Traditional Chinese medicine: saponins, critical micellar concentrations and partition coefficients

Traditional Chinese medicine (TCM) revolves around complex mixtures bound to specific roles within the formulation, among which saponin-containing plants with alleged properties of harmonising or detoxifying other compounds present in the preparations.

This article deals with the study of these interactions with, as a model, the interaction between saponins and selected active principles.

The measurement of the partition coefficient between water and octanol (logP) was used as an indicator and determined by nuclear magnetic resonance (NMR) for these active principles in the presence of saponins. For each compound, a graph was constructed showing the evolution of logP with increasing concentrations of saponins.

Four distinct patterns of interactions were distinguished. Pattern A showed a constant decrease of logP, pattern B showed a decrease followed by a plateau, in pattern C the logP did not vary until the critical micellar concentration (CMC) and decreased afterwards, and pattern D exhibited an increase of logP. These properties were linked to the ability of saponins to form micelles in water once the CMC is reached. The interaction of aconitine and saponins followed pattern D, thus explaining the detoxification of herbal preparations using Aconitum with licorice. The licorice facilitated the extraction of the notoriously water-insoluble artemisinin from Artemisia annua.

This investigation confirms that the physical properties of micelle forming saponins are intimately linked to a modification of behaviour of the other molecules in solution, as seen with the alteration of logP and the four types of interactions presented.