Difference in formation mechanism of inclusion complex between configuration isomers of gallate-type catechin and β-cyclodextrin

Abstract

The formation mechanism of inclusion complex between (-)-epigallocatechin gallate (EGCg), which is the main catechin in tea leaves, or (-)-gallocatechin gallate (GCg), which is an isomer of EGCg, and β-cyclodextrin (βCD) was studied by isothermal titration microcalorimetry (ITC), NMR and molecular modeling calculation (MMC). ITC measurements revealed that EGCg or GCg interacted with βCD at 1:1 molar ratio driven by enthalpy. Comparing the values of binding constant (7.2 × 103 1/M for EGCg with βCD and 3.9 × 104 1/M for GCg with βCD), it was suggested that GCg interacted with βCD more strongly rather than EGCg. In their ROESY NMR spectra, the cross-peaks were observed between the protons of A, B and B′ ring of EGCg and the protons in the cavity of βCD, and between the protons of B and B′ ring of GCg and the protons in the cavity of βCD. MMC in water showed that EGCg had one kind and GCg had two kinds of most stable conformation (GCg(E) and GCg(A)) in energy. In the stable conformation of EGCg, B ring was coordinated equatorially to C ring, and B′ ring was axially coordinated to C ring. On the other hand, in GCg (E), the coordination of B ring and B′ ring to C ring was both equatorial, and in GCg (A), the coordination of B ring and B′ ring was both axial. MMC also revealed B or B′ ring of GCg(A) was easy to be included in the cavity of βCD more deeply than each ring of EGCg or GCg(E) since the steric hindrance of B ring or B′ ring was small. Therefore, it was found that the difference of B ring’s configuration of gallate-type catechin greatly affected the formation of inclusion complex with βCD.