Molecular Complex of Glycyrrhizic Acid Monoammonium Salt with Cholesterol

Background: Glycyrrhizic Acid (GA) is the major triterpene saponin of licorice roots. The most important derivative of GA is its monoammonium salt (glycyram, GC). Some pharmacological properties of triterpene saponins explain their molecular complexation with Cholesterol (Chol). However, the molecular complexation of GC with Chol has not been proven. The functional groups of GA and GC involved in the interactions with Chol were not identified. Methods: The complexation has been investigated by the method of isomolar series in the spectrophotometric version, IR, and 13C NMR spectroscopy. The constant is calculated on the basis of isomolar curves. Results: The molecular complex of GC with Chol has been prepared for the first time. It has been shown that GC forms a 1 : 1 complex with Chol having a stability constant Ks of (3.3 ± 0.2)x10⁵ (mol/L)-1 (in 70% aqueous EtOH at 18 °C). Conclusion: Intermolecular interaction in the complex is carried out by hydrogen bond formation between C=O group of GC (in carboxyl group of the terminal residue of glucuronic acid in the carbohydrate part) and 3β-hydroxyl group of Chol: -C=O...H-O-. Hydrophobic contacts of the aglycone part of GC with a lipophilic Chol molecule are possible.