The inclusion complex of rosmarinic acid into beta-cyclodextrin: A thermodynamic and structural analysis by NMR and capillary electrophoresis

This work focuses on the characterization of the rosmarinic acid (RA)-β-cyclodextrin (CD) complex in aqueous solution by ¹H NMR (1D- and 2D-ROESY), completed with studies by capillary electrophoresis (CE). From the ¹H NMR data, the stoichiometry of the complex was determined by a Job's plot and the binding constant was estimated from a linear regression (Scott's method). At pH 2.9, the results showed that RA binds CD with a 1:1 stoichiometry and a binding constant K_b of 445 (±53) M^-1 or 465 (±81) M^-1 depending on the CD protons (H-5 or H-3) selected for the evaluation. The K_b value was also calculated from the CD-induced chemical shifts of each RA proton in order to collect information on the structure of the complex. The pH dependence of K_b revealed that the RA carboxylic form displays the highest affinity for CD. An investigation by capillary electrophoresis fully confirmed these results. 2D ROESY analysis provided detailed structural information on the complex and showed a strong correlation between H-3 and H-5 of CD and most RA protons. In conclusion, RA, an efficient phenolic antioxidant from rosemary with a marketing authorization, spontaneously forms a relatively stable inclusion complex with CD in water.