Phenolics.pdf

PHENOLICS LSB Lecture 5  Introduction A secondary product that contains a phenol group a hydroxyl functional group on an aromatic ring. Phenolics are a chemically diverse group: many different properties and functions.  Plant Phenolic Compounds A...

PHENOLICS LSB Lecture 5  Introduction A secondary product that contains a phenol group a hydroxyl functional group on an aromatic ring. Phenolics are a chemically diverse group: many different properties and functions.  Plant Phenolic Compounds About 8,000 phenolic structures known Account for 40% of organic carbon circulating in the biosphere Evolution of vascular plants: in cell wall structures, plant defense, features of woods and barks, flower color, flavors etc They can be: Simple, low molecular weight, single aromatic ringed compounds OR Large and complex polyphenols Plant Phenolic Compounds  Plant Phenolic Compounds Primarily derived from the: Phenylpropanoid pathway and acetate pathway (and related pathways)  Plant Phenolic Compounds - origin Plant Phenolic Compounds - origin Phenylpropanoid and phenylpropanoid- acetate pathways provide various phenolic compounds  Biosynthesis of Phenolics Shikimic acid pathway is most common in plants. Converts simple carbohydrates into aromatic amino acids. Not present in animals.  Biosynthesis of Phenolics Most plant phenolics are derived from cinnamic acid formed from phenylalanine by phenylalanine ammonia lyase (PAL) enzyme. PAL PAL activity is inducible: fungal infection, low nutrient levels, high light  Plant Phenolic Compounds As in other cases of secondary metabolites, branches of pathway leading to biosynthesis of phenols are found or amplified only in specific plant families Commonly found conjugated to sugars and organic acids Phenolics can be classified into 2 groups: Flavonoids Non-flavonoids  Plant Phenolic Compounds - Classification Flavonoids - two aromatic rings, 2 pathways anthocyanins, flavones/flavonols Simple phenolics - e.g. coumarins Lignin - 2nd most abundant compound in plants Condensed tannins - polymerized flavonoids Hydrolyzable tannins made of phenolic acids and sugars smaller molecules than condensed tannins  Flavonoids - General Largest group of phenols: 4500 Major role in plants: color, pathogens, light stress Very often in epidermis of leaves and fruit skin Potential health promoting compounds- antioxidants A large number of genes known  Flavonoids - Structure Composed of diphenylpropane units C6-C3-C6 15 carbons: 2 aromatic rings connected by a 3 carbon bridge Flavonoids differ through substitution on the rings Typically OH or OCH3 groups Typical to have OH on positions: 5 and 7 3’, 4’ and 5 The Flavane Nucleus  Flavonoids - Structure The basic flavonoid skeleton can have a large number of substitutions on it: Hydroxyl groups Sugars Rhamnose Methylation Prenylation (farnesyl group) Acylation Glucose Sugars and hydroxyl groups increase the water solubility of flavonoids Methyl and isopentyl groups make flavonoids lipophilic If no sugar - AGLYCONE , With sugar - GLYCOSIDE  Flavonoids - Classification O O O O OH OH O O O O Flavones Flavonols Flavanones Flavanoes Flavanonols O O O OH OH OH O o Catechins Leucoanthocyanidins Chalcones Aurones Flavonoids - Classification Flavonoids - Biosynthesis  Flavonoids - Anthocyanins  Anthocyanidins and anthocyanins are pigments that give plant tissues red, blue, and purple color.  Used in Pollinator attraction Disperser attraction  Structure: A positive charge on the C ring; Two double bonds in the C ring  Flavonoids - Anthocyanins Plant pigments : Anthocyanins, Carotenoids, Chlorophylls Flavonoids - Anthocyanins Anthocyanins - Fruit color Anthocyanins - Flower color Anthocyanins - Leaves and Root Color Anthocyanins - Biosynthesis  Flavonoids - FlavAnols Structures are very similar to those of anthocyanidins: NO positive charge on the oxygen atom and no double bonds in the C ring Flavan-3-ol + Anthocyanidin  FlavAnols - Catechin & Epicatechin Catechin is a common flavan-3-ol that occurs in many plants. It's found in Green tea, Cocoa powder, Red wine It is also a common subunit of proanthocyanidin polymers such as Procyanidin C2. Epicatechin is another common example; it differs from Catechin only in the spatial orientation of its -OH group.  The flavAnols in green tea Green tea contains high levels of flav-3-ols such as (-) Epigallocatechin gallate Flav-3-ols, such as epicatechin, catechin and epigallocatechin (and procyandins their polymers) are:  Powerful antioxidants  Have beneficial effects on cardiac health, immunity and longevity  Levels of flav-3-ols decline on roasting FlavAnols - Biosynthesis  Flavonoids - FlavOnols  Double-bonded oxygen atom attached to position 4 (that’s why flavOnols).  Still "-ols" because they retain the - OH group at position 3 like the FlavOnol flavAnols  Double-bonded oxygen atom, makes them like another class of flavonoids known as "flavones"  Double bond in between C2 and C3 (C FlavAnol ring) involved in UV screening, due to their strong absorbance in UV-A (325-400nm) and UV-B (280-325 nm) wavelengths  FlavOnols - Uses  UV absorbing  protection against UV (280 - 320 nm)  insect pollinator attraction How we see the golden eyes… How honeybees see golden eyes… UV absorbing flavonols are present in the inner part of petals  The FlavOnols- Quercetin  The most abundant flavonol in the diet and is found in hundreds of herbs and foods.  Onions are especially rich in Quercetin.  It has proven antioxidant effects  FlavOnols  FlavOnols are mostly found as O-glycosides  More than 200 different sugar conjugates of Kaempferol !!  Flavonoids - Flavones  Close to the flavOnols but not so widespread (celery, parsley and some herbs)  - BUT without the "-ol." there is no longer an -OH group at position 3 on the central ring Flavones Flavonol  The Flavones- Apigenin Apigenin, a flavone with -OH groups added to positions 5, 7, and 4'  Another flavone luteolin, found in sweet red peppers  Both act as signaling molecules that induce NOD factors in compatible interaction with Rhizobium bacteria (nitrogen fixing root nodules) in legumes (e.g. alfalfa)  Flavonoids - Flavonones No double bond between carbons 2 and 3 of the flavone structure A highly reactive structure (a lot of substitutions) Flavone Flavanone  Flavonones - Naringenin An antioxidant flavanone from citrus species Has - OH groups attached at positions 5, 7, and 4' Studies have indicated that it has anti- inflammatory, anti-cancer, and liver protective effects  Flavanones - in citrus - High concentrations in citrus fruit Naringin, Neohesperidin, grapefruit peel, bitter orange, intense bitter taste intense bitter taste Hesperidin, citrus peel, tasteless  Flavonoids - Isoflavones (Isoflavonoids) -Isoflavones are also known as isoflavonoids -Very similar to flavones, except that the B ring is attached to position 3 of the C ring, rather than to position 2 as in the flavones Isoflavones Flavones (Isoflavonoids)  Isoflavones (Isoflavonoids) Found almost exclusively in leguminous plants with highest concentrations in soybean Possess antimicrobial properties also involved in signaling e.g. attracting rhizobia - Genistein, daidzein are phyto-estrogens (can effect reproduction of grazing animals) - Structure similar to the steroidal hormone estradiol which blocks ovulation - Low isoflavonoid producing varieties are being fed to animals  Genistein and Daidzein Daidzein Genistein  Simple Phenolics (C6-C3)  Also Hydroxycinnamates or cinnamic acids  Generated from cinnamic acid  They are phenylpropanoids  Most common: p-coumaric acid, caffeic and ferulic acids  Involved in defense against insect herbivores and fungi, some may have allelopathic function.  Simple Phenolics  Caffeic acid & ferulic acid implicated in allelopathy. Simple Phenolics Psoralen is one of several phototoxic furanocoumarins, (UV activated)  Lignins One of the most abundant compounds in plants. Highly branched polymer of phenylpropanoid groups (benzene- C3)  Lignins Often found in vessel elements, tracheids, and stems; confers structural support. Primary structural role! Secondary role as a herbivore deterrent by reducing digestibility of plant matter Also difficult for microbes to degrade; its presence slows litter decomposition.  Tannins Condensed formed by polymerization of flavonoid units common in woody plants Hydrolyzable contain phenolic acids: gallic acid, & simple sugars smaller molecules than condensed tannins more easily hydrolyzed and degraded Tannins reduce growth and survival of many different kinds of herbivores Also act as antioxidants  Tannins Many foods contain tannins (e.g. tea, red wine) and have some healthy side effects for humans (e.g. disallowing constriction of blood vessels) Tannins also make protein less digestible. Animals can sense high levels of tannins in their food and opt for another food resource (e.g. mule deer, beavers). High levels of tannins in diet can actually kill some animals.  Tannins Tannins bind to salivary proteins and making the astringency taste In fruit- Astringency in Persimmon, strawberry (boser) Astringency (mild) enhances the taste of wine and tea Animals such as apes and dear will not eat fruit with high tannins In fruit - tannins decline in ripening - evolution for seed dispersal  Condensed Tannins Condensed tannins are formed through the polymerization of flavan- O 3-ol (catechin) and flavan–3,4-diols OH (leucoanthocyanidins) OH In tree, polymerization through Flavan-3,4-diols (Leucoanthocyanidins) acidic enzymatic non-oxidative coupling O 2-50 units (Typically 2-8) Linkages can be through a variety OH of sites Flavan-3-ols (Catechins) Condensed Tannins Condensed tannins are polymerized flavonoids. Responsible for astringency in many foods and medicinal herbs  Condensed Tannins OH O O HO OH OH OH OH HO Catechins HO (flavan-3-ols) O OH O OH O HO OH OH OH OH OH Leucoanthocyanidins Typical Structure of Condensed Tannins  Condensed Tannins Type-B proanthocyanidins (formed from epicatechin and catechin) -Antifeedant proanthocyanidin in red sorghum -These condensed tannins deter birds from feeding on the seed -White sorghum deficient in these compounds is eaten by birds  Hydrolyzable Tannins Hydrolyzable Fig. 13.15 tannins are made of phenolics and sugars.  Tannins The term “tannin” is derived from the tanning process in which raw animal hides are preserved by rubbing tannins on them. The tannins help to complex the proteins and keep them from degrading. This protein-binding property of tannins lends them their toxicity to herbivores. tannins can bind digestion enzymes in the gut of herbivores. tannins also form complex polymers when bound to proteins which are difficult to digest, thus decreasing the nutritional value of the plant material. Tannins can reduce nutritional value of tissues by binding to proteins, making them less digestible. Fig. 13.16 Care for a spot of milk in your tea? After life effects of phenolic compounds. Plant litter decomposition, and release of nutrients from decomposing litter, are strongly influenced by the chemical composition of the litter. Litter higher in tannins and lignin decomposes more slowly. Decomposition rate Lignin/Nitrogen ratio  Stilbens - Polyphenolic like flavonoids - They are phytoalexines, produced in response to fungal, bacterial, viral attack - Resveratrol, the most common stilbene - Major source: grape, wine, peanut products and soya - trans-resveratrol and its glucoside are the active agents in the famous Itadori root ("well being" in Japanese) - Cardio protective effects of red wine, can inhibit LDL oxidation which is the initial stage of atherosclerosis