The Chemistry and Biology of Bioflavonoids

 

Amit Tapas¹*, Dinesh Sakarkar¹, Rajendra Kakde²

 

¹Sudhakarrao Naik Institute of Pharmacy, Pusad- 445204, Dist.-Yavatmal (MS) India

²University Department of Pharmaceutical Sciences, R.T.M. Nagpur University, Nagpur-440033 (MS) India

 

ABSTRACT

Phenolic compounds form one of the main class of secondary metabolites. They display a large range of structures and

are responsible for the major organoleptic characteristics of plant-derived-foods and beverages, particularly color and taste properties and they also contribute to the nutritional qualities of fruits and vegetables. Among these compounds flavonoids constitute one of the most ubiquitous groups of all plant phenolics. Owing to their importance in food organoleptic properties and in human health, a better understanding of their structure, biosynthesis, chemical synthetic methodologies and various biological activities appears essential to make them as potential therapeutic agents and also to predict  and  control  food  quality.  The  purpose  of  this  work  is  an  overview  of  structural  diversities,  synthetic methodologies and shed a light on biological activities. The first part of this review concerns the structural aspects, biosynthetic and synthetic schemes for different classes of flavonoids. Regarding the synthesis of flavonoids various schemes have been highlighted to obtain variety of higher potent flavonoid derivatives. These schemes can be applicable at regular laboratory levels. The second part of this review concerns the various biological properties of flavonoids on plant as well as on mammalian cells. Recently this group of natural products has gained much interest as bioactive compounds. Because of the variety of pharmacological activities on mammalian body flavonoids are more correctly referred as “Nutraceutical”.

 

 

INTRODUCTION:

Phenolic compounds form one of the  main class of

secondary metabolites. They display a large range of structures  and  are  responsible  for  the  major organoleptic characteristics of plant-derived-foods and beverages, particularly color and taste properties and they also contribute to the nutritional qualities of fruits and vegetables. Among these compounds flavonoids constitute one  of  the  most  ubiquitous groups of  all plant phenolics. The most important natural pigments are carotenoids, tetrapyrrole derivatives of naturally occurring phenolic compounds ubiquitously distributed in plant kingdom. So far over 8,000 varieties of flavonoids have been identified¹. Until ~50 years ago, information on the working mechanisms of flavonoids was scare. But it has been widely known for centuries that   compounds   of   plant   origin   possesses   broad spectrum of biological activity². In 1930, Szent- Gyorgyi isolated a new substance from oranges and classifies it as a vitamin, Vitamin P. But latter it came clear that this substance was a flavonoid. Flavonoids have created a more attention of researcher with the discovery of the French Paradox, i.e. decrease incidence of cadio-vascular disease observed in Mediterranean population, in association with  red  wine  consumption  and  a  greater  amount  of saturated fat than average diet in other countries³. In this review it  is  attempted to  describe current  knowledge on structural aspects, biosynthetic and synthetic routes of flavonoids.

 

STRUCTURE AND CLASSIFICATION OF FLAVONOIDS:

Flavonoids occur as aglycones, glycosides and methylated derivatives⁴. In plants, flavonoids aglycones (i.e. flavonoids without attached sugar) occur in a variety in a variety of structural forms. All contain fifteen carbon atoms in their

basic nucleus: two six membered rings linked with a three carbon unit which may or may not be a part of third ring⁵. For convenience the rings are labeled A, B, and C. The individual carbon atoms are based on a numbering system, which uses ordinary numeral for the A and C and “primed” numerical  for  B-ring  (1).  Primed  modified  numbering system is not used for chalcones (2) and the isoflavones derivatives (6):  the  pterocarpans and  the  rotenoids⁶.  The different ways to close this ring associated with the different oxidation degrees of ring A provide the various classes of flavonoids. linkage is normally located at position 3 or 7 and carbohydrate unit can be L-rhamnose, D-glucose, glucorhamnose, galactose or arabinose⁹. and   “Acetate  Malonate”  pathways,  the   first   flavonoid initially formed in the biosynthesis is the chalcone and all other forms are derived from this variety of routes¹⁰. Further

Figure 1. Chemical structure of some representative flavonoids

 

 

The six member ring condensed with the benzene ring is either a γ-pyrone (flavones (1) flavonols (3)) or its dihydroderivative (flavanones (4) and flavan-3-ols (5)). The position of benzenoid substituent divides the flavonoids into two classes as flavonoids (1) (2- position)  and  isoflavonoids  (6)  (3-position). Biflavonoid   (9),   (10)   is   dimmer   having   similar

_{(biflavone,   biflavanone)   or   dissimilar   (flavanone-}

modification  of  flavonoid  may  occur  at  various  stages

resulting in additional (or reduced) hydroxylation, methylation of  hydroxyl  groups or  of  flavonoid  nucleus, isoprenylation of hydroxyl groups or of the flavonoid nucleus, methylenation of ortho-dihyroxyl groups, dimerisation (to produce biflavonoids), bisulphate formation, and most importantly, glycosylation of hydroxyl groups (to produce flavonoid o-glycosides) or of flavonoid nucleus (to produce flavonoid C-glycosides)^11-12.

 

 

Flavonoids   are   synthesized   via   the   phenylpropanoid

₁₃

_{flavone,      flavone-isoflavone,      flavanone-chalcone)}

pathway

.  Phenylalanine ammonia lyase (PAL) catalyzes

_{14, 15}

_{flavonoid moieties in their structure. These monomer}

the conversion of phenylalanine to cinnamate

. PAL also

_{units usually bound to one another through C-6 or C-}

shows  activity  with  converting  tyrosine  to  p-coumarate,

₁₆

₈⁷_{.      In      prenylated      flavonoids      (11),      (12)}

albeit to a lower efficiency

. The cinnamate 4-hydroxylase

_{prenyl/isopentenyl or their furano/dimethyl chromano}

(C4H) catalyzes the synthesis of p-hydroxycinnamate from

₁₇

_{derivatives, as well as the geranyl and farnesyl side}

cinnamate and 4-coumarate

:CoA ligase (4CL) converts p-

attachment is present. Generally, the most flavonoids are C-prenylated and more frequently on ring ‘A’ at position 6/8 as well as on position 3’/5’ especially in flavanones and flavones⁸. Most flavonoids occurs naturally associated with sugar in conjugated form and within any one class may be characterized as monoglycosidic, diglycosidic, etc. the glycosidic

coumarate to its coenzyme-A ester, activating it for reaction

with malonyl CoA¹⁸.

 

The flavonoid biosynthetic pathway starts with the condensation  of  one  molecule  of  4-coumaroyl-CoA  and three molecules of malonyl-CoA, yielding naringenin chalcone¹⁹. This reaction is carried out by the enzyme

 

Figure 3. Examples showing structures of some  prenylated flavonoids. General Structure

 

Group	3	5	6	7	8	3’	4’	5’	C2=C3
Flavones
Apigenin	H	OH	H	OH	H	H	OH	H	+
Diosmin	H	OH	H	Oru	H	OH	OH	H	+
Luteolin	H	OH	H	OH	H	OH	OH	H	+
Flavonol
Quercetin	OH	OH	H	OH	H	OH	OH	H	+
Kaempferol	OH	OH	H	OH	H	H	OH	H	+
Galangin	OH	OH	H	OH	H	H	H	H	+
Fisetin	OH	H	H	OH	H	OH	OH	H	+
Myricetin	OH	OH	H	OH	H	OH	OH	OH	+
Vitexicarpin	OCH3	OH	OCH	OCH3	H	OH	OCH3	H	+
Flavanone
Naringenin	H	OH	H	OH	H	H	OH	H	-
Eriodictyol	H	OH	OH	OH	H	OH	OH	H	-
Pinocembrin	H	OH	H	OH	H	H	H	H	-
Liquiritigenin	H	H	H	OH	H	H	OH	H	-
Flavanonol
Taxifolin	H	OH	H	OH	H	OH	OH	H	-
Isoflavone
Genistein	-	OH	H	OH	H	H	OH	H	+
Tectorigenin	-	OH	OCH3	OH	H	H	OH	H	+
Daidzein	-	H	H	OH	H	H	OH	H	+
Formononetin	-	H	H	OH	H	H	OCH3	H	+
Flavan-3-ols
(+) Catechin	βOH	OH	H	OH	H	OH	OH	H	-
(-) Epicatechin	αOH	OH	H	OH	H	OH	OH	H	-
(-) Epigallocatechin	αOH	OH	H	OH	H	OH	OH	OH	-
Flavylium Salts
Cyanidin	OH	OH	H	OH	H	OH	OH	H	-
Pelargonidine	OH	OH	H	OH	H	H	OH	H	-

 

chalcone synthase (CHS)¹⁹. Chalcone is isomerised to a flavanone by the enzyme chalcone flavanone isomerase (CHI)²⁰. From these central intermediates, the pathway diverges into several side branches, each resulting in a different class of flavonoids. Flavanone 3-hydroxylase (F3H) catalyzes the stereospecific 3β-hydroxylation of (2S)-flavanones to dihydroflavonols^21-24. For the biosynthesis of  anthocyanins, dihydroflavonol reductase (DFR) catalyzes the reduction of dihydroflavonols to flavan-3,4-diols (leucoanthocyanins)²⁵, which are converted to anthocyanidins by anthocyanidin synthase (ANS)^26-28. The formation of glucosides is catalyzed by UDP glucose-flavonoid 3-o-glucosyl transferase (UFGT), which stabilize the anthocyanidins by 3-O- glucosylation (Harborne 1994, Bohm 1998)²⁹. The overview of the flavonoid pathway is presented in Figure 5. There is evidence that the enzymes involved in flavonoid metabolism might be acting as membrane- associated multienzyme complexes, which has implications on the overall efficiency, specificity, and regulation of the pathway (Stafford 1991, Winkel-Shirley 1999, 2001)³⁰.

 

 

 

Figure 4. Interrelationship between flavonoids for biosynthetic pathways.

 

 

CHEMICAL SYNTHETIC ROUTES FOR FLAVONOIDS

Flavonoids have been a topic of research for more than

one century. Some scientist have been trying to get their extracts from different plants and different parts of plants, and some started a course of synthetic experiments to afford the flavonoids. The isolation of bioflavonoids is carried out via an extraction process, but usually the plants contain a variety of derivatives in low concentration. Thus a large amount of dried raw material and laborious chromatographic purification schemes are needed to isolate quantities of individual compounds. So synthetic routes may be useful to get selective flavonoid compounds. In this review, we have tried to focus various synthetic schemes available for flavonoids synthesis. Various methods have been reported regarding the synthesis of flavonoids. The interest in these methods is that these syntheses allow the establishment with certainty the structure of naturally occurring flavonoids by preparing them unequivocally from known starting materials. It was also  enabled  the  workers  to  elucidate  biosynthetic

_31-33

heterocycle ring directly while in other cases some interconversion is necessary. Four classical flavonoid synthetic routes are reported to  be used namely Claisen- Schmidt, Baker-Venkataraman, Allan-Robinson and Algar- Flynn-Oyamada methods.

 

The most important intermediates to obtain

Chalcones

2’-hydroxy   chalcones   (15)    are    the    most    important

intermediates for the synthesis of flavonoids like flavones, flavonols, 3-hydroxyflavanones and aurones. The formation of chalcone was reported to involve the  Claisen-Schmidt condensation of aromatic aldehyde (14) with acetophenone (13) in the presence of alkali as a catalyst. (Figure 6) flavonoids can be prepared to study their usefulness as flavoring/coloring additive or as potential drugs.

Two major pathways

The  synthesis of  flavones was first reported around

1900. The methods those have synthetic importance are divided into two chemical routes (Figure 6).

1. Substitution of phenol with an α,β-unsaturated acyl chloride.

2. Substitution     of     an     acetophenone     with     a benzaldehyde, a benzoyl anhydride, or        benzoyl

chloride.

 

Table 2: Reactive oxygen species that can be scavenged or whose formation can be inhibited by flavonoids^69-70.

 

O3(Superoxide anion)	One-electron  reduction  product  of  O2.  Produced  by  phagocytes,  formed  in  autoxidation  reactions (flavoproteins, redox cyclin._g), and generated by oxidases (heme proteins).
HO2	Potonated form of O2.
H2O2 (Hydrogen Peroxide)	Two-electron reduction product of O2  formed from O2  by dismutation or directly from O2. Reactivity of O  and H O  is amplified in the presence._of heme proteins.       ._         ._ 2                2    2
OH (Hydroxy radical)	Three-electrons reduction product of O2  generated by Fenton reaction, transition metal (iron, copper)- catalysed Haber-Weiss reaction; also formed by decomposition of peroxynitrite produced by the reaction of O2 with NO. (Nitric oxide radical).
RO. (Alkoxy radical)	Example: Lipid radical (LO.).                                           ._
ROO. (Peroxyl radical)	Example: Lipid peroxy radical (LOO.) produced from organic hydroperoxide (e.g. lipid hydroperoxide, LOOH), ROOH by hydrogen abstraction.
1O2	Singlet oxygen

 

Table 3: Characteristics of flavonoids structure for most effective radical-scavenging activity ^71-73.

 

•    The catechol (O-dihydroxy) group in the ring confers great scavenging ability.

•      A pyrogallol (trihydroxy) group in ring B of a catechol, as in myricetin, produces even higher activity. The C2-C3 double bond of the C ring appears to increase scavenger activity because it confers stability to the phenoxy radical produced.

•      The 4-oxo (keto double bond at position 4 of the C ring), especially in association with the C2-C3 double bond, increases scavenger activity by delocalizing electrons from B-ring.

•      The 3-OH group on the C ring generates an extremely active scavenger; in fact, the combination of C2-C3 double bond and 4-oxo group appears to be the best combination on the top of the catechol group.

•    The 5-OH and 7-OH groups may also add scavenging potential in certain cases.

 

 

In addition to their contribution to color, flavonoids also  known to  provide UV-B  protection to  plants⁶⁰. UV-B  is  band  of  lowest  wavelength  and  highest energy. It can penetrate the ozone layer in the stratosphere and hence potentially cause damage to plant life. Resistance to UV-B may take many forms, but one type of resistance could lie in flavonoid pigments, which are known to always present in green leaves. These flavonoids generally absorb in the 280-

315 nm region and thus capable of acting as UV filters. One of the undisputed functions of flavonoids and related polyphenols is their role in protecting plants against microbial invasion. This not only involves their presence in plants as constitutive agents but also their accumulation as phytoalexins in response to microbial attack⁶¹. The majority of flavonoids recognized as constitutive antifungal agents in plants are either isoflavonoids, flavans, or flavanones.

 

Role of Flavonoids in Animals

As these compounds have beneficial role in plants they

also known to posses variety of pharmacological activities on mammalian cells. Because of the variety of pharmacological activities on mammalian body flavonoids are more correctly referred as “Nutraceutical”.

Flavonoids As Nutraceutical

“Nutraceutical”  is  a  term  coined  in  1979  by  Stephen

DeFelice. It  is  defined  “as  a  food  or  parts  of  food  that provide medical or health benefits, including the prevention and treatment of disease.” Nutraceuticals may range from isolated nutrients, dietary supplements, and diets to genetically engineered “designer” food, herbal products, and processed products, such as cereals, soups, and beverages. The increasing interest in nutraceuticals reflects the fact that consumers hear about epidemiological studies indicating that a specific diet or component of the diet is associated with a lower   risk   for   a   certain   disease.   The   major   active nutraceutical ingredients in plants are flavonoids. As is typical for phenolic compounds, they can act as potent antioxidants and metal chelators. They also have log been recognized to possess anti-inflammatory, antiallergic, hepatoprotective, antithrombotic, antiviral, and anticarcinogenic activities, discussed below separately.

 

Antioxidant Activity

The  best-described  property  of  almost  every  group  of

flavonoids is their capacity to acts as antioxidants. The flavones and catechins seem to be the most powerful flavonoids for protecting the body against reactive oxygen species (ROS). Body cells and tissues are continuously threatened by the damage caused by free radicals and ROS,