Aglycone flavonoids, separation

The chromatographic conditions as mentioned above must in most cases be adapted to the matrix under study, i.e., to the predominant flavonoid classes. Paganga et al. studied aglycone flavonoids in apples, onions, and tomatoes, and were able to develop another solvent gradient enabling very good separation of the different aglycones (Table 2.3.2). 

The leaf flavonoids of the cruciferous species such as Camelina sativa, Crambe abyssinica, Crambe hispanica, Thlaspi arvense, Brassica napus and Sinapis alba were separated and identified with the combination of HPLC, TLC and paper chromatography. Llavonoid aglycones were extracted by cutting fresh three-week-old leaves in tiny pieces and boiled in 50 ml of 2 M HC1 for 45 min. 

G. Kitanov, A method for separation of flavonoid and xanthone aglycones with monohydrox-yphenyl and o-dihydroxyphenyl moiety. Acta Pharm. Zagreb 50 (2000) 69-73. 

Normal phases (unmodified silica gel) are rarely employed, except for the occasional separation of weakly polar flavonoid aglycones, polymethoxylated flavones, flavanones, or isoflavones. The polymethoxylated flavones present in citrus fruits can, for example, be separated on silica gel columns. The big drawback is that solvent gradients cannot normally be run with normal phases. 

In three of the volumes of The Flavonoids, Advances in Research, published between 1975 and 1993, flavanones and dihydroflavonols were part of the chapter on Minor Flavonoids, expertly written by Professor Bruce Bohm. These Minor Flavonoid chapters also included chalcones, dihydrochalcones, and aurones. The term Minor Flavonoids was first used by Harborne in 1967 to encompass not only flavanones, chalcones, and aurones, but also isoflavonoids, biflavonyls, and leucoanthocyanidins, because so few compounds belonging to each of these flavonoid classes were known at that time. For example, only about 30 flavanone and dihydroflavonol aglycones, 19 chalcones, and 7 aurones were known in 1967. The number of known minor flavonoids increased considerably in the next two decades, so that when the checklist for The Flavonoids, Advances in Research Since 1980 was published in 1988, 429 known flavanones and dihydroflavonols (including glycosides) were listed, 268 chalcones and dihydrochalcones, and 29 aurones. In the last 15 years, the total number of known compounds in these flavonoid classes has more than doubled, so that the term minor flavonoids is no longer appropriate. Consequently, it has been decided that separate chapters should be devoted to the flavanones and dihydroflavonols (this chapter), and chalcones, dihydrochalcones, and aurones (Chapter 16). 

An HPLC separation method with diode array detector and mass spectrometric (MS) detection equipped with atmospheric pressure ionization (API) was developed to determine flavone, flavonol, and flavanone in various vegetables, including green bean, broccoli, brussels sprouts, celery, kale, leek, onion, parsley, pepper (green, yellow, and red), and tomato (118). The flavonoids were analyzed as aglycones after acid hydrolysis. The extraction and acid hydrolysis conditions are based on previous work by Hertog et al. (119). Quercetin is the overall major flavonol, followed by kaempferol. The flavones, apigenin and luteolin, were found only in limited foods,... 

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