Inclusion of Alkali Metal Ions by Permethylated Inulin Studied by Electrospray Ionization Mass and Multinuclear Nuclear Magnetic Resonance Spectrometric Approaches

Abstract

Inclusions of alkali metal ions by permethylated inulin (MeInu) were examined by electrospray ionization (ESI) mass spectrometry and multinuclear nuclear magnetic resonance (NMR) spectroscopy. Complexes of MeInu with metal ions were observed as multicharged ions in ESI mass spectra. The number of fructofuranose units of MeInu binding to one metal ion was 12, 6, and 7 for Li⁺, K⁺, and Cs⁺, respectively, on the basis of ESI mass spectra. However, the number estimated on the basis of multinuclear NMR spectra was 2 and 3-4 for Li⁺ and K⁺, respectively. The binding ability of MeInu to Li⁺ is very weak, so metal ions dissociate from the complex in the ionization process. As a result, the ratio of fructofuranose units to Li⁺ ions might have been estimated in excess. In the case of K⁺ and Cs⁺, the number of fructofuranose units binding a metal ion is found to be in good agreement with that of the oxyethylene units of crown ethers in solution.