Pharmacognosy: Module 1–3 Highlights and Core Concepts

Module 1: Introduction to Pharmacognosy

  • Definition and etymology
    • Pharmacognosy derives from the Greek words “pharmakon” (drug) and “gnosis” (knowledge) or “gignosco” (to acquire knowledge). It literally means “knowledge of drugs or pharmaceuticals.”
    • It has been part of healing arts since mankind began treating illnesses.
  • Scope of Pharmacognosy (comprehensive, applied science)
    • Biological aspects: taxonomy, morphology, anatomy, physiology, genetics, biochemistry, etc.
    • Chemical aspects: isolation, purification, and characterization of active constituents.
    • Economic aspects: commercial production and trade of natural drugs and their constituents.
    • Comprehensive view: acquisition of drug knowledge from every point of view (Fluckiger’s definition).
  • Plant chemistry and phytochemistry
    • Plant Chemistry: study of plant constituents; part of natural product and organic chemistry; includes biosynthesis explaining how plants synthesize constituents from simple molecules.
    • Phytochemistry: study of composition of plant principles with extraction, biosynthesis, and identification.
    • Scheele: regarded as father of modern Plant Chemistry.
  • Historical development of pharmacognosy and related disciplines
    • Earliest days: materia medica — traditional compilation of drugs and their usage in western culture.
    • Early 19th century division: materia medica split into Pharmacology (drug actions) and Pharmacognosy (drug information from natural sources) with pharmacognosy placing lesser emphasis on action.
    • Late 19th century: three main drug-related disciplines established:
    • Pharmacology (drug actions)
    • Pharmacognosy (drugs from natural sources)
    • Medicinal chemistry (synthetic drugs)
  • Crude drugs and natural substances (definitions)
    • Crude drugs: vegetable/animal drugs consisting of natural substances merely collected and dried; not yet processed beyond essential packing and preventive decay.
    • Natural substances include:
    • Whole plants and herbs; anatomic parts; plant saps, extracts, secretions and other constituents.
    • Whole animals and anatomic parts; glands/organs; extracts and secretions.
    • Substances found in nature with no changes to their molecular structure.
  • Pharmaceutically, biologic drugs (categories)
    • 1. Crude drugs — examples: chondrus, cascara sagrada, cochineal, thyroid.
    • 2. Drug constituents — chief constituents and their derivatives from biologic origin or synthetic/semi-synthetic prep — examples: sucrose, menthol, progesterone.
    • 3. Other natural products — plant juices, exudates, secretions, extracts — examples: aloe, acacia, orange oil.
    • 4. Biologics
    • 4.1 Antigenic matter or antibody preparations for immunity — examples: vaccines, toxoids, antitoxins, anti-rabies serum.
    • 4.2 Diagnostic aids — examples: mumps skin test antigen, tuberculin.
    • 4.3 Biologics related to human blood — examples: blood grouping serum, whole blood, human albumin.
  • Materia medica and historical authorities
    • Materia medica: medical matter/medicinal substances; early science describing drugs and their usage.
    • De Materia Medica: ancient five-volume treatise by Pedanios Dioscorides (circa 1st century AD) covering ~600 plant drugs plus animal/mineral products; remained an authority for ~15 centuries.
    • Pedanios Dioscorides: authored De Materia Medica (78 AD) describing ~600 plants; some entries like belladonna, ergot, colchicum, aloe, hyoscyamus, opium still in use today.
    • J. A. Schmidt: first to introduce the word pharmacognosy in his title Lehrbuch der Materia Medica.
    • C. A. Seydler: coined the term Pharmacognosy in Analectica Pharmacognostica.
  • Historical figures in pharmacognosy and related texts
    • Galen: Greek physician/pharmacist; described preparation methods for plant and animal drugs (Galenical preparations).
    • Pen-ts’ao kang mu: Chinese drug encyclopedia compiled by Li Shih-Chen (1596 AD) listing >2000 drugs of natural origin.
    • Vedas of India: collection predating 1000 BCE, including >1000 healing herbs.
    • Egyptian Papyrus Ebers (c. 1550 BC): 110 pages, 20 m long; important for historical context of drugs.
    • Pseudopharmacognostic writings: literature on natural drugs based on beliefs/opinions of laypersons.
  • Preparation and extraction of drugs
    • Drugs may be used crudely or their principles (active constituents) extracted and used as medicinal agents.
    • Common extraction/separation methods:
    • a) Maceration
    • b) Infusion
    • c) Digestion
    • d) Decoction
    • e) Percolation
    • f) Distillation
    • g) Expression
  • Solvents used in extraction (common solvents)
    • a) Petroleum ether — fats, fixed oils, waxes, pigments, resins
    • b) Ether/chloroform — alkaloids, resins, glycosides
    • c) 95% EtOH — glycosides, tannins, saponins, resins
    • d) 80% EtOH — similar to 95% EtOH; preferred in phytochemical screening
    • e) Water — glycosides, sugar, salt, gums, proteins
    • f) 1% HCl — alkaloids, salts of vegetable acids
    • g) 5% NaOH — pentosans and hemicelluloses
  • Plant metabolites: primary vs secondary
    • Primary metabolites: CHON, CHO, lipids; abundant in plants but generally lower in value.
    • Secondary metabolites: unique to a species; often therapeutic (alkaloids, steroids, flavonoids).
  • Terminology: derivatives, solvents, and related terms
    • Derivatives/extractives: principles separated from crude drugs by extraction; chief constituent after extraction.
    • Menstruum: solvent used in extraction that is selective in action.
    • Marc: undissolved portion of the drug remaining after extraction.
  • Geographic source and habitat
    • Indigenous plants: native to their home country.
    • Naturalized plants: grown in foreign lands but established there.
  • Preparation for commercial market: collection, harvesting, drying, garbling, packaging, storage
    • Collection: best time where drug content of active principles is highest; material dries well and looks good.
    • Harvesting methods: hand labor or mechanical devices; skilled hand labor important for plant part selection.
    • Rules for collection (examples by plant part):
    • Leaves: mature
    • Flowers: as they begin to open
    • Underground parts: after vegetative phase ceases
    • Bark: before vegetative processes begin
    • Gums and resins: during dry weather
    • Leaves/tops: not dew or rain-covered; do not collect if insect-attacked
    • Fruits: mature but not ripe
    • Seeds: mature but before fruit opens
    • Drying: methods include sun-drying, artificial heat (40–60°C), or shade-drying to retain color.
    • Goals: remove moisture to preserve quality, facilitate comminution, prevent enzymatic action, inhibit mold/bacteria, prevent chemical changes, fix constituents, convert drug to a more convenient form for handling.
    • Curing: a modified drying process that converts inert constituents to active form (example: vanilla).
    • Garbling: final step; remove extraneous matter (other plant parts, dirt, adulterants).
    • Packaging, storage, and preservation: choose packaging to protect from moisture, light, air, insects, high temperature; ensure economy of space.
  • Post-harvest quality threats and controls
    • Moisture: increases weight and reduces active content; encourages enzymatic activity and fungal growth.
    • Light: affects highly colored drugs; can cause unattractive appearance and changes in constituents; photosensitive drugs are especially vulnerable.
    • Air/Oxygen: increases oxidation of constituents, especially with oxidases.
    • High temperature: harmful to thermolabile substances and volatile constituents (e.g., essential oils).
  • Insects and protection strategies
    • Principal orders attacking drugs: Lepidoptera, Coleoptera, Diptera.
    • Protection methods: heat treatment at 65°C (effective); fumigation with methyl bromide; store in tight, light-resistant containers and use small amounts of CHCl3/CCl4.
  • Animal drugs
    • Sources: wild animals (e.g., whale, musk deer); domesticated animals (sheep, hogs, honeybee, cattle).
    • Source of glands, enzymes, and other products from slaughterhouses.
    • Processing and purification vary by drug.
  • Evaluation of drugs: quality and purity or identification
    • Quality definition: intrinsic value; amount of medicinal/active principles/constituents.
    • Evaluation methods:
    • 1) Organoleptic: macroscopic appearance, odor, taste, sound, feel.
    • 2) Microscopic: essential for detecting adulterants and identifying powdered drugs.
    • 3) Pharmacologic/Biologic assay: bioassays on living animals or organs (or microorganisms) to indicate strength.
    • 4) Chemical assay: best method for determining official potency; examples include colorimetric tests.
    • 5) Physical tests: solubility, specific gravity, optical rotation, refractive index, melting/congealing points, moisture.
    • 6) Instrumental methods: UV-VIS, IR (fingerprinting), Mass Spectrometry, NMR.
  • Classification of drugs (vegetable drugs)
    • Classifications include:
    • 1) Alphabetical — by Latin or vernacular names.
    • 2) Morphological — grouped by plant/animal part represented.
    • 3) Taxonomic — based on botanical classification or phylogeny.
    • 4) Pharmacologic/Therapeutic — based on therapeutic use or action of the most active constituent.
    • 5) Chemical/Biogenetic — based on chemically or biologically active constituents.
  • Plant constituents: active vs inert; two main groups
    • Active constituents: responsible for therapeutic effect (e.g., glycosides, alkaloids, terpenes).
    • Inert constituents: do not exert therapeutic activity (e.g., cellulose, lignin, suberin, cutin, starch, albumin, plant coloring matters; in animals, keratin, chitin, muscle fiber, connective tissue).
  • Two classes of active constituents
    • Pharmaceutically active: cause precipitation or chemical changes in a preparation (e.g., tannin).
    • Pharmacologically active: responsible for therapeutic activity (e.g., glycosides, alkaloids).
  • Constituents can be single substances or mixtures
    • Single substances: sugars, starches, plant acids, enzymes, glycosides, steroids, alkaloids, proteins, hormones, vitamins.
    • Mixtures: fixed oils, fats, waxes, volatile oils, resins, oleoresins, oleo-gum-resins, balsams.
  • Factors influencing secondary constituents
    • Heredity (genetics): phenotypic changes reflect genotype; identical genetic composition leads to similar constituents.
    • Ontogeny (developmental stage): constituent identity varies with plant development.
    • Environmental factors: soil, climate, flora, cultivation methods.
  • Drug biosynthesis / biogenesis
    • Study of biochemical pathways leading to secondary constituents; aims to understand pathways and interrelationships.
  • New plant family names (old → new)
    • 1) Cruciferae → Brassicaceae (Mustard)
    • 2) Graminae → Poaceae (Grasses)
    • 3) Labiatae → Lamiaceae (Mint)
    • 4) Palmae → Arecaceae (Palm)
    • 5) Umbelliferae → Apiaceae
    • 6) Leguminosae → Fabaceae
    • 7) Compositae → Asteraceae
    • 8) Guttiferae → Clusiaceae

Module 2: Carbohydrates

  • What carbohydrates are
    • Group of compounds composed of carbon, hydrogen, and oxygen in the same proportion as water; general formula expressed as (CH<em>2O)</em>n(CH<em>2O)</em>n (hydrated carbon).
    • Origin of term: historical misbelief that carbohydrates are hydrates of carbon; many substances have a CX(H2O)Y formula like glucose C</em>6H<em>12O</em>6C</em>6H<em>{12}O</em>6, sucrose C<em>12H</em>22O11C<em>{12}H</em>{22}O_{11}, etc.
  • Drawbacks of the simple formula definition
    • 1) Not all organic compounds with H and O in water proportion are carbohydrates (examples: formaldehyde H<em>2COH<em>2CO, acetic acid C</em>2H<em>4O</em>2C</em>2H<em>4O</em>2, lactic acid C<em>3H</em>6O3C<em>3H</em>6O_3).
    • 2) Many carbohydrates lack the usual H:O ratio (e.g., rhamnose C<em>6H</em>12O<em>5C<em>6H</em>{12}O<em>5, cymarose C</em>7H<em>14O</em>4C</em>7H<em>{14}O</em>4, digitoxose C<em>6H</em>12O4C<em>6H</em>{12}O_4).
    • 3) Some carbohydrates contain nitrogen or sulfur in addition to C, H, and O.
  • Chemical definition of carbohydrates
    • Polyhydroxy aldehydes or polyhydroxy ketones or compounds that hydrolyze to yield either aldehyde or ketone forms.
    • Carbohydrates are among the first products of photosynthesis and form a large portion of plant biomass.
    • Roles: cellulose provides cell wall structure; starch serves as energy reserve; sugars form glycosides and secondary metabolites.
  • Physical properties by molecular weight
    • Low molecular weight carbohydrates: crystalline, water-soluble, sweet (e.g., glucose,fructose,sucroseglucose, fructose, sucrose).
    • High molecular weight carbohydrates (polysaccharides): amorphous, tasteless, less soluble; examples include starch,cellulose,inulinstarch, cellulose, inulin.
  • Classification and nomenclature
    • Simple sugars (Saccharides): Monosaccharides, Disaccharides, Trisaccharides, Tetrasaccharides.
    • Bioses (sugar counts): Trioses (3 C), Tetroses (4 C), Pentoses (5 C), Hexoses (6 C).
    • On the basis of glycone (sugar part): glucosides (glucose as glycone), fructosides (fructose), glucuronides (glucuronic acid), etc.
    • On the basis of glycosidic linkage:
    • O-glycosides: sugar attached to phenol or OH of aglycone (example: amygdaline, salicin, arbutin, senna glycosides).
    • N-glycosides: sugar attached to nitrogen of amine in aglycone (example: nucleosides, e.g., adenosine).
    • S-glycosides: sugar attached to sulfur (thiol) of aglycone (example: sinigrin).
    • C-glycosides: sugar attached directly to a carbon atom of aglycone (examples: aloin, cascarosides, certain anthraquinone glycosides).
  • Examples of glycosides by glycone and linkage basis
    • Glucosides: glycone = glucose; e.g., Rhein-8-glucoside from rhubarb.
    • Fructoside: glycone = fructose (examples less common in current notes).
    • Glucuronides: glycone = glucuronic acid.
    • O-glycosides: Rhein-8-glucoside; Salicin; Arbutin; Salicin-derived examples; Cardiac glycosides; Anthraquinone glycosides.
    • N-glycosides: Nucleosides (e.g., Adenosine).
    • S-glycosides: Sinigrin (from Brassica juncea, Brassicaceae).
    • C-glycosides: Aloin (Aloe), Cascarosides (Cascara).
  • Biosynthesis of glycosides (two-step model)
    • Step 1: Sugar phosphates condense with UTP to form UDP-sugar (sugar-UDP complex) via uridyl transferase.
    • Reaction: ext{UTP} + ext{Sugar-1-P}
      ightarrow ext{UDP-Sugar} + ext{PP}_i
    • Step 2: UDP-sugar transfers the sugar to the acceptor (aglycone) via glycosyl transferase, yielding Sugar-Acceptor glycoside and UDP.
    • Subsequent steps can add additional sugar units, producing di-, tri-, tetraglycosides, etc., via further actions of glycosyltransferases on the previously formed glycoside.
  • Anthracene glycosides
    • Found mostly in dicot plants; aglycones include anthraquinones, anthranols, anthrones or their dimers.
    • Anthrones are insoluble in alkali and less fluorescent; anthranols are alkali-soluble and fluoresce strongly; reduced anthraquinones are more active.
    • Fresh drugs typically contain reduced anthraquinones; storage oxidation/hydrolysis can alter activity.
  • Sterol or cardiac glycosides
    • Cardiac glycosides are important heart drugs (glucosideates) used in treatment of congestive heart failure, atrial fibrillation, and flutter.
    • Structural features: steroid nucleus with hydroxyl groups at 3- and 14-positions; sugar attaches at the 3-OH; a lactone at C-17 is essential.
    • Two natural classes: cardenolides (with an unsaturated butyrolactone) and bufadienolides (with a pyrone ring).
    • Lactone specifics: cardenolides have a five-membered lactone ring (C23 steroids); bufadienolides have a six-membered lactone ring (C24 steroids).
  • Saponin glycosides
    • Saponins are glycosides with foaming properties (natural detergents); they lower surface tension and can be hemolytic; often bitter; may have antifungal/antimicrobial activity.
    • Usually large molecular weight, amorphous powders; often water-soluble and glucosidic, with some solubility in alcohols; generally insoluble in nonpolar solvents.
    • Steroidal saponins relate to cardiac glycosides; diosgenin is a key steroid sapogenin.
    • Triterpenoid saponins are sapogenins; used as foaming agents and have antifungal, antimicrobial, and adaptogenic properties.
  • Cyanogenic glycosides
    • Hydrolysis yields hydrocyanic acid (HCN) along with benzaldehyde and sugars; medicinal activity largely due to hydrocyanic acid release.
    • Examples: dhurrin (Sorghum spp.), prunasin (found in cherry bark and several Rosaceae families).
  • Isothiocyanate glycosides (glucosinolates)
    • Sulfur-containing glycosides found in Cruciferae; hydrolysis yields isothiocyanate (-NCS) group; often irritants used externally as counterirritants.
    • Examples: sinigrin (black mustard, Brassicaceae), sinalbin (white mustard), gluconapin (rapeseed).
  • Flavone glycosides
    • Contain a phenylbenzopyrone (flavone) ring system; can be free or glycosidic forms (O- or C-glycosides) with derivatives such as flavone, flavonol, flavanone, isoflavone, chalcones.
    • Examples: rutin, quercitrin, hyperoside, diosmin, hesperidin, vitexin.
  • Coumarin and furanocoumarin glycosides
    • Aglycone is coumarin; many exhibit fluorescence under alkaline conditions; coumarins have anticoagulant precursors (warfarin).
    • Furanocoumarins: toxic compounds often in Rutaceae, Umbelliferae, and Leguminosae.
    • Biosynthetic pathways involve hydroxylation, glycolysis, cyclization, and shikimate/mevalonate linkages.
  • Aldehyde glycosides
    • Vanillin and cinnamic aldehyde are examples; Vanilla pod contains glucovanillin; cinnamon bark contains cinnamic aldehyde.
  • Phenol glycosides
    • Common plant phenolics with glycosidic linkages; therapeutic uses include antipyretic, antiseptic, diuretic effects.
    • Notable examples and sources include Salicin (Salix), Arbutin (Populus, Ericaceae), Phloridzin (Populus), Gentianose (Gentiana), Coniferin (Coniferae), Gaultherin (Gaultheria), Hamamelitannin (Hamamelis).
    • Table of hydrolysis products lists various sugar moieties and aglycones.
  • Bitter and miscellaneous glycosides
    • Bitter glycosides influence digestion by stimulating gastric secretions and appetite.
  • Biosynthesis of glycosides (summary of specific pathways)
    • Anthracene glycosides: Emodin pathway via a poly-β-ketomethylene acid intermediate; Alizarin pathway via shikimic/mevalonate mediators; involved in Rubiaceae family.
    • Phenol glycosides (arbutin): shikimic acid → phenylalanine → cinnamic acid → hydroquinone → arbutin.
    • Steroid glycosides: biotransformation studies using plant cell cultures show acetate → mevalonate → isopentenyl pyrophosphate → squalene → steroid skeleton; branching leads to spiroketal steroids (steroidal saponins) or pentacyclic triterpenoids (e.g., beta-amyrin).
    • Flavonoid glycosides: biosynthesis has two major inputs (acetate pathway and shikimic acid pathway); aromatic rings originate from shikimate/phenylpropanoid pathway; one 6-carbon fragment from acetate becomes attached to a 9-carbon fragment from shikimate, yielding C6-C3-C6 framework; more than 20 glycosides identified from parsley cells.
    • Coumarin/furanocoumarin glycosides: two routes to bezopyran nucleus via polyketide and shikimate-chorismate pathways; linear vs angular furanocoumarins produced; root systems sometimes essential for coumarin biosynthesis (grafting studies in Melilotus alba and parsley/fenugreek).
    • Cyanogenic glycosides: cyanogenesis arises from glycosides of hydroxynitriles; enzymes hydrolyze glycosides to generate hydroxynitriles; notable examples: dhurrin, prunasin; multiple plant families listed with genus examples.
    • Thioglycosides (isothiocyanate glycosides): mustard family; aglycones typically isothiocyanates; Sinigrin example.
    • Saponin glycosides biosynthesis: head-to-tail coupling of acetate units; branching after squalene formation yields spiroketal steroids or pentacyclic triterpenoids; key sapogenins include diosgenin, β-amyrin.
    • Aldehyde glycosides: cinnamic aldehyde to vanillin proposed in shikimate pathway; key steps shown from shikimic acid to phenylalanine and cinnamic acid to vanillin.
    • Thioglycosides and cyanogenic glycosides are important examples of specialized glycosides with characteristic biological activities.

Module 3: GLYCOSIDES

  • What is a glycoside?
    • A glycoside is a molecule in which a sugar (glycone) is bound through its anomeric carbon to another group (aglycone) via a glycosidic bond.
    • Glycosidic bond forms between the hemiacetal group of a saccharide (or derivative) and the hydroxyl of the aglycone; hydrolysis (acid or enzymes) can separate glycone and aglycone.
  • Components
    • Glycone: sugar part; may be a single sugar or multiple sugars.
    • Aglycone (genin): non-sugar part; typically soluble in organic solvents; may be hydrolyzed by water, enzymes, or minerals acids.
  • Natural characteristics
    • Glycosides are crystalline or amorphous and water/alcohol soluble; insoluble in benzene/ether.
    • Aglycone is soluble in organic solvents (benzene, ether).
    • Glycosides are optically active; natural sugars are typically β-type; sugars in natural glycosides are usually β-anomers.
  • Classification basis
    • By glycone (which sugar forms): e.g., glucoside, fructoside, glucuronide, etc.
    • By glycosidic linkage:
    • O-glycosides: sugar attached to phenolic OH or other OH groups on aglycone.
    • N-glycosides: sugar attached to nitrogen of amine in aglycone (nucleosides).
    • S-glycosides: sugar attached to sulfur (thiol).
    • C-glycosides: sugar directly attached to carbon in aglycone.
  • Notable glycoside classes and examples
    • O-Glycosides: Rhein-8-glucoside (rhubarb); Salicin; Arbutin; Sennosides (anthraquinone glycosides); Cardiac glycosides.
    • N-Glycosides: Adenosine (nucleoside).
    • S-Glycosides: Sinigrin (Brassicaceae).
    • C-Glycosides: Aloin (Aloe); Cascarosides (Cascara).
  • Biosynthesis of glycosides (two-step process, detailed)
    • Step 1: Formation of UDP-sugar via uridine diphosphate sugar (UTP-sugar) with sugar-1-P.
    • Enzyme: Uridyl transferase.
    • Reaction: ext{UTP} + ext{Sugar-1-P}
      ightarrow ext{UDP-Sugar} + ext{PP}_i
    • Step 2: Transfer of sugar from UDP-sugar to aglycone by glycosyl transferase to form the glycoside (acceptor + UDP sugar → glycoside).
    • Additional glycosylation steps can occur to yield di-, tri-, tetra-glycosides via successive glycosyltransferases.
  • Anthracene glycosides (as a subset)
    • Anthracene glycosides are based on aglycones like anthraquinones, anthranols, anthrones or their derivatives.
    • Anthrones are less reactive with alkali; anthranols are alkali-soluble and fluoresce strongly; reduced anthraquinones are more active.
  • Anthracene glycosides biosynthesis and related topics
    • Emodin and related derivatives: proposed biosynthesis via a poly-β-ketomethylene acid intermediate derived from 8 acetate units; intramolecular condensation yields anthraquinones (emodin, etc.).
    • Alizarin: proposed pathway via shikimic acid–mevalonic acid mediators (Rubiceae family examples).
  • Saponin glycosides (expanded)
    • Saponins can be steroidal or triterpenoid (sapogenins).
    • Pathways involve head-to-tail coupling of acetate units; squalene formation marks a branching point toward spiroketal steroids (steroidal saponins) and pentacyclic triterpenoids (β-amyrin).
  • Cyanogenic glycosides (expanded)
    • hydrolysis yields hydrocyanic acid (HCN), benzaldehyde, and sugars; dhurrin and prunasin are common examples across multiple families.
    • Tyrosine/phenylalanine are linked to shikimic acid and phenylalanine pathways in cyanogenesis in plants.
  • Isothiocyanate glycosides (glucosinolates)
    • Contain sulfur; upon hydrolysis yield isothiocyanate groups.
    • Common in Cruciferae; examples: sinigrin, sinalbin, gluconapin.
  • Flavone glycosides
    • Flavones with glycosidic forms; derivatives include flavone, flavonol, flavanone, isoflavone, chalcones.
    • Examples: rutin, quercitrin, hyperoside, diosmin, hesperidin, vitexin.
  • Coumarin and furanocoumarin glycosides
    • Coumarin glycosides: aglycone coumarin; fluorescence under alkaline conditions; important anticoagulant precursors (warfarin).
    • Furanocoumarins: toxic; found in Rutaceae, Umbelliferae, Leguminosae; two forms: linear and angular.
  • Aldehyde glycosides
    • Examples: glucovanillin (vanilla), cinnamic aldehyde (cinnamon bark).
  • Phenol glycosides
    • Therapeutically significant; many plant-derived analgesics, antiseptics, diuretics; often linked to tannins and other phenolics.
    • Notable sources and hydrolysis products listed (e.g., Salicin → salicyl alcohol, glucose).
  • Bitter and miscellaneous glycosides
    • Bitter glycosides play a digestive role; traditional use as stomachics and tonics; stimulate appetite and digestive secretions.
  • Biosynthesis of anthracene glycosides (in-depth example)
    • Emodin pathway via Penicillium islandicum demonstrates intermediate poly-β-ketomethylene acids leading to anthraquinones.
    • Alizarin pathway via shikimic/mevalonate mediators observed in Rubiaceae.
  • Biosynthesis of phenol glycosides (arbutin example)
    • Arbutin biosynthesis from shikimic acid via phenylalanine → cinnamic acid → hydroquinone → arbutin.
  • Biosynthesis of steroid glycosides (cardiac glycosides)
    • Plant cell culture studies show biotransformation of steroids and cardiac glycosides; acetate → mevalonate → isopentenyl pyrophosphate → squalene → steroid nucleus; branching to cardenolides (spiroketal) and bufadienolides (pyrone).
  • Biosynthesis of flavonoid glycosides
    • Enzymatic studies confirm steps involving acetate and shikimic acid pathways; many flavonoid glycosides arise from similar aglycones with substitution mainly at C-3 or C-3’ positions.
    • Relationship to primary aromatic biosynthesis (acetate and shikimate pathways).
  • Biosynthesis of coumarin and furanocoumarin glycosides
    • Grafting experiments show roots may be essential for coumarin biosynthesis in certain species; two main routes to coumarin nucleus (polyketide and shikimate/chorismate).
    • Linear vs angular furanocoumarins distinguished.
  • Biosynthesis of cyanogenic glycosides
    • Tyrosine and phenylalanine derived via shikimic acid/phenylalanine pathway; detailed schemes show relationships between shikimate, prephenic acid, tyrosine, and prunasin/dhurrin formation.
  • Biosynthesis of thioglycosides
    • Mustard family glycosides with isothiocyanate aglycones; Sinigrin example produced from Brassica juncea.
  • Biosynthesis of saponin glycosides
    • Saponins derived from squalene via acetate/mevalonate pathway; branching yields two major classes: steroidal saponins (spiroketal) and pentacyclic triterpenoids (β-amyrin).
  • Biosynthesis of aldehyde glycosides
    • Evidence supports vanillin formation from cinnamic acid via shikimate pathway and phenylalanine transformations.
  • Summary of glycoside biosynthesis themes
    • Sugar donors: UDP-sugars formed via UTP-dependent pathways; transfer to aglycone by glycosyltransferases.
    • Aglycone diversification via multiple biosynthetic routes (shikimate, mevalonate, acetate pathways).
    • Cross-talk between primary metabolism (central carbon metabolism) and secondary metabolite formation.

Module 4: TANNINS

  • Definition and historical context
    • Tannins are complex, non-nitrogenous plant products with astringent properties; term first used by Seguin (1796) to describe substances capable of tanning hides.
  • Classification by Goldbeater’s skin test
    • True tannins: give a positive tanning test and bind to hide powder.
    • Pseudotannins: do not fully bind to hide powder; partly dissociate in tannin tests.
  • True tannins vs pseudotannins and polymerization
    • True tannins are typically high molecular weight; complex polyphenolics produced by polymerization; may form complex glycosides or exist as polymers.
  • Hydrolysable tannins
    • Hydrolyzable by mineral acids or enzymes (e.g., tannase).
    • Structures involve gallic, ellagic, or hexahydrodiphenic acids esterified to a central glucose.
    • Subtypes:
    • Gallotannins: composed of gallic acid units.
    • Ellagitannins: contain hexahydrodiphenic acid; upon hydrolysis yield ellagic acid.
    • Also referred to as pyrogallol tannins because phenolic acids yield pyrogallol derivatives on dry distillation.
    • Solubility: soluble in water; aqueous solutions turn blue with ferric chloride.
  • Nonhydrolysable (condensed) tannins
    • Also called proanthocyanidins; not readily hydrolyzed to simpler molecules.
    • Derived from flavonoid units (e.g., catechin is flavan-3-ol; leucoanthocyanidins are flavan-3,4-diol).
    • Can bind to carbohydrates or proteins to form tannin complexes; when treated with acids/enzymes, polymerize to yield phlobaphens (red pigments).
    • Solubility: water-soluble and often yield green color with ferric chloride.
  • Pseudotannins
    • Simple phenolic compounds of lower molecular weight; do not respond to the tanning test.
    • Examples: gallic acid, chlorogenic acid, catechin, etc.
  • Characteristics of tannins
    • Mostly colloidal in water; non-crystalline; soluble in water, alcohol, dilute alkali, glycerin; sparingly soluble in ethyl acetate; insoluble in most organic solvents except acetone.
    • Molecular weight ranges from ~500 to >20,000.
    • Typically oligomeric with multiple phenolic units; can bind to proteins to form complexes.
  • Biosynthesis of tannins (overview)
    • Tannins belong to the phenolics class; formed via the shikimic acid pathway (phenylpropanoid pathway).
    • Hydrolysable tannins formed via gallic/ellagic acid esters with glucose; ellagitannins formed via hexahydroxydiphenic acid esters.
    • Proanthocyanidins (condensed tannins) arise from leucocyanidins (flavan-3,4-diol) and are linked to flavonoid biosynthesis.
    • Gallic acid is derived from quinic acid; ellagitannins arise from oxidative coupling of gallic acid units with a glucose core; proanthocyanidins polymerize from catechin and related flavonoids.
  • Selected glycoside-tannin associations (examples)
    • Cascara sagrada, Frangula, Aloe, Rhubarb, Senna, Danthron, Glycyrrhiza, Dioscorea, Ginseng, Vanilla, and others appear in tannin-related glycoside discussions.
    • The table of glycosides (Table 21.1) lists common glycosides and their sources and hydrolysis products (e.g., Salicin from Salix, Hamamelitannin from Hamamelis, Phloridzin from Malus, etc.).
  • Practical notes
    • Tannins have wide pharmacological and industrial relevance due to their astringent properties, complex formation with proteins, and various biological effects including antimicrobial, antifungal, and antioxidative activities.

Summary and connections

  • Pharmacognosy integrates historical lore with modern science: taxonomy, chemistry, biochemistry, pharmacology, and industry to understand natural drugs.
  • The course emphasizes: preparation and quality control of crude and derived products, biosynthesis of secondary metabolites, and classification frameworks for plant-derived drugs and their constituents.
  • Glycosides and tannins illustrate the complexity of plant chemistry: from simple glycosides to polyphenolic tannins, with multiple subtypes and biosynthetic pathways influenced by genetics and environment.
  • The material links to real-world relevance in drug discovery, quality assurance of herbal medicines, and the rationale behind standardization and pharmacological evaluation of natural products.
  • Ethical and practical implications include sustainable collection, biosafety in handling plant and animal drugs, and the need for rigorous evaluation standards for traditional medicines.

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  • Glossary of terms to remember:
    • Glycone, Aglycone, Glycosidic bond, O-/N-/S-/C-glycosides, Cardiac glycosides, Saponins, Tannins (hydrolysable vs condensed), Flavonoids, Coumarins, Shikimic acid pathway, Mevalonate pathway, UDP-sugars, Uridyl transferase, Glycosyl transferase.