Purification of Alkaloids by Countercurrent Chromatography

Alkaloids are nitrogen-containing compounds of the secondary metabolism of plants and microorganisms, which can also be found in animals, insects, and marine organisms. Because of the presence of one or more nitrogen atoms in the molecule, these compounds can form salts in the presence of acids, which are soluble in water and not in organic solvents. This characteristic is useful for their extraction from the matrix source and is explored in liquid-liquid partitioning techniques like countercurrent chromatography. Solvent systems used in classic purifications of alkaloids by this separation technique consist of an organic solvent such as chloroform or dichloromethane and a water buffer where the pH varies along the purification. The addition of other organic solvents like methanol and other alcohols to the solvent system is discussed, as well as the use of less polar systems. Recently developed techniques in countercurrent separations, such as pH-zone-refining countercurrent chromatography, are also presented. This comprehensive review covers the early work on separation of alkaloids with the Craig and Post apparatus and the evolution in the use of modern equipment for the isolation and purification of this class of bioactive natural products.