Solution structures of 1:1 complexes of oxyphenonium bromide with beta- and gamma-cyclodextrins

The solution structures of complexes of oxyphenonium bromide (OB) with beta- and gamma-cyclodextrins (beta- and gamma-CDs, respectively) in deuterium oxide have been investigated by 500 MHz proton NMR spectroscopy and molecular mechanics calculations. The chemical shifts induced by complex formation provide the 1:1 binding constants and the chemical shift variations, DeltadeltaOB-CD, with complexation for the protons of OB and the CDs. The observed binding constants are very close to those obtained by other methods and are in the following order: beta-CD > gamma-CD > alpha-CD. Initial structures of the complexes are constructed on the basis of the ROESY spectra and the DeltadeltaOB-CD values and are optimized by molecular mechanics calculations. The intermolecular distances between the protons of OB and CD calculated for these structures are well-correlated with the observed ROESY intensities. The cyclohexyl group of OB penetrates deeply into a beta-CD cavity, and the phenyl group is close to the wide rim of the cavity. The phenyl and cyclohexyl groups of OB are both incorporated into a gamma-CD cavity. Furthermore, these structures of the complexes are consistent with the suppression of bitter taste and basic hydrolysis of OB by CDs and the polarity of binding sites of OB.