants in the same group.

Alkaloids are distributed quite unevenly in terrestrial flora. In some families (Papaveraceae, Solanaceae, Ranun-culaceae, Leguminosae, and so on) they appear in many species; in others they have not yet been observed. Several structurally similar alkaloids usually appear in a given species, and the number may be as great as 15–20 (the opium poppy, cinchona bark). The number varies depending on the growth conditions of the plant. Alkaloid content varies from fractions of one percent up to several percent and is determined by the character of the soil, agricultural practices, altitude, water regime, length of day, intensity of solar radiation, and other factors.

Alkaloids play a part in the chemistry exchange reactions of plant organisms. In ripening tobacco seeds, for example, a steady disappearance of nicotine is found to accompany the accumulation of protein.

A mixture of alkaloids is extracted from dried, ground vegetable raw material with the use of dilute acids or spirits. In some cases the vegetable material is processed with a mixture of an alkaline aqueous solution and benzol, di-chloroethane, or other organic solvents which are immiscible in water. Various methods—fractional crystallization of salts, chromatographing, and others—are used to separate a mixture of alkaloids. In the production of various tonic and narcotic preparations (from tobacco, tea, coffee, and others) whose action depends on their alkaloid content, special branches of agriculture and industry have been developed.

A number of alkaloids, such as caffeine, theobromine, ephedrine, and arecoline, are produced synthetically. In some cases, separated natural alkaloids are transformed into more effective medicinal substances. Industry produces effective synthetic medicinal substances which are structurally close to alkaloids; novocaine (to replace cocaine) and acri-quine (to replace quinine) are examples.

The majority of alkaloids are valuable medicines; many are powerful poisons as well. Some alkaloids exercise a selective action on various parts of the nervous system, the blood vessels, muscles, and so forth. Strychnine and caffeine excite the central nervous system, but morphine and scopolamine have a sedative effect; pilocarpine intensifies gland secretion, and atropine retards such secretion; quinine kills malaria plasmodia; apomorphine is an emetic, theobromine is a diuretic; adrenaline raises blood pressure; and so on. Alkaloids may act against the cause of an illness (as does quinine in the case of malaria), or they may merely alleviate individual symptoms (as do morphine and opium used as somnifacients or pain killers, or caffeine as a stimulant). Medically, alkaloids may be used in pure form (codeine), in the form of their salts (quinine hydrochloride), or in the form of their derivatives (apomorphine).

In addition, alkaloids are used in agriculture as insecticides.

REFERENCES

Orekhov, A. P. Khimiia alkaloidov, 2nd ed. Moscow, 1955.
Henry, T. A. Khimiia rastitel’nykh alkaloidov. Moscow, 1956. (Translated from English.)
Preobrazhenskii, N. A., and E. I. Genkin. Khimiia organicheskikh lekarstvennykh veshchestv. Moscow-Leningrad, 1953.