NTA Level 5 • Semester 2 • PST05210

Classification, Uses and Extraction of Alkaloids

Basic Pharmacognosy • Source Session/Topic 19

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Session 19: Classification, Uses and Extraction of Alkaloids

Total Session Time: 120 minutes

Prerequisites

• None

Learning Tasks

By the end of this session students are expected to be able to:

• Classify alkaloids

• List pharmacological actions and uses of alkaloids

• Explain extraction, purification and isolation of alkaloids from powdered

plants

• Explain chemical tests for alkaloids

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

• LCD projector and laptop

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning objectives and clarify

ASK students if they have any questions before continuing.

STEP 2: Classification of Alkaloids (40 minutes)

• Alkaloids differ widely in their botanical and biochemical origin, in

chemical structure and their pharmaceutical action

• Therefore, there many different systems for classification of alkaloids

• Major classification systems include

o On the basis of basicity

▪ Alkaloids may be classified into weak bases e.g. caffeine, strong

bases e.g. atropine, amphoteric alkaloids e.g. phenolic alkaloids

like morphine and neutral alkaloids e.g. colchicine

o Biological origin

o Chemical structure

o Biosynthetic pathway

• Classification based on chemical structures (the Hegnauer’s

classification) is more useful and practical

• By the Hegnauer’s Classification, alkaloids are grouped into:

o Typical/True Alkaloids

▪ They are derived from amino acids and contain a nitrogen in a

heterocyclic ring e.g. Atropine

▪ They are toxic

▪ Show a wide range of physiological activity

▪ They are almost invariably basic

▪ They are biosynthesized from amino acids

▪ They normally occur in the plant as salt of an organic acid

▪ Colchicine and quaternary alkaloids are not basic in nature and

do not have heterocyclic ring in their structures

▪ They are divided into the following groups based on their ring

structures

• Pyrrole and Pyrolidine e.g. hygrine, strychnine

• Pyrrolizidine e.g. symphitine, echimidine etc.

• Pyridine and Piperidine e.g. Nicotine etc.

• Tropane (Piperidine/N-methyl-pyrrolidine) e.g. hyoscyamine,

atropine, etc.

• Quinoline e.g. quinine etc.

• Isoquinoline e.g. emetine, morphine, codeine, etc.

• Aporphine (reduced isoquinine/ naphthalene) e.g. boldine

• Quinolizidine e.g. cytisine etc

• Indole or Benzopyrrole e.g. Ergometrine, ergotamine etc.

• Indolizidine e.g. Castanospermine etc.

• Imidazole or Glyoxaline e.g. pilocarpine

• Terpenoid e.g. Aconitine

o Proto-alkaloids/Atypical Alkaloids

▪ Derived from amino acids but do not contain nitrogen in the

heterocyclic ring

▪ They are also called biogenic amines

▪ They are simple amines biosynthesized from amino acids and are

basic

▪ Examples of proto-alkaloids include mescaline, ephedrine,

erythromycin, taxol

o Pseudoalkaloids

▪ They are not derived from amino acids

▪ There are two major series in this class:

• The steroidal alkaloids e.g. Conessine

• The terpenoid alkaloids e.g. Purines, Caffeine, Theophylline and

Theobromine

[pic]

STEP 3: Pharmacological Actions and Uses of Alkaloids (15 minutes)

|Activity: Brainstorming (5 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|What are the uses of alkaloids? |

| |

|ALLOW few students to respond? |

| |

|WRITE their responses on the flip chart/ board |

| |

|CLARIFY and SUMMARIZE by using the content below |

• Alkaloids have marked pharmacological activities

o Analgesics and narcotics e.g. morphine and codeine

o CNS stimulants e.g. caffeine and strychnine

o Anticancer activity e.g. vincristine, vinblastine and taxol

o Mydriatics e.g. atropine

o Anti-asthmatic activity e.g. ephedrine

o Antitussive e.g. codeine

o Expectorant effect e.g. lobeline

o Anti-hypertensive e.g. reserpine

o Smooth muscle relaxants e.g. atropine and papaverine

o Skeletal muscle relaxants e.g. d-tubocurarine

o Anthelmintic activity e.g. pelletierine and arecoline

o Antiparasitic action e.g. quinine and emetine

o Antibacterial activity e.g. berberine

STEP 4: Extraction, Purification and Isolation of Alkaloids (25 minutes)

• Stas-Otto Method

o This technique involves the distribution of alkaloid bases between

acid or aqueous solution and immiscible organic solvent

o First Stage

▪ Powdered plant material is moistened with water and mixed with

alkali like sodium and potassium carbonate, ammonia, calcium

hydroxide. A paste is made with water, dried and re-powdered

• The lime combines with acid, tannins and other phenolic

substances and sets free the alkaloids

o Second Stage

▪ The free alkaloids are extracted by hot continuous percolation

with chloroform or other organic solvent

• The free alkaloids dissolve together with other substances

soluble in solvent

o Third stage

▪ Chloroform solution is agitated with successive portions of

dilute sulphuric acid separating the aqueous layer before adding

the next portion

▪ The alkaloids are converted into alkaloidal sulphates, which

being soluble in water, pass into the aqueous layer

o Fourth Stage

▪ The mixed aqueous liquid is made alkaline with ammonia, and

precipitates that form are collected, washed with water and dried

• Ammonia decomposes the alkaloidal sulphates forming ammonium

sulphates, soluble in water and the free alkaloid which being

practically insoluble in water is precipitated

• Volatile alkaloids are extracted by steam distillation i.e.

Plant material + water + Fixed alkali [pic] steam containing

alkaloids

Heat received in

acidic solution

• Alkaloids are purified by;

o Direct crystallization from solvent

o Steam distillation

o Chromatography techniques

o Gradient pH techniques

STEP 5: Chemical Tests for Alkaloids (20 minutes)

• Alkaloids can be identified by chemical tests

• Precipitation reactions:

o Reagents used for carrying out chemical tests for alkaloids

include;

▪ Reagents that form double salts:

• Mayer’s or Valser’s Reagent (Potassium Mercuric Iodide)

o Produces white or yellow colour precipitates

• Dragendorff’s or Krauts Reagents (Potassium Iodide +

bismuth nitrate)

o Produces orange red precipitates

▪ Reagents Containing Halogens:

• Wagner’s Reagent (Iodine-Potassium Iodide)

o Produces brown or reddish-brown precipitates

▪ Organic Acids:

• Hager’s Reagent (Picric Acid)

o Gives yellow crystalline precipitates

• Tannic Acid solution

o Give buff coloured precipitates

• Colour reactions

o Van-Urks test

▪ For ergot alkaloids and gives a blue colour

o Vitali-Morin test

▪ For solanaceous alkaloids and give a violet colour

STEP 6: Key Points (5 minutes)

• Alkaloids differ widely in their botanical and biochemical origin, in

chemical structure and their pharmaceutical action hence classified by

using various methods

• Common chemical tests for presence of alkaloids include; Mayer's reagent

i.e. potassiomercuric iodide solution, Dragendorff's reagent i.e.

solution of potassium bismuth iodide, Wagner’s reagent i.e. iodine in

potassium iodide and Hagers reagent i.e. picric acid

• Alkaloids are tested chemically by using precipitation reactions such as

by using the Wagner’s reagent or by colour reaction tests such as Vitali-

Morin test

STEP 7: Evaluation (10 minutes)

• What are true alkaloids?

• What are proto alkaloids?

• How are alkaloids extracted?

References

Trease, G. E., Evans, W. C., & Evans, D. (2009). Trease and Evans

pharmacognosy. London: Saunders.

Joanne Barnes et al (2002), Herbal medicines 3rd Edition: Pharmaceutical

Press

Wallis, T. E. (2005). Textbook of pharmacognosy. New Delhi: CBS.

Robbers, J. E., Speedie, M. K., Tyler, V. E., & Tyler, V. E. (1996).

Pharmacognosy and pharmacobiotechnology. Baltimore: Williams &

Wilkins.

Heinrich, M., Kinghorn, A. D., Maizels, D., Gibbons, S., & Phillipson, J.

D. (2012). Fundamentals of pharmacognosy and phytotherapy. Edinburgh:

Churchill Livingstone/Elsevier.

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