Anthocyanins chalcone

Flavonoid Compounds based on two benzene rings linked by a three-carbon chain. They are important pigments and nutraceuticals due to their antioxidant capacity. Flavonoids include anthocyanins, chalcones, flavones, flavonols, and isoflavones. 

As frequently mentioned in the literature, anthocyanins co-exist in equilibrium in four different forms. The pH conditions shift this equilibrium toward a variety of structural forms, with the direct consequences of color changes of these pigments. As a rule, at pH above 4, yellow compounds (chalcone form), blue compounds (quinoid base), or colorless compounds (methanol form) are produced. Anthocyanins have the highest stabilities at a pH between 1 and 2 since the flavylium cation is the most stable predominant form. 

Four anthocyanin species exist in equilibrium under acidic conditions at 25°C/ according to the scheme in Figure 4.3.3. The equilibrium constant values determine the major species and therefore the color of the solution. If the deprotonation equilibrium constant, K, is higher than the hydration constant, Kj, the equilibrium is displaced toward the colored quinonoidal base (A), and if Kj, > the equilibrium shifts toward the hemiacetalic or pseudobase form (B) that is in equilibrium with the chalcone species (C), both colorless." - Therefore, the structure of an anthocyanin is strongly dependent on the solution pH, and as a consequence so is its color stability, which is highly related to the deprotonation and hydration equilibrium reaction constant values (K and Kj,). 

Attempts to stabilize anthocyanins by complex inclusion with a- and P-cyclo-dextrins failed on the contrary, a discoloration of anthocyanin solutions was observed.Thermodynamic and kinetic investigations demonstrated that inclusion and copigmentation had opposite effects. In the anthocyanins, the cw-chalcone colorless structure is the best species adapted to inclusion into the P-dextrin cavity, shifting the equilibrium toward colorless forms. "... 

An anthocyanin occurs in solution as a mixture of different secondary structures, a quinonoidal base, a carbinol pseudobase, and a chalcone pseudobase. ° hi addition, different mechanisms for the stabilization of anthocyanins lead to the formation of tertiary structures such as self-association, inter-, and intra-molecular co-pigmentation. ... 

Havonoids are made up of a number of classes of very similar groups in which two phenyl rings are connected by a three-carbon unit [10], The open structure members are yellow in color and termed as chalcones, and simple cyclization to a furanoid structure deepens the color to the orange aurones. The most usual flavonoids are, however, the pale-yellow flavones and flavonols, or 3-hydroxyflavones, which will be treated here, and the red-blue anthocyanins, which will be treated in the next section. Figure 13.4 shows examples of these main classes and their structural relationships. The natural compounds of all classes often occur as glycosides and as methyl ethers. 

Recently, a new polyketide biosynthetic pathway in bacteria that parallels the well studied plant PKSs has been discovered that can assemble small aromatic metabolites.8,9 These type III PKSs10 are members of the chalcone synthase (CHS) and stilbene synthase (STS) family of PKSs previously thought to be restricted to plants.11 The best studied type III PKS is CHS. Physiologically, CHS catalyzes the biosynthesis of 4,2, 4, 6 -tetrahydroxychalcone (chalcone). Moreover, in some organisms CHS works in concert with chalcone reductase (CHR) to produce 4,2 ,4 -trihydroxychalcone (deoxychalcone) (Fig. 12.1). Both natural products constitute plant secondary metabolites that are used as precursors for the biosynthesis of anthocyanin pigments, anti-microbial phytoalexins, and chemical inducers of Rhizobium nodulation genes.12... 

This chapter, therefore, aims to present a brief unified summary of general techniques, with reference to the different categories of structure flavones and flavonols (and their glycosides), isoflavones, flavanones, chalcones, anthocyanins, and proanthocyanidins. 

The APCI source (Table 2.8) has been used for the analysis of various flavonoids, especially flavonols, flavones, flavanones, and chalcones (Table 2.11). APCI is based on gaseous-phase ionization, and is most suitable for compounds that are partially volatile and have a medium polarity. Thus, the application of APCI with respect to analysis of condensed tannins and anthocyanins is more limited. Compared with ESI, APCI produces more fragment ions in the spectrum due to the harsher vaporization and ionization processes. More information about ESI and APCI can be found in Section 1.4.5. 

Absorption spectra have also been used in the reexamination of pH-dependent color and structural transformations in aqueous solutions of some nonacylated anthocyanins and synthetic flavylium salts." ° In a recent study, the UV-Vis spectra of flower extracts of Hibiscus rosasinensis have been measured between 240 and 748 nm at pH values ranging from 1.1 to 13.0." Deconvolution of these spectra using the parallel factor analysis (PARAFAC) model permitted the study of anthocyanin systems without isolation and purification of the individual species (Figure 2.21). The model allowed identification of seven anthocyanin equilibrium forms, namely the flavylium cation, carbinol, quinoidal base, and E- and Z-chalcone and their ionized forms, as well as their relative concentrations as a function of pH. The spectral profiles recovered were in agreement with previous models of equilibrium forms reported in literature, based on studies of pure pigments. 

With 6 -hydroxychalcones, such as naringenin chalcone, the isomerization reaction can readily occur nonenzymically to form racemic (2R,2S) flavanone. This occurs easily in vitro and has been reported to occur in vivo to the extent that moderate levels of anthocyanin can be formed. However, 6 -deoxychalcones are stable under physiological conditions, due to an... 

The PAs, or condensed tannins, are polymers synthesized from flavan-3-ol monomer units. The phlobaphenes are 3-deoxy-PAs formed from flavan-4-ol monomers. The biosynthesis of both types of PAs follows the biosynthetic route of anthocyanins from chalcones through to the branch points to flavan-3-ol and flavan-4-ol formation. In this section, the specific enzymes forming the monomers are discussed, along with a discussion on the polymerization process. Although the chemistry of tannins is described in detail elsewhere in this book, it is useful to briefly mention the nature of the monomer subunit types and the polymer forms. 

Joung, J.Y. et al., An overexpression of chalcone reductase of Pueraria montana var. lobata alters biosynthesis of anthocyanin and 5 -deoxyflavonoids in transgenic tobacco. Biochem. Biophys. Res. Common., 303, 326, 2003. 

Anthocyanins are usually represented as the red flavylium cations (Figure 5.1, left). However, this form is predominant only in very acidic solvents (pH < 2) such as those used for HPLC analysis. In mildly acidic media, the flavylium cations undergo proton transfer and hydration reactions, respectively, generating the quinonoidal base and the hemiketal syn carbinol) form (Figure 5.1, right) that can tautomerize to the chalcone. Thus, at wine pH, malvidin 3-glucoside occurs mostly as the colorless hemiketal (75%), the red flavylium cation, yellow chalcone, and blue quinonoidal base being only minor species. 

Brouillard, R. and Lang, J., The hemiacetal-cis-chalcone equilibrium of malvin, a natural anthocyanin. Can. J. Chem. 68, 755, 1990. 

Each anthocyanidin is involved in a series of equilibria giving rise to different forms, which exhibit their own properties including color. One- and two-dimensional NMR have been used to characterize the various forms of malvidin 3,5-diglucoside present in aqueous solution in the pH range 0.3 to 4.5 and to determine their molar fractions as a function of pH. In addition to the flavylium cation, two hemiacetal forms and both the cis and trans forms of chalcone were firmly identified. In a reexamination, the intricate pH-dependent set of chemical reactions involving synthetic flavylium compounds (e.g., 4 -hydroxyflavylium) was confirmed to be basically identical to those of natural anthocyanins (e.g., malvidin 3,5-diglucoside) in... 

Flavonoids can be classified according to their biosynthetic origins. Some flavonoids are both intermediates in biosynthesis and end-products, e.g. chalcones, flavanones, flavanon-3-ols and flavan-3,4-diols. Other classes are only known as the end-products of biosynthesis, e.g. anthocyanins, flavones and flavonols. Two further classes of flavonoids are those in which the 2-phenyl side-chain of flavonoid isomerizes to the 3-position (giving rise to isoflavones and related isoflavonoids) and then to the 4-position (giving rise to the neoflavonoids). The major classes of flavonoids, with specific examples, are summarized helow. 

Herbicides are the biocides most likely to affect the metabolism of plants, including secondary metabolism [107]. The synthesis of hydroxyphenolics and anthocyanin in plants can be influenced by a variety of environmental and chemical stimuli. Some herbicides were found to raise the levels of these compounds in plants [108] whilst others had the opposite effect [109]. The products of secondary metabolism are controlled by enzymes, including PAL and chalcone isomerase (Cl), and several herbicides appear to intensify the activities of those enzymes involved in the accumulation of hydroxyphenolic compounds and anthocyanin biosynthesis in several plant species [109-111] whereas others depress this activity [112]. For example,... 

Fig. 13 Equilibrium distribution of four anthocyanin forms of malvidin-3-glucoside as a function of pH The red flavylium cation (AH+), the blue quinonoidal base (A), the colorless carbinol pseudobase (B), and the colorless chalcone (C). (From Ref. 138.)...

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