Glycosides

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GlycosidesDefinition:

Glycosides are (usually) non-reducing compounds, on hydrolysis by reagents or enzymes yield one or more reducing sugars among the products of hydrolysis.

1- Alcoholic or phenolic (aglycone): e.g., O-Glycoside

2- Sulphur containing compounds: e.g., S-Glycoside

3- Nitrogen containing compounds: e.g., N-Glycoside

4- C-Glycoside

1- Sugars exist in isomeric and forms. Both and Glycosides are theoretically possible.

2- All natural glycosides are of the Type.3- Some linkage exists in sucrose, glycogen and starch. Also the glycoside K-strophanthoside (strophanthidin-linke to strophanthotriose (Cymarose + -glucose + - glucose).

1- According to the type of glycosidic linkage: - glycoside (-sugar) and -glycosides (-sugar).

2- According to the chemical group of the aglycone involved into the acetal union:a. O-glycoside (OH group)

b. S-glycoside (SH group).

c. N-glycoside (NH group).

d. C-glycoside (C group).

3- According to the nature of the simple sugar component of the glycoside:a. Glucosides (the glycone is glucose).

b. Galacosides (the glycone is galacose).

c. Mannosides (the glycone is mannose).

d. Arabinosides (the glycone is arabinose).

4- According to the number of the monosaccharides in the sugar moiety:a. Monoside (one monosaccharide) e.g., salicin.

b. Biosides (two monosaccharide) e.g., gentobioside.

c. Triosides (three monosaccharide) e.g., strophanthotriose.

5- According to the physiological or pharmacological activity therapeutic classification)

a. Laxative glsycosides.b. Cardiotonic glycosides.

6- according to the correlation to the parent natural glycoside:

a. primary glycosides e.g., amygdalin, purpurea glycoside A,

b. Secondary glycosides e.g., prunasin, digitoxin.

7- According to the plant families.

8- According to the chemical nature of the aglycone:a. Alcoholic and phenolic glycosides (aglycones are alcohols or phenols)b. Aldehydic G (aglycones are aldehydes).

c. Cyanogenic G (aglycones are nitriles or derivatives of hydrocyanic acid).

d. Anthracene or anthraquinone G (aglycones are anthracene der.).

e. Steroidal G (aglycones are steroidal in nature, derived from cyclopentanoperhydrophenanthrene) .f. Coumarin G (aglycones are derivative of benzo -pyrone).

g. Chromone glycosides (aglycones are derivatives of benzo--pyrone)

h. Flavonoidal G (aglycones are 2-phenyl chromone structure).

i. Sulphur containing or thioglycosides (aglycones are contain sulphur).

j. Alkaloidal glycosides (aglycone is alkaloidal in nature) e.g., glucoalkaloids of solanum species.

Sugars in glycosides:

1- Monosaccharide (glucose in salicin, rhamnose in ouabain)

2- Disaccharides (gentiobiose in amygdalin).

3- Trisaccharides (strophanthotriose).

4- Tetrasaccharides (purpurea glycosides)

5- Rare sugers (deoxy sugers)

6- Sugar linked in one position to the aglycone rarely in 2 positions as sennosides.

A- 6-deoxy sugars

e.g., 1- methylpentoses

2- -L-rhamnose.

B- 2,6-deoxy sugars (called rare sugars)

e.g., 1- D.digitoxose 2- D.cymarose 3- diginose

C- 2-deoxy sugars

e.g., 2-deoxy-D-ribose

Characteristic of 2-deoxy sugers:

1- Give positive Schiffs test for aldehydes.

2- Positive Keller-Kelliani test.

Diversity in structure makes it difficult to find general physical and chemical properties:1- A- Most glycosides are water soluble and soluble in alcohols.

B- Either insoluble or less soluble in non polar organic solvents.

C- More sugar units in a glycoside lead to more soluble in polar solvents. 2- Glycosides do not reduce Fehlings solution, but when are susceptible to hydrolysis give reducing sugars (C-glycosides are exceptions).

1- Acid hydrolysis:

a- Acetal linkage between the aglycon and glycone more unstable than that between two individual sugars within the molecule.b- all glycosides are hydrolysable by acids non specific (except C-glycosides).

c- Glycosides containing 2-deoxy sugars are more unstable towards acid hydrolysis even at room temperature.

d- C-glycosides are very stable (need oxidative hydrolysis).

2- Alkali hydrolysis:

1- mild alkali

2- strong alkali3- Enzyme hydrolysis:

1- Enzymatic hydrolysis is specific for each glycoside there is a specific enzyme that exerts a hydrolytic action on it.

2- The same enzyme is capable to hydrolyze different glycosides, but and sterio-isomers of the same glycoside are usually not hydrolysed by the same enzyme.3- Emulsin is found to hydrolysed most -glycoside linkages, those glycoside are attacked by emulsin are regarded as -glycosides.

4- Maltase and invertase are -glycosidases, capable of hydrolyzing -glycosides only.

1- Water mixed with different proportions of methanol or ethanol (most suitable extracting solvent).2- Non-polar organic solvents are generally used for de-fating process.

3- Glycosides are not precipitate from aqueous solutions by lead acetate.

1- Destruction of hydrolysing enzymes.a. Drying for 15-30 min. at 100 C.

b. Place plant in boiling water or alcohol 10-20 min.

c. Boiling with acetone.

d. Cold acid pH treatment.

e. Extract at very low temperature.

2- De-fating or purification of the plant material (in case of seeds).3- Extraction of the glycosidal constituents by alcohol, water or dilute alcohols. Some times ether saturated with water for dry material.4- Concentrate the alcoholic extract (to get rid of the organic solvent). Add water (or hot water) filter any precipitate.

5- Purify aqueous extract:

a- Extract non glycosidal impurities by org solvent.

b- Water soluble impurities precipitate by lead acetate.

6- Precipitate excess lead salts.

7- Isolation of the glycosides from the purified aqueous solution, by crystallization.

They do not themselves reduce Fehlings. but reducing sugars upon hydrolysis.

To test for the presence of glycosides Estimate reducing sugars before and after hydrolysis. (by acids or enzymes)

1- Steroidal or cardiac glycosides:Give positive Liebermanns test (steroidal structure).

2- Anthraquinone glycosides and/or aglycone:

Give positive Borntragers test, characteristic reddish coloration with alkalies.

3- Flavonoidal glycosides and/or aglycones:

Characteristic color with, NH4OH, AlCl3, FeCl3.

4- Cyanogenetic glycosides give upon hydrolysis hydrocyanic acid can be easily tested by change Na bikrate paper (yellow) to red color.

5- Sulphur containing glycosides give black precipitate of silver sulphate upon treatment with AgNO3 solution.

1- Keller Killianis test for 2-deoxy sugers:Specificity of action of the hydrolyzing enzymes is often applied for the identification of the sugar moieties of glycosides or even the glycoside as alcohol.

1- Scillarin A [acid hydrolysis] Scillaridine A + Scillabiose

Scillabiose [Scillabiase] Rhamnose + glucose.

2- Prunasin [Prunase] glucose + HCN + 3- Amygdalin [amygdalase] Prunasin + glucose 4- Myrosin enzyme is specific for thio D- glucosides e.g., Sinigrin and sinalbin.

Determination of the glycosidic linkages:

1- By the use of and glycosidases.

2- By acid hydrolysis of glycosides, immediate optical activity measurement of the resulting solution.Color reactions based on the sugar moiety [2-deoxy sugars]:

1- Keller Killiani:

glacialacetic acid containing + FeCl3 + H2SO4 brown ring free from red (acetic acid a quire blue).

2- Xanthydrol:

xanthydrol in glacial acetic containing 1% HCl + glycoside [heat] red color.

N.B. Stability indicating after extraction. U.S.P.

Medicinal importance of glycosides:

1- Cardiac drugs: cardiotonic glycosides e.g., digitalis glycosides, strophanthus, squill.

2- Laxatives e.g., anthraquinone glycosides of senna, aloes, rhubarb, cascara, frangula.

3- Counter irritants e.g., thioglycosides and their hydrolytic products allylisothiocyanate

4- Analgesics e.g., methylsalicylate a hydrolytic product of gaultherin.

5- Anti rheumatic e.g., salicin.

6- Some glycosides are claimed to reduce the capillary fragility e.g., flavonoidal glycosides, rutin, hisperidin.7- Anti-inflamatory: e.g., the glycoside glycyrrhizin has a demulcent, expectorant and antispasmodic action.

8- More recently as an anticancer agent e.g., amygdalin known in the U.S. as Laetrile.

1-The genins of all cardiac glycosides are steroidal in nature, that act as cardiotonic agents.2-They are characterized by their highly specific action cardiac muscle, increasing tone, excitability and contractility of this muscle, thus allowing the weakened heart to function more efficiently.

All cardio active glycosides are characterized by the following structural features:

1- The presence of -OH at position C-3, which is always involved in a glycosidic linkage to a mono, di, tri, OR tetra saccharide.

2- The presence of another -OH group at C-14.

3- The presence of unsaturated 5 or 6- membered lactone ring at position C-17, also in the configuration.

4- The A/B ring junction is usually (cis), while the B/C ring junction is always (trans) and the C/D ring junction is in all cases (cis).5- Additional OH groups may be present at C-5, C-11 and C-16.1- Cardiac glycosides that - unsaturated 5-membered lactose ring in position C-17 are known as cardenolides. These are represented by the digitalis and straphanthus group.

2- Digitalis glycosides contain angular methyl group at C-10, while strophanthus glycoside are characterized by presence of either an aldehydic (CHO) or primary alcoholic (C`H2OH) group at C-10.

CardenolidesDigitalis glycosides R=CH3Strophanthus glycosides R=CHO OR CH2OH

3- Cardiac agents that have doubly unsaturated 6-membered lactone ring in position C-17 are referred to as Bufadienolides.

4- This group includes the squill glycosides and the toad venom, Bufotoxin.

Bufadienolides

Squill glycosides R1=OH, R2=H

Bufotoxin R1 & R2 = ester group5- The glycone portion at position C-3 of cardiac glycosides may contain four monosaccharide molecules linked in series. Thus, from a single genin one may have a monoside, a bioside, a trioside or a tetroside.

6- With the exception of D-glucose and L-rhamnose, all the other sugars that are found in cardiac glycosides are uncommon deoxy-sugars e.g., Digitoxose, Cymarose, Thevetose.

Digitoxose Cyamarose ThevetoseIsolation difficulties:1- Major difficulty in the isolation of 1ry glycosides from the crude drug.. why? because 1ry glycosides are converted into secondary glycosides by hydrolysable enzymes.

2- Other difficulty is the existence of several closely related glycosides in the same drug, which are extremely difficult to separate and purify.

Origin: D. purpurea, D. lanata, D. lutea and D. thapsiThe structures of the common aglycones of the digitalis group are indicated below:

Compounds R1 R2

Digitoxigenin H H

Gitoxigenin H OH

Digoxigenin OH H

DX = Digitoxose, DX (AC)=Acetyldigitoxose,G = Glucose.

1- Glycosides derived from Digitoxigenin:a- Lanatoside A = Digitoxigenin---DX---DX----DX(AC)---G.

b- Acetyl-digitoxin = Digitoxigenin---DX---DX----DX---(AC).

c- Digitoxin = Digitoxigenin------DX---DX----DX.

d- Purpurea gly A = Digitoxigenin---DX---DX----DX---G

2- Glycosides derived from Gitoxigenin:a- Lanatoside B = Gitoxigenin---DX---DX----DX(AC)---G.

b- Acetyl-gitoxin = Gitoxigenin---DX---DX----DX---(AC).

c- Gitoxin = Gitoxigenin------DX---DX----DX.

d- Purpurea gly B = Gitoxigenin---DX---DX----DX---G

3- Glycosides derived from Digoxigenin:a- Lanatoside C = Digoxigenin---DX---DX----DX(AC)---G.

b- Acetyl-digoxin = Digoxigenin---DX---DX----DX---(AC).

c- Digoxin = Digoxigenin------DX---DX----DX.

d- Deslanoside = Digoxigenin---DX---DX----DX---G

1- The 1ry glycosides Lanatoside A, Lanatoside B, Lanatoside C are acted by specific enzyme which split the terminal glucose, give the 2ry glycosides acetyldigitoxin, acetylgitoxin and acetyldigoxin respectively.

2- The deacetyl-lanatosides A, B and C can be obtained by the alkaline hydrolysis of the corresponding lanatosides.

3- Digitoxin, gitoxin and digoxin are obtained by the action of alkali on their acetyl-derivatives.

1- The glycoside K-strophanthoside (a trioside), K-strophanthin B (bioside) and cymarin (a monoside) were isolated from different strophanthus species.

2- The 1ry glycoside K-strophanthoside gives by hydrolysis one molecule of glucose and the 2ry glycoside K-strophanthoside B or K- strophanthin B.3- The later gives by hydrolysis one molecule of glucose and the tertiary glycoside cymarin, which on turn hydrolyze into the genin K-strophanthidin and the deoxysugar cymarose.

The seeds of Strophanthus gratus contains another glycoside named Ouabain or (G-strophanthin), which yield on hydrolysis rhamnose and the aglycone ouabagenin. Ouabagenin differs from K-strophanthidin in having 2 additional (OH) groups at C-1 and C-11 and having a 1ry alcoholic group at C-10 instead of the aldehydic group.

Ouabain (G-strophanthin)

This group of cardioactive agents includes the squill glycosides (the scillarins) and the Toad poison (Bufotoxin).

The genins of squill glycosides differ from those of the cardenolides in two important aspects:

1- They have six membered doubly unsaturated lactone ring in position C-17.2- They have at least one double bond in the steroid nucleus.

The Bufadienolides of SquillName of glycosides Structure

Glucoscillarin Scillaridin A ---RHG---G

Scillarin A Scillaridin A ---RHG

Proscillaridin A Scillaridin A ---RH

* The different cardiac glycosides show different solubilities in aqueous and organic solvents. They are usually soluble in water or aqueous alcohol and insoluble in the fat solvents with exception of chloroform and ethylacetate.

* The higher number of sugar units in the molecule, the greater solubility in water but lower soluble in chloroform.

* Alcohols are good solvents for both the glycosides and the aglycones. Therefore, they are considered as the solvents of choice for the extraction of all CG from drugs.

* pet.ether and ether are used for defatting process of drug, they do not dissolve CG.

1- Acid hydrolysis cleavage of the glycosides into aglycones and sugar residues.

2- Specific enzyme usually coexist with CG in plants, which may split the primary G into G with less sugar units. Thus, CG deteriorate during drying and storage unless special precautions are taken.3- So it is required by many pharmacopoeias that CG containing drugs must contain not more than specified moisture content and that these drugs should be stored in sealed containers over dehydrating agents. 4- It is recommended to heat stabilize these CG, by destroying the enzymes at higher temperatures. At higher temperature, the tertiary OH gp at C-14 may split off as water, leading to formation of an inactive anhydro-form of CG.

5- The gitoxin has in addition to tertiary OH at C-14 another secondary OH at C-16. Both OH gps split as water by the action of H2SO4 with the formation of two additional double bonds. These with the double bond of the lactone ring from a conjugated double bond system that makes the compound fluorescent in UV light.

The detection of gitoxin in other digitalis G is based on the above mentioned reaction.

1- CGs are steroidal in nature, give +Ve with Liebermanns and Salkoviskis test.

2- CG that contain deoxy-sugars are distinguished by Keller Kilianis test, e.g., digitoxose and cymarose.

3- Cardenolides are distinguished from the scillarins by a group of color reagents, that are all alkaline solutions of aromatic nitro compounds, namely, Keddes reagent, 3,5 dinitrobenzoic, Raymonds reagent, metadinitrobenzene, Baljets reagent, picric acid, Legals test, alkaline solution of sodium nitroprusside.

4- All these nitrocompounds react with the active methylene of the five membered lactone ring (in alkaline medium) to give characteristic colors.

1- Cardiotonics, CHF, rheumatic heart disease, atherosclerosis, HTN.2- Diuretics (capillary of the kidneys are dialated).

1- The glycone part displays a great influence on the solubility and the rate of absorption and distribution of the glycosides to the site of action.

2- Small change in the molecules such as a change of the location of the OH gp, modify the cardiac activity or even eliminate it completely.

3- The saturation and/or cleavage of the lactone ring, destroys the cardiac activity.

Therefore, the closely related CG, differ greatly in the rate of absorption, duration of action and their cumulative effect.

1- digitalis leaf (digitalis tablets)

2- digitoxin tablets 200g/tablet

3- digoxin injection contain 0.0025% digoxin

4- digoxin tablets contain 250g/tablet

5- gitalin, lanatoside C, deslanoside, strophanthus, strophanthin, ouabain and squill.

1- O-glycosides where the aglycone moiety is 1,8 dihydroxyanthraquinone derivatives, e.g.,

2- O-glycoside where the aglycone moiety partially reduced 1,8 dihydroxy anthraquinone, e.g., Oxanthrone-type.

Emodin-oxanthrone-9-glucoside3- C-glycoside where the aglycone structure (anthrone der.)

Barbaloin4- O-glycosides where the aglycone moiety is di-anthrone der. (i.e., dimmer) e.g., Sennosides where there is C-C bridge between the anthranol units. Sennoside A&B

The most widely used drugs that contain anthracene compounds are:

Consists of the dried leaflet of Alexandrian or Khartoum senna, Cassia senna (C.acutifolia), Tinnevelly senna (C.angustifolia).Constituents:

Dimeric anthracene glycosides derived from two anthrones moieties which may be:

1- Similar anthrone moiety (Homo-dianthrones) i.e., 2 rhein anthrone moieties condensate through two C-10 atomes. Thus it can be exist in two optical forms, Sennoside A (L- form) & Sennoside B (meso form).

Sennosides A &B2- Or different (Hetero-dianthrones) i.e., one rhein-anthrone & one emodin anthrone, Sennoside C (L- form) and Sennoside D (meso form).

Sennoside C&D

The dried bark of Rhamnus purshiana Family Rhamnaceae. B. P. specified that the collection must be made at least one year before the bark is used (fresh bark contains an emetic principle).Constituents:

A- Four primary glycosides:

1- cascarosides A&B (glycosides of barbaloin)

2- cascarosides C&D (glycosides of chrysaloin)

B-Two aloins (secondary glycosides):Barbaloin derived from (C-10-C-glycoside) of aloe-emodin anthrone and chrysaloin derived from (C-10-C-glycoside) of chrysophanol anthrone.

C- A number of O- glycosides:

e.g., derived from emodin, emodine oxanthrone, aloe emodin and chrysophanol.

E- Free anthraquinones:

Aloe emodin, chysophanol and emodin.

1- Frangulin (frangula emodin rhamnoside).

2- Glucofrangulin (frangula emodin glucorhamnoside).

3- hydrolysis of glucofrangulin yields frangulin and glucose.

4- Hydrolysis of frangulin gives frangula emodin and rhamnose.

1- Consist of glycoside of rhein, rhein anthrone, chrysophanol and aloe emodin.

2- Dianthrones of heteroanthrone types are palmidin A, B, C, Rheidins, sennosides A&B and their oxalate esters (sennosides E&F).

3- The presence of tannins in rhubarb makes the drug constipating. So in small doses, rhubarb exerts no purgative action but acts only as intestinal astringent, but large doses cause purgation.

Cascara is a purgative, mainly in the form of liquid extract, elixir or as tablets prepared from a dry extract.The laxative action of the crude drugs is always higher than from their content of anthracene der. The different anthracene der. contained by the crude drug are said to exert a synergistic action. Thus, the naturally occurring anthracene glycosides were found superior to the synthesis of numerous hydroxyl anthracene der. Some of these synthetic compounds act too drastically and also caused kidney damage.The only compound which is used to some extent in current medicine is danthrone. It is also used as a standared in colorimetric assays of anthraquinone glycosides.

Danthrone

Note:

1- The 1ry glycosides are more active than the aloins while the free anthraquinon have little purgative activity.

2- C-C glycosides, aloins are very resistance to hydrolysis and are not easily hydrolysed (like other anthrones and anthranols) to corresponding anthraquinones.3- Aloin type glycosides are present in aloes and other anthracene bearing drugs of the family liliaceae.

1- Glycosilation:The purgative action of anthracene bearing drugs is owed to their anthracene glycosidal content rather than their content of free anthracene aglycones (i.e., glycosylation is the main requirement for activity, as the sugar moiety serve to transport the aglycone to the site of action in the large intestine).

2- Hydroxylation:

Hydroxylation of C-1, C-8 is essential for activity. Increase hydroxylation leading to increase solubility.

3- Oxidation level:

The degree of oxidation at positions C-9 & C-10 plays an important role in the pharmacological activity. Higher oxidation level at C-9 & C-10 caused lowering of activity. i.e., anthrones and anthranols are more potent than their corresponding oxanthrones, which in turn more active than their corresponding anthraquinones. Complete reduction of C-10 &C-9 lead to complete loss of activity.4- The nature of substances at C-3:

Derivative with CH2OH (as in aloe emodin) are more active than those with CH3 substitution. The latter more active than derivative with COOH substitution at C-3.Anthraquinone glycosides containing adimer more active than a monomer.

5- Effect of storage on the active of anthracene glycosides:a- Prolonged storage of anthracene bearing drugs may bring oxidation of anthranols and anthrones to give the less active anthraquinones. Thus, the activity of drugs decreases by time. However, anthraquinone glycosides do not cause any griping action (like anthranol and anthone), thus no antispasmodic such as belladonna is prescribed with them.b- Drugs as senna, Aloe and cascara preparations retain their activity for a long time.

c- Cascara and frangula must be aged for one year before it is used for medicinal preparation.WHY?Stability is achieved as follows:

1- In senna, there is dimeric glycoside in which a C-C bridge between two anthrone units is formed (the C-10 position of one anthrone is involved in a C-C-covalent bonding with C-10 of the other anthrone). Thus, the C-10 position can not be easily oxidized and the anthrone structure is stabilized.2- In the aloe, the aloins (barbaloin & chrysaloin) contain C-C glycosidic linkage (anhydroglycosides) stabilise the anthrone structure.4- In cascara, cascarosides have an additional O-glycosidic linkage (beside the C-10-C glycosidic linkage. The solubility of cascarosides is increased and thus, produce higher pharmacological activity.

The glycosides are extracted and hydrolyzed by boiling the drug with acids. The aglycones are extracted from the acidic solution with ether or benzene. Upon shaking the ether or benzene layer with aqueous alkali or ammonia solution, the aqueous layer assumes a deep red color, because of the formation of anthraquinone salts.

Borntragers reaction can distinguish anthraquinones from anthrones and anthranols which do not give the test unless they are converted to anthraquinone by oxidation with mild oxidants such as hydrogen peroxide or ferric chloride. Official anthraquinone drugs in B.P and U.S.P.:

1- Senna leaf & senna fruit (pod).

2- Aloes.

3- Cascara tablets, elixir, dry exract, liquid extract.

4- Rhubarb powdered, tincture.

5- Danthrone

6- Frangula bark

Flavonoidal compounds are considered as the largest group of naturally occurring phenols.

Flavonoidals constitute the majority of the yellow colored plant pigments.

Many flavonoidal compounds present as a glycosidic or as a free forms.

All derived from the same parent nucleus, 2-phenyl-benzopyran (flavan), thus they have a basic C-15 skeleton.

Flavonoidal compounds are classified according to the oxidation level of central pyran ring they are classified into flavones, isoflavones, flavonols, flavanones, (true flavanoids) anthocyanidins, chalcones and aurones.

True flavones, are 2-phenyl chromones (2-phenyl benzopyrone), while isoflavones are 3-phenyl chromones der.

Flavonols are 3-hydroxyflavones, while flavanones are 2,3-dihydro der. of flavones (2,3-double bond is lacking).(2-phenylbenzopyran) (2-phenylbenzopyrone)

Anthocyanidines, chalcones and aurones are lack the typical flavone structure. Anthocyanidins and its glycosides (anthecyanins) are ionic oxonium salts. This is responsible for the permanent blue, purple, violet, mauve, and red color of flower, fruits and leaves of higher plants. Anthocyanidins and anthecyanins are soluble in polar solvents.

Cyanidin chloride is an example of anthocyanidines .

Chalcones, have no central pyrone ring, so they are not true flavonoidal compounds. The parent compound chalcone, is chemically phenyl-styryl ketone, or benzylidene acetophenone.

Aurones are oxidized forms that are obtained by enzymatic oxidation. Instead of the central pyrone ring of the normal flavonoidal structure, aurones have five membered ring.

Chalcon Aurone

Flavonoids dissolve in alkalis give intense yellow color solution, on the addition of acid become colorless.Flavonoids exhibit strong fluorescence under UV light.Flavonoidal glycosides are soluble in water and alcohol. Ethylacetate is the solvent of choice for the extraction of flavonoids from aqueous solution.Flavonoids compounds may be characterized through the investigation of their UV Spectra, that usually show two main bands,

1- Band at higher wavelength (band I) which is attributed to the cinnamoyl fraction of the flavonoidal structure Why?.2- Band at lower wavelength (band II) which is due to the benzoyl fraction of the flavonoidal structure.

Band I >> 300 nm If R= H R=OH R=O-substitution

Flavones flavonols 3-sub flavonol

Band I: 304-350 nm Band I: 352-385 Band I: 328-357Band II