Binding characteristics of native cyclofructan 6 and its derivatives with metal ions

The binding ability of native, isopropylated and permethylated cyclofructan 6 (CF6) with various cations was characterised in water and hydro-organic solvents. Association constants (K_ass) of 24 cations including alkali metals, transition metals and lanthanides were measured using NMR titrations and a nonlinear least square treatment of the data. Ba²⁺ and Pb²⁺ showed the highest binding abilities with native CF6 in both D₂O and 40% methanol-d₄. Metal ion binding increased with an increasing in methanol content. The weaker binding of cations with CF6 in acetonitrile-d₃ compared with those of methanolic solvents was noted and indicated the importance of dehydration of the host and guest to binding affinity. The effect of counteranions such as chloride, nitrate, perchlorate and acetate on binding was examined. Isopropyl-CF6 provided slightly higher binding affinities with cations than native CF6 in D₂O and 40% methanol-d₄, whereas permethylated CF6 only associated with K⁺ cation in 80% methanol-d₄. There are distinctly different metal ion-binding selectivities between native and derivatised CF6.