Alkaloids:
alkaloids are basic nitrogenous compounds usually obtained from plants which are physiologically active.
Alkaloids are extremely difficult to define because they do not represent a single homogeneous group of compounds from chemical, biochemical or physiological bases.
Due to presence lone pair of electrons on Nitrogen alkaloids are basic. Basicity is not the same because nitrogen is present in different forms like Alkyl amine, alkyl imine, and Alkyl nitrile etc.
The quaternary amines have four organic groups covalently bonded to nitrogen having a positive charge which is balanced by some negative ion.
Plants are the rich source of alkaloids however they are also found in animals, fungi & bacteria. Principally all have been reproduced in labs by chemical synthesis. Alkaloids have a restricted distribution in certain plant families and genera.
For example, labiateae and Rosaceae are almost free of alkaloids. Some alkaloids occur in Free State in nature e.g caffeine.
Few alkaloids also occur as glycosides e.g with aglycone part, which alkaloidal in nature and the alkaloids are called glycoalkaloids.
Few alkaloids also occur as glycosides e.g with aglycone part, which alkaloidal in nature and the alkaloids are called glycoalkaloids.
The concentration of alkaloid in a plant depends on the season, the age of the plant, locality etc. It is interesting to note that the closely related alkaloids genera occur together in the same plant.
It is also observed that different genera of the same family contain same or structurally related alkaloids. E.g different genres of the family Solanaceae contains hyoscyamine.
Alkaloids can occur in any part of the plants. Here are some examples of medicinal plants and their part used where alkaloids are concentrated.
It is also observed that different genera of the same family contain same or structurally related alkaloids. E.g different genres of the family Solanaceae contains hyoscyamine.
Alkaloids can occur in any part of the plants. Here are some examples of medicinal plants and their part used where alkaloids are concentrated.
- Nuxvomica .. seeds
- Black pepper, conilum… fruits
- Hyoscyamus…leaves
- Cinchona …bark
- Belladonna, aconite roots…roots
- Ipecac… roots and rhizome.
Papaveraceae, Solanaceae, Ranunculaceae, Rubiaceae, Leguminosae are the outstanding alkaloids yielding plant families. Usually, alkaloids are present in salt form because they react with acids of plants. Freebase alkaloids are soluble in organic solvents while salts are soluble in water.
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Important features of alkaloids:
- In addition to hydrogen and carbon, all alkaloids contain nitrogen and generally oxygen as well.
- Alkaloids are basic due to a lone pair of electrons on nitrogen.
- The degree of basicity varies greatly depending upon the structure of molecule present and location of other functional groups.
- Most alkaloids are solid in nature where they may be crystalline or amorphous solids. However, some are liquids like nicotine, coniine, spartein etc.
- Liquid alkaloids usually lack oxygen in their molecular structure.
- Alkaloids usually possess a bitter taste.
- Alkaloids are generally white but some are coloured like berberine, which has a yellow colour.
- Alkaloids are physiologically active, even some are being extremely poisonous.
- Alkaloids salts are soluble in water and free bases are soluble in organic solvents.
- However, there are few exceptions to the above generalization e.g. caffeine base is soluble in water. Solubility, however, varies with the type of salts formed e.g quinine sulphate one part is soluble in 1000 parts of H 2 o while quinine hydrochloride is soluble in less than 1 part of water.
Alkaloids function in plants:
Alkaloids act as poisonous agents thus protecting the plant against insects and herbivores also because of bitter taste.
alkaloids are end products of detoxifications reactions thus representing a metabolic lockup of compounds otherwise harmful to plants. May act as growth regulatory factors.
Alkaloids may act as reserve food material, capable of supplying N or other elements necessary for plant growth.
Types:
- Following are the types of alkaloids;
- True alkaloids
- Proto-alkaloids
- Pseudo-alkaloids
True alkaloids:
True alkaloids include nicotine, atropine and morphine.
There is a nitrogen-containing heterocyclic ring. The nitrogen is amino acids derived.
Proto-alkaloids:
In protoalkaloids the nitrogen is outside the ring and derived from amino acids.
e.g ephedrine, mescaline, cathinone are proto alkaloids.
Pseudo-alkaloids:
The nitrogen in pseudo-alkaloids is derived from a non-amino acid source. Examples of pseudoalkaloids are caffeine, theobromine, theophylline, steroidal alkaloids etc.
Classification of alkaloids:
Alkaloids may be classified in any of the following ways.
- Biosynthetic classification
- Chemical classification
- Pharmacological classification
- Taxonomic classification
Biosynthetic classification:
It is quite convenient logical to classify alkaloids on basis of their precursor molecule. Such alkaloids may be of different taxonomic classification or pharmacological activities.
For example;
· Indole alkaloids are derived from tryptophan.
· Piperidine alkaloids derived from lycine.
· Pyrrolidine alkaloids derived from tyrosine.
· Imidazole alkaloids derived from histadine.
Chemical classification:
The criteria for chemical classification are the presence of a basic chemical structure. This is probably the most widely accepted and common mode of classification of alkaloids.
Examples are;
1) Pyridine and piperidine alkaloids e.g arecoline, labeline and nicotine.
2) Tropane alkaloids e.g atropine, hyoscyamine and hyoscine.
3) Quinolones alkaloids e.g quinine, quinidine, cinchonine, cinchonidine.
4) Isoquinoline alkaloids e.g hydrastine, tubocurarine, emetine and opium alkaloids.
5) Indole alkaloids e.g ergonovine, reserpine and strychnine.
6) Imidazole alkaloids e.g pilocarpine.
7) Purine bases e.g caffeine and theobromine.
8) Steroidal alkaloids e.g protoveratrine.
Pharmacological classification:
The alkaloids exhibit a broad spectrum of pharmacological actions on the basis of which it is classified into different classes; such as analgesics, cv drugs, CNS stimulants and depressants, mydriatics, anticholinergics, antimalarials & purgatives.
However, such a classification is not quite common and broadly known.
Taxonomic classification:
This particular classification essentially deals with the taxon i.e taxonomic category. A few typical examples of plant families and the various species associated with them are stated below, namely;
· Cannanabinaceous alkaloids e.g cannabis sativa.
· Rubiaceous alkaloids e.g cinchona sp.
· Solanaceous alkaloids e.g atropa belladonna L.
