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Resin Adsorption Technology in Extraction of Alkaloids

Alkaloids, also known as organic bases, are widely distributed in many plant materials, and most of them have significant physiological activities. There are many kinds of alkaloids, and the structure is very complex. It has a lipophilic alkaloid and a hydrophilic alkaloid, but its commonality is alkaline and can be salty with acid. Its alkalinity can be characterized by the value of its conjugated acid pKa, and the pKa value of alkaloids is characterized. The pKa value of alkaloids can be changed from 0 to 13, indicating that their alkalinity is very different.

The molecular characteristics of 10-hydroxycamptothecin usually have large hydrophobic parts and are adsorbed according to the hydrophobic mechanism. On the one hand, the amino group has a certain alkalinity. On the other hand, the nitrogen atom has unpaired electrons, which can form hydrogen bonds with the hydrogen on the hydroxyl group, and can also be complex with some metal ions. It can be imagined that the alkaloids can have a variety of separation methods, extraction, ion exchange or adsorption are alternative methods, and there are many alternative resins in the adsorption process. In principle, the extraction process is more complicated, and the ion exchange method can not be eluted by simple organic solvent. The adsorption method is a better method.
The separation of camptothecin is a typical example. In terms of molecular structure, camptothecin is a weak base and has strong hydrophobicity under neutral and alkaline conditions. It is feasible to use non polar resin to adsorb. The extraction method is to crush the fruit of Camptotheca acuminata, extract, filter, and recover ethanol. The pH of the aqueous solution is 8, to reduce its water solubility. Then, the water solubility of the tree base is poor with the adsorption of AB-8 resin. The desorption effect of acetic acid solution is very poor, and the desorption rate of the chloroform / ethanol solution is high, and the elution peak is very concentrated. When the pH of desorption solution is adjusted to 3, the desorption rate can reach 96%. The concentration of camptothecin with purity above 90% can be obtained by condensation, drying and recrystallization with chloroform / methanol (1:1). 
Camptothecin can also be adsorbed by sulfonic acid cation exchange resin. Because of strong adsorption capacity, it must be eluted by alkaline organic solvent. The adsorption of weak acid carboxylic acid resin is similar to that of general adsorption resin. The advantage of using ion exchange resin adsorption is to reduce the adsorption of hydrophobic impurities. However, impurities such as amino acids and hyaluronic acids in the extract may be adsorbed. Considering the effect of practical application, no matter which resin should be used, it is necessary to match the corresponding process so as to give full play to its advantages and avoid its disadvantages.
The separation of berberine can also be used by adsorption resin method. Berberine, is a quaternary amine alkali, with strong alkalinity, widely distributed in Coptis chinensis, three needles, and many other plants. Because of its strong alkalinity and water solubility, it can be extracted with water. For example, 10 grams of three needle coarse powder is percolated by 10%HzS04 to a weak alkaloid reaction, collecting about 160mL liquid, neutralizing to neutral by 10%NaOH, and after filtration about 200mL, through the column containing 5g non polar adsorption resin, after the flow is washed to the effluent, it is alkaloid reaction. Berberine was washed out with water without alkaloid reaction. The yellowish powder o.59g is steamed from the eluate. With methanol elution, the brown rubber o.33g was obtained. The content of berberine in the two products was 97% of the crude drug content.
Scopolamine is the main alkaloid of belladonna root and leaf. The water solution of scopolamine is adjusted to pH>6, the sediment is filtered and adsorbed with XAD-4 resin. Then, the pure products of scopolamine can be extracted with chloroform.
Caffeine is a stimulant of central nervous system. Caffeine is removed when tea polyphenols are extracted from tea leaves. Tea polyphenols contain --OH, while caffeine contains amino and carbonyl groups, so using the adsorption resin containing --OH and caffeine to form hydrogen bonds, the phenol hydroxyl - containing polystyrene adsorption resin can be used to adsorb caffeine. Resins containing --S03 H can also form hydrogen bonds with caffeine.