Beneficial Effects of Flavonoids on Neurological Disorders

Abstract

Flavonoids are polyphenolic compounds found in fruits, vegetables, and beverages derived from plants. Humans consume about 1–2 g of flavonoids daily. Information about the occurrence of more than 5,000 different flavonoids has been reported in the literature. The six major subclasses of flavonoids include the flavones (e.g., apigenin, luteolin), flavonols (quercetin, myricetin), flavanones (naringenin, hesperidin), catechins or flavanols (epicatechin, gallocatechin), anthocyanidins (cyanidin, pelargonidin), and isoflavones (genistein, daidzein) (Fig. 4.1). Minor dietary flavonoids include dihydroflavonols, flavan-3,4-diols, coumarins, chalcones, dihydrochalcones, and aurones (Crozier et al. 2009). The basic structure of flavonoids consists of two benzene rings (A and B) linked through a heterocyclic pyran or pyrone (with a double bond) ring (C) in the middle. The carbon atoms are identified with ordinary numerals for A- and C-rings and “primed numerals” for the B-ring, although a modified number system is used for chalcones (Fig. 4.2). In vivo and in vitro studies indicate that flavonoids produce antioxidant and anti-inflammatory effects (Middleton et al. 1998; Havsteen 2002). The antioxidant activity of flavonoids depends on the structure and the substituents of the heterocyclic and B-rings, in particular the presence of an o-di-OH structure on the B-ring, a 2,3-double bond in conjugation with a 4-oxo function, and the additional presence of 3- and 5-OH groups on the heterocyclic ring. Thus, flavanones contain a carbonyl group in the C₄ position of the C-ring, flavanols possess a hydroxyl group in the C₃ position of the C-ring, flavones have a carbonyl group in the C₄ and a double bond between C₂ and C₃ of C-ring, flavonols contain a carbonyl group in the C₄, a double bond between C₂ and C₃, and also a hydroxyl in the C₃ position of the C-ring, anthocyanidins have flavylium cation derivatives, and in isoflavonoids the B-ring is bound to the C₃ of the C-ring instead of bound to the C₂ (Croft 1998; Gutierrez-Merino et al. 2011). The antioxidant activity of flavonoid depends on the arrangement of functional groups on its core structure. It is suggested that both the configuration and total number of hydroxyl groups substantially influence the mechanism of the antioxidant activity (Heim et al. 2002). The B-ring hydroxyl configuration is the most significant determinant of ROS scavenging activity (Burda and Oleszek 2001), whereas substitution of the rings A and C has little impact on superoxide anion radical scavenging rate constants (Burda and Oleszek 2001; Amić et al. 2007). Most of the flavonoids present in plants are attached to sugars (glycosides), although occasionally they are found as aglycones.