Determination of binding constant of molecular complex between β-cyclodextrin and bisphenol a by using 1H NMR spectroscopy

Abstract

The inclusion complexation behaviour of bisphenol A (BPA) into hydrophobic cavity of β-cyclodextrin (β-CD) was investigated by using 1H NMR spectroscopy in deuterium oxide by varying the molar ratios between β-CD and BPA from 1:0 to 1:2. The ideal molar ratio for the β-CD:BPA complex was determined as 1:1. In addition, the inclusion of BPA into β-CD produced significant changes in the chemical shifts of H5 and H3 protons, which were located inside the cavity of cyclodextrin. On the other hand, the H2, H4 and H6 protons that were located at the exterior surface of β-CD did not result in any significant changes in the chemical shift, and thus confirmed the formation of the β-CD:BPA inclusion complex. The observed chemical shifts of H5 and H3 protons, when BPA interacted with the β-CD cavity, were utilised to determine the binding association constant (Ka) and maximum chemical shift difference (Δmax). From the nonlinear calculation, the Ka for H3 proton, i.e. 4.10 x 103 M-1, was shown to be stronger than that of H5, i.e. 3.62 x 103 M-1. However, the H5 proton gave a higher Δmax than the H3 proton, which were 0.1412 ppm and 0.0573 ppm, respectively.