Aggregate formation of glycyrrhizic acid

Glycyrrhizic acid is derived from the licorice root. The compound is amphiphilic with a large hydrophobic aglycone group, and two glucuronic acids and a carboxyl group constituting the hydrophilic part. Therefore, glycyrrhizic acid can form aggregates or lower the surface tension of aqueous solutions owing to its specific amphiphilic structure. The aqueous solubility of glycyrrhizic acid is relatively low (0.15 mM) and dependent on the solvent pH due to the weak acidity resulting from the carboxylic groups. Glycyrrhizic acid is quite soluble in a buffer solution at pH 4.5, and its ability to affect the surface tension of the buffer solution increases from pH 5. The critical micelle concentration (cmc) was estimated to be 2.9 mM (γcmc = 55.2 mN/m) and 5.3 mM (γcmc = 56.8 mN/m) at pH 5 and 6, respectively. The surface tension also decreased gradually at pH 7, but the critical point was not observed in the curve. At pH 7, the pyrene fluorescent probe method and light scattering measurements did not show a clear cmc. Small angle X-ray scattering experiments revealed that the aggregates were rod-like micelles with an estimated radius and length of 1.5 nm and 21 nm, respectively, at 5 mM in a pH 5 solvent. Transmission electron microscopy confirmed that glycyrrhizic acid forms rod-like micelles. These results suggest that glycyrrhizic acid has potential applications as a biosurfactant in various fields.