Alkaloids are a class of natural compounds containing nitrogen with physiological effects, most of which have a more complex nitrogen heterocyclic structure, and a few are non-nitrogen heterocyclic organic amines. Although certain vitamins, amino acids and peptides of natural origin are also nitrogen-containing compounds, they are customarily not included in the category of alkaloids. Alkaloids are produced by a wide variety of organisms, including bacteria, fungi, plants and animals. Generally speaking, alkaloids are more distributed in dicotyledonous plants, less in monocotyledonous plants, gymnosperms and lambsquarters or lower plants are very few. Plants that contain alkaloids often contain multiple alkaloids with similar chemical structures. Plants with similar family affinities often contain alkaloids with similar, and in some cases identical, structures.
There are several ways to classify alkaloids, either by plant origin, physiological effect, or chemical structure. The common types are pyrroles, pyridines, quinolines, isoquinolines, indoles, imidazoles, purines, scopolamine, terpenoids, steroids, organic amines, etc.
There are many methods to extract alkaloids, generally the crushed sample is extracted with water, dilute acid water or ethanol first, the extract is concentrated, alkalized and then the total alkaloids are extracted with organic solvent. Alternatively, the crushed sample is first alkalinized, and then the total alkaloids are extracted with organic solvents. The total alkaloids were separated by various methods, such as chromatography and preparation of derivatives. A few alkaloids that are volatile or sublimable can be purified by water vapor distillation or sublimation.
Alkaloids have a wide range of pharmacological activities, including antimalarial, antiasthmatic, anticancer, cholinomimetic, vasodilator, antiarrhythmic, analgesic, antibacterial, and hypoglycemic agents. It also has psychotropic and euphoric effects and has been used in refreshment rituals or as a recreational drug.
All alkaloid components (with a few exceptions) react with a variety of alkaloid precipitation reagents in acidic solutions to produce a precipitation reaction. The operation is as follows.
Take four copies of the acidic solution infusion, respectively, drop by drop iodine-potassium iodide, potassium mercury iodide reagent, potassium bismuth iodide reagent, silicotungstic acid reagent. If all four or most of them have precipitation reaction, indicating that the sample may contain alkaloids, and then the next test, further identification.
a. Mercury potassium iodide reagent (Mayer's reagent): the acid-water extract was added with potassium mercury iodide reagent drop by drop to produce a white precipitate.
b. Potassium bismuth iodide reagent (Dragendorff's reagent): Acid water extract is added with potassium bismuth iodide reagent drop by drop to produce an orange or red-brown precipitate.
c. Iodine-potassium iodide reagent (Wagner's reagent): Acid-water extraction solution is added with iodine-potassium iodide reagent drop by drop to produce a brown precipitate.
d. Silicotungstic acid reagent: Acid-water extract is added dropwise with silicotungstic acid reagent to produce a light yellow or off-white precipitate.
This acid aqueous extract with the above four reagents are (or mostly) produced precipitation reaction, that is, the sample is predicted to contain alkaloids.