Page 38 - Phytochemistry -1 (PG404) / Clinical Pharmacy 2nd level students ( 2019 )

Clinical pharmacy PharmD program Third level Phytochemistry-1 (PG-504)

Glycosides

Glycosides are plant constituents which yield on hydrolysis by acids or
enzymes a sugar part and a non-sugar part (aglycone or genin).

Hydrolysis

• Acid hydrolysis: strong acids break the glycosidic linkage while weak

acids hydrolyze only glycosides containing 2-deoxy sugar.

• Alkali hycholysis: alkalies do not affect the glycosidic linkage, but

lactone ring in cardiac glycosides is opened by strong alkali and ester or
amide by weak alkali.

• Enzymatic hydrolsis: each glycoside has a specific enzyme but all β-

glycosides hdrolyzed by-------- emulsin

all S-glycosides hydrolyzed by-----------myrosin.

Qualitative tests for Glycosides

General tests for all glycosides

1- Molisch's test:

Mix 2 ml of the aqueous solution of the glycoside with 0.2 ml of
alcoholic α-naphthol (20%). Pour carefully 2 ml of sulfuric acid on the
walls of the test tube, so as to form a layer below the aqueous solution. A
bluish-violet zone at the junction of the two liquids will be obtained with

all glycosides.

2- Fehling's test after hydrolysis:

Mix 0.1 g of the glycoside with 10 ml of water and 2 ml of dilute H 2SO 4.
Boil for 10 minutes. Render the solution alkaline with sodium hydroxide

and add 5 ml of Fehling's solution. Heat to boiling A. red precipitate of
copper oxide is formed indicating the presence of reducing sugar

produced by the hydrolysis of the glycoside This test is negative if carried
out before hydrolysis of the glycoside.
38