Abstract
A natural pentacyclic triterpenoid, oleanolic acid 1 and its biotransformed metabolites 2 and 3 are potential alpha-glucosidase inhibitors. To elucidate the inhibitory mechanism of compounds 1-3 against alpha-glucosidase, (i) their electronic and optical properties were calculated using DFT and TD-DFT methods at the B3LYP/6-31G(d) level of theory in gas and IEF-PCM solvent; and (ii) their binding energies to alpha-glucosidase were determined via a docking study. DFT results showed that the alpha-glucosidase inhibtion is mainly dependent on the polarity parameters of the studied compounds. Docking results revealed that the activity is increased with binding energies (i.e. the stability of ligand-receptor complex). The NMR spectroscopic data revealed that C-13 and H-1 chemical shifts of 1 and its hydroxylated metabolites 2 and 3 are well predicted with R-2 of 99% and 90%, respectively. The UV/vis spectra of substrate 1 and its transformed products 2 and 3 are well reproduced. The detailed assignments of H-1 and C-13 chemical shifts, and bathochromic shift of lambda(MAX) absorption bands have been presented.