Quercetin glycosides structures

Figure 8.1 Structures of main flavonol aglycones and quercetin glycosides present in plant foods.

NMR is also a non-destructive technique, and a small number of sequential applications have been published. Wilson and co-workers [147] used HPLC-DAD-NMR-MS to characterize plant extracts. Hanson and co-workers [148] used a similar approach to examine another plant extract of pharmaceutical interest. In both cases, the complementary nature of the data provided quantitation of both major and minor constituents and aided in the structural identification of several of the minor components, including chiral isomers. Lommen et al. [149] describe a similarly configured system. The DAD and MS outputs were used to detect peaks, which were then transferred for NMR. They examined glycosides found in apple peel. They identified six quercetin glycosides and two phloretin glycosides, with the NMR data providing the definitive conformational data to differentiate the isomers. 

Condensed tannins, when present in a woody plant, may not always exhibit a constant structure throughout the plant. There are a number of examples of this phenomenon known Acacia species commonly elaborate both 5-oxy and -deoxy-flavanoid tannins (for example, profisetinidins and procyanidins) in the wood (34) and bark (140), and exclusively 5-oxy-flavanoids and associated flavanoids (for example, quercetin glycosides) in the leaves (20, 138). Other examples include many Ribes species that commonly contain prodelphinidins. These are predominantly of the gallocatechin-4 (6) type in the leaves, but with the contrasting epigallocatechin-4 (3) stereochemistry in the fruits (37). Further, the tannins of Pinus radiata display contrasting structures between the bark in which 2,3-trans stereochemistry, catechin-4 (5), procyanidin polymers predominate, and tannins in the phloem, needles, male cones, and winter buds, in which a variety of mixed procyanidin and prodelphinidin polymers exist with predominantly 2,3-c/5 stereochemistry (Thble 7.7.5). 

HPLC analysis of the antioxidant ethyl acetate extract of whole cranberries prepared by Method 1 showed the sample to be composed primarily of flavonoids in particular, five compounds which were identified mainly as glycosides of quercetin with a small percentage (8%) of myricetin-3-P-D-galactoside. The exact structures of the quercetin glycosides are currently under investigation. Table III indicates the total flavonoid and anthocyanin composition in the whole berry and its parts. Percent composition of flavonoid glycosides in each part of the cranberry is given in Table IV. 

Flavonoids constitute a large class of polyphenols found in fruits and vegetables that share a common skeleton of phenylchromane. This basic structure allows a large number of substitution patterns leading to several subclasses of flavonoids, such as flavonols, flavones, flavanones, flavanols, anthocyanidins, isoflavones, dihydroflavonols, and chalcones. Among the diverse flavonoid subclasses, flavonols (especially quercetin) and flavanols (catechins) are the most abundant in our food. Flavonols are present in foods as diverse glycosides, whereas flavanols are usually found as aglycones. 

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