Spectroscopic analysis on structure-affinity relationship in the interactions of different oleanane-type triterpenoids with bovine serum albumin.

Oleanane-type triterpenoids serve as an important group of plant secondary metabolites with a variety of biological activities and the C-3 position substitution pattern is a significant structural feature for their biological activities. Three selected oleanane-type triterpenoids (glycyrrhizin, glycyrrhetinic acid, and carbenoxolone) bearing different substituents (glucuronic acid dimer, hydroxyl, and succinyl groups) at the C-3 position were studied for their affinities to bind bovine serum albumin (BSA) by steady-state fluorescence, synchronous, three-dimensional fluorescence and ultraviolet-visible (UV-vis) absorption spectra. The binding mechanism of the triterpenoids to BSA is due to the formation of the triterpenoids-BSA complex and the binding affinity is strongest for carbenoxolone and ranked in the order carbenoxolone > glycyrrhetinic acid > glycyrrhizin. The thermodynamic parameters calculated at different temperatures showed that triterpenoids binding to BSA primarily depended on hydrophobic interaction and hydrogen bonding. The distance between the bound triterpenoid and BSA was determined on the basis of the Förster's energy transfer theory. Displacement experiments using phenylbutazone and ibuprofen showed the binding site of triterpenoids on BSA at subdomain IIA (Sudlow's site I). The effect of triterpenoids on BSA conformation was analyzed by UV-vis absorption, and synchronous and three-dimensional fluorescence spectra. These results revealed that the C-3 position substitution pattern significantly affects the structure-affinity relationships of oleanane-type triterpenoid binding to BSA and further affects the bioavailability of triterpenoids in the blood circulatory system.