**2. The classification and biosynthesis of flavonoids**

Flavonoids can be classified according to biosynthetic origin. Flavonoids are characterized by the presence of 15 carbon atoms in their basic skeleton, arranged in the form C6-C3-C6, which corresponds to two aromatic rings A and B linked by a unit of three carbon atoms, which may or may not give rise to a third ring. The rings are labeled A, B and C [15, 16]. The initial step in the biosynthesis of most flavonoids is the condensation of one *p*-coumaroyl-CoA molecule (shikimate derived, B ring) with three molecules of malonyl-CoA (polyketid origin, A ring) to give chalcone (2′, 4′, 6′, 4-tetrahydroxychalcone). This reaction is carried out by the enzyme chalcone synthase (CHS) [14–16]. Chalcone is subsequently isomerized by the enzyme chalcone flavanone isomerase (CHI) to flavanone. From these central intermediates, the pathway diverges into several side branches, each yielding a different class of flavonoids (**Figure 1**) [14, 16, 17].

Although the central pathway for flavonoid biosynthesis was conserved in plants, depending on the species, a group of enzymes, such as isomerases, reductases, hydroxylases, modifies the basic flavonoid skeleton, leading to the different flavonoid classes [1, 16], including chalcones and flavanones which are intermediary compounds in biosynthesis and final products present in various parts of the plant. Anthocyanins, proanthocyanidins, flavones and flavonols are other classes only known as end products of biosynthesis. The other important class is the isoflavonoids, which are formed by migration reaction of 2-aryl side chain to 3-position mediated by isoflavone synthase [1, 16, 18].

The retrochalcones are unusual flavonoids and have reversed A and B rings. The biosynthesis is not yet clearly defined but is likely to be derived from the common C15 intermediate of general flavonoid biosynthesis, more specifically from the reduction of dibenzoylmethanes [19–21] or by 2-hydroxylation of a flavanone [22, 23]. These compounds are restricted to relatively few plant species and have been isolated from some species of the families Leguminosae [24], Annonaceae and Basellaceae [25–28].

Flavonoids: Classification, Biosynthesis and Chemical Ecology http://dx.doi.org/10.5772/67861 5

and ultraviolet (UV) radiation [10]. They resemble in their regulatory properties most of the lipid-soluble vitamins but serve, in addition, due to their color, as communicators with the environment. Flavonoids are recognized by pollinators, for example, insects, birds and animals, which contribute to the dispersion of seeds [11]. They act as symbionts, as allelochemicals, as antimicrobial and antiherbivory factors [10, 12]. Many studies have shown that flavonoids exhibit biological and pharmacological activities, including antioxidant, cytotoxic, anticancer, antiviral, antibacterial, anti-inflammatory, antiallergic, antithrombotic, cardioprotective, hepatoprotective, neuroprotective, antimalarial, antileishmanial, antitrypanosomal

The topics that will be discussed in this chapter describe the regulation of flavonoid biosynthesis, the roles of flavonoids in flowers, fruits and roots and mechanisms involved in pollina-

Flavonoids can be classified according to biosynthetic origin. Flavonoids are characterized by the presence of 15 carbon atoms in their basic skeleton, arranged in the form C6-C3-C6, which corresponds to two aromatic rings A and B linked by a unit of three carbon atoms, which may or may not give rise to a third ring. The rings are labeled A, B and C [15, 16]. The initial step in the biosynthesis of most flavonoids is the condensation of one *p*-coumaroyl-CoA molecule (shikimate derived, B ring) with three molecules of malonyl-CoA (polyketid origin, A ring) to give chalcone (2′, 4′, 6′, 4-tetrahydroxychalcone). This reaction is carried out by the enzyme chalcone synthase (CHS) [14–16]. Chalcone is subsequently isomerized by the enzyme chalcone flavanone isomerase (CHI) to flavanone. From these central intermediates, the pathway diverges into several side branches, each yielding a different class of flavonoids

Although the central pathway for flavonoid biosynthesis was conserved in plants, depending on the species, a group of enzymes, such as isomerases, reductases, hydroxylases, modifies the basic flavonoid skeleton, leading to the different flavonoid classes [1, 16], including chalcones and flavanones which are intermediary compounds in biosynthesis and final products present in various parts of the plant. Anthocyanins, proanthocyanidins, flavones and flavonols are other classes only known as end products of biosynthesis. The other important class is the isoflavonoids, which are formed by migration reaction of 2-aryl side chain to 3-position

The retrochalcones are unusual flavonoids and have reversed A and B rings. The biosynthesis is not yet clearly defined but is likely to be derived from the common C15 intermediate of general flavonoid biosynthesis, more specifically from the reduction of dibenzoylmethanes [19–21] or by 2-hydroxylation of a flavanone [22, 23]. These compounds are restricted to relatively few plant species and have been isolated from some species of the families Leguminosae [24],

and antiamebial properties [13–15].

4 Flavonoids - From Biosynthesis to Human Health

(**Figure 1**) [14, 16, 17].

tion and their specific functions in the plant.

mediated by isoflavone synthase [1, 16, 18].

Annonaceae and Basellaceae [25–28].

**2. The classification and biosynthesis of flavonoids**

**Figure 1.** A diagram of the flavonoid biosynthetic pathway. Key enzymes catalyzing some reactions: PAL, phenylalanine amonialyase; C4H, cinnamate 4-hydroxylase; 4CL, 4-coumaroyl-coenzyme A ligase; CHS, chalcone synthase; CHI, chalcone flavanone synthase; F3H, flavanone 3β-hydroxylase; DFR, dihydroflavonol 4-reductase; FLS, flavonol synthase; IFS, isoflavonoid synthase; AS, anthocianin synthase and UF3GT, UDP glucose: flavonoid 3-O-glucosyltransferase. Adapted from Ref. [17].

Several species reported the presence of chalcone dimers bound by a cyclobutane (**Figure 2**) [29–35]. The phytochemical study of the roots of *Dahlstedtia grandiflora* was observed, and for the first time, the occurrence of dimerization in retrochalcones was noted [24]. The mechanisms of [2 + 2] cycloaddition involved in the formation of these compounds are suggested [30, 35]. In spite of the lack of biosynthetic studies of these natural products, much effort has been made in elucidating the biosynthetic pathways of flavonoids from a genetic perspective.

**Figure 2.** Chalcone dimmers isolated from *Combretum albopunctatum* [31], *Helichrysum zivojinii* [29] and *Agapanthus africanus* [30].

Flavonoids occur naturally as compounds associated with sugar in conjugated forms (glycosides), without attached sugar as aglycones [1, 36]. They are often hydroxylated in positions 3, 5, 7, 3′, 4′ and 5′. Some of these hydroxyl groups are frequently methylated, acetylated or sulfated. Prenylation usually occurs directly at a carbon atom in the aromatic rings, but O-prenylation has also been found [11]. When glycosides are formed, the glycosidic linkage is normally located in position 3 or 7, and the carbohydrates are commonly L-rhamnose, D-glucose, glucose rhamnose, galactose or arabinose [1, 11]. These changes often alter their solubility, reactivity and stability. The majority of flavonoids are present in the form of glycosides under natural conditions [1].

#### **2.1. The chemical structure of flavonoids**

The chemical nature of flavonoids varies according to the hydroxylation pattern, conjugation between the aromatic rings, glycosidic moieties, methoxy groups and other substituents [37–39]. Flavonoids contain conjugated double bonds and groups (hydroxyl or other substituents) that can donate electrons through resonance to stabilize the free radicals, which originate in the electronic spectra of flavonoids [40].

Studies on flavonoids by UV spectroscopy have shown that most flavonoids consist of two major absorption maxima: band II (240–285 nm) which corresponds to the benzoyl system of the A ring, while band I (300–400 nm) represents the cinnamoyl system of the B ring (**Figure 3**) [36, 41].

Functional groups attached to the flavonoid skeleton may cause a shift in absorption. The application of standardized UV (or UV-Vis) spectroscopy has for years been used in analyses of flavonoids [11].

Flavonoids have the ability to sequester free radicals, are natural antioxidants derived from plants and are commonly found in foods and beverages [40]. The main structural features of flavonoids required for antioxidant activity can be determined by three fundamental factors: (1) a 3′,4′-dihydroxy (catechol) structure in the B ring favors the electron delocalization (**A**), (2) an unsaturated 2-3 bond in conjugation with a 4-keto group provides electron delocalization from the B ring (**B**) and (3) hydroxyl groups at positions 3 and 5 form intramolecular hydrogen bonding to the keto group (**C**) (**Figure 4**). These effects lead to the increases of the radical scavenging by delocalization of electrons or by donation of hydrogen [42].

Flavonoids: Classification, Biosynthesis and Chemical Ecology http://dx.doi.org/10.5772/67861 7

**Figure 3.** Band II absorption (originated from A-ring benzoyl system) and band I (from the B-ring cinnamoyl system). Adapted from Ref. [41].

**Figure 4.** Structural groups for radical scavenging [42].

Flavonoids occur naturally as compounds associated with sugar in conjugated forms (glycosides), without attached sugar as aglycones [1, 36]. They are often hydroxylated in positions 3, 5, 7, 3′, 4′ and 5′. Some of these hydroxyl groups are frequently methylated, acetylated or sulfated. Prenylation usually occurs directly at a carbon atom in the aromatic rings, but O-prenylation has also been found [11]. When glycosides are formed, the glycosidic linkage is normally located in position 3 or 7, and the carbohydrates are commonly L-rhamnose, D-glucose, glucose rhamnose, galactose or arabinose [1, 11]. These changes often alter their solubility, reactivity and stability. The majority of flavonoids are present in the form of glycosides under natural conditions [1].

**Figure 2.** Chalcone dimmers isolated from *Combretum albopunctatum* [31], *Helichrysum zivojinii* [29] and *Agapanthus* 

The chemical nature of flavonoids varies according to the hydroxylation pattern, conjugation between the aromatic rings, glycosidic moieties, methoxy groups and other substituents [37–39]. Flavonoids contain conjugated double bonds and groups (hydroxyl or other substituents) that can donate electrons through resonance to stabilize the free radicals, which

Studies on flavonoids by UV spectroscopy have shown that most flavonoids consist of two major absorption maxima: band II (240–285 nm) which corresponds to the benzoyl system of the A ring, while band I (300–400 nm) represents the cinnamoyl system of the B ring (**Figure 3**) [36, 41].

Functional groups attached to the flavonoid skeleton may cause a shift in absorption. The application of standardized UV (or UV-Vis) spectroscopy has for years been used in analyses

Flavonoids have the ability to sequester free radicals, are natural antioxidants derived from plants and are commonly found in foods and beverages [40]. The main structural features of flavonoids required for antioxidant activity can be determined by three fundamental factors: (1) a 3′,4′-dihydroxy (catechol) structure in the B ring favors the electron delocalization (**A**), (2) an unsaturated 2-3 bond in conjugation with a 4-keto group provides electron delocalization from the B ring (**B**) and (3) hydroxyl groups at positions 3 and 5 form intramolecular hydrogen bonding to the keto group (**C**) (**Figure 4**). These effects lead to the increases of the

radical scavenging by delocalization of electrons or by donation of hydrogen [42].

**2.1. The chemical structure of flavonoids**

6 Flavonoids - From Biosynthesis to Human Health

of flavonoids [11].

*africanus* [30].

originate in the electronic spectra of flavonoids [40].

Flavonoids have different activity mechanisms such as free radical scavenging, inactivation of peroxides and other reactive oxygen species, chelation of metals and quenching of secondary lipid oxidation products [40]. The radical scavenging properties associated with the structure of flavonoids defend against oxidative stress and in doing so reduce heart disease, prevent cancer and slow down the aging processes in cells responsible for degenerative diseases [40, 42].
