Biochemistry of Natural Products Lecture 3: Flavonoids

Study Notes

Biochemistry of Natural Products, Lecture 3: Flavonoids

Flavonoids are polyphenolic secondary plant metabolites.
They comprise 15 carbons (2 aromatic rings connected with a 3-carbon bridge) and numerous hydroxyl groups (OH).
Flavonoids exhibit antioxidant properties by acting as free radical scavengers, singlet oxygen quenchers, hydrogen donors, and metal chelators.
Flavonoid sources include fruits and vegetables, tea, red wine, citrus fruits, cocoa, and berries. Specific flavonoid types within these sources include: flavanols, flavonols, flavanones, isoflavones, and anthocyanins (depending on the food type).
Flavonoids have several important biological properties.
- Anti-ulcer effects: promote mucus secretion and inhibit H. pylori bacteria.
- Anti-atherosclerotic effects: suppress LDL oxidation and inflammatory progression in arteries. Elevated LDL is a primary risk factor for atherosclerosis and coronary artery disease. Oxidized LDL reacts with free radicals, becoming more reactive and causing tissue damage; it also attracts immune cells (e.g. macrophages) and platelets to the damaged area exacerbating the process.
- Anti-inflammatory effects: inhibit the cyclooxygenase (COX) pathway and inflammatory mediators like arachidonic acid.
- Hepatoprotective activity: many flavonoids (e.g., silymarin, apigenin, quercetin, naringenin) protect the liver.
- Memory improvements: through various mechanisms like anti-neuroinflammatory actions, reducing metal ion neurotoxicity, promoting autophagy, reducing oxidative stress, mitigating mitochondrial dysfunction, and reducing amyloid-β and tau levels.

Physicochemical Properties, Extraction, Characterization, and Quantification

Flavonoid glycosides are water-soluble and soluble in alcohols.
Glycosides are extracted primarily using acetone or alcohols (ethanol, methanol) mixed with water, often at high temperatures.
Aglycones are soluble in apolar organic solvents (like dichloromethane).

Extraction and Characterization of Flavonoids

Extraction methods include maceration, Soxhlet, ultrasound-assisted extraction (UAE), supercritical fluid extraction (SFE), and accelerated solvent extraction (ASE).
Chromatographic techniques (HPLC, TLC) are used for analysis.

Detection and Quantification of Flavonoids

Flavonoids can be identified and quantified using various chemical tests.
- Shinoda, Vanillin-HCl, Zinc, and Ammonia tests produce specific color changes in the presence of flavonoids.