Complexation characteristics of permethylated cycloinulohexaose, cycloinuloheptaose, and cycloinulooctaose with metal cations

Abstract

The host–guest complexation behavior of cyclic oligosaccharides, permethylated cycloinulohexaose 1, permethylated cycloinuloheptaose 2, and permethylated cycloinulooctaose 3 with metal cations has been characterized by means of UV-visible, NMR, and electrospray ionization (ESI) mass spectrometry. In the crystal state, the structures of 1·K⁺, 1·Rb⁺, and 1·Cs⁺ were same as that of the 1·Ba²⁺ complex which has a u-u-d-u-u-d (u = up, d = down) furanose ring arrangement for the plane of the crown ring moiety. The association constants (K_S) in THF and in [²H₆]acetone at 298 K were evaluated. The binding ability of host 1 with metal cations was of the same degree as that of calix[6]arene derivative 4 and much higher than those of hosts 2 and 3. The thermodynamic parameters of the complexation of host 1 with metal cation in THF were determined, and it was suggested that the entropy change for the solvation of the metal cations was one of the important factors in the complexation equilibrium. It was clarified that the structure of the host 1·K⁺ complex in solution at low temperature (furanose ring arrangement: u-d-u-d-u-d) was different from that in the crystal state (u-u-d-u-u-d arrangement) by the coalescence behavior in ¹H-NMR. The relative peak intensity of the complex ions of host 1 or 2 with two alkali metal ions in ESI mass spectrometry (in acetone) showed a correlation in the first order approximation with the ratio of the corresponding complex ion concentrations estimated from the K_S values.