Sources and Chemistry of Flavonoids; their Biological and Therapeutic Potential

Flavonoids are natural polyphenolic compounds, which are responsible for the taste and colors of medicinal plants, herbs, fruits, vegetables. Fruits (e.g., Berries, cherries, plums, apples, lemons, oranges, and grapes etc) and vegetables (e.g., broad beans, olives, onions, spinach, shallot etc) are the main sources of flavonoids. They also exist abundantly in cocoa products, black and green tea, red wine, red pepper, chamomile, celery, parsley, ginkgo, and mint. Flavonoids derivatives can also be synthesized through esterification, halogenation, alkoxylation, alkylation, aromatic hydroxylation, acylation and conjugation with various organic compounds. Flavonoids can be supplemented in a staple of food as nutraceutical agents and have an important role in human diet. They possess diverse biological activities including anti-inflammation, anti-oxidation, anti-cancer, anti-diabetes, anti-obesity, antimutagenic, neuroprotective and also have beneficial effects on oxidative stress, inflammation, insulin resistance, lipid metabolism and neurodegenerative diseases (e.g., amyotrophic lateral sclerosis, Huntington’s disease, Parkinson’s disease and Alzheimer’s disease). Flavonoids contain a 15-carbon skeleton; the basic structure consists of a flavan nucleus, a combination of two benzene and one pyran rings. Flavonoids are divided into eight important groups i.e., flavones, flavanols, isoflavones, flavan-3-ols, flavanonols, anthocyanidins, chalcones, and flavanones. A structure-activity relationship exists between flavonoids and their antioxidant activities. Flavonoids are effective in chelating metal ions and scavenging free radicals. The antioxidant properties of flavonoids are governed by their –OH groups, differences in hydrophobicity and molecular planarity.