Remarkable selectivity of per-O-methylated tricationic 6A,6C,6E-tripyridinio-6A,6C,6E-trideoxy-α-cyclodextrin for basic anions over non-basic anions

Abstract

The per-O-methylated tricationic 6^A,6^C,6^E-tripyridinio-6^A,6^C,6^E-trideoxy-α-cyclodextrin shows a high affinity for basic anions, especially highly charged phosphate and pyrophosphate anions, at near-neutral pH with association constants of 7 000 and 9 000 M⁻¹, respectively, but it did not bind non-basic anions, such as I⁻, ClO₄⁻, SCN⁻ or even doubly charged SO₄²⁻, in contrast to the non-methylated counterpart. ¹H NMR spectral studies verify a plausible structure, in which the bound anion is located more closely to the pyridinio meta and para positions rather than to the ortho positions, due probably to the greater MeO- vs. HO-group hydrophobicity, which rejects deep intrusion of the anion into the positive cavity. Molecular mechanics calculations supported the above conclusions.