Plant Constituents- Phenols and Tannins       
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Polyphenols  
Their distribution is widespread amongst all classes of plant. They are tertiary alcohols which act like weak acids.  These include pyrogallol and hydroquinone.

   
Sumac, chestnut, oak galls and oak-wood contain pyrogallol. Pyrogallol is used for tanning leather.
Uva- ursi (arbutin-hydroquinone beta glucoside) and Bilberry contain hydroquinone. Hydroquinone is used for bleaching the skin to remove skin discoloration and pigmentation.

Phenylpropanoids  
These are C₆C₈ compounds made up of a benzene ring with a three carbon side chain. The most important are the hydroxy cinnamic acids: Caffeic acid, p-coumaic acid, ferulic and sinapic acids. They can be derived from different stages of the shikimic acid pathway. These acids are of much benefit therapeutically and are non-toxic. They may also occur as glycosides.

Caffeic acid- hydroxycinnamic acid: This yellow solid consists of both phenolic and acrylic functional groups. It is found in all plants because it is a key intermediate in the biosynthesis of lignin, one of the principal components of plant biomass and its residues.
Caffeic acid can be found in the bark of Eucalyptus globulus. It can also be found in the freshwater fern Salvinia molesta or in the mushroom Phellinus linteus.
Coffee contains 40-200 mg caffeine per 100 gram- depending on the extraction method.
Caffeic acid is an inhibiter of the enzymes DPA-decarboxylase and 5-lipoxygenase. It is analgestic and anti-inflammatory, and poromotes intestinal motility. It is of widespread occurence and is found in green and roasted coffee beans.

Cynarin (1,5 dicaffeoyl-D-qinic acid)- the major active principle of Globe Artichoke- Cynaria scolymus, is formed from the bonding of two phenolic acids caffeic and quinic acids. Cynarin is a proven hepatoprotective agent.









Curcumin: the yellow pigment from Turmeric rhizome- Curcuma longa. Curcumin and its derivative are diaryheptanoids. They have significant anti-inflammatory, hypotensive and heptoprotective properties.

Decarboxylation of caffeic acids results in formation of simple phenols - eg hydroxybenzoic acid - - > hydroquinone.

Upon glucolysation arbutin- a simple phenol glycoside is formed. Arbutin occurs in leaves of the Pear tree- Pyrus comunis and Artostraphylos- Uva- ursi- a urinary tract antiseptic and diuretic. Arbutin is hydrolised to hydroquinone in alkaline urine- this effect is strictly localised. It is indicated for urinary tract infections ie cystitis, urethritis, prostatitis.Arbutin is both an ether and a glycoside; a glycosylated hydroquinone extracted from Bearberry plant in the genus Arctostaphylos. It inhibits tyrosinase and thus prevents the formation of melanin. Arbutin is therefore used as a skin-lightening agent. Arbutin is found in wheat, and is concentrated in pear skins. It is also found in Bergenia crassifolia

Salicylates & Salicins  

Salicylic acid is a carboxylated phenol. It is rarely found free in plants, but usually occurs as glycosides (salicins), esters and salts. These derivatives are converted to salicylic acid in the human body.

Salicylic acid was first prepared in pure form from Meadowsweet- Filipendula ulmaria in 1838. It was first synthesised by Kolbe, a German chemist in 1860. The subsequent synthesis of acetylsalicylic acid was performed in 1899 by the Bayer company who developed aspirin

Main derivatives of salicylic acid.

Glycosides
     Salicin- found in Willow bark, Poplar bark and Cramp Bark- Viburnum opulus.

     Populin- Poplar bark.
     Gaultherin- Wintergreen
     Siraein- Meadowsweet
Esters
     Methyl salicylate- found in Meadowsweet, Wintergreen.
     Salicyladehyde- Meadowsweet.
Acetylsalicylic acid (ASA)- Aspirin- the synthetic derivative of salicylic acid.

Properties of salicins and salicylates
Analgesic- there has been a long history of use for relief of headaches in European and North American fold medicine.
There is depression of central nervous system and an influence on prostaglandin metabolism.
Antipyretic- acting to increase peripheral blood flow and sweat production by direct action on the thermogenic section in the hypothalamus.
Anti-inflammatory- used in rheumatic conditions.
Anti-clotting effect- present in ASA(Aspirin) but not demonstrated with salicins. through its acetyl group ASA blocks cylooxygenase in the blood platelets, thus irresversibly inhibiting thromboxane synthesis^[2]. So salicin containing herbs have no anti-clotting effect, in the way Aspirin is presently used in western medicine.
Toxicity: A well known tendency to gastric hemorrhage is asociated with the use of Aspirin. However theer is no evidence of this effect in salicin containing herbs.

Lignans  
These are dimeric compounds in which phenylpropane C₆C₃ units are linked to form 3-dimensional networks. They are distinct from lignin which is a high molecular weight polymer based on the C₆C₃ compound coniferyl alcohol.
Lignans have demonstrated significant therapeutic benefit to humans.

The simple lignan nordihydroguaiaretic acid (NDGA) from Chaparral is a potent antioxidant.

Shizandrins from Schizandra chinensis reverse destruction of liver cells by inducement of cytochrome P-450 while other lignans are antiviral and antineoplastic.

Podophyllotoxin, a lignan dervied from the resin of the mayapple polophyllum peltatum, is an anti-mitotic (inhibits cell divisions) and a caustic used topically for warts and papillomas, though extreme care is require due to the highly irritant nature of the compound. The anticancer drugs atposide and teniposide were synthesised from podophyllotoxin.

Flavonolignans from St Mary's Thistle- Silybum marianum are known as hybrid lignans since they are also classed amounst the flavonoids. The mixture of flavonolignans collectively known as silymarin which are found in the fruits of this thistle, have well documented hepoprotective actions.

Coumarins  

Coumarin is the lactone of 0-hydroxy-cinnamic acid, having a cyclized C₆C₅ skeleton.

It occurs as colourless, prismatic crystals and has a characteristic fragrant odour and a bitter, aromatic burning taste. It is soluble in alcohol and can be readily synthesised in the laboratory.
Classification and structures: Most simple coumarins are substituted with OH or OCH₃
They often occur in glycosidic form. For instance aesculin is the glycoside of aesuletin.
Furocoumarins have a furan ring at C6 and C7 as in psoralen, or C7 and C8 as in angelican, of the coumarin ring system, however they are not phonlic in structure. The linear furanocoumarins psoralen and begapten have photosensitising properties which have been utilised in treatments of vitiligo and psoriasis where the subject is concurrently exposed to solar radiation. These coumarins are found in Ammi maju and Angelica archangelica, Ruta graveolens and Citrus species as well as the common Fig.
The furanochromone khellin is the active constituent of Ammi visnaga, a significant antispasmodic and antiasthmatic herb, which also has a beneficial action on coronary blood vessels. Pyranocoumarins, which contain a pyran ring fused at C7 and C8 are also present in Ammi visnaga.
The distribution of coumarins is widespread, Orignally isolated from Tonka Beans, they are abundant in particular plant families eg Rubiaceae- Asperula, Poaceae- Avena, Fabaceae- Medicargo, Melolotus, Rutaceae- Tua, Murraya, Apiaceae- Angelica, Ammi.

Properties of Coumarins
Anti-coagulant, antimicrobial, fungicidal.
Antispasmodic- visnadin, khellin from Ammi visnaga- Calcium channel blockers.
Antifertility agents- chalepensin from Rue- Ruta graveolens.
Photosensitising- psoralen from Ammi majus.

Quinones  
Quinon itself is benoquinone (C₆H₄O₂), a diketone, which undergoes reversible oxidation-reduction (redox) reaction in the presence of reductase enzymes. The reduced from of quinone is hydroquinone which, as noted above, occurs as the glycoside arbutin. Many of the more complex compounds including some naphoquinones and anthraquinones have phenolic structures.
Quinones from an important component of the electron transport system in plants and mammals, Ubiquinol, the reduced form of C0-enzyme Q10 and menaquinone or Vitamin K have significant anti-oxidant properties, playing a mojor role in protecting cells from free-radical damage. There two compound are virtually universal and hence not classed amoung the secondary metabolites. any of four different metabolic pathways may be involed in quinone biosynthesis.

Many quinones are known to induce contact dermatitis and repiratory rections in susceptible people.
Napthoquinones including lapachol and other found in the Ebonaceae family have been associated with these reactions.

Napthoquinones
These are dark yellow pigments. The hair dye henna is derived from the plant Lawsonia inermis, which contains the napthoquinone lawsone linked to a sugar (ie a glycoside).

Other napthoquinones have antimicrobial and antifungal properties. These include juglone from the Walnut- Butternut- Julglans cineria, and Butternut which occurs in leaves and stain derived from the fresh plants. The leaves are also rich in hydrolysable tannins, hence they are of benefit for pile and venous insufficiency. Juglone is a laxative and vermifuge agent.

Other napthoquinones with potent antimicrobial properties are plumbagin from the Sundew- Drosera rotundifolia and lapachol from Pau D'arco- Tabebuia impertiginosa.

Phenolic compounds in foods
Anthrocynanin pigments (condensed tannins) are found in red/blue/black fruits. Ribe, Rubus & Vaccinium fruits were shown to possess supoxide radical scavenging and antilipoperoxidant activities. Grape seeds are a major source of the condensed tannins known as olgomeric proanthocyanidins (OPCs).
Globe artichoke- contains the cinnamic acid depsides cynarin and chlorogenic acid.

Tannins  

Tannins represent the larest group of polyphenols. They are widely distributed in the bark of trees, insect galls, leves, stems and fruit. Tannins were originally isolated from the bark and insect galls of oak trees- Quercus spp.
Tannins are non-crystalline compounds which in water produce a mild acid reaction. Their ingestion gives rise to a puckering, astringent sensation in the mouth. The taste is sour. Thkey often occur as glycosides. Their ability to precipitate proteins into insoluble complexes enable humans to "tan" animal hides and covert them to leather. It is also the basis of their astrngent effects. Due to protein precipitration the tannins exert an inhibitory effect on many enzymes, hence contributing an antipathogenic protective function in bark and heartwoods of woody plant species. Tannins also form precipitates with polysaccharides and some alkaloids including caffeine.
Tannins are high molecular weight compounds MW500-5000 containing sufficient phonolic hydroxyl groups to permit the formation of stable cross links with proteins, and as a result of this cross-linking enzymes may be inhibited. Almost all tannins are classified as either hyrolysable tannins or condensed tannins, Some plants contain both kinds eg Oak- Quercus robur.
Hydrolysable tannins: These are drived from simple phonolic acids, particularly gallic acid, which is linked to a sugar by esterification. The gallic acid groups are usually bonded to form dimers such as ellagic acid.
Hydrolysable tannins break down on dyolysis to give gallic acid and glucose. They are readily soluble in water and alcohol. Botanicals containing hydrolysable tannins include Geranium maculatum, Agrimonia eupatoria and Artostaphylos uva-ursi.
Condensed tannins: Or phlobotannins are polymers of flavan-3-ols (catechins) and flavan-3,4-diols.
Upon hydrolysis condensed tannins from phlobaphenes, insoluble red residues also known as "tanners red". They are only partically soluble in water and alcohol, the addition of glycerine aids solubility.
Botanicals with condensed tannins include Acacia spp and tea- Camellia sinensis.
Astringent action
Astringents cuase contraction of tissue, blanching and wrinkling of mucous menbranes, and diminished exudations. When applied to wound they form a thin protective surface, cuting down on secretion of exudates, precipitation of proteins and/or polysaccharides on the surface resulting in hardening of the epidermis, reducing absorption of toxins, and protecting agains irritants. Tannins display antimicrobial properties. May are able to constrict blood vessels, thereby reducing bleeding.
Uses of tannins
1. Protecting inflammed mucous membranes
2. Drying effect on mucous membranes, reduces hypersecretions.
3. Reduces inflammation and swelling which accompany infections.
4. Prevents bleeding from small wounds.
5. Reduce uterine bleeding eg menorrhagia, metrorrhagia.
6. Binding effect in the gut_ relieves diahhea, dysentery.
7. Used externally as douches, snuffs, eyewash.

Tannins as digestive inhibitors
Consumption of tannins may lead to reduced absorbtion of proteins and other nutrients. For this reason it is not considered wire to drink strong tea with meals. This can also lead to problems in compounding herbal medciiens since there is a tendency to cause precipitates in some cases.