Introduction

Alkaloids constitute a class of essential, nitrogen-containing plant products that have compound structures and possess significant pharmacological properties. The term alkaloid, meaning 'alkali-like,' was 1^st proposed via the pharmacist W. Meissner in the early nineteenth century before anything was recognized about the chemical structures of the compounds. 

The term alkaloid has been utilized to designate the compounds of plant origin having one or more basic nitrogen atoms in heterocyclic ring systems that induce pronounced physiological activity in animals and man.

The above definition of the alkaloids is via no means perfect and doesn't cover all compounds classed as alkaloids for reasons these as; piperine, the alkaloid of pepper, isn't essential and has practically no physiological activity and ephedrine is a straight-chain alkaloid that is generated via animal glands and has marked physiological activity. 

Actually, no exact description of the term 'alkaloid' is possible but in common it designates compounds having the subsequent general features: 

(a)   They are originated in plants, although a few are of animal origin.

(b)   They are essential in nature and show marked physiological activity.

(c)   They have heterocyclic rings containing nitrogen as a part of their structures.

Occurrence

Alkaloids take place essentially in plants of the dicotyledons families and are localised in seeds, leaves, bark or root of the plant. Each site might enclose closely related alkaloids. They happen mainly as salts of general plant acids as acetic acid, oxalic acid, lactic acid, malic acid, tartaric acid, citric acid or of assured special organic acids.

Nomenclature

There is no systematic nomenclature for alkaloids but several of the techniques adopted in their naming are revealed below:  

• According to their source: Several are named according to the family in that they are found, for instance papavarine from papaveraceae family, punarnavin, ephedrin.
• According to their physiological response: Others attained their names from their physiological response; for instance morphine means God of dreams; emetine means to vomit.
• According to their finding: The pelletierine group has been named after its discoverer, P.J. Pelletier.

Isolation of Alkaloids

The extraction of alkaloids is based upon their essential quality and solubility pattern. The normal procedures followed are to treat moistened drug through alkali so as to set free the base as it exists in salt form and then to divide the free base by organic solvent. Although the processes of extraction fluctuate, the subsequent universal process is applied for small scale extraction of alkaloids. For extraction of alkaloids, the plant substance is macerated. If the substance is rich in fat 1^st it is defatted through ligroin or petroleum ether, particularly in case of seed and leaf forms of drugs. Before applying this treatment the alkaloid should be tested for its solubility in petroleum ether. Or else, the drug should be pretreated through acid so as to convert alkaloids into the salts. This occurs in case of extraction of ergotamine from ergot. 

In the 2^nd stage the drugs might be extracted through polar solvents as water, ethanol, methanol, aqueous alcohol mixture or by acidified aqueous solution and the cellulose substance divided via filtration. By this treatment, alkaloid salts are transferred to polar solvent. It as well assists in removing pigments, sugar and other organic constituents in the subsequent stage. The filtrate is evaporated to thick syrup to give the crude plant extract. This is then dissolved in dilute acid and is subjected to separating between aqueous acid solution and an organic solvent these as ether. After continuous extraction through organic solvent for several times the aqueous phase is made alkaline by either sodium carbonate or ammonia and extracted by ether. Evaporation of ether solution provides a solid mixture of crude alkaloids. It is then subjected to fractional crystallisation for division into individual pure alkaloids. In modern practice, the isolation is resulted via column chromatography, gas chromatography and via counter current distribution. The common scheme for extraction of alkaloids is demonstrated in Fig.

Fig: General Scheme for Extraction of Alkaloids

Properties

Physical properties

Through few exemptions, all the alkaloids are colourless, crystalline solids by sharp melting points or decomposition range. Several alkaloids are amorphous gum, while others these as coniine, spartine and nicotine are liquid and volatile in nature. Several alkaloids are coloured in nature, for instance betanidin is red, berberine is yellow and its salts are copper-red in colour. 

In common, the free bases of alkaloids are soluble in organic, non-polar solvents. The salts of most alkaloids are soluble in water. In contrast, free bases are insoluble in water and their salts are as well sparingly soluble in organic solvents. The alkaloids enclosing quaternary bases are only water soluble. Several of the pseudoalkalods and protoalkaloids show higher solubility in water. For instances, colchicine is soluble in alkaline water, acid and water and caffeine (free base) is freely soluble in water. Quinine hydrochloride is highly soluble in water; for example one part of quinine hydrochloride is soluble in less than one part of water, while only one part of quinine sulphate is soluble in 1000 parts of water. The solubility of alkaloids and their salts is helpful in pharmaceutical industry for the removal and formulation of final pharmaceutical products.

Chemical properties 

Most of the alkaloids are basic in reaction, due to availability of lone pair of electrons on the nitrogen in the heterocyclic ring. The basic character of alkaloids is enhanced if the attached functional groups are electron releasing. The alkaloid turns to be neutral or acidic when the attached functional groups are electron withdrawing like amide group which reduces availability of lone pair of electrons. Free alkaloids exhibit basic characters and are very much prone to decomposition and this causes a problem during their storage. Their salt formation through inorganic acid prevents many a time their decomposition. The alkaloids may contain one or more number of nitrogen and it might exist in the form of primary (R-NH₂), for example mescaline, secondary amine (R₂-NH), for example ephedrine; tertiary amine (R₃-N), for example atropine; and quaternary amine (R₄N+X-), for example tubocurarine chloride. In the last kind, their properties fluctuate from other alkaloids, owing to quaternary nature of nitrogen. Alkaloids exist in nature either in free form, as amine or as salt through acid or alkaloids N-oxides. 

Chemical Tests for Alkaloids

• Test by Dragendorff reagent (potassium-bismuth-iodide solution): Alkaloids provide reddish-brown precipitate through this reagent.
• Test by Mayer reagent (potassium-mercuric-iodide solution): Alkaloids provides cream colour precipitate with this reagent.
• Test by Wagner reagent (iodine-potassium-iodide solution): Alkaloids provide Brown colour precipitate with this reagent.
• Test by Hager reagent (saturated solution of picric acid): alkaloids give yellow colour precipitate by this reagent.
• Test by Tannic acid: Alkaloids provides buff colour precipitate with this acid.
• Test by Picrolonic acid: Alkaloids provide yellow colour precipitate with this acid.

Classification

Alkaloids are so various and engage these varieties of molecular structure that their rational categorization is hard. Though, the best approach to the problem is to group them into families, depending on the kind of heterocyclic ring system present in the molecule. Extremely frequently, a number of alkaloids having similar structure are originated in the equivalent plant and as well demonstrate resemblance in the properties. Alkaloids are as well named as a group after the name of the plant from that they are isolated. For historical reasons as well since of their structural complexities, the nomenclature of alkaloids has not been systematized. The names of individual members are, therefore, generally derived from the name of the plant in which they occur, or from their characteristic physiological activity.

The diverse classes of alkaloids according to the heterocyclic ring system they enclose are listed below: 

• pyrrolidine
• piperidine (tropane)
• pyridine-pyrrolidine
• pyridine-piperidine
• quinoline
• isoquinoline
• Indole alkaloids.

In the subsequent treatment we will discuss the above classes taking instances of the familiar members. In most of such cases we will as well give concisely the structure, the source and the characteristic physiological activity.  

Pyrrolidine alkaloids

They contain the pyrrolidine (tetrahydropyrrole) ring system, e.g. Hygrine and Cuscohygrine.

Hygrine is isolated from the leaves of Peruvian coca shrub. Cuscohygrine is found in 'cusco leaves'.  Piperidine alkaloids

They have piperidine (hexahydropyridine) as the heterocyclic structural unit, e.g., Coniine, Isopelletierine, Lobeline and Piperine. Coniine occurs in the oil of hemlock and is poison to humans. Isopelletierine is isolated from bark of pomegranate tree. Lobeline, obtained from the seeds of Indian tobacco is used in medicine as a respiratory stimulant and as a tobacco substitute.

Pyrrolidine-pyridine alkaloids

The heterocyclic ring system present in these alkaloids is pyrrolidine-pyridine. Examples are nicotine or mysomine. Nicotine is isolated from tobacco leaf. It is extremely toxic which in awfully small doses reasons respiratory stimulation and in huge doses causes respiratory failure and death.  Mysomine as well happens in tobacco, and the aroma of tobacco smoke is due to it.

Pyridine-piperidine alkaloids

This family of alkaloids contains a pyridine ring system connected to a piperidine ring system. The simplest member is Anabasine, the chief alkaloid isolated from the poisonous Asiatic plant,  Anabasis aphyllan. Another alkaloid of this class is Anatabine.

Quinoline alkaloids

Such have the basic heterocyclic ring system quinoline; for instance, quinine and primaquinine. Qiunine happens in the bark of cinchona tree. It has been utilized for centuries for treatment of malaria. Synthetic drugs these as primaquinine have mainly swapped quinine as an anti-malarial. A recently isolated alkaloid of this group is camphtotecin establish in Chinese tree Camphotheca acuminata Nyssaceae. This is recognized to possess anti-leukemic and anti-tumor activities in animals.

Isoquinoline alkaloids

They enclose the heterocyclic ring system isoquinoline; for example papverine, narceine, emetine, cephaline and morphine. Papaverine is isolated from the unripe seed capsule of the opium poppy. It discovers employs as medicine in the treatment of spasm of the stomach or intestines. Morphine is isolated from opium. It is employed in medicine as hypnotic (sleep producing) and analgesic (pain relieving). Unfortunately, prolonged intake of morphine leads to addiction and therefore, its employ is sharply restricted.

Indole alkaloids

Alkaloids depend upon the indole skeleton are widely allocated in nature. The physiological action of such substances might rest in the fact that derivatives of indole play significant roles in the chemistry of the brain. Lysergic acid, isolated from Claviceps purpurea, is the best recognized alkaloid of this class. In small regulated amounts, it is utilized in the treatment of migraine headaches as well for the induction of uterine contraction in child birth. Another indole alkaloid is strychnine that is isolated from the seed of Strychnos nuxvomica. It is tremendously poisonous and is utilized for exterminating vermin. Others comprise yohimbine, aspidospermine, vinblastine and vincristine.

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