Flavonoids - a detailed scientific overview of a large group of natural compounds found in plants.
- Definition and Occurrence: Flavonoids are phenolic substances, with over 8,150 different types reported, found in vascular plants (fruits, vegetables, nuts, seeds, leaves, flowers, tea). They are a major coloring component of flowering plants and are integral to human and animal diets. Humans cannot synthesize them, so they must be obtained from plants.
- Structure: The basic flavonoid structure consists of a C6-C3-C6 skeleton, with two aromatic rings (A and B) linked by a three-carbon chain that often forms a heterocyclic C-ring.
- Classification: Flavonoids are classified based on the position of the B-ring linkage and the degree of oxidation/saturation in the C-ring. Major groups include flavones, flavonols, flavanones, anthocyanidins, isoflavones, chalcones, and aurones. Further categories include biflavonoids (two flavonoid units linked) and polyflavonoids.
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Biological and Pharmacological Activities: Flavonoids exhibit a wide range of beneficial properties, including:
- Antioxidant: Scavenging free radicals and metal complexation.
- Antimicrobial: Antibacterial, antiviral, antifungal, antimalarial, antileishmanial, antitrypanosomal, antiamebial.
- Cytotoxic/Anticancer: Potential to inhibit cancer cell growth.
- Protective: Cardioprotective, neuroprotective, cell- and organ-protective (e.g., liver, stomach, retina).
- Other: Influence on food quality (color, taste), potential positive effects against Parkinson's disease, memory improvement in elderly, antihypertensive effects.
- Chemical Characteristics: Most are yellow, but anthocyanidins are responsible for red/blue flower colors. They can be modified by hydroxylation, glycosylation (attachment of sugars, commonly at position 3 or 7), alkylation, and methoxylation. Over 62,000 glycosides of flavones and flavonols have been isolated.
- Spectral Characteristics: Flavones and flavonols show two major UV absorption bands (Band I for B-ring, Band II for A-ring), which can shift based on functional groups. Flavanones have a saturated C-ring and distinct UV profiles. Anthocyanins show distinctive peaks in the 450-560nm range.
- Nomenclature: Flavonoids are named using trivial names (often related to plant source or structure), semi-systematic chemical names based on parent structures, or systematic chemical names (rarely used due to complexity).
- Biosynthesis and Synthesis: Flavonoid biosynthesis is part of the phenylpropanoid pathway, starting from phenylalanine and malonyl-CoA. Chalcones are the first flavonoids formed. Various laboratory methods exist for synthesizing different flavonoid classes, including flavones, aurones, and dihydroflavonols, often involving specific chemical reactions like aldol condensations and oxidations.
- Solubility and Toxicity: Flavonoid aglycones have low water solubility, which, combined with short intestinal residence time and lower absorption, limits acute toxic effects in humans (except for rare allergies). This low solubility can pose challenges for medicinal applications.