Liquid - liquid extraction of tropane alkaloids


Alkaloids are a diverse group of compounds that are found primarily in plants and contain basic nitrogen atom. The basic nature of these compounds makes them exists mostly as salts.

Choosing extraction conditions for isolation of a product from its natural source depends on both the properties of the compound and the composition of the source. Tro
pane alkaloid free bases are soluble in organic solvents and acids, but poorly soluble in water. It is possible to extract these alkaloids from plant parts e.g. leaves by solid/liquid extraction to hot water.

Designing an efficient extraction scheme requires analyzing the major components of leaves. These include cellulose, proteins and amino acids, tannins, pigments and saponins.

  • Cellulose

The major component of leaves is cellulose, which serves as a rigid and insoluble structural element. Cellulose is a linear polymer composed of D-glucopyranose units
connected through carbons 1 and 4. The fully equatorial conformation of β-linked glucopyranose residues stabilize the chair structure of the ring and minimize its flexibility. Although cellulose carries many hydroxyl groups, it is not water soluble due to its high molecular weight.

  • Tannins

Tannins are polyphenolic compounds with molecular weights of 50-20,000. Tannins are soluble in water and therefore extracted from the leaf. The core structure is D-glucose, to which several units of gallic acid are attached, via ester bonds. It is important to note the gallic acid is poly functional, and able to form multiple ester bonds.


The presence of soluble tannins in leaves complicates the isolation of alkaloids, as low molecular weight tannins are also soluble in methylene chloride. However, a advantage can be taken of the chemical reactivity of the ester bonds in hydrolyzble tannins. When tea leaves are boiled in the presence of weak bases, such as CaCO3, the ester bond is cleaved. This cleavage produces glucose and a calcium salt of gallic acid. These very polar compounds will stay in water and will not be extracted into the methylene chloride. Additionally, the base also converts alkaloid molecules the may be present as salts to the free base, increasing its solubility in methylene chloride.

  • Proteins and pigments
Proteins and pigments are very soluble in water and therefore do not present problems to the separation of alkaloids by extraction.

  • Saponins
Saponins are compounds that have amphiphilic structure, i.e. molecules having a polar water-soluble group attached to a water-insoluble hydrocarbon moiety. This amphiphilic nature gives saponins detergent-like properties, which increases the solubility of organic molecules in water. Saponins may therefore induce the formation of emulsions during extraction, complicating the separation. Emulsions can be broken by addition of salt to the aqueous layer (therefore increasing the polarity of the aqueous phase and reducing the solubility of organic molecules in it) or by centrifugation.

Process:

  1. The powdered or fresh material is moistened with water and mixed with lime which combines with acids, tannins and other phenolic substances and sets free the alkaloids (if they exist in the plant as salts).

  2. Insoluble materials like cellulose are removed in filtration.
  3. Extraction is then carried out with organic solvents such as methylene chloride, chloroform, ether etc. It is a good practice to repeat the extraction 3-4 times to ensure most of the alkaloids are in the organic liquid.
  4. The water portion is discarded.

  5. The concentrated organic liquid is evaporated and alkaloids are left behind.    

Fig. Schematic representation of tropane alkaloid extraction