Alkaloids and its types distribution and its occurrence, Summaries of Pharmacognosy

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ALKALOIDS

Defined as- Physiologically active basic

compounds of plant origin, in which at least one

Nitrogen atom forms a part of a cyclic system

Definition: the term “alkaloid” (alkali-like) was proposed by- W. Meissner (1819) for and is commonly used to designate basic heterocyclic nitrogenous compounds of plant origin that are physiologically active****.

Distribution in Plant:

• (^) All Parts e.g. Datura.
• (^) Barks e.g. Cinchona
• (^) Seeds e.g. Nux vomica
• (^) Roots e.g. Aconite
• (^) Fruits e.g. Black pepper
• (^) Leaves e.g. Tobacco
• (^) Latex e.g. Opium

Since the alkaloids are basic in nature, it is

expected that in plants they exist as salt of

plant acids such as: acetic, oxalic, citric, malic,

lactic, tartaric, tannic, aconitic acids etc.

Some alkaloids are also found to occur

• (^) As free e.g. Nerceine and Nicotine
• (^) As Glycoside e,g Solanam and Veratrum

groups

• (^) As Amides e,g Piperine
• (^) As Esters e,g Tropane and Cocaine

• (^) They may act as reservoir for protein

synthesis.

• (^) Like harmones, they may function as plant

stumilants or regulators in activities like

growth metoblolism and reproduction.

Classification:

• (^) True (Typical) alkaloids that are derived from

amino acids and have nitrogen in a

heterocyclic ring. e.g. Atropine

• (^) Protoalkaloids that are derived from amino

acids and do not have nitrogen in a

heterocyclic ring. e.g. Ephedrine

• (^) Pseudo alkaloids that are not derived from

amino acids but have nitrogen in a

heterocyclic ring. e.g. Caffeine

• (^) False alkaloids are non alkaloids give false

positive reaction with alkaloid reagents.

The basicity of alkaloids depends on the availability of the lone pair of e- on the N2 atoms: e- donating groups enhance basicity, while e-withdrawing groups decrease it. Because some alkaloids have a carbonyl group on the amide, they can also be neutral (colchicine & piperine). Basic characteristic renders complex alkaloids unstable, so that in solution they are sensitive to heat, light & oxygen. Basic character of alkaloids also allows them to form salts with mineral acids (such as hydrochlorides, nitrates and sulphates) or inorganic acids (tartrates, sulfamates).

Tests for Alkaloids Most alkaloids are precipitated from neutral or slightly acidic solution by  (^) Mayer's reagent (potassiomercuric iodide solution)  Cream coloured precipitate.  (^) Dragendorff's reagent (solution of potassium bismuth iodide) orange coloured precipitate.  (^) Wagner’s reagent (iodine in potassium iodide)  red-brown precipitate  (^) Hagers reagent (picric acid)  yellow precipitate

Pharmacological Action & Uses CNS Action: stimulants (caffeine) or depressants (morphine) ANS: sympathomimetics (ephedrine) or sympatholytics (yohimbine, ergot alkaloids), parasympathomimetics (pilocarpine), anticholinergics (choline), ganglioplegics (nicotine). Also: local anesthetics (cocaine), defibrillation (quinidine), anti-tumour agents (ellipticine), anti- malarial (quinine), anti-bacterials (berberine), and amoebicides (emetine).

These actions lead to the extensive use of alkaloid containing herbs and drugs. Although some are used as galenicals (belladonna, datura, and henbane), most are used as starting materials for industrial extraction (morphine from poppy straw or opium, and quinine from Cinchona bark.