NMR spectroscopic characterization of inclusion complexes comprising cyclodextrins and gallated catechins in aqueous solution: cavity size dependency

Abstract

The structure of inclusion complexes of gamma-cyclodextrin (gamma-CD), (-)-gallocatechin gallate (GCg), and (-)-epigallocatechin gallate (EGCg) in D(2)O was investigated using several NMR techniques. GCg formed a 1:1 inclusion complex with gamma-CD in which the A and C rings of GCg were inserted deep at the head of the A ring into the gamma-CD cavity from the wide secondary hydroxyl group side. In the 1:1 inclusion complex with GCg and gamma-CD, the GCg moiety maintained a conformation in which the B and B' rings of GCg took both pseudoequatorial positions with respect to the C ring. The structure of the inclusion complex of GCg and gamma-CD obtained from NMR experiments supported well that determined from PM6 semiempirical SCF MO calculations. However, (1)H NMR experiments suggested that EGCg did not form any inclusion complex with gamma-CD in D(2)O. The marked difference between GCg and EGCg in inclusion behavior toward gamma-CD may be explained in terms of the stabilization energy calculated with the PM6 method. .