1. Flavonoids: Introduction

2. Flavonoids
The flavonoids are polyphenolic compounds possessing 15 carbon atoms; two benzene rings joined by a linear three carbon chain having the carbon skeleton C6 - C3 - C6 and they are the plant pigments and they are having polar in nature and soluble in methanol and water.
Flavonoids constitute one of the most characteristic classes of compounds in higher plants. Many flavonoids are easily recognized as flower pigments in most angiosperm families (flowering plants).
However, their occurrence is not restricted to flowers but include all parts of the plant.
They are secondary metabolite and effective in CNS disorders.

3. Tests for flavonoids
The extracts were dissolved in ethanol, filtered and subjected to following tests.
Shinoda test: The dried extracts were dissolved in 95% ethanol (5ml) and few drops of concentrated hydrochloric acid (HCL) were added. Then the magnesium turnings were put into the solution and observed for appearance of pink color.
Lead acetate solution test: To small quantity of above residue, lead acetate solution was added and observed for appearance of formation of yellow colored precipitates.

4. The Most Important Classes of Flavonoids and their Biological Significance
number of known members
biological significance (so far as known)
anthocyanin(s)
250
red and blue pigments
chalcons 60
yellow pigments
aurones 20
flavones
350
cream-coloured pigments of flowers
flavonols
feeding repellents (?) in leaves
dihydrochalcons 10
some taste bitter
proanthocyanidins 50
astringent substances
catechins 40
some have properties like those of tannins
isoflavonoids 15
oestrogen effect, toxic for fungi
Flavonoids
Polyphenolic compounds with 15 C atoms, 2 benzene rings on linear 3 C chain
Over 4,000 flavonoids
Easily recognized as flower pigments in most angiosperm plants but are not always flower pigments
In plants they repair damage and shield from environmental toxins

5. Core structures and nomenclature
The nomenclature of flavonoids proper is straight-forward with the aromatic ring A
condensed to the heterocyclic ring C and the aromatic ring B most often attached at the C2 position. The various substituents are listed first for the A and C ring and - as primed numbers - for the B ring (note that the numbering for the aromatic rings of the open-chained precursor chalcones is reversed).
(Harborne JB, ed. (1988) The Flavonoids. Advances in Research. Chapman & Hall.)

6. Flavonoids example Flavone:- Luteolin, Apigenin, Tangeritin
Flavonol:- Quercetin, Kaempferol, Myricetin,
Fisetin, Isorhamnetin,
Pachypodol, Rhamnazin
Flavanone:- Hesperetin, Naringenin,
Eriodictyol, Homoeriodictyol
Flavanonol- Taxifolin, Dihydrokaempferol

7. Other flavonoids Chalcone
Chalcone is derived from three acetates and cinnamic acid as shown below.
Anthocyanidin
is an extended conjugation made up of the aglycone of the glycoside anthocyanins. Next to chlorophyll, anthocyanins are the most important group of plant pigments visible to the human eye.

8. The anthocyanodins constitute a large family of differently coloured compounds and occur in countless mixtures in practically all parts of most higher plants. They are of great economic importance as fruit pigments and thus are used to colour fruit juices, wine and some beverages.
The anthocyanidins in Hydrangea, colours it RED in acid soil and BLUE in alkali soil.

9. ISOLATION OF FLAVONOIDS
Ground plant material
Maceration (3 times) Purification with Chloroform (3 times) Dropwise addition of 10% NaCl solution and centrifugation
Aqueous layer
Aqueous layer
Flavonoids in supernant liquid
Precipitate of tannins
Partioning with ethyl acetate
Flavonoid obtained by evaporation of ethylacetate layer