FLAVONOIDS: BIOCHEMISTRY, BIOPHYSICS, MEDICINE - Tarakhovsky Yu. S. - 2013

PART 1. MAJOR FLAVONOIDS

1.1. Flavonoids: Plant Polyphenols

Phenols are defined as substances whose molecules contain at least one aromatic ring substituted with one or more hydroxyl groups. Approximately 10,000 distinct structures of Phenolic Compounds are known; they are widely distributed throughout the plant kingdom and are also present in food products. The Classification of phenols is based on the structural analysis of their phenolic moiety, although their diversity is largely determined by carbohydrate molecules, organic acids, and other entities attached to the aromatic backbone. The simplest naturally occurring phenolic compounds can be classified According to the number and arrangement of hydroxyl groups attached to the aromatic ring (Fig. 2).

Class="center">Fig. 2. Simple natural phenols. Molecules containing two hydroxyl groups (pyrocatechol and resorcinol) are classified as benzenediols. Molecules containing three hydroxyl groups (pyrogallol and phloroglucinol) are classified as benzenetriols.

Flavonoids/flavonoids.files/image002.jpg" width="294"/>

Components of simple phenols can serve as building blocks for more complex polyphenol structures that contain multiple hydroxyl groups, as well as potentially multiple aromatic rings. These rings can be linked together in various ways, resulting in molecules that typically possess a planar Structure (Fig. 3).

Fig. 3. Typical modes of aromatic ring linkage, including molecular models of these compounds shown in three projections.

Other molecules can be attached to the polyphenol backbone, such as sugars, Polypeptides, and secondary metabolites formed at various stages of plant metabolic pathways. Examples of such compounds include Tannins or tannic acids, which are composed of gallic acid oligomers (hydrolyzable tannins), flavone (non-hydrolyzable tannins), or phloroglucinol found in brown Algae.

Tannins also incorporate sugars in their composition. These substances form stable complexes with Alkaloids, certain Proteins and polypeptides, as well as various metal salts. Lignins exhibit an even more complex structure, representing polymers of phenolic units such as guaiacylpropane and syringylpropane subunits. These Materials function as excellent sorbents and are promising candidates for the storage and Controlled Release of biologically active compounds and Pharmaceuticals.

One of the most extensively studied classes of polyphenolic compounds is flavonoids—substances found in all Plant Tissues and characterized by an immense structural diversity. The phenolic backbone of flavonoid molecules comprises 15 carbon atoms forming two aromatic rings (A and B) connected via a three-carbon bridge. Typically, the general formula of flavonoids is represented as C₆-C₃-C₆. The classification of flavonoids is based on structural variations within the three-carbon chain connecting the rings, designated as positions 2, 3, and 4 in the scheme (Fig. 4). Distinctive features of this group include the potential presence of a double bond, the attachment of carbonyl or hydroxyl groups, and The ability to form a five- or six-membered heterocyclic ring C. Furthermore, the aromatic rings may attach at positions other than the terminal carbons of the C₃ chain.

Fig. 4. Classification of flavonoids based on the structural CHARACTERISTICS OF THE molecule in the region of carbon atoms 2, 3, and 4. The highlighted portion of the molecule is used for classification purposes (see also metabolomics.jp/ and www.phenol-explorer.eu/).