An overview of the two-phase solvent systems used in the countercurrent separation of phenylethanoid glycosides and iridoids and their biological relevance

Phenylethanoid glycosides (PhGs) and iridoids are two vast groups of water soluble secondary metabolites widely distributed in plant kingdom and well known for their important biological activities. Their purification by conventional chromatography is time consuming, uses large amounts of organic solvents and requires repeated steps. Moreover, the sample recovery is low, because the hydroxyl groups of PhGs and iridoids make them strongly adsorbed onto the solid support (silica gel, Sephadex LH-20) during separation. Being a liquid–liquid based technology, countercurrent separation (CCS) comes as an alternative tool to overcome the issues associated with solid-phase adsorbents. It has been successfully used for the separation of many groups of specialized plant metabolites. This is the first extensive review describing the application of CCS for purification of iridoids and PhGs, based on the research papers from the last 20 years (1998–2018) that used countercurrent chromatography and centrifugal partition chromatography technologies. In total, 65 papers described the isolation of 84 different phytochemicals (28 PhGs and 56 iridoids) from 42 plant species belonging to 16 distinct families that were separated with 59 different biphasic solvent systems. Since PhGs and iridoids are highly polar molecules, more than half of the employed systems (56%) were composed of various ratios of ethyl acetate–alcohol–water, as well as binary systems, such as ethyl acetate–water and n-butanol–water. The current review may be used as starting point for CCS users on their sinuous road of isolating known or waiting to be discovered PhGs and iridoids.