Flavonoids individual compounds

When light from all three channels excites the fluorescence of crystalline individual compounds such as allelochemicals flavonoids quercetin and rutin or pigments of plant cells - azulene, chlorophyll and carotenoids fluoresce in different regions of the spectra in yellow and red or blue, red and yellow-orange, respectively (Fig. 7). It compares the light emission of the substances within cellular structures. 

HPLC separation, as described above, is restricted to rather simple compounds that represent only a small proportion of flavonoids. Indeed, proanthocyanidin analysis becomes increasingly difficult as their molecular weight increases, due to the larger number of possible structures, smaller amounts of each individual compound, and poorer resolution of the chromatographic profiles. This is especially true of grape proanthocyanidins, which, unlike those of apple or cacao consisting only of epicatechin units, are based on four major... 

In column Plant species, these are grouped by families and are listed alphabetically for each flavonoid. For individual compounds of single publications, a maximum of two species per genus is noted more than two species are abbreviated by spp. ... 

In keeping with the rapid development of 13C NMR spectroscopy, numerous publications containing 13C NMR data of flavonoids have appeared since ca. 1975. In many cases, the information is specific to an individual compound and it is not possible to cover all such work in this account. However, several studies which are more general in approach are discussed below and should direct the reader to data for each flavonoid system. 

Procyanidins can usually not be completely separated from each other, from phenolic acids and from accompanying flavonoids by sample clean up and/or chromatographic procedures. The differentiation between these individual compounds can therefore only by accomplished by the selectivity of a suitable detection method. UV-DAD (ultra violet diode array detection), electrochemical and mass spectrometry detection have been used for the identification and quantitative determination of procyanidins. 

After biosynthesis the different groups of flavonoids may undergo further modifications which lead to a large number of individual compounds. In this respect the following reactions are of importance ... 

Epicatechin and catechin (Fig. 74.4) represent the basis on which to build more complex molecules such as procyanidin polymeric forms. Proanthocyanidins are polymer chains of flavonoids such as flavan-3-ols. Mainly, two primary forms of procyanidins occur in plants A-type and B-type, which differ by the linkage between individual compounds. A-type procyanidins form 4—8 and 2-7 cross-links and has... 

The various classes of flavonoids differ in the level of oxidation and substitution pattern of ring C, while individual compounds within a class differ in the arrangements of hydroxyl, methoxyl, and glycosidic side groups. Dietary flavonoids exist primarily as 3-0-glycosides and polymers [12]. 

Knowledge of the identity of phenolic compounds in food facilitates the analysis and discussion of potential antioxidant effects. Thus studies of phenolic compounds as antioxidants in food should usually by accompanied by the identification and quantification of the phenols. Reversed-phase HPLC combined with UV-VIS or electrochemical detection is the most common method for quantification of individual flavonoids and phenolic acids in foods (Merken and Beecher, 2000 Mattila and Kumpulainen, 2002), whereas HPLC combined with mass spectrometry has been used for identification of phenolic compounds (Justesen et al, 1998). Normal-phase HPLC combined with mass spectrometry has been used to identify monomeric and dimeric proanthocyanidins (Lazarus et al, 1999). Flavonoids are usually quantified as aglycones by HPLC, and samples containing flavonoid glycosides are therefore hydrolysed before analysis (Nuutila et al, 2002). 

Identification of flavonoids Quantification of individual flavonoids depends heavily on the availability of standard references. Only a limited number of common flavonoids are commercially available as standards. Standard references for flavonoid glycosides are particularly difficult to find thus direct quantification of the native glycosides is nearly impossible. Analysis of the aglycones after acid or enzymatic hydrolysis is therefore common practice. When standard flavonoids are not available, or when unknown compounds are encountered in a particular fruit or vegetable, use of a DAD... 

Antioxidant capacity of fruits and vegetables depends on the total concentrations of phytochemicals, mainly ascorbic acid, phenolic compounds (including flavonoids), and carotenoids. However, as previously stated, the individual contribution of each compound to the total antioxidant capacity varies widely and is difficult to quantify in a whole food product. 

At present, considerable interest is drawn to the use of natural mixtures of antioxidants isolated from various vegetable materials. Some authors claim that such mixtures manifest stronger antioxidant effects than individual components due to synergistic interactions. It is of course quite possible, but it should be noted that synergistic interactions are not a single mechanism of the interaction between components for example, the simultaneous presence of the antioxidant and prooxidant flavonoids might diminish summary antioxidant effect of the mixture. Furthermore, natural mixtures contain, as a rule, some unknown compounds, which affect the summary effect by unknown manner. 

The application of standardized UV (or UV-Vis) spectroscopy has for years been used in analyses of flavonoids. These polyphenolic compounds reveal two characteristic UV absorption bands with maxima in the 240 to 285 and 300 to 550 nm range. The various flavonoid classes can be recognized by their UV spectra, and UV-spectral characteristics of individual flavonoids including the effects of the number of aglycone hydroxyl groups, glycosidic substitution pattern, and nature of aromatic acyl groups have been reviewed in several excellent books. ... 

---