Pharmacognosy · Short notes

Glycosides: Short Notes & Cheat Sheet

Glycoside structure and linkages, and the cardiac, anthraquinone, saponin, cyanogenetic, flavonoid and coumarin classes with their tests.

Short notes

Read these before you attempt the topic's MCQs.

The aglycone decides the class and the activity; the sugar mostly decides solubility.

A glycoside is a sugar (glycone) joined to a non-sugar part (aglycone or genin) by a glycosidic bond. The aglycone gives the activity; the sugar affects solubility and absorption.
The link can be O- (most common), C- (e.g. aloin; resists acid hydrolysis), S- (sinigrin) or N- (nucleosides).
Glycosides are hydrolysed by acids or specific enzymes (emulsin for β-glycosides, myrosinase for sinigrin).
They are classified by the chemistry of the aglycone.

Cardenolides have a five-membered unsaturated lactone ring: digitoxin (Digitalis purpurea), digoxin (Digitalis lanata), ouabain (Strophanthus).
Bufadienolides have a six-membered lactone ring: squill (Urginea) and toad venoms.
Tests: Keller–Killiani detects the deoxysugar (reddish-brown ring at the junction); Legal, Baljet and Kedde tests detect the lactone ring.
Anthraquinone glycosides are stimulant laxatives: senna (sennosides A and B), cascara (bark aged at least a year to reduce griping), aloe (aloin, a C-glycoside) and rhubarb.
Borntrager's test gives a pink-red colour in the ammonia layer. C-glycosides need the modified test with ferric chloride to break the C–C bond.

Saponins foam in water and haemolyse red cells. Steroidal saponins (diosgenin from Dioscorea) are starting materials for steroid hormones. Triterpenoid saponins include glycyrrhizin (liquorice).
Cyanogenetic glycosides release hydrogen cyanide: amygdalin (bitter almond, wild cherry), linamarin (cassava). Detected with sodium picrate paper (brick-red; Guignard test).
Isothiocyanate glycosides: sinigrin in black mustard releases allyl isothiocyanate when crushed with water (myrosinase).
Flavonoid glycosides (rutin, hesperidin) reduce capillary fragility; the Shinoda test (magnesium and HCl) gives pink–red.
Coumarins: psoralens from Ammi majus (methoxsalen with UVA for vitiligo); dicoumarol from spoiled sweet clover led to warfarin.
Phenolic glycosides: salicin (willow bark) and arbutin (bearberry, urinary antiseptic).

High-yield facts in one place. Cover the right-hand column and test yourself.

Class	Example · test
Cardiac	Digoxin · Keller–Killiani, Legal, Kedde
Anthraquinone	Senna, aloe · Borntrager's
Saponin	Liquorice, Dioscorea · froth, haemolysis
Cyanogenetic	Amygdalin · sodium picrate (Guignard)
Isothiocyanate	Sinigrin (black mustard) · pungent odour
Flavonoid	Rutin · Shinoda (Mg/HCl)
Coumarin	Psoralen · blue fluorescence in UV
Phenolic	Salicin, arbutin

Drug	Plant
Digitoxin	Digitalis purpurea (leaf)
Digoxin	Digitalis lanata (leaf)
Sennosides	Cassia angustifolia / acutifolia
Cascara	Rhamnus purshiana (bark)
Glycyrrhizin	Glycyrrhiza glabra (root)
Diosgenin	Dioscorea (tuber)

Keller for the Kind of sugar: Keller–Killiani detects the 2-deoxysugar; Legal and Kedde detect the lactone ring.

Aloin is a C-glycoside, so ordinary Borntrager's test fails; the modified test is needed.
Digoxin comes from Digitalis lanata, digitoxin from Digitalis purpurea.
Cascara bark is aged before use to reduce its griping anthrones.
Saponins are haemolytic, so they are toxic if injected but poorly absorbed orally.

Attempt the Glycosides MCQs while the notes are fresh. Anything you get wrong is saved to your incorrect list for the next revision round.