Pharmacognosy and Natural Products Chemistry
Pharmacognosy and Natural Products Chemistry tests the chemical classes, biosynthesis, extraction, and quality control of medicinal plants. Expect questions linking a plant source to a marker compound, a pharmacological class, and an analytical method.
Core rules
- Major classes: alkaloids are nitrogenous and basic, such as morphine, quinine, and vincristine; terpenoids are isoprene-derived, such as artemisinin, taxol, and digoxin; phenolics arise from shikimate or phenylpropanoid pathways and include flavonoids and tannins.
- Biosynthetic routes: mevalonate and MEP pathways for terpenes; shikimate for aromatic phenolics; amino acids for many alkaloids.
- Alkaloid detection uses Dragendorff, Mayer, or Wagner reagents; flavonoids use aluminum chloride; tannins use ferric chloride; cardiac glycosides use Keller–Kiliani.
- Extraction should match polarity: maceration or percolation, Soxhlet for heat-stable compounds, supercritical carbon dioxide for nonpolar thermolabile compounds.
- Quality control uses botanical identity, HPLC or UPLC fingerprinting, marker quantification, heavy metals, pesticide residues, and aflatoxins.
Worked example
A leaf extract gives a positive Dragendorff test and a major HPLC peak. Reasoning: Dragendorff suggests alkaloids, but it is not specific enough for identification. Confirm with LC–MS/MS, compare against a reference standard, and check marker stability. If the active constituent is unknown, a fingerprint plus bioassay is more defensible than quantifying an arbitrary marker.
Traps
Assuming the marker compound is the active compound; confusing closely related species; ignoring seasonal and geographic variability; using a single color test as proof of identity.