Liquid chromatographic study of two structural isomeric pentacyclic triterpenes on reversed-phase stationary phase with hydroxypropyl-β-cyclodextrin as mobile phase additive

Retention behavior of two structural isomeric pentacyclic triterpenic acids, maslinic acid and corosolic acid, was investigated by reverse phase high performance liquid chromatography (HPLC) with hydroxypropyl-β-cyclodextrin (HP-β-CD) as mobile phase additive. Inclusion complexation of maslinic acid, corosolic acid with hydroxypropyl-β-cyclodextrin was evaluated under different concentration of hydroxypropyl-β-cyclodextrin. Apparent formation constant (Km) between methanol and hydroxypropyl-β-cyclodextrin was determined to be 13.82 L mol−1 under 25 °C using UV-spectrophotometry. Two retention models were employed individually for evaluation of inclusion complexation between the two pentacyclic triterpenic acids and hydroxypropyl-β-cyclodextrin. It was found that a higher apparent formation constant (Kf) for corosolic acid and hydroxypropyl-β-cyclodextrin was obtained, 19115 L mol−1, indicating that a greater affinity of hydroxypropyl-β-cyclodextrin with corosolic acid was produced compared with that of maslinic acid, 11775 L mol−1, in the selected mobile phase, and stoichiometric ratio for both of inclusion complex was found to be 1:1. Thermodynamic analysis showed that a negative standard enthalpy change (ΔH) and an entropy change (ΔS*) for analyte transfer were obtained, where ΔH of maslinic acid and corosolic acid was found to be −10.188 kJ mol−1 and −10.650 kJ mol−1, ΔS* of two compounds was −2.092 and −2.180, respectively, indicating that transfer of structural isomers from mobile phase to stationary phase was enthalpically driven. Meanwhile, positive values were obtained for standard enthalpy change and standard entropy change, 136 kJ mol−1 and 274 kJ mol−1 and 536 J mol−1 K−1and 1004 J mol−1 K−1, for inclusion complexation between maslinic acid, corosolic acid and hydroxypropyl-β-cyclodextrin, while negative values were obtained for Gibbs free energy during formation of inclusion complex, −160 kJ mol−1 and −299 kJ mol−1, indicating a spontaneous inclusion reaction happened.