Unit 4: Phytochemistry and Analytical Pharmacognosy
Active principles and methods of their testing
Active Principles (Phytochemicals/Secondary Metabolites)
Active Principles: These are the specific chemical compounds found in a medicinal plant that are responsible for its therapeutic (healing) effect.
They are also called secondary metabolites because they are not essential for the plant's basic survival (like primary metabolites - carbs, proteins), but are instead produced for defense, attraction, or signaling.
Major classes of active principles include:
Alkaloids: Nitrogen-containing, basic compounds. Often have powerful physiological effects. (e.g., Morphine from Opium, Quinine from Cinchona, Reserpine from Sarpagandha).
Glycosides: A molecule where a sugar (glycone) is bound to a non-sugar (aglycone). The "aglycone" part is usually the active component. (e.g., Digitoxin from Foxglove).
Flavonoids: A large group of phenolic compounds. Often responsible for plant pigments (yellow, red, blue). Many are strong antioxidants. (e.g., Quercetin in onions).
Terpenoids: A large class of lipids built from isoprene units. (e.g., Menthol from mint, Taxol from yew, Cannabinoids from cannabis).
Phenolic Compounds: Contain a phenol ring. (e.g., Tannins, which are astringent; Salicylic acid from Willow bark).
Methods of their Testing (Extraction & Analysis)
Testing involves extracting these compounds from the plant and then identifying/quantifying them.
Extraction: The plant material (crude drug) is dried, powdered, and then mixed with a solvent (e.g., alcohol, water, ether) to dissolve the active principles. The liquid extract is then filtered and concentrated.
Analysis & Testing:
Phytochemical Screening: Simple, qualitative test-tube reactions to check *if* a class of compound is present (see topic 6).
Chromatography: The primary tool for separation and identification.
TLC (Thin Layer Chromatography): A simple, fast method to separate compounds on a coated plate. Used for identification.
HPLC (High-Performance Liquid Chromatography): A sophisticated, high-pressure, quantitative method. It can separate, identify, and tell you *how much* of a compound is present. This is the industry standard for quality control.
Spectroscopy: Used to determine the chemical structure of an isolated compound (e.g., UV-Vis Spectroscopy, Mass Spectrometry).
Management of herbal medicines
This refers to the proper handling of medicinal plants from collection to consumption to ensure safety, quality, and efficacy.
Collection: Plants must be collected at the right time of year and day, and from the correct species. They must be free from pollution (e.g., pesticides, heavy metals).
Drying: Crude drugs must be dried properly (e.g., shade-drying, sun-drying) to reduce moisture, prevent microbial spoilage, and preserve the active principles.
Storage: Dried drugs must be stored in cool, dry, dark conditions in airtight containers to protect them from moisture, insects, rodents, and light (which can degrade compounds).
Processing: Proper cleaning, garbling (removal of foreign matter), and size reduction (powdering).
Standardization: Ensuring that every batch of the final herbal product (e.g., a capsule) has the same quality and the same concentration of active principles.
Drug adulteration (types)
Adulteration: The practice of substituting the original crude drug with other, similar-looking substances that are of inferior quality, less effective, or harmful. This is usually done for financial gain.
Types of Adulteration
Substitution with Inferior Varieties: Using a related, but cheaper, species of the plant that has fewer active principles.
Substitution with Exhausted Drugs: Using a drug where the active principles have already been extracted (e.g., using "spent" ginger or cloves that have already had their essential oils removed).
Addition of Foreign Organic Matter: Mixing the drug with worthless plant parts (e.g., adding stems to a leaf drug, or adding mango-seed powder to amla powder).
Addition of Artificial Substitutes: Using man-made materials to mimic the drug (e.g., adding small, artificially-colored pebbles to look like cardamom seeds).
Addition of Harmful Substances: Adding toxic materials to enhance color or weight (e.g., adding brick powder to chili powder, or lead chromate to turmeric).
Methods of drug evaluation
Drug evaluation is the process of quality control to confirm the identity, purity, and quality of a crude drug and to detect adulteration.
Organoleptic Evaluation:
Uses the sensory organs (sight, smell, taste, touch).
Checks the drug's color, odor, taste, size, shape, and texture.
Example: Smelling cinnamon bark, tasting a licorice root for sweetness.
Microscopic Evaluation:
Uses a microscope to examine the fine cellular structure of the drug.
Looks for specific, diagnostic features (e.g., types of stomata, trichomes (hairs), stone cells, calcium oxalate crystals).
Example: Identifying senna leaf powder by its unique unicellular trichomes.
Physical Evaluation:
Uses physical constants.
Examples: Measuring moisture content (should be low), ash value (total ash, acid-insoluble ash – high ash indicates soil/sand adulteration), and extractive value (how much dissolves in a specific solvent).
Chemical Evaluation:
Simple chemical tests (phytochemical screening) to identify the class of active principles (see topic 6).
Quantitative chemical assays to determine the *amount* of active principle (e.g., total alkaloid content).
Chromatographic & Spectroscopic Evaluation:
The most advanced method. Uses techniques like TLC and HPLC to create a chemical "fingerprint" of the drug.
This fingerprint can be compared to a standard, authentic sample to confirm identity and purity.
WHO guidelines for the assessment of herbal medicines
The World Health Organization (WHO) has established guidelines to help countries regulate and assess herbal medicines, ensuring they are safe, effective, and of good quality.
Key points from the guidelines include:
Quality Control: The herbal product must be fully characterized. This includes:
Botanical Identity: Proper species identification (macroscopic, microscopic, and chemical).
Purity: Must be tested for and be within safe limits for contaminants like pesticides, heavy metals (lead, mercury), microbial contamination (bacteria, fungi), and mycotoxins (aflatoxins).
Assay: The *quantity* of the main active principles should be determined and standardized (e.g., "each capsule contains 5 mg of reserpine").
Safety & Efficacy:
Traditional Use: If a product has been used traditionally for a long time without harm, this can be accepted as proof of safety.
Clinical Trials: For new herbal products or new claims, scientific evidence (toxicology studies, clinical trials) is required, similar to modern drugs.
Labeling: All products must be labeled accurately with the botanical name of the plant(s), the part used, the quantity, and clear indications for use and contraindications.
Phytochemical screening tests for secondary metabolites
These are simple, rapid, qualitative (test-tube) tests used in the lab to detect the *presence* of different classes of secondary metabolites in a plant extract.