Definition Alkaloids are Organic products of natural or synthetic
origin which are basic in nature, normally contain two or more nitrogen with
heterocyclic nature and shows specific pharmacological action on any human
or animal in small dose.
- True alkaloids: These are toxic in nature contains heterocyclic nitrogen derived from amino acids and basic in nature true alkaloids are normally present in plants at salt of organic acid.
- Pseudo-alkaloids: We include mainly steroidal and terpenoid alkaloid and purine not derived from amino acids they do not show typical character of alkaloids but gives standard qualitative test for alkaloid for example lenses caffeine.
- Proto alkaloids: Proto alkaloids are also called as amino is colloid are simple amines in which nitrogen is not a heterocyclic ring they are basic in nature and prepared in plants from amino acids for example mescaline n-dimethyl tryptamine colchicine ephedra
Occurrence
Alkaloids generally occurs in angiosperms plants and also found in some
gymnosperm plants like ephedra, families like Leguminosea, Papaveraceae,
solanaceae , rubiaceae , berberidaceae, liliaceae, Lauraceae,
ranunculaceae, rutaceae,apocynaceae ,etc mostly contains alkaloids. Alkaloids found in some parts of plants like seeds of nux vomica, roots of rauwolfia, leaves of Coca plant conium fruit, also found in whole plants like vinca and cinchona.
Physical properties
Physical properties of alkaloids are colourless some shows coloured
alkaloids like betanidine red, berberine yellow salts of sanguinarine are
copper red in colour.
They are mostly crystalline solids but some may amorphous gum or some are
liquid and volatile oils for example nicotine, serpentine, Coniine.
Solubility
The free bases alkaloids are soluble in organic nonpolar, immiscible
solvents. the salt’s of most alkaloids are soluble in water. Some of the
pseudo alkaloids and protoalkaloids show high solubility in water for
example colchicine soluble in alkaline water or acid or water, caffeine and
quinine hydrochloride are soluble in water. That is one part of quinoline
hydrochloride is soluble in less than one part of water while only one part
of quinoline sulphate is soluble in thousand parts of water
Melting point: Sharp melting point.
Chemical properties
Basicity: Alkaloids are basic due to the presence of lone pair on
nitrogen. Basicity can be increased by adding electron releasing group and
can be decreased by adding electron withdrawing group to adjacent
functional group.
But the basicity decreases the stability of alkaloids causing problems during storage. salt formation with inorganic acids prevents decomposition and increase stability. In nature they exist other in free state as amine are the salt with acid or alkali and N-oxide
Optical activity most of the alkaloids are optically active and
laevorotatory for example quinine is dextrorotatory papaverine is inactive
and resonant is neutral
function in plants
- Alkaloids maybe used as a growth factor in plants
- They also used in defense mechanism
- Alkalis are reservoir of Nitrogen in most plants
- As a career for acids
- Alkaloids are the products of detoxification process in plants
Uses
Alkalis are highly potent medicament with curative property for example,
- Morphine opium -as a narcotic
- Reserpine rauwolfia as a tranquilizer
- Srychine nux vomica -as a nervine stimulant
- Cocaine coca leaf- as a local anesthetics
- Hyoscine belladonna as a antispasmodic
In high dose or high concentration alkaloid are toxic.
Qualitative chemical test used for identification of alkaloids are based on the characters of alkaloids to form precipitate as salts of organic acids or with compounds of heavy metals like mercury gold or Platinum etc.
| Different Reagents used | Colour of Precipitate |
|---|---|
| Mayer’s reagent (potassium mercuric iodide solution) | Cream coloured precipitate |
| Dragendorff’s reagent potassium Bismuth iodide solution | Reddish brown precipitate |
|
Wagner’s reagent iodine potassium iodide solution |
Reddish brown precipitate |
|
Hager's reagent picric acid or picrolonic acid |
Yellow coloured precipitate |
Alkaloids gives colour or precipitate with certain specific reagents.
Chemical test with heavy metals are not limited to alkaloids some proteins, coumarine and Alpha pyrone also give precipitates with these reagents, it is also noted that some alkaloids like caffeine(highly water soluble ) not give such test hence tests with heavy metals are not much reliable. The specific tests for individual alkaloids are more important for qualitative evaluation.
- Stas-Otto process (normal procedure)
- Firstly the moistened drug is treated with alkali to set free bases from salts form
- Free bases are separated with organic solvent.
- For small scale
- Firstly the plant defatted with petroleum Ether (specially on leaves pr seed form of drug) if soluble and if not treated with acid to form salts for example ergotaimine extraction from ergot
- Drugs extracted by polar solvent like water, ethanol etc
- To transfer the alkaloidal salts to polar solvents.
- By evaporating solvent partition/purification of aqueous acid solution and organic solvent
- Aqua solution is made alkaline with sodium carbonate or ammonia and
- Followed by drying for separation
The purification of crude extract of alkaloids is done by following method first direct crystallization of solvent it is a simple method of isolation.
Steam distillation it is specially employed for volatile liquid alkaloids but not suitable for high molecular weight alkaloids.
Chromatographic techniques this is mostly used methods for separation the different techniques of chromatography are used for the separation of individual alkaloids from Complex mixtures.
Gradient pH technique in this technique the crude alkaloidal mixture is dissolved in two person tartaric acid solution and extracted with Benzene first extraction contains neutral are very weakly basic alkaloids pH of the aqua solution is increased gradually by 0.5 increments up to 9 pH and extraction is carried out at each pH level with organic solvent. This way alkaloids with different busy cities are extracted strongly basic alkaloids extracted at end
Classification of alkaloids pharmacological classification
Depending on the physiological actions alkaloids are classified under the
various pharmacological categories like central nervous system stimulants,
depressants, sympathomimetics, analgesics, purgatives etc. It does not
depend on chemical nature of crude drugs. Sometimes individual alkaloids may
exhibit different actions.
For example Morphine is a narcotic analgesics, cinchona quinine is
antimalarial, while quinidine is a cardiac depression.
Taxonomical classification
This type of classification is based on the distribution of alkaloids in
various plant families like solanaceae rubiaceae alkaloids. example ephedra
cinchona etc. from this classification chemo-taxonomic classification has
further derived.
Biosynthetic classification
This is based on the precursor from which alkaloids are biosynthesized in
plants. two different alkaloids from different classes can brought under the
same group if they are derived from same precursor.
For example all indole alkaloids are derived from tryptophan are group
together
Chemical classification
This is most accepted way of
classification in alkaloids it is mainly based on type of fundamental ring
structure present in alkaloids they are categorised into two divisions
Heterocyclic alkaloids, true alkaloids
They are derived into 12
groups according to the nature of their heterocyclic ring
| Types | Examples | Ring structures |
|---|---|---|
| Pyrrole & pyrrolidine | Hygrine coca species |
|
| Pyridine & piparidine | Arecoline, anabasine, conine, lobeline, pelletierine, trigonelline |
 |
| Pyrrolizidine | Echimidine, senecionine, seneciphylline, symphitine |
 |
| Tropane piperidine, n-methyl pyrrolidone | Atropine, hyoscine, hyoscyamine, cocaine, pseudo-pelletierine |
 |
| quinoline | Quinine, quinidine, cinchonine, cinchonidine cupreine |
 |
| Isoquinoline | d-tubocurarine, berberine, emetine, cephaeline, papaverine, narcotine |
 |
| Phenanthrene | Morphine, codeine, hydrastine |
 |
| Aporphine reduced isoquinoline naphthalene | Boldine |
 |
| Indole/benzopyrrole | Ergometrine, ergotamine, reserpine, vincristine, vinblastine, strychnine |
 |
| Imidazole | Pilocarpine, Isopilocarpine, Pilosine |
 |
| Norlupinane | Cytisine, laburnine, lupanine, sparteine |
 |
| Purine imidazole/pyrimidine | Caffeine, theobromine, theophylline |
 |
| Steroidal (cyclopentanol perhydro phenanthrene ring) | Protoveratrine, solanidine, conessine, funtumine |
 |
Non heterocyclic alkaloids:
| Diterpene | Aconite, Aconine, hypoaconitine, |
| alkylamine alkaloid | Ephedrine pseudoephedrine, Mescaline, colchicine |
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