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Dr. Pran Kishore Deb

Dr. Balakumar ChandrasekaranAssistant Professor

Pharmaceutical Medicinal Chemistry

Faculty of Pharmacy, Philadelphia University-Jordan

Email: pdeb@Philadelphia.edu.jo

Learning Outcomes

At the end of this lesson students will be able to

Define flavonoids

Explain the chemistry of flavonoids

Classify flavonoids

Outline examples of various classes of flavonoids

Describe the biosynthesis, extraction and isolation of flavonoids

Outline the significance of flavonoids

Outline natural distribution of flavonoids

Explain the mechanism of actions of flavonoids

Explain the botanical name, family name, medicinal uses of

Rutin, Hesperidin, Genistein, Daidzein2

FLAVONOIDS

Represents diverse class of naturally occurring polyphenolic

secondary metabolites

Found either in free form or as glycosides

About 10,000 flavonoids have been recorded

Represents the third largest group of natural products

Following the alkaloids (12,000) and terpenoids (30,000)

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FLAVONOIDS

Flavonoids = from the Latin word flavus meaning YELLOW

Also contribute to many other colours found in nature

Responsible for the colour of flowers, fruits, leaves

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• They have 15-carbon atoms in their basic skeleton

• Arranged in the form C6-C3-C6

• Consists of two aromatic rings (A & B) linked by 3 carbon atoms

• 3 carbon atoms (C3) may be an aliphatic chain (e.g. chalcone)

• 3 C-atoms may give rise to a third ring C (mostly pyran / pyrone ring)

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Chemistry of Flavonoids

• Classification based on the

• Position of phenyl ring B

• Degree of oxidation and

• Saturation present in the heterocyclic C-ring

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Classification of Flavonoids

• Based on the position of aromatic (phenyl) ring B to benzopyran moiety

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Classification of Flavonoids

8

Classification of Flavonoids

9

Classification of Flavonoids

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Examples of Different Classes of Flavonoids

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Examples of Different Classes of Flavonoids

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Examples of Different Classes of Flavonoids

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Which of the following structure belongs to flavonol group?

√

×Flavone

group

×Flavanonol

group

×FlavAnol

group

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Which of the following represents an isoflavone structure?

A B

C D

√

×

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Biosynthesis of Flavonoids

Biosynthesis of Flavonoids

• Flavonoids are products of mixed biosynthesis

• Cinnamoyl-CoA (C6C3, precursor from the shikimate pathway)

• Three (3) molecules of malonyl-CoA (from acetate pathways)

• Triketide starter unit undergoes cyclization by the enzyme chalconesynthase to generate the chalcone group of flavonoids

• Cyclization of chalcone then occur to give a pyranone ringcontaining flavanone nucleus

• Flavonone can either undergo

• Oxidation of C2-C3 bond of pyranone ring to give the flavone or

• Hydroxylation at C3 of pyranone ring to give the flavanonol

• Flavanonol may then further oxidized to give anthocyanidin

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Biosynthesis of Flavonoids

Glycosides are formed by introduction of sugar

unit/s to the hydroxyl groups of flavonoids

Examples of Flavonoid Glycosides

Pelargonin

Naringin (Bitter)

Naringin dihydrochalcone

(1000 times sweeter than sucrose)

Rutin

capillary fragility,

dermatitis

Hesperidin

Vitamin P

Extraction and Isolation of Flavonoids

• Extraction: extracted from plants with ethanol, methanol or water

• Since they are highly hydroxylated

• Purification: with alkaline solution or precipitation with lead acetate

• Separation: Different classes of flavonoids can be separated by

• Column chromatography

Significance of Flavonoids

Ecological importance in nature

Aid to plant pollination and seed dispersal

An essential step in the reproductive process of plants

Pollination depends on

Action of abiotic forces (20%): wind and water

Rely on animals (80%): flies, birds, beetles and other insects

Flavonoids act as colour attractants to insects and birds

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FLAVONOIDS

Even the colourless flavonoids absorb light in the UV spectrum

due to their extensive chromophores - visible to many insects

Human recognizes the colour of a compound by perceiving

reflected or transmitted light of wavelength 380 and 730 nm

Insects can recognise the light of shorter wavelengths

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FLAVONOIDS

Studies on flavonoids by UV spectroscopy

Most flavonoids consist of two major absorption maxima

Band II (240–285 nm) - benzoyl system of the A ring

Band I (300–400 nm) - cinnamoyl system of the B ring

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Band II

(240–285 nm)

Band I

(300–400 nm)

Significance of Flavonoids

Taste of foods

Naringin (flavanone glycoside) - bitter and astringent

Occurs in the peels of grapefruit

Naringin dihydrochalcone (no pyranone ring) - exceptionally sweet

1000 times sweeter than table sugar (sucrose).

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Naringin (Bitter) Naringin dihydrochalcone

(1000 times sweeter than sucrose)

Significance of Flavonoids

Plants defense system

Protection against fungal parasites, pathogens, herbivores, UV

radiation (e.g. Rotenone)

Allelopathy

Definition: It is a process by which compounds are released from

the metabolism of a plant, preventing the germination, growth and

development of other neighboring plants.

Flavonoids influence, in the vegetation and succession of plants,

by inhibiting the germination and growth of other seedlings

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Significance of Flavonoids

Examples - Flavonoids with allelopathic properties

Plant organism – Oryza sativa L. (rice)

Family - Poaceae

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Flavonoids Functions

Allelopathic inhibitor of weeds and pathogens

Weeds of species – Cyperus difformis, C. iria

Allelopathic inhibitor of seed germination

Significance of Flavonoids

Allelopathy of Weeds

Impose limitations on the development of agricultural activity

Difficult to eradicate

Able to produce, store and release chemicals with

allelopathic properties to the environment

Biopesticides based on flavonoids display allelopathic properties

against weeds

Considered as an efficient natural defense against them

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Significance of Flavonoids

Dietary significance

Acts as strong natural antioxidant (being polyphenolic)

Many diseases can be aggravated due to the presence of free

radicals

Flavonoids scavenge free-radicals, inactivates peroxides

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Significance of Flavonoids

Biological and pharmacological activities

antioxidant, antidiabetic

cytotoxic, anticancer, antifungal

antibacterial, antiviral, antimalarial

antileishmanial, antitrypanosomal and antiamebial

anti-inflammatory, antiallergic

antithrombotic, cardioprotective

hepatoprotective, neuroprotective28

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Class Flavonoids Dietary source

Flavanols Catechin, epicatechin,

epigallocatechin,

chrysin, epigenin

Tea, red wine, cocoa,

chocolates, grapes, blackberry

etc.

Flavones Rutin, luteolin, luteolin

glycoside

Fruit skins, red wine,

buckwheat, red pepper,

tomato skin

Flavonols Kaempferol, quercetin,

myricetin, temarixetin

Onion, red wine, olive oil,

berries, grapefruits

Flavanones Naringin, naringenin,

taxifolin, hesperidin

Citrus fruits, grapefruits,

lemon, oranges

Isoflavones Genistin, daidzin Soyabean

Anthocyanidins Apigenidin, cyanidin Cherry, strawberry, blue berry

Classification and Food Sources of Some

Dietary Flavonoids

• Rutin was first isolated from the leaves of Ruta graveolens

• Family - Rutaceae

• Rutin is usually isolated from Fagopyrum esculentum or Buckwheat

• Family - Polygonaceae

Fagopyrum esculentumRuta graveolens

RUTIN

Rutin

(5,7,3’,4’-tetrahydroxy-flavonol-3-rhamnoglucoside)

Rutinose

6-O-α-L-rhamnosyl-D-glucose

(C12H22O10)

Hydrolysis

Rhamnodiastase

Uses of Rutin1. Decrease capillary fragility and permeability

In cases of hypertension and radiation injuries

2. Anti-inflammatory properties

Beneficial for various types of dermatitis

3. According to the University of Maryland Medical Center:

50 to 250 mg of a rutin supplement two or three times a day

Relieve symptoms of allergic skin reactions, and eczema

Seborrheic dermatitis which causes dandruff

4. Treatment of various types of erythema

• Usually found in the peels of species of citrus fruits like

• Citrus aurantium (Bitter Orange)

• Citrus sinensis

• Family: Rutaceae

Citrus aurantium Citrus sinensis

HESPERIDIN

HESPERIDIN

• It is also called citrin or Vitamin P or permeability vitamin

• Chemically it is hesperitin-7-rutinoside (flavone glycoside)

GENISTEIN

• It is an isoflavone

• Found in plants of Genista species (Genista tinctoria, Dyer's Broom)

• Glycine max, (Soybean in USA and Europe)

• Family: Fabaceae or leguminosae

Uses of Genistein

• Genistin is a phytoestrogen

• Given as a diet supplement

• Contrast the side effect caused by lack of estrogen

• Specially for women during their menopause

• It prevents osteoporosis

• It prevents estrogen dependent cancer like breast cancer

• It also has anti-atherosclerotic and anti-viral properties

DAIDZEIN

• An isoflavone

• Also called phytoestrogen

• Structurally similar to estrogen

• It has similar effects as the Genistein

• Source - leguminous plants such as soybean and mung bean

SoyabeanSoyabean Mung bean

Daidzein – Mechanism of Actions

• Binds to estrogen receptors (ER)

• Shows weak estrogenic effects (weak agonists or partial agonists)

• Shows weak anti-estrogenic effects (antagonist)

• Exerts protective effects against some diseases which are linked to the

regulation of estrogen

Breast cancer

Daidzein – Mechanism of Actions

• Prostate cancer

• Growth of prostate cancer is androgen dependent

• Daidzein has anti-androgen activity

• Prostate androgen-regulated transcript-1 gene (PART-1) is a new gene

• responsive to androgens

• used as a biomarker of prostate cancer

• Daidzein inhibits dihydrotestosterone (DHT)-induced expression

of the PART-1 dose-dependently

• Diabetes - Regulate glucose and lipid metabolism

• Activation of peroxisome proliferator-activated receptors (PPAR)

Glucoside

(inactive)

• Stomach (acid hydrolysis)

• Intestine (bacterial enzymes)

Metabolized

(intestinal bacteria)

Aglycone

(active)

• More potent antioxidant than daidzein

• Binds to ER with a greater affinity

• Anti-atherosclerotic effects

Chemo-preventive role in UVB

induced non-melanoma skin cancer

• Anticancer activity

• Breast cancer

• Prostate cancer

• Other cancer (colon, skin)

• Anti-cardiovascular disease

• Anti-osteoporosis

• Anti-diabetic activity

• Anti-oxidant activity

• Anti-inflammatory activity

• Neuroprotective activity

Soyabean

Mung bean

Reference: Sun et al., Afr J Tradit Complement Altern Med. (2016), 13(3):117-132

DAIDZEIN

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The French Paradox and the Mediterranean Diet

• French population - low incidence of CHD despite of high fat intake

• Difference between French and non-Mediterranean diet

• Higher consumption of Red wine - a rich source of flavonoids

• Mediterranean Diet, also contains other flavonoids and polyphenol-

rich foods such as olive oil, fruits, and nuts

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The French Paradox and the Mediterranean Diet

• Red wine - reported mechanisms of action:

• Attenuation of myocardial ischemic reperfusion injury

• Inhibition of platelet aggregation

• Promotion of endothelial nitric oxide synthase (eNOS), inhibition

of thromboxane synthesis, modulation of lipoprotein secretion,

• Inhibition of low-density lipoprotein (LDL) oxidation,

phospholipase A2 (PLA2), COX and phosphodiesterase (PDE)

• Inhibition of protein kinases involved in cell signaling

• Inhibition of carcinogenesis

Reference - Fernandes, et al. (2017). Wine flavonoids in health and disease

prevention. Molecules, 22, 292. https://doi.org/10.3390/molecules22020292

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Mechanism of Actions of Flavonoids

Antioxidant activity

• Protects body against free radicals and reactive oxygen species

(ROS)

• Free radicals and ROS:

• Associated with large number of human diseases

• Flavones & catechins:

• Most powerful flavonoids protecting against ROS

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Antioxidant activity of flavones: require three factors

1. A 3′,4′-dihydroxy (catechol) structure in the B ring

• favors the electron delocalization

2. An unsaturated 2-3 bond in conjugation with a 4-keto group

• provides electron delocalization from the B ring

3. Hydroxyl groups at positions 3 and 5

• forming intra-molecular hydrogen bonding to the keto group

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Antioxidant activity of flavonoids: require three factors

1. A 3′,4′-dihydroxy (catechol) structure in the B ring

• favors the electron delocalization

2. An unsaturated 2-3 bond in conjugation with a 4-keto group

• provides electron delocalization from the B ring

3. Hydroxyl groups at positions 3 and 5

• forming intra-molecular hydrogen bonding to the keto group

• These 3 effects lead to the increase of the radical scavenging

• Either by delocalization of electrons or by donation of hydrogen

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Mechanism of Actions of Flavonoids

Clinical study example

• Quercetin reduces plasma urate, in moderately hyperuricemic men

• By inhibition of xanthine oxidoreductase

• A major producer of intracellular superoxide and

• Driver of uric acid synthesis

• Reference: Shi, Y., & Williamson, G. (2016). Quercetin lowers uric acid in pre-hyperuricaemic males: A randomized,

double-blinded placebo-controlled, cross-over trial. British Journal of Nutrition, 115, 800–806.

https://doi.org/10.1017/S0007114515005310

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Mechanism of Actions of Flavonoids

Anti-inflammatory Activity

• Inhibit the expression of inducible nitric oxide synthase (iNOS),

cyclooxygenase (COX-2), and lipooxygenase (LOX)-

• Inhibit the production of nitric oxide (NO), prostanoids, leukotrienes

• Inhibit other mediators of the inflammatory process - cytokines,

chemokines

Inhibits COX-2, LOX, iNOS Inhibits NF-kꞵInhibits IL-6, 8 production &

prostaglandin synthesis

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Mechanism of Actions of Flavonoids

• Flavonoids inhibits phosphodiesterase (PDE) enzymes

• PDEs breaks phosphodiester bonds (e.g. cAMP & cGMP)

• Important regulators of signal transduction

• There are many other families of PDEs

• Phospholipases C and D, autotaxin, sphingomyelin

phosphodiesterase, DNases, RNases, & restriction endonucleases

(break the phosphodiester backbone of DNA or RNA)

cGMPcAMP

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Mechanism of Actions of Flavonoids

• Flavonoids inhibits PDEs - acts as potential therapeutics

• Pulmonary arterial hypertension, coronary heart disease, asthma,

COPD, protozoal infections (including malaria), dementia,

depression and schizophrenia

• Examples: Naringenin and 3,5,7,4-tetrahydroxy-flavanone

• Inhibits phosphodiesterase-1 (PDE1) which is involved in

airway smooth muscle activity and airway inflammation

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Anticancer Activity of Flavonoids

• Downregulation of mutant p53 protein

• lead to arrest the cancer cells in the G2-M phase of the cell cycle

• Mutations of p53 - most common genetic abnormalities in human

cancers

• Inhibition of heat shock proteins

• Heat shock proteins form a complex with mutant p53

• Allows tumor cells to bypass normal mechanisms of cell cycle

arrest and apoptosis

• Allows improved cancer cell survival under different bodily

stresses

Mechanism of Actions of Flavonoids

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• Inhibition of EGFR-Tyrosine kinase (e.g. quercetin)

• Inhibition of Estrogen receptor binding capacity

• Inhibition of expression of Ras-proteins

• Consumption of onions and /or apples - major sources of quercetin

• Inversely associated with the incidence of cancers of the

• Prostate, lung, stomach, and breast

Anticancer Activity of Flavonoids

Mechanism of Actions of Flavonoids

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Mechanism of Actions of Flavonoids

Antiviral Activity

• 5,6,7-Trihydroxyflavone-7-O-glucoside

• Inhibits the human T-cell leukemia virus type-1 (HTLV-1)

• Effective against human immunodeficiency virus (HIV)

• Inhibition of HIV-1 reverse transcriptase (e.g. flavone-o-glucoside)

• Inhibition of HIV-1 proteinase (e.g. gardenin A and robinetin)

• Inhibition of DNA polymerase (e.g. catechins)

• Effective against dengue virus type-2 (DENV-2)

• Inhibition of viral polymerase

• E.g. quercetin, hesperetin, diadzein

1. Williamson G, Colin D.K, and Alan C. The Bioavailability, Transport, and

Bioactivity of Dietary Flavonoids: A Review from a Historical Perspectiv.

Comprehensive Reviews in Food Science and Food Safety, 2018, 17, 1054–1112.

2. Katarzyna Małgorzata Brodowska. Natural flavonoids: classification, potential

role, and application of flavonoid analogues. European Journal of Biological

Research 2017, 7 (2): 108-123.

3. Shashank K, and Abhay K.P. Chemistry and Biological Activities of Flavonoids:

An Overview. The Scientific World Journal (Hindawi), 2013, 162750, 1-16.

4. Oyvind M. Andersen and Kenneth R. Markham (Eds.). BOOK: FLAVONOIDS

Chemistry, Biochemistry and Applications. CRC Press, Taylor & Francis,

London, 2006.

5. Erich Grotewold (Ed.). BOOK: The Science of Flavonoids. Springer, United

States of America, 2006.

REFERENCES

1. Meng-Yao Suna, Ying Ye, Ling Xiao, Khalid Rahman, Wei Xia, Hong Zhang.

DAIDZEIN: A review of pharmacological effects. Afr J Tradit Complement

Altern Med., 2016, 13(3):117-132.

2. Shi, Y., & Williamson, G. Quercetin lowers uric acid in pre-hyperuricaemic

males: A randomized, double-blinded placebo-controlled, cross-over trial.

British Journal of Nutrition, 2016, 115, 800–806.

https://doi.org/10.1017/S0007114515005310

3. Iva Fernandes, Rosa Pérez-Gregorio, Susana Soares, Nuno Mateus and Victor de

Freitas. Wine flavonoids in health and disease prevention. Molecules, 2017, 22,

292. https://doi.org/10.3390/molecules22020292

4. https://www.herbs2020.com/herbs/herbs_soy.htm

REFERENCES