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CHAPTER – 10
ALKALOIDS
Today, more than 10,000 alkaloids are known to occur from more
than 4000 species of Angiosperms. Ladenberg defined alkaloid as :
“It can be defined as the naturally occurring basic, complex,
nitrogenous organic, heterocyclic ring compound of plant origin
with physiological and pharmacological properties.”
TWO BASIC PROPERTIES:
1. Heterocyclic ring structure with N in the ring structure.
2. Basic in nature.
COMPOSITION OF ALKALOIDS:
They are heterocyclic compounds having C,H,O and N in their
structure ( exception : Nicotine without oxygen). They are mainly
composed of pyrol, pyridine, quinoline, isoquinoline and tropane structure.
Morphine contains phenanthrene without nitrogen.
GENERAL PROPERTIES:
They are colourless and non-volatile solids. They are slightly
soluble in water (Coniine and nicotine are highly soluble in water) but
soluble in most organic solvent. Some are colored substances (Barberine is
yellow). Most alkaloids are optically active and are leavo rotatory but few
are dextrorotatory too. While, some are optically inactive.
Almost all are bitter in taste. They are used in pharmacy. They are
active at small concentration. At higher doses, they may cause death. They
are basic in nature and form insoluble salts with picric acid, potassium
mercuric iodine etc.
OCCURRENCE:
Alkaloids are found in vascular plants. They rarely occur in
Cryptogram, gymnosperms or mono cotyledons. They occur abundantly in
di-cotyledons, Apocynaceae, Papaveraceae, Papilonaceae, Rubiaceae,
Solanaceae, etc. families. Papaveraceae is an unusual family that all its
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members contain alkaloids. They are isolated from roots, leaves, bark of
the stem or seeds of the plants. In leaves of some plants, it constitutes 0.5
to 8.0 % of dry weight. They are stored in the vacuoles or in the cytoplasm
in the solid form. Their quantity depends upon the age of plant, cultivation,
type of growing, place of growing and climate.
Alkaloids are basic in character so they exist as salts of plant
acids like oxalic, citric, malic, lactic, tannic, acetic acid etc. Most alkaloids
occur in combined form with other compound however, glycolides, amides
occur free.
NOMENCLATURE:
Although, their scientific nomenclature is not possible due to their
complex structures. Nomenclature can be given according to following
general criteria.
1. According to their plant source. e.g. Papaverine, berberine
2. According to their physiological actions e.g. Morphine, Narcotine,
Emetine.
3. According to the name of scientist who discovered it e.g. Palatrin
which was discovered by P.J.Pelletior.
4. By adding prefix or suffix in the name of plant source e.g. Cinchonine
from Cinchona, quinine
5. Same isomeric alkaloids have been designated by adding prefix iso,
neo, epi, α and β.
6. Some times Greek words are used in the nomenclature.
CLASSIFICATION: They are classified by two systems:
1. Classification based on the genera of plants in which they occur.
Cinchonine from Cinchona spp.
2. Classification based on the nature (type) of the heterocyclic nucleus
present in the structure which was given by Henry.
1.
PHENYL
ETHYL
AMINO
GROUP
PHENYLIALKYL AMINO GROUP
OR
The examples of this group are:
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Hordinine
2.
Tyramine
L-Ephedrine
PYROLLIDINE GROUP. :
It was isolated Hygrine from Paruvian coke in 1814. It is
extremely basic and boils at 194 0 C.
Hygrine ( C8 H15 O N )
3.
PYRIDINE GROUP :
Stachydrine ( C7 H14 O N )
They are sub- classified in three
groups.
a) Hemlock alkaloid (Dhatura) : Five alkaloids are included in this
group. The main alkaloid of this group is Coniine was used to kill political
prisoners in olden times. ( Socrates was given cup of Dhatura
Alkaloid) CONIINE ( C8 H17 N )
b)
Pepper Alkaloid: There are two members in this group. I) Piperine
and II) Chavicine. It is found with isomer Chavicine in black and white
peppers ( 5 to 8 % ) . Taste is pungent. It is less toxic than other alkaloid.
They are solid, optically inactive, and soluble in water. Melting Point ( M.P.)
= 128 0 C.PIPERINE ( C17 H19 O2 N )
C) Ricidine : They are isolated from castor oil. They are less toxic ,
contain cyanide group in its structure. It is present in castor seeds,
optically inactive, weak base.
RICIDINE ( C8 H8 O2 N2 )
4.
PYROLLIDINE- PYRIDINE GROUP:
It is known as TOBACCO ALKALOID. Important group is NICOTINE.
NICOTINE: It is found in the leaves of tobacco. They occur as malic
and citric acid salts. Dry leaves contain around
4 %. They are strong basic. Optically active, found in leavo (L)
form, water soluble, boiling point is 242 0 C. It is most toxic
alkaloid at 40mg or more. It may cause death. It can also act as
good insecticide. It is also used for the manufacturing of niacin and
nicotinic acid. It does not have oxygen in its structure.
NICOTINE ( C10 H4 N2 )
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5. TROPANE ALKALOID:
There
Solanaceous alkaloid and B) Coca alkaloids.
are
two
groups.
A)
a) Solanaceous alkaloids: They are obtained from Solanaceae
family. ATROPINE is the main alkaloid in this group.
ATROPINE ( C17 H23 N O3 )
They are isolated from Atropa belladonna. They are
optically inactive, alkaline, bitter in taste, having Melting Point of 115 0 C.
If it is taken orally, it can depress the Central Nervous System (CNS). It is
used to dilate the pupils (widening of Iris) of eyes during the eye surgery.
Extraction of Atropine:
Juice of Atropa belladonna Spp. is treated with K2CO3 and
extracted in chloroform. The chloroform is evaporated and alkaloid atropine
is extracted with dilute H2SO4 . This is made alkaline by addition of K2CO3.
The precipitated atropine is purified by washing and crystallization. Tropine
and tropic acid can also be converted to atropine with heating in HCl.
b)
Coca Alakloid: The important member of this group is
COCAINE.
COCAINE: ( C17 H21 N O4 ) : Cocaine is extracted from leaves from
coca which is grown in Java, Sri Lanka, Peru countries. It is colourless,
alkaline and less soluble in H2O. Its Melting Point is 98 0 C. It is used as
local anesthetic.
6.QUINOLINE GROUP:
It is also known as Cinchona alkaloid. Main alkaloids are
Cinchonine and Quinine. These are isolated from the bark of Cinchona
plant Spp. IN middle of 17th century; the barks of Cinchona were utilized to
cure fever. It was later confirmed as Quinine (for malarial fever). More
than 35 alkaloids have been extracted from Cinchona. Some of them are
Strychnine,
cinchonine,
quinine,
quinidine,
cuperine,
epiquinine,
epiquinidine etc.
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a) QUININE:
It is obtained from the bark of Cinchona plant. It is
having bitter taste, crystalline, solid substance. Its Melting Point is 175 0C,
semi soluble in ether and alcohol. It is used as quinine sulphate. The
precursor for quinine synthesis is tryptophan.
Tryptophan →
Corynanthrene →
Cinchonamine →
Quinine
IT IS USED AS ANTI-MALARIAL DRUG
QUININE ( C20 H24 N2 O2 )
b) CINCHONINE:
It is present in the bark of grey Cinchona plant. The precursor
of its synthesis is tryptophan. It has bitter taste, less toxic than quinine,
water soluble, optically active, dextro-rotatory, Melting Point is 265 0C.
Tryptophan
Cinchonine
→
Corynanthrene
→
Cinchonamine
→
CINCHONINE ( C19 H22 N O2 )
7. ISOQUINOLINE
PHENANTHRENE GROUP:
ALKALOID
AND
Isoquinoline contain several alkaloids including Morphine,
Thebaine, Papaverine, Narcotine, Narceine etc. Almost all these
alkaloids are obtained from Poppy Spp. (Papaver somaniferum).They are
isolated from green capsules.
a) Papaverine : ( C20 H21 N O4) It is an important alkaloid of opium
group.
b) MORPHINE: Opium contains 10-20 % morphine. It was isolated
from Sertwener plants in 1806. It is insoluble in water having bitter taste,
optically active, leavo –rotatory, Melting Point 255 0C. It is used as pain
killer (analgesic) and sometimes as anaesthat, also used in
medicine. The molecular formula is C17 H19 N O3.
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c) Berberine: Isolated from the roots of Berberine plants. The
molecular formula is C20 H9 O5 N .
8.PHENANTHRENE ALKALOID:
The morphine alkaloid constitute a separate sub-group of
opium alkaloids. Phenanthrene nucleus is found in their structure. e.g.
Morphine, Codeine, Thebaine (10-15 %)
9. INDOLE ALKALOIDS:
There are three classes on the basis of
occurrence and mode of action.
a)
Ergot alkaloid: Their main source is ergot found on the
grasses and rye isolated from fungi. There are more than 20
members.e.g. Ergotamine, Ergotaminine, Ergosine, Ergosinine.
Ergot has been used is medicine as an activator in working of
Uterus.
Strychnos alkaloid: e.g. Strychine, Bruchine: They are used as antihelmentics (to kill Worms and insects). It affects respiration and
CNS of animals.
10. RAUWOLFIA ALKALOID:
e.g. raserpine which is used in
the treatment of hypertension, tension, fevers, dizziness, and insomnia.
11. MISCELLANEOUS GROUPS:
It is also called Tropolane
alkaloids. Colchicine which is present in the seeds of Colchieum is used in
artificial production of polyploidy. Molecular formula is C22 H25 N O6 .
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USES AND IMPORTANCE OF ALKALOIDS:
(A) BIOLOGICAL SIGNIFICANCE:
1. Protect the plants against fungi, insects and animals.
2. Excretory products of plants.
3. Some scientists proposed that alkaloids serve as reservoir of N
and help the protein synthesis.
4. They behave as growth regulators like hormones.
5. Colchicine is used in the multiplication of chromosome by which
new varieties of crops are developed.
6. Helpful to neutralize the harmful effects of acids in plants.
7. Morphine is essential in formation of poppy seeds.
8. Alkaloids inhibit enzyme activity and seed germination. Some
remove bad effects of tannins.
9. Alkaloids are known as co-product in the metabolism of protein
in plants.
10. Alkaloids protect the plants from infection of plant
disease.
MEDICINAL SIGNIFICANCE:
1. Alkaloids are used as medicine in several diseases.
2. Quinine is used as anti-malarial drug.
3. Morphine is used as pain killer.
4. Atropine is used to extend the size of Iris in the treatment of eye
ailments.
5. Ergotine is used to stimulate the function of uterus.
6. Several alkaloids are used as insecticides and pesticides (Nicotine
sulphate from tobacco and azadirachtin from neem).
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7. Several compounds of opium are useful in medicines.
8. Colchicine is used to control rheumatic disease
9. Some are used to control hypertension to improve Central Nervous
System.
10. Amitine is used as cough expectorant and prevent vomiting.
REMOVAL OF ALAKLOID TOXICITY:
If some one takes alkaloids orally in high concentrations, he
should be directed for vomiting immediately and drink dilute tannic
acid. The tannic acid precipitates out the alkaloid and thus acts as
Antidote. This is the reason that tea or coffee 9containing tannin)
act as antidote to alkaloid poisoning.
REFERENCE BOOKS:
1. A TEXT BOOK OF BIOCHEMISTRY (1997) BY R.P.S.
DHAKA AND NANAK SINGH , AMAN PUBLISHING
HOUSE, MEERUT.
DEFINE:- Alkaloid
Classification of alkaloid (Only enlist)
What is biological significance of alkaloid (
any five)
QUININE IT IS USED AS ANTI-MALARIAL DRUG
NICOTINE: It is found in the leaves of tobacco.
MORPHINE: It is used as pain killer (analgesic)
and sometimes as anaesthat, also used in
medicine.
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