Secondary Metabolites: Glycosides and Other Compounds PDF

Ergastic cell contents Primary metabolites Secondary metabolites (Food reserves) (Metabolism by-products Carbohydrates Alkaloids Proteins Volatile oils Lipids Tannins Glycosides...

Ergastic cell contents Primary metabolites Secondary metabolites (Food reserves) (Metabolism by-products Carbohydrates Alkaloids Proteins Volatile oils Lipids Tannins Glycosides 1  Secondary metabolites Glycosides A glycoside is an organic compound, usually of plant origin. It consists of a sugar portion linked to a non-sugar part (aglycone or genin) in a particular way (glycosidic linkage). Aglycone Sugar Glycoside (Genin)  when the sugar portion of a glycoside is glucose, the glycoside is known as glucoside.  Glycosides can be hydrolysed either by acids or enzymes into aglycone (genin) and sugar(s). The enzyme emulsin hydrolyses most naturally occurring glycosides. 2 Secondary metabolites Glycosides Glycosides do not reduce Fehling’s solution except after hydrolysis. Glycosides are classified into different groups according to: 1- Number of sugar molecules : Monoglycosides, diglycosides, triglycosides.............. 2- Linkage between the sugar and aglycone: O- glycosides, C- glycosides, S- glycosides..... 3- Nature of the glycosides: primary glycosides, secondary glycosides, tertiary glycosides.............. 4- Chemical nature of the aglycone 3  Secondary metabolites Glycosides Number of sugar units Aglycone - glucose - glucose - glucose - glucose ONE sugar: Monoglycoside TWO sugars: Diglycoside THREE sugars: Triglycoside FOUR sugars: Tetraglycoside 4  Secondary metabolites Glycosides Types of glycosidic Linkage The usual linkage between the sugar and aglycone is an oxygen linkage such glycosides called O-glycosides and are the most numerous ones found in nature. Other linkages: ✓ C - glycoside : e.g. cascaroside ✓ S-glycoside : e.g. sinigrin 5 Genin Sugar Glycoside agly-OH HO-C6H11O5 agly-O-C6H11O5 O –glycoside e.g. arbutin agly-CH HO - C6H11O5 agly C- C-6H11O5 C - glycoside E.g.cascaroside agly -SH HO -C6H11O5 agly- S-C6H11O5 S-glycoside e.g. sinigrin 6  Secondary metabolites Glycosides Nature of glycosides (i) Primary glycosides: Primary glycosides are the unchanged form of glycosides which occur naturally in fresh plant. (ii) Secondary glycosides: Secondary glycosides are those glycosides which result from the hydrolysis of other glycosides (i.e. primary glycosides). 7  Primary glycoside Aglycone - glucose - glucose- glucose - glucose Hydrolysis - glucose Secondary glycoside Aglycone -- glucose -- glucose -- glucose Hydrolysis - glucose Tertiary glycoside Aglycone -- glucose -- glucose 8  Secondary metabolites Glycosides Chemical nature of the aglycone ✓Phenolic glycosides ✓Anthracene glycosides ✓Flavone glycosides ✓Cyanogenic glycoside ✓Thio –glycosides (glucosinolates) ✓Steroidal/Cardiac glycosides ✓Saponin glycosides 9  Secondary metabolites Glycosides Phenolic glycosides The aglycones of this group of glycosides contain phenolic group e.g. arbutin Arbutin is the main active constituent of Uva ursi leaves, Uses as diuretic and antiseptic in urethritis and cystitis, and as a skin lightening agent (arbutin yields hydroquinone upon hydrolysis) 10  Secondary metabolites Glycosides Anthracene glycosides/ Anthraquinone glycosides Glycosides in which the aglycones are anthracene derivatives. When alkali is added to powdered drugs containing anthraquinone derivatives, a red or purple colour is formed (Screening of anthracenes) Examples of anthracene glycosides: Sennosides A,B,C and D from Senna leaf and Cascarosides from cascara bark  Uses: As Cathartic, purgative and laxative 11  Secondary metabolites Glycosides Modified Borntrager’s test for Anthraquinone glycosides 1. Boil the powdered drug with alcoholic potassium hydroxide solution then filter (hydrolysis). 2. Acidify the filtrate with dilute hydrochloric acid and then filter (to liberate the free aglycones). 3. Cool and shake with ether or benzene (extract the free aglycones) 4. Separate the yellowish ethereal layer into a clean test-tube and shake with dilute solution of ammonium hydroxide; a pink to violet colour is observed in the aqueous layer. 12  Secondary metabolites Glycosides Flavone glycosides The flavonoid group may be described as a series of C6-C3-C6 compounds O C6 C6 C3 Flavonoids are often yellow (Latin. flavus, yellow) and they dissolve in alkalies giving yellow solutions.  test: Addition of potassium hydroxide to powdered plant gives a yellow colour which on addition of acid become colourless. 13  Secondary metabolites Glycosides Examples of flavone glycosides: - Diosmin from Buchu leaf - Hesperidin & Rutin (citrus) Diosmin Uses of flavone glycosides: ❑ They are used to reduce capillary fragility e.g. diosmin and rutin ❑ As Antioxidant eg. Green tea ❑Improve cerebral circulation e.g Ginko biloba 14  Secondary metabolites Glycosides Cyanogenic (Cyanophore) glycosides Cyanogenic glycosides are glycosides yielding hydrocyanic acid upon hydrolysis e.g. ▪ prunasin ▪ amygdalin ▪ linamarin  Test for cyanogenic glycosides 1-The material is well-broken and placed in a small flask with sufficient water 2-In the neck of the flask a strip of filter paper impregnated with sodium picrate (yellow) is by means of a cork. 3- After incubation and hydrolysis the colour of the sodium picrate paper is changed into brick- red(sodium isopurpurate) 15  CHO Emulsin/acid HOH + HCN + 2 glucose Amygdalin Benzaldehyde (Cyanogenic glycoside) (vo) (Bitter almond oil) HOH amygdalase - glucose CHO prunase glucose + HCN + glucose CH O HOH CN Prunasin Benzaldehyde (Cyanogenic glycoside) (vo) 16  Secondary metabolites Glycosides Thioglycosides (Glucosinolates) The aglycones of this group of glycosides contain sulphur, sometimes called thiocyanate or sulphurated glycosides. NOSO3X Glucosinolates have the general structure: R C S C6H11O5 A number of plants of the family Brassicaceae yield glucosinolates e.g. Brassica nigra (black mustard seed) yields sinigrin glycoside. Brassica alba (white mustard seed) yields sinalbin glycoside  Uses: Black mustard seed is used as rubefacient or counterirritant 17  Secondary metabolites Glycosides Steroidal / Cardiac glycosides The aglycones of this group have a steroidal structure The glycosides are used to increase the tone and contractility of cardiac muscle for the treatment of congestive heart failure. Cardiac glycosides occur in small amount in: D C ▪ Digitalis purpura e.g. Digitoxin A B ▪ Squill ▪ Strophanthus Test for cardiac glycosides (steroidal nucleus): Liebermann’s test few drops of acetic anhydride and one ml of concentrated sulphuric acid from the sides gives reddish ring at the junction of 2 layers. 18  Secondary metabolites Glycosides Saponin glycosides Saponin glycosides share in varying degrees two common characteristics:: They foam (froth) in an aqueous solution They cause haemolysis of red blood cells The aglycones of saponin glycosides are referred to as sapogenins. Two kinds of saponin are recognized according to the structure of the aglycone or sapogenin: i- Steroidal type ii- Triterpenoid type 19  Secondary metabolites Glycosides Saponin glycosides i-Steroidal type Steroidal saponins are of great importance owing to their relationship with such compounds as the sex hormones, cortisone, vitamin D and cardiac glycosides They are used as starting materials for the synthesis of such compounds. Examples of drugs containing steroidal saponins: - Fenugreek, ginseng ii-Triterpenoidal type Examples of drugs containing triterpenoidal saponins Quillaia, liquorice, senega ✓Quillaia root is used as emulsifying agent 20 ✓ Senega root is used as expectorant in chronic bronchitis.  Secondary metabolites Alkaloids They are basic compounds (alkali-like). Contain one or more nitrogen atoms usually in a heterocyclic ring. The name proto-alkaloid or amino-alkaloids is sometimes applied to compounds such as ephedrine and colchicine which have their nitrogen in a side chain. Names of alkaloids should end with the suffix (ine). 21  Secondary metabolites Alkaloids Properties: Most alkaloids are unite with acids to form salts. In plant they may exist in the free state or as salt Alkaloidal base + Acid → Alkaloidal salt Insoluble in water Soluble in water Soluble in chloroform Insoluble in chloroform Alkaloids contain the elements C, H , N & O. 22  Secondary metabolites Alkaloids Chemical Tests for Alkaloids: (Precipitating reagents) 1.Mayer’s reagent : creamy precipitate. 2.Wagner’s reagent : reddish brown precipitate. 3.Dragendorff’s reagent : reddish-brown precipitate. 4.Tannic acid solution: It gives a white precipitate. 5.Picric acid solution: It gives yellow precipitates. Caffeine and some other alkaloids do not give these precipitates. (negative result). 23  Secondary metabolites Volatile Oils Volatile or essential oils, as their name implies, are volatile in steam. They are generally mixture of terpene hydrocarbons (C10H20) and oxygenated terpenes compounds derived from these hydrocarbons. They are secreted in oil cells, secretion ducts or cavities or in glandular hairs. They are frequently associated with other substances such as gums and resins and they tend to resinify on exposure to air, therefore, they should be stored in a cool dry place, preferably in full amber glass containers. 24  Secondary metabolites Volatile Oils Properties: Volatile oils dissolve in alcohol, sparingly soluble in water. They resemble fixed oils in their behavior towards: osmic acid, Sudan III, and alkanna tincture, but are not saponified when treated with caustic alkalis. Uses: 1- For their therapeutic action e.g. oils of eucalyptus, clove. 2- Flavouring agents e.g. oil of cinnamon and oil of cardamom 3- Perfumery e.g. oil of lavender and oil of rosemary. 25  Secondary metabolites Tannins They are water soluble polyphenol compounds present in plant extracts They have high molecular weight ranging from 500-5000 amu They are able to combine with protein of animal hides preventing their putrefaction and convert them into leather. 26  Secondary metabolites Tannins They are classified into: 1. Hydrolysable Tannins: - These may be hydrolyzed by acids or enzymes such as tannase. - They are formed from several molecules of phenolic acids such as gallic and ellagic acids. - Their solution turn blue with iron salts. Ellagic acid 27  Secondary metabolites Tannins They are classified into: 2. Condensed Tannins (Proanthocyanidins): - These include all other true tannins, their molecules are more resistant to breakage than hydrolysable tannins. - With acids or enzymes they are decomposed into red insoluble compounds known as “phlobaphenes”. Phlobaphenes give the characteristic red colour to many drugs such as red cinchona bark. - On dry distillation, they yield catechol and these tannins are therefore sometimes called catechol tannins. Like catechol itself, their solutions turn green with ferric chloride. - Condensed tannins appear to be formed of the polymerization of flavans e.g. catechin. 28  Secondary metabolites Tannins They are classified into: 3. Pseudotannins: These are compounds of lower molecular weight than true tannins and they do not respond to the gold beater’s skin test. Uses of tannins: Tannins are used in diarrhea and as haemostatic in hemorrhoids. Commercial tannins are used in leather industry and in manufacture of ink. 29  Secondary metabolites Tannins Tests for tannins: Goldbeater’s Skin test: 1. Soak a piece of goldbeater skin (membrane prepared from the intestine of the ox which behaves similarly to an untanned hide) in 2% HCl, then rinse with water. 2. Place the soaked piece in the solution to be tested for 5 min. 3. Wash with distilled water and transfer to a 1% solution of ferrous sulphate. →brown or black colour on the skin indicates the presence of tannins. Ferric chloride solution: Tannins are extracted from powdered plant with hot water. Add few drops of ferric chloride solution, a blue colour or precipitate is produced with hydrolysable tannins and green colour or precipitate with condensed tannins. 30