Flavonoids in nature

Hofenk de Graff, J. H. and W. G. T. Roelofs (1978), The analysis of flavonoids in natural yellow dyestuffs occurring in ancient textiles, Proc. bit. Council of Museums Committee for Conservation, 5th Trienial Mtg., Zagreb. 

The widespread occurrence of flavonoids in nature has resulted in the adoption of a trivial nomenclature that is exemplified by the common names flavene 10 and isoflavene 11. The corresponding benzopyrylium species is commonly called flavylium 12. 

Ngadjui et al. (87) isolated three novel flavonoids namely hoslundin, hoslundal and hoslunddiol, from the twigs. They further isolated the flavonoids oppositin, 5-0-methylhoslundin, tectochiysin, hoslunddiol and hoslundin from the methanol extract of the twigs. The isolation of oppositin and 5-0-methylhoslundin was the first report of the two pyrone-substituted flavonoids in nature. Another novel pyrone-substituted flavonoid hosloppin was isolated from the methanolic extract of the leaves (99). 

Acceptors with more than one reactive group are often differentially glycosylated in different plants. Glycosides may also act as acceptors for further glycosylation. For example, quercetin, one of the most widely observed flavonoids in nature,... 

IsofLavones-. These flavonoids are among the less-widely distributed flavonoids in nature, but they are the compounds most frequently tested for in humans [13]. The only important amount of these compounds were found in leguminous species, especially in soybeans, black beans, and green peas, the most common isoflavones being genistein, daidzein, and glycitein. In addition, these compounds undergo various reactions such as methylation, hydroxylation, or polymerization that lead from simple (isoflavanones, isoflavans, and isoflavanols) to more complex structures (rotenoids, pterocarpans, and coumestans) [12]. 

The second half of this book provides examples of applications of UHPLC in bioanalysis, including analysis of drugs of abuse in human biological matrices, analysis of isoflavones and flavonoids in natural products and biological samples. 

Biosynthesis of Tea Flavonoids. The pathways for the de novo biosynthesis of flavonoids in both soft and woody plants (Pigs. 3 and 4) have been generally elucidated and reviewed in detail (32,51). The regulation and control of these pathways in tea and the nature of the enzymes involved in synthesis in tea have not been studied exhaustively. The key enzymes thought to be involved in the biosynthesis of tea flavonoids are 5-dehydroshikimate reductase (52), phenylalanine ammonia lyase (53), and those associated with the shikimate/arogenate pathway (52). At least 13 enzymes catalyze the formation of plant flavonoids (Table 4). 

Coumarins and flavonoids are common in nature and find their main applications as pharmaceuticals, fragrances, and agrochemicals. 

Perhaps the most useful contribution made by flavonoids in this group of plants, however, was the assistance they provided in studies of natural hybridization between Dubautia scabra and D. ciliolata (Crins et al., 1988a). Dubautia scabra is a pioneer plant that colonizes new lava moderately D. ciliolata occurs on somewhat older lava. Where newer flows overlap older ones, one can often find plants with features intermediate between the two species. This phenomenon is readily observed in the vicinity of Kilauea Volcano on the island of Hawaii, where one finds D. scabra subsp. scabra on newer substrate and D. ciliolata subsp. glutinosa on the older substrate. Since the two taxa exhibit different flavonoid profiles, and we know that flavonoid profiles are inherited in an additive fashion, hybrids would be expected to exhibit flavonoid markers inherited from each parent. A major advantage in this study was the availability of known Fj hybrids that had been produced from controlled crosses at the University of Hawaii (by G. D. Carr). Analysis of the... 

Lairon, D. and Amiot, M.J., Flavonoids in food and natural antioxidants in wine, Curr. Opin. Lipidol., 10, 23, 1999. 

Several general characteristics of the results compiled in Table I are worthy of mention. Compared to the variety of chemicals postulated to be involved in allelopathy (1), few specific compounds have been tested for inhibition of mineral absorption. The most extensively studied compounds are the phenolic acids, probably because of their being ubiquitously found in nature (1). Also, several flavonoids are inhibitory to mineral absorption (Table I). Both of these groups of compounds are often cited as being responsible for allelopathic interactions between plants. 

The chromophores contained in natural dyes are mainly flavonoids, antrachinoids, indigoids and gallotannins. Other molecular types include carotenoids, benzochinons and anthocyanidins. 

The raison d etre for the proliferation of flavone and flavonol glycosides in nature continues to intrigue plant scientists. The ability of UV-B radiation to damage DNA, RNA, and proteins as well as to impair processes like photosynthesis is well known. Most flavonoids are very efficient antioxidants and chemoprotectants, and are therefore used in a plethora of food supplements and nutraceuticals [30]. Some flavonoids, however, have a Janus face - they could be excellent antioxidants but also have negative features. 

Nogata, Y. et al.. High-performance liquid chromatographic determination of naturally occurring flavonoids in Citrus with a photodiode-array detector, J. Chromatogr., 667, 59, 1994. 

In a recent paper, the interaction of various simple flavonoids with an anionic surfactant, sodium dodecyl sulfate (SDS) in aqueous solution, has been studied through absorption spectroscopy as a function of the concentration of the surfactant above and below the critical micelle concentration.The approximate number of additive molecules (flavonoids) incorporated per micelle was estimated at a particular concentration of SDS. Incorporation of flavonoids in micelles shifted the UV absorption bands toward higher wavelengths, and the bathochromic shifts also depended upon the nature of the surfactant head group. 

Fjelkner-Modig, S. et al., Flavonoids in vegetables — the influence of processing, Proceedings of the 2nd International Conference on Natural Antioxidants and Anticarcinogens in Nutrition, Health and Disease, 1998. 

Tsai, S.M. and Phillips, D.A., Flavonoids released naturally from alfalfa promote development of symbiotic Glomus spores in vitro, Appl. Environ. Microbiol, 57, 1485, 1991. 

Most reported anthocyanins are monomeric in nature however, new types of flavonoids consisting of an anthocyanin moiety covalently linked to another flavonoid unit have been reported in the period of this review. One class includes one anthocyanin unit and one flavone or flavonol unit attached covalently to each end of a common dicarboxylic acid. The other class involves one anthocyanin moiety covalently linked directly to a flavanol unit. 

The main remit of this chapter is to provide reference and plant source details of new flavone and flavonol 0-glycosides discovered since 1991, i.e., covering the years 1992 to 2003. A checklist of all (as far as possible) known structures is also included in Appendices A and B. A series of reviews, which include most of the data on new 0-glycosylflavones and flavonols presented here, have appeared in Natural Product Reports and cover the years 1992 to 1994, 1995 to 1997, and 1998 to 2000, and with a fourth (2001 to 2003) " in press. Other useful sources of data are The Phytochemical Dictionary,The Handbook of Natural Flavo-noids, and for general background reading Jeffrey Harborne s Comparative Biochemistry of the Flavonoids. 

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