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Flavonoids are a large group of structurally related compounds with a chromane-type skeleton, with a phenyl substituent in the C2 or C3 position. The main flavonoid subclasses are depicted in Fig. 1. Flavonoids are often hydroxylated in positions3, 5, 7, 3’, 4’ and/or 5’. Frequently, one or more of these hydroxyl groups are methylated, acetylated, prenylated or sulphated. In plants, flavonoids are often present as O- or C-glycosides; O bonding in flavonoids occurs far more frequently thanCbonding. TheO-glycosides have sugar substituents bound to a hydroxyl group of the aglycone, usually located at position 3 or 7, whereas the C-glycosides have sugar groups bound to a carbon of the aglycone, usually 6-C or 8-C. The most common carbohydrates are rhamnose, glucose, galactose and arabinose. Flavonoid-diglycosides are also frequently found. Two very common disaccharides contain glucose and rhamnose, 1→6 linked in neohesperidose and 1→2 linked in rutinose. The sugars are often further substituted by acyl residues such as malonate and acetate [1]. Flavonoids are referred to as glycosides when they contain one or more sugar groups (or glucosides in case of a glucose moiety), and as aglycones when no sugar group is present. Flavonoid classification and nomenclature are not always straightforward, therefore, of the flavonoids discussed in the paper the structure can be derived from Fig. 1. Throughout the paper the most common trivial names are used. Given the above structural variety, it will come as no surprise that there is an extremely large number of flavonoids.Typical quotations include “>4000 known flavonoids comprising 12 subclasses” [2], “more than 3000 flavones and more than 700 known isoflavones exist in plants” [3] and “almost 6500 different flavonoids are known” [4]. Consequently, the separation, identification and trace-level determination of flavonoids is challenging. They receive considerable attention in the literature,
specifically because flavonoids are of biological and physiological importance. Flavonoids are one of the largest groups of secondary metabolites, and they play an important role in plants as defence and signalling compounds in reproduction, pathogenesis and symbiosis [5,6]. Plant flavonoids are involved in response mechanisms against stress, as caused by elevatedUV-B radiation [7–10], infection by microorganisms [11] or herbivore attack [12]. Flavonoids are also involved in the production of root nodules as a nitrogen fixation system after infection by Rhizobium bacteria in a variety of leguminous plants [13] – theyare pigment sources for flower colouring compounds – [14] and play an important role in interactions with insects [15]. They
also affect human and animal health because of their role in the diet, which is ascribed to their antioxidant properties [16] or their estrogenic action [17], and to a wide range of antimicrobial
and pharmacological activities [18,19]. Many different enzymes involved in intracellular signalling can be affected by flavonoids.Especially, the effects of flavonoids on protein kinases are of great interest since they directly influence immune functions in the host [20]. The above described spectrum of functions explains why recently quite a number of reviews have been published on the properties of flavonoids [8,21–24] and on the state-of-the-art analysis of flavonoids (Table 1).
An important aspect of flavonoid analysis is whether to determine the target analytes in their various conjugated forms or as the aglycones. In biological fluids (serum, plasma and urine)
flavonoids exist as glucuronide and sulphate conjugates. In most cases, only the total aglycone content is determined; therefore, a hydrolysis step is used. However, in plants, medicine and food
products, researchers are usually interested in the intact conjugates.For example, for the classification of plant species, intact flavonoid profiles in plants are determined [25–27]. In that case, analyses become much more complicated, because the number of target analytes increases significantly: much more selective and sensitive analytical methods are now required. In Fig. 2 the principal strategies for the determination of flavonoids in biological fluids, drinks, plants and food – the main sample types – are schematically depicted. The various steps in this flow chart
will be considered in some detail below, with attention to both routine procedures and recent developments. Of course, in view of the complexity of the problem (almost) all analytical methods dealing with flavonoids include a high-performance separation method. The choice of the method depends on the sensitivity required for the purpose at hand, the complexity of the biological matrix – which is related to the time spent on sample pretreatment prior to analysis – the required chromatographic resolution and the preferred detection method. To give a general indication of the attention devoted to flavonoid analysis in the last 5 years, over 300 papers were written on the analysis of plants, mainly to characterize and quantify their constituents for medicinal or taxonomical reasons. Most reviews listed in Table 1 also deal with the determination of flavonoids in plants. A further 50 papers reported on the analysis of human and animal body fluids. The main goal of these studies was to monitor flavonoid metabolism. Some 30 papers and several reviews (cf. Table 1) were devoted to the analysis of flavonoids in food and drinks, in most cases to determine their anti-oxidant activity and, in the case of juices, also to check them
for possible adulterants. As was already briefly indicated above, the present review intends to discuss the determination of a wide variety of flavonoids – aglycones as well as conjugates – in many different sample types by means of routine or more recently developed analytical techniques. In all instances, selected real-life applications will be included to illustrate the practicability, and the scope and limitations of the various approaches. Because of the increasing interest in structure elucidation of flavonoids,special attention will be devoted to the use of tandem-mass spectrometric (MS/MS) techniques for the characterization of several important sub-classes, and to the potential of combined diode-array UV (DAD UV), tandem-MS and nuclear magnetic resonance (NMR) detection for unambiguous identification. The structures of the main flavonoids discussed in the following sections are listed in Fig. 1.
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