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📚 Glycosides - Study Notes

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1. Isolation of Glycosides

• Dried plant material → ground to coarse powder → extracted in Soxhlet apparatus with aqueous ethanol
• Non-glycosidal impurities removed by precipitating with lead acetate solution
• Excess lead acetate removed by passing hydrogen sulphide gas → lead precipitates as lead sulphide → filtered out
• Filtrate contains glycosides → solvent removed under reduced pressure
• Further purification by column chromatography

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2. Anthraquinone Glycosides

• Aglycone = polyhydroxy anthraquinone derivative
• Pharmacological action: cathartic activity
• Examples: Cascara, Frangula, Aloe, Rhubarb, Senna
• Common derivatives: Chrysophanic acid, Aloe emodin, Frangula emodin, Rhein
• Appear orange-red coloured in medullary rays of rhubarb and cascara
• Detected by Bontrager's test
• Free anthraquinone derivatives have little therapeutic activity; most active in combination with sugar

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3. Cyanophore Glycosides

• Also called cyanogenetic glycosides
• Yield hydrocyanic acid (HCN) on hydrolysis
• Mainly found in family Rosaceae
• Most widely distributed: Amygdalin (found in bitter almonds, apricots, cherries, peaches, plums)
• Amygdalin hydrolysis → 2 glucose + benzaldehyde (C₆H₅CHO) + hydrocyanic acid
• Prunasin - another cyanophoric glycoside in wild cherry bark
• Drug used in cough preparations (syrup): gives pleasant taste + sedative expectorant property

• Drug + water (moisten) + few drops dilute sulphuric acid → filter paper with sodium picrate solution suspended → flask gently heated → brick-red colour = positive

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4. Isothiocyanate Glycosides

• Also called glucosinolate compounds
• Aglycone contains isothiocyanate (-NCS) group
• Principal example: Sinigrin (from black mustard)
  • Hydrolysis of Sinigrin → allyl isothiocyanate + potassium acid sulphate + glucose
• Sinalbin (white mustard) and Gluconapin (rapeseeds) are other examples

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5. Therapeutic Uses of Glycosides

• Pharmacological actions depend on nature and source of glycosides
• Digitalis glycosides → steroidal aglycone → cardiotonic action → collectively called cardiac glycosides
• Senna, rhubarb, aloe glycosides → anthraquinone aglycone → laxative properties
• Aglycone may have independent medicinal value:
  • Glycyrrhetinic acid (aglycone of saponin glycyrrhizin from Liquorice) → treatment of peptic ulcer

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6. ALOE

Basic Info

• Synonyms: Aloe, Musabbar, Kumari, Kuvarpathu
• Biological source: Dried juice from transversally cut leaves of various Aloe species
• Family: Liliaceae

Species Table

History

• Arabic word "alloch" = shining bitter substance
• vera = true, ferox = wild, spicata = flowers in spikes, barbadense/Africana = habitat
• Folklore: leaf mucilage rubbed on skin to reduce perspiration and mask odour (protection from wild animals)
• Aloe barbadensis used with burnt alum for healing sore eyes

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7. Collection and Cultivation of Aloe

• About 200 species of Aloe exist
• Leaves: subulate, succulent, sessile; strong spine at apex; lower portion rounded, upper slightly concave
• Propagation: root suckers
• Soil: Poor grade | Climate: Dry environment
• Root suckers planted 50 cm apart in rows
• Waterlogging must be prevented; roots do not penetrate deep
• Manure: mixture of nitrogen, potassium and phosphorus
• Leaves first cut in 2nd year of cultivation; drug obtained for 12 years
• After 12 years: plants uprooted, land prepared for replantation
• Collection: cut near leaf base → juice from pericyclic parenchymatous cells exudes due to pressure from mucilage cells → single incision drains all juice from pericycle

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8. Preparation of Aloe (by variety)

Barbados/Curacao Aloe

• Cut leaves placed along sides of V-shaped wooden troughs
• Spiny leaves → placed in kerosene tins → brought to wooden troughs in tilted position → juice drains
• Juice boiled in large copper pans → latex evaporates → juice thickens
• Thick juice poured into gourds or metal containers → hardens

Cape Aloe

• Transversely cut leaves arranged in circular manner in basin-shaped depression (lined with goat skin or canvas)
• Leaves overlapped at cut ends → kept for 5-6 hours → juice collected in goat skin
• Juice transferred to large iron kettle → boiled and stirred with wooden paddle
• Attains desired concentration → poured into wooden cases → becomes solid mass

Socotrine Aloe

• Juice collected in goat skin → allowed to become semisolid

Zanzibar Aloe

• Juice poured into skins of carnivorous animals → small solidifies → packed in wooden boxes
• Also called "monkey-skin aloe" (though skin is not from monkeys)

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9. Characteristics Comparison Table

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10. Microscopic Characteristics of Aloe

• Mesophyll encloses vascular bundles covered with pericyclic layer
• Inside pericycle: few large elongated thin-walled aloeitic cells containing highly viscous yellow juice (aloe gel)
• Few calcium oxalate crystals present in parenchyma
• Aloes exists as crystalline and amorphous forms:
  • Amorphous form = Cape aloe
  • Crystalline form = Curacao aloe
• When mounted in cresol under polarized light:
  • Crystalline aloe (Curacao) → insoluble in cresol → shining bright colour
  • Amorphous aloe → quickly dissolved → remains invisible

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11. Chemical Constituents of Aloe

• All varieties are major sources of anthraquinone glycosides
• Principal active component: Aloin (mixture of glycosides)
  • Chief constituent: Barbaloin (chemically: aloe-emodin anthrone C-10 glucoside)
  • Barbaloin is a C-glycoside - NOT hydrolyzed by heating with dilute acids or alkali
  • Ferric chloride decomposes barbaloin by oxidative hydrolysis → aloe-emodin-anthrone + aloe-emodin + glucose
• Other constituents: Isobarbaloin, B-barbaloin, aloe-emodin, resins

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Key Memory Points:

• Glycoside isolation uses Soxhlet + lead acetate + H₂S
• Anthraquinone → cathartic; detected by Bontrager's test
• Cyanophore → HCN; detected by sodium picrate (brick-red)
• Isothiocyanate → -NCS group; Sinigrin from black mustard
• Aloe drug: 12-year cultivation cycle, 4 commercial varieties
• Barbaloin is a C-glycoside (acid/alkali stable)